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Changes to Visual Parameters Following Virtual Reality Gameplay

Article in British and Irish Orthoptic Journal · June 2022

DOI: 10.22599/bioj.257

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Changes to Visual
Parameters Following
Virtual Reality Gameplay RESEARCH

SANJOG BANSTOLA
KERRY HANNA
ANNA O’CONNOR
*Author affiliations can be found in the back matter of this article

ABSTRACT CORRESPONDING AUTHOR:

Sanjog Banstola
Introduction: Virtual reality (VR) gameplay is popular with a range of games and
University of Liverpool, GB
educational resources available. However, it puts high demands on the visual system.
s.banstola@student.liverpool.
Current evidence shows conflicting impacts on visual parameters. Therefore, this study
ac.uk
explores the changes to vision following VR gameplay.

Methods: The study was conducted at the School of Health Sciences, University of
Liverpool. All participants had binocular vision with good visual acuity and no manifest KEYWORDS:
strabismus. Participants were assessed before and after playing 15 minutes of the Virtual Reality; Gameplay;
VR game Beat Saber, which incorporated convergence and divergence movements. Visual Changes; Vision
Clinical assessments including near point of convergence (NPC) and near point
of accommodation (NPA) using the RAF rule; accommodative convergence to TO CITE THIS ARTICLE:
accommodation (AC/A) ratio; motor fusion using the prism fusion range (at 33cm), Banstola, S, Hanna, K and
accommodation facility using +2.00/–2.00DS flipper lenses, and stereoacuity using the O’Connor, A. 2022. Changes to
Visual Parameters Following
Frisby stereo test were assessed before and after playing.
Virtual Reality Gameplay.
Results: Seventy-eight participants (19–25 years old) were included in the study, with British and Irish Orthoptic
16 males and 41 females respectively. The breakpoint of convergence reduced by Journal, 18(1), pp. 57–64. DOI:
https://doi.org/10.22599/
0.5 cm (p = 0.001). The binocular accommodative facility improved by 2 cycles per
bioj.257
minute (cpm); p = 0.004. The mean, near horizontal prism fusion range (PFR) base
break and recovery points both worsened by of 5.0 dioptres (p = 0.003), whereas the
mean near horizontal PFR base in recovery point improved by of 4.0 dioptres (p = 0.003).

Discussion: The study validated previous findings as VR gameplay over-exercised and

fatigued convergence muscles, but to a small degree. The VR experience improved the
participants’ ability to change focus quickly and improve accommodation, as well as
the divergence function of the eye. However, as the participants were retested directly
after the VR gameplay, the findings were limited to short term effects on vision.
Banstola et al. British and Irish Orthoptic Journal DOI: 10.22599/bioj.257 58

INTRODUCTION has been seen with stereopsis as dichoptic training in

patients with amblyopia (viewing of separate fields by
The use of virtual reality (VR) is becoming more each eye) has shown improved stereoacuity from 263.3
widespread, with the potential of the technology (±135.1) to 176.7 (±152.4) seconds of arc (p < 0.01)
ranging from entertainment and education to medical (Žiak et al. 2017), whilst a different study on healthy
interventions (Izard et al. 2018; Izard et al. 2017). VR participants has shown no improvement of stereopsis
is a computer-generated simulated experience that is from VR technology (Yoon et al. 2020). Therefore,
traditionally created by a head-mounted display with a it is likely that where patients have deficient visual
small screen for viewing (Martin 2017). It typically uses parameters at baseline, VR can support improvement
auditory and video feedback to give the user a sense of in clinical measures, whilst those with normal levels of
control over their virtual environment. The technology vision appear to be unchanged after VR use.
can be distinguished into immersive and non-immersive Besides vision function tests, previous research
experiences. For the purpose of this research immersive has found conflicting vergence and accommodation
VR technology is used, where the user is able to interact relationship associated with virtual reality (Iskander et
with the environment by moving around within it. al. 2019). Use of VR can lead to physical visual stress by
Given the visual demands of VR, there is an increasing affecting visual acuity and causing greater symptoms
body of evidence evaluating the impact on the visual such as headache, diplopia, blurred vision, sore eyes and
system, with studies showing impacts on monocular and eyestrain for the user (Yoon et al. 2020; Tychsen and
binocular vision, as well as eye alignment. Positive impacts Foeller 2020). The conflict arises due to the inability of the
have been shown in terms of eye alignment, where VR eyes to focus on the projection screen whilst converging
gameplay has shown the potential to treat intermittent on the virtual object. Furthermore, a study has shown
exotropia by improving the patients’ exotropia from strong correlation between symptomatic patients and
21.44 (±13.25 SD) to 7.2 (±8.54 SD) prism dioptres poor vision function tests, such as increased exophoria
(Δ) on the prism cover test (Li et al. 2019). Regarding and reduced AC/A ratio, following VR exposure (Morse
monocular vision, it has shown to improve visual acuity and Jiang 1999).
in the amblyopic eye by improving the best correct visual With the true effect of virtual reality still needing more
acuity from a mean value of 0.58 (±0.35) logMAR to 0.43 clarification, with previous studies showing improvements
(±0.38) logMAR (p < 0.01) (Žiak et al. 2017). However, of some visual parameters whilst worsening others,
the technology has shown to cause negative effects on finding how this technology affects vision remains an
monocular vision, as a study reported that it significantly objective that needs to be addressed. Hence, this study
worsened the near point of accommodation (NPA) of its will aim to provide greater clarification of the changes
users by 1.5cm (p < 0.01) in both of their dominant and to visual function measures that have shown conflicting
non-dominant eyes (Yoon et al. 2020). findings in previous literature (NPC, NPA, accommodative
With regards to binocular vision, VR gameplay facility, AC/A ratio, stereoacuity, and prism fusion range)
has shown conflicting effects on accommodation by by observing the changes to visual parameters before
improving binocular accommodative facility amongst and after 15 minutes of VR gameplay.
users with normal binocular function, where the eyes
were not exposed to fatigue, whilst also worsening the
accommodative-convergence/accommodation (AC/A) MATERIALS AND METHODS
ratio in others due to continual change in depth leading STUDY DESIGN AND SETTING
to an exo-shift of the horizontal phoria (Munsamy et A prospective observational cohort study was conducted
al. 2020; Mohamed Elias et al. 2019). Similarly, the VR using data from a year two undergraduate research
technology has been shown to impact the near point project module where eight student-led subgroups
of convergence (NPC) in convergence insufficiency (CI) collected the data, with close supervision from two
patients (Irving et al. 2017), where the training methods university lecturers. Volunteer participants were
require individuals to repeatedly converge and re-diverge. university students recruited using advertisements on
This has resulted in further improvements of one’s email, posters, and social media between February
binocular vision (the recovery point in near prism fusion and March 2019. The study involved collecting baseline
range from 27.1 (+ 14.2) Δ to 34.9 (+ 13.5) Δ). Therefore, information to screen for inclusion in the study, followed
the findings from this study suggest that VR could be by a visual assessment before and after 15 minutes of
useful in managing patients with CI, namely those with virtual reality gameplay, observing any changes to the
neurological and neurodegenerative conditions. visual function parameters. Fifteen minutes was the
However, it has also worsened NPC by 2 cm (p < 0.01) chosen duration, as this was both clinically feasible within
in healthy users with good binocular vision (Yoon et al. the constraints of this research study and was informed
2020; M. Y. Boon 2017). Furthermore, the same pattern by previous literature, which reported significant results
Banstola et al. British and Irish Orthoptic Journal DOI: 10.22599/bioj.257 59

to vision after only 10 minutes of VR gameplay (Szpak PFR using handheld prism bars, with blur, break, and
et al. 2020). The participants played Beat Saber, a VR recovery point measured for base in (BI) and base out
game where the user is required to slash blocks as they (BO); binocular accommodative facility using +2.00/–
come towards them (Games 2021). As a result, the game 2.00DS flipper lenses; AC/A ratio (calculated using the
required the person to make binocular convergence and gradient method); and the Frisby stereoacuity test (Frisby
redivergence movements. 2014). All measures were chosen based on previous
evidence outlining impact from VR use; however, the
PARTICIPANTS near horizontal prism fusion range has limited previous
Participants of the study were included following a evidence but was included to explore the impact and
screening process. The inclusion criteria consisted of: provide greater understanding.

• Aged between 18 and 40 years old to exclude cases DATA ANALYSIS

of presbyopia (Fricke et al. 2018) After the formation of the central database, the
• Presence of stereoacuity and no manifest strabismus spreadsheet was exported and analysed using Statistical
on cover test assessment. Package for the Social Sciences (SPSS) version 25 (IBM
• Good and equal visual acuity (VA), defined as a 2021). Analysis showed the data were not normally
difference of VA less than 0.1 logMAR between the distributed (Shapiro-Wilk), thus descriptive analysis
two eyes. Each eye must also achieve 0.2 logMAR. was conducted using medians and interquartile ranges
• Had mental capacity and the ability to consent. and illustrated with the number and percentage of
the demographical and screening data. Furthermore,
ETHICAL APPROVAL the visual parameters were analysed using a matched
All participants of the study were able to provide a non-parametric test, the Wilcoxon signed-rank test,
written informed consent after reading the patient exploring any significant changes due to VR gameplay.
information sheet before enrolling onto the experiment. Additionally, the Bonferroni correction method was
The study was approved by the Research Ethics Approval used to counteract the possibility of false significance
Committee of the University of Liverpool. due to multiple comparisons of the data (Curtin and
Schulz 1998). As 14 comparisons were planned to be
DATA COLLECTION carried out (thus producing the calculation 0.05/14 =
The study gathered data from eight subgroups, each 0.004), a p-value of less than or equal to 0.004 was
of which aimed to enroll 10 participants. Demographic considered statistically significant for the purposes of
data collected were age, gender, and past ocular history, this study.
whilst the clinical data included visual acuity, cover test,
and interpupillary distance (IPD). As part of the basic
visual assessment before and after VR gameplay, based RESULTS
on existing scientific evidence, each subgroup chose at DEMOGRAPHICS AND SCREENING
least three appropriate visual function parameters to test A total of 79 participants were recruited in the study and,
on their volunteers. The student assessors were required after the application of inclusion criteria and accounting
to select at least three outcome measures to meet the for one subgroup failing to recruit 10 participants, 78
needs of the undergraduate module assessment. The participants were included in the study. In addition,
module teaching staff, co-author (KH), closely supported various aspects of participant’s demographical and
the students to ensure relevant outcome measures were screening data were missing (Table 1); however, enough
selected for this study, supported by previous evidence data was recorded to be included in the analysis.
obtained earlier in the module through literature Furthermore, the screening measures between those
searching. However, as each group chose different visual with missing demographic data (gender and age) and
function tests, it was expected that each parameter would those without missing data produced no statistically
have missing values at the analysis stage. Each subgroup significant findings, therefore allowing for further
collected the data in a Microsoft Excel spreadsheet, from analysis of the experimental measures (VA right eye
which a central database containing all the screening p = 0.99, VA left eye p = 0.99, CT near p = 0.80, CT distance
and experimental data was later formed for the current p = 0.091). The median age of the participants was 20
study analysis. years (IQR 1.0 years) and there was a considerably greater
number of females compared to male participants (41
OUTCOME MEASURES vs 16 participants, respectively). Of the 38 participants
Changes in the visual parameters before and after VR that had their past ocular history recorded, the majority
gameplay included: NPC and NPA at break and recovery of the individuals did not have any history to disclose,
points using the RAF rule (Adler 2004); near horizontal whilst four reported a manifest strabismus (n = 1) and
Banstola et al. British and Irish Orthoptic Journal DOI: 10.22599/bioj.257 60

AFTER EXCLUSION* MISSING VALUES,

N = 78 N (%)

Age in year (median, IQR) 20.0 (1.0) 22 (27.5)

Gender, N (%) Male 16 (20.5) 22 (27.5)

Female 41 (52.6)

Past Ocular History, N (%) Present 4 (51.3) 40 (51.2)

Absent 34 (43.6)

Visual Acuity, logMAR (median, IQR) Right eye 0.0 (0.2) 1 (1.3)

Left eye 0.0 (0.1)

Near Cover Test, N (%) Esophoria 9 (11.5) 1 (1.3)

Exophoria 39 (50.0)

Vertical Phoria 1 (1.3)

NAD 29 (37.2)

Distance Cover Test, N (%) Esophoria 3 (3.8) 21 (26.3)

Exophoria 14 (17.9)

Vertical Phoria 1 (1.3)

NAD 40 (51.3)

IPD, mm (median, IQR) 60 (4.5) 33 (41.3)

Table 1 Outline of the participant demographics and screening data.

* After exclusion accounts for data after exclusion criteria applied and missing values removed.
N = number of participants, IQR = interquartile range, logMAR = the logarithm of the Minimum Angle of Resolution, NAD = no
abnormality detected.

a convergence insufficiency (n = 1), with the remaining (p = 0.004). However, one subgroup (n = 10) measured
two histories being unrecorded. It is important to note accommodative facility monocularly and revealed
that three participants (n = 1 CI and n = 2 unknown) worsening of 3 cpm in each eye, but the findings were
had recovered from their presenting complaints and statistically insignificant (p = 0.888 right eye and p = 0.482
demonstrated binocular vision with good and equal left eye). The AC/A ratio and stereoacuity measures were
visual acuity at the time of the experiment, and thus not statistically different after VR gameplay (p = 0.389
were included in the study. and p = 0.317, respectively).
In terms of the screening data, the cohort showed For the BO near horizontal PFR (Table 3), the median
good visual acuity with median values of 0.0 logMAR blur point worsened from 19.0Δ to 16.0Δ in 48 participants
(IQR 0.1 and 0.2 logMAR) across both eyes. The near and but was not statistically significant (p = 0.008). The
distance cover test confirmed one individual to have median break point (observation of double vision) also
a manifest strabismus and they were excluded from worsened from 35.0Δ to 30.0Δ in 68 participants of the
the study. The median IPD was 60mm (IQR 4.5mm), study and was statistically significant (p = 0.003). The BO
although 33 patients did not have this data recorded. near horizontal PFR recovery point, which was recorded
in 48 out of 78 participants of the study, also worsened
EXPERIMENTAL RESULTS from 30.0Δ to 25.0Δ (p = 0.003).
One variation between groups was the number of On the other hand, the BI near horizontal PFR showed
repetitions of the clinical measurements made, but for improvement following VR gameplay. Although the BI
analysis purposes the first attempt is used to ensure there blur point improved from a median of 12.0Δ to 13.0Δ
is no fatigue effect. The changes to visual parameters in 48 participants of the study, it was not statistically
before and after VR gameplay can be found in Table 2. significant (p = 0.006). However, the median BI break
The NPC break and recovery values worsened by 0.5cm, point improved from 14.0Δ to 16.0Δ in 68 participants
but only the break of convergence was statistically and was statistically significant (p = 0.024). Lastly,
significant (p = 0.001). The NPA was only carried out by the median BI near horizontal PFR recovery point also
one subgroup (10 individuals), and showed no statistically significantly improved from 12.0Δ to 16.0Δ (p = 0.003)
significant changes before and after VR gameplay. in 48 participants. In fact, the median recovery point
Binocular accommodative facility, recorded in 58 was equal to the median break point after VR gameplay
out of 78 participants, improved from 11.0 to 13.0 cpm (16.0Δ).
Banstola et al. British and Irish Orthoptic Journal DOI: 10.22599/bioj.257 61

VISUAL PARAMETERS BEFORE VR AFTER VR P-VALUE VALUES INCLUDED,

GAMEPLAY GAMEPLAY** N (%)

NPC, cm Break 6.0 (1.0) 6.5 (3.0) 0.001* 68 (87.2)

(median, IQR)
Recovery 6.5 (2.8) 7.0 (3.0) 0.526 20 (25.6)

NPA, cm Break 8.0 (3.0) 8.0 (7.0) 0.026 10 (12.8)

(median, IQR)

Accommodative Facility, cpm Both eyes 11.0 (5.0) 13.0 (5.0) 0.004* 58 (74.3)
(median, IQR)
Right eye 12.5 (8.5) 9.5 (15.25) 0.888 10 (12.8)

Left eye 15.0 (8.5) 12.0 (10.5) 0.482 10 (12.8)

AC/A ratio (median, IQR) 2.0 (2.0) 1.5 (2.3) 0.389 58 (74.3)

Frisby, log10secs of arc 1.74 (1.2) 1.74 (1.2) 0.317 20 (25.6)

(median, IQR)

Table 2 Summary of the changes to visual parameters before and after VR gameplay.
* Significant values after Bonferroni correction applied.
** After 15 minutes of playing Beat Saber using a VR headset.
N = number of participants, IQR = interquartile range, cpm = cycles per minute.
p-value calculated using Wilcoxon signed rank test followed by Bonferroni correction resulting in a statistically significant level of
p-value < 0.004.

NEAR HORIZONTAL PFR, BEFORE VR AFTER VR P-VALUE VALUES INCLUDED,

DIOPTRES (MEDIAN, IQR) GAMEPLAY GAMEPLAY** N (%)

Base Out Blur 19.0 (9.0) 16.0 (11.0) 0.008 48 (61.5)

Break 35.0 (20.0) 30.0 (15.0) 0.003* 68 (87.2)

Recovery 30.0 (18.8) 25.0 (17.0) 0.003* 48 (61.5)

Base In Blur 12.0 (4.0) 13.0 (4.0) 0.006 48 (61.5)

Break 14.0 (6.0) 16.0 (8.0) 0.024 68 (87.2)

Recovery 12.0 (5.5) 16.0 (8.0) 0.003* 48 (61.5)

Table 3 The changes to near horizontal PFR before and after VR gameplay.
* Significant values after Bonferroni correction applied.
** After 15 minutes of playing Beat Saber using a VR headset.
N = number of participants, IQR = interquartile range.
p-value calculated using Wilcoxon signed rank test followed by Bonferroni correction resulting in a statistically significant level of
p-value < 0.004.

DISCUSSION study could have been due to fatigue of the eye muscles
after carrying out convergence movements during the
The study explored changes to vision following VR game, rather than a long-term effect of VR gaming.
gameplay by measuring visual parameters before and The current study found that VR gameplay worsened
after playing 15 minutes of the VR game Beat Saber. other convergence measures such as the near
The findings showed that the VR game, which primarily horizontal PFR BO break and recovery points. This was
utilised binocular convergence and redivergence, contradictory to a study carried out in participants with
could be attributable to a slight worsening of motor CI, where individuals followed up for three weeks after
components of vision such as breakpoint of convergence. VR gameplay showed improved BO near horizontal PFR
This supported previous findings which also suggested (M. Y. Boon 2017). The difference in findings could have
worsening NPC following VR gaming on healthy been due to the alternative VR game played, in which
volunteers (Munsamy et al. 2020). However, when VR users exercised convergence more than 50% of the
gaming is applied to participants with CI for a longer time (Boon et al. 2017), compared to that played in the
period of time, such as three weeks, it has the capability current study where performance was not quantified.
to improve this function, possibly due to suggestions that In addition, as the study consisted of 15minutes of VR
neuroplasticity can be altered after repeated exposure, gaming repeated over three weeks (equating to 103 ± 76
compared to just one exposure of VR gaming (Li et al. minutes of training in total) compared to just one 15
2019). Therefore, the worsening of convergence in this minutes session in the present study, the participants
Banstola et al. British and Irish Orthoptic Journal DOI: 10.22599/bioj.257 62

were exposed to greater convergence training. A further of 4Δ of near horizontal PFR BI recovery point are all
study measuring the effect on visual parameters after VR significant results, but it does not reflect a clinical change
gameplay also showed the changes that were observed to vision where the user is able to notice the difference
after 10 minutes returned to baseline values by 50 (Abraham et al. 2015; Yekta et al. 2017). Therefore,
minutes (Szpak et al. 2020), suggesting that duration of further studies investigating symptoms and visual
gameplay is a key factor in change to vision long-term. complaints from VR users need to be undertaken to
The current study showed improvement of divergence assess the clinical consequences of VR gameplay.
as the BI near horizontal PFR recovery point improved The study emphasised the potential harmful impact
following VR gameplay. The improvement of only to vision for children and young adults who typically
divergence could be due to the lower demands on use the technology more regularly and for a much
the eye to carry out divergence (relaxation) than longer period of time (Vailshery 2018). In addition, for
convergence (constricting) movements. Hence, the individuals undergoing training to improve a CI, the study
muscles assisting divergence were not over-exercised suggests advice should be given to reduce the use of
and were able to recover and refocus quicker. Boon et al. this technology. On the other hand, as studies including
(2017) also investigated the changes of this parameter CI participants with longer exposure time to VR gaming
in CI participants and showed no significant findings resulted in no improvement of visual parameters (Boon
even after three weeks of VR gaming. However, the et al. 2017), it highlights the need to find a relationship
authors suggest this could be due to a ceiling effect in between duration of VR gameplay and impact on visual
the negative fusional vergence ranges. Further studies function. Therefore, before any guidelines on the use of
exploring changes of these visual parameter on healthy VR gameplay are established further research with longer
individuals need to be carried out to produce conclusive exposure and follow-up period should be carried out.
findings.
The present study suggested some improvement STRENGTHS
of accommodation measures as the binocular One of the main strengths of this study was the inclusion
accommodative facility improved after the VR of multiple visual parameters. Unlike previous studies,
experience, which was consistent to previous findings which focused on limited number of outcome measures,
on participants with normal binocular vision (Munsamy this study investigated a wide range of theses outcomes
et al. 2020). Having to continually focus on a different ranging across convergence, accommodation, and
target during the game may have helped exercise stereoacuity. This helped ensure all the changes to vision
accommodation. Monocular accommodative facility after VR gameplay were explored in the study. Moreover,
was not altered significantly. However, there were fewer Bonferroni correction was used to exclude any significant
participants included in this subsample. Similarly, other results due to chance. This increased the power of the
results such as the recovery point of NPC and near significant findings of the study and ensured any changes
horizontal PFR BO blur point from the current study may to vision were as a result of VR gameplay.
have been impeded from a smaller subsample compared
to the significant results found using a larger subsample LIMITATIONS
of the participants. Therefore, a recommendation from There were several study limitations that should be
this research is for future studies to assess the impact noted. The VR headset was not adjustable below an
on these visual parameters after VR gameplay within a IPD of 60mm. Therefore, approximately half of the
larger cohort for accuracy. participants were wearing an oversized headset. Recent
Due to limited evidence exploring the impact of research has shown participants with a mismatch of IPD
VR gameplay on stereoacuity, stereoacuity was also may experience a lower image quality with less accurate
investigated in this study. The results revealed no changes depth perception and greater eye discomfort (P. Hibbard
to this parameter following 15 minutes of VR gameplay. 2020). All of this could have played a confounding role
A previous study showed improvement of stereoacuity in and affected the experimental results, hence reducing
adults with amblyopic eyes; however, a contrasting study the credibility of the findings. This was a restriction caused
carried out on healthy individuals had shown no changes by the technology and could not be altered manually,
to this visual function (Žiak et al. 2017; Yoon et al. 2020). highlighting a flaw in the VR technology design.
As the current study was also carried out on healthy Another factor that could have affected the results
individuals, it adds to previous findings by showing no of the study is the use of a different examiner for each
effects on stereoacuity following VR headset use. subgroup. Despite all the examiners being orthoptic
Although the study has highlighted statistically undergraduate students, the inconsistency between
significant changes to vision following VR gameplay, it the subgroups was not accounted for during the
is important to consider the clinical significance of these experiment. Furthermore, the use of students rather than
results (Peeters 2016). For the purpose of this study, a professional opticians may have reduced the accuracy of
worsening of 0.5 cm of NPC, improvement of 2 cpm of the test results due to their lack of clinical judgment and
binocular accommodative facility, and improvement experience. Future studies comparing accuracy of clinical
Banstola et al. British and Irish Orthoptic Journal DOI: 10.22599/bioj.257 63

assessments by a qualified orthoptist could provide AUTHOR AFFILIATIONS

clarity on the reliability of these findings. Sanjog Banstola orcid.org/0000-0001-6302-1923
The timing of the post-VR visual assessment may have University of Liverpool, GB
impacted on the reliability of the results, where fatigue of Kerry Hanna orcid.org/0000-0001-7357-7749
the eye following continuous convergence and divergence University of Liverpool, GB
movements may have skewed the results and masked Anna O’Connor orcid.org/0000-0002-0376-9670
the long-term effects to vision. Therefore, having a longer University of Liverpool, GB
follow-up time may have helped observe more accurate
changes to vision. In addition, due to the autonomous
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TO CITE THIS ARTICLE:

Banstola, S, Hanna, K and O’Connor, A. 2022. Changes to Visual Parameters Following Virtual Reality Gameplay. British and Irish
Orthoptic Journal, 18(1), pp. 57–64. DOI: https://doi.org/10.22599/bioj.257

Submitted: 20 October 2021 Accepted: 27 May 2022 Published: 27 June 2022

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 RESEARCH ARTICLE

Impact of various types of near work and time

spent outdoors at different times of day on
visual acuity and refractive error among
Chinese school-going children
Hongyu Guan1‡, Ning Neil Yu ID2☯*, Huan Wang2‡, Matthew Boswell2‡, Yaojiang Shi ID1‡,
Scott Rozelle2☯, Nathan Congdon3,4,5☯

1 Center for Experimental Economics for Education, Shaanxi Normal University, Xi’an, China, 2 Freeman
a1111111111
Spogli Institute of International Studies, Stanford University, Stanford, CA, United States of America,
a1111111111 3 Centre for Public Health, Queen’s University Belfast, Belfast, United Kingdom, 4 Orbis International, New
a1111111111 York, NY, United States of America, 5 Zhongshan Ophthalmic Center, Guangzhou, China
a1111111111
a1111111111 ☯ These authors contributed equally to this work.
‡ These authors also contributed equally to this work.
* ningyu@stanford.edu

OPEN ACCESS Abstract

Citation: Guan H, Yu NN, Wang H, Boswell M, Shi
Y, Rozelle S, et al. (2019) Impact of various types
of near work and time spent outdoors at different
times of day on visual acuity and refractive error Background
among Chinese school-going children. PLoS ONE
Various types of near work have been suggested to promote the incidence and progression
14(4): e0215827. https://doi.org/10.1371/journal.
pone.0215827 of myopia, while outdoor activity appears to prevent or retard myopia. However, there is a
lack of consensus on how to interpret these results and translate them into effective inter-
Editor: Yingfeng Zheng, Sun Yat-Sen University
Zhongshan Ophthalmic Center, CHINA vention strategies. This study examined the association between visual acuity and time allo-
cated to various activities among school-going children.
Received: October 17, 2018

Accepted: April 9, 2019

Published: April 26, 2019 Methods

Copyright: © 2019 Guan et al. This is an open Population-based survey of 19,934 students in grade 4 and 5 from 252 randomly selected
access article distributed under the terms of the rural primary schools in Northwest China in September 2012. This survey measured visual
Creative Commons Attribution License, which acuity and collected self-reported data on time spent outdoors and time spent doing various
permits unrestricted use, distribution, and
types of near activities.
reproduction in any medium, provided the original
author and source are credited.

Data Availability Statement: All relevant data are Results

available from the Harvard Dataverse, https://doi.
org/10.7910/DVN/VRJXIG. Prolonged (>60 minutes/day) computer usage (-0.025 LogMAR units, P = .011) and smart-
Funding: The authors are supported by the 111
phone usage (-0.041 LogMAR units, P = .001) were significantly associated with greater
Project, grant number B16031. This study was refractive error, while television viewing and after-school study were not. For time spent out-
funded by OneSight (Mason, OH), Luxottica-China doors, only time around midday was significantly associated with better uncorrected visual
(Shanghai). Dr. Guan is funded by National Natural
acuity. Compared to children who reported no midday time outdoors, those who spent time
Science Foundation of China, grant number
71803107, Prof Congdon is funded by the outdoors at midday for 31–60 minutes or more than 60 minutes had better uncorrected
Ulverscroft Foundation (UK). The funders had no visual acuity by 0.016 LogMAR units (P = .014) and 0.016 units (P = .042), respectively.

PLOS ONE | https://doi.org/10.1371/journal.pone.0215827 April 26, 2019 1 / 13

 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

role in the design and conduct of the study; Conclusions

collection, management, analysis, and
interpretation of the data; preparation, review, or Use of smart phones and computers were associated with declines in children’s vision,
approval of the manuscript; and decision to submit while television viewing was not. Statistically significant associations between outdoor time
the manuscript for publication. at midday and reduced myopia may support the hypothesis that light intensity plays a role in
Competing interests: Luxottica-China (Shanghai) the protective effects of outdoor time.
is one of the funders of this study. The funders had
no role in the design and conduct of the study;
collection, management, analysis, and
interpretation of the data; preparation, review, or
approval of the manuscript; and decision to submit Introduction
the manuscript for publication. Funding from this
this commercial funder does not involve any
Various types of near work[1–4] have been suggested to promote the incidence and progres-
agreement related to employment, consultancy, sion of myopia[2,5–8], while outdoor activity appears to prevent or retard myopia[9]. How-
patents, products in development, marketed ever, there is a lack of consensus on how to interpret these results and to translate them into
products, etc. We confirm that this does not alter effective intervention strategies[10–14].
our adherence to PLOS ONE policies on sharing One difficulty lies in differentiating the direct effects of more time spent on one activity, as
data and materials. There are no restrictions on
well as the substitution effects of having less time left for other activities[10]. Spending more
sharing of data and/or materials.
time outdoors may reduce myopia because children spend less time on near work, while
spending more time on near work may exacerbate myopia. Attempts have been made to
account for this difficulty in a few studies, but it remains relevant to interpreting the body of
literature on the impact of time allocation. Another difficulty lies in elucidating the effect on
myopia of different factors underlying an activity. For example, time spent outdoors may lead
both to increased light exposure and a tendency to focus on more distant objects, each of
which might potentially reduce myopia risk.
The current study reports data from a large population-based survey of rural primary
school students in Northwest China, during which visual acuity was measured and a widely-
used form[15] was utilized to collect self-reported data on time spent outdoors and in various
types of near activities.
We used a diary approach to collect self-reported data on time spent outdoors; this not only
allowed us to identify potential substitution effects, but also to distinguish between time spent
outdoors before school, around midday, and after school in order to test the hypothesis that
brighter light, rather than a tendency to focus at greater distance, is responsible for the protec-
tive effect of outdoor activities against myopia[16]. This is because the level of illumination
from the sun is strongest around midday, and weaker in the morning and late afternoon hours
before or after school, while the tendency to focus at greater distance is presumably unchanged
throughout the day.
This paper also examines the association between myopia and uncorrected visual acuity,
and between myopia and various types of near activities, including schoolwork and the use of
smartphones, computers and televisions, in order to guide potential interventions to reduce
the visually-harmful impact of such activities.

Methods
Setting and sampling
This study utilizes data collected during a randomized trial of glasses provision on educational
outcomes among 19,934 students at 252 primary schools in Northwest China, in the fall of
2012.
Our sample schools are located in A prefecture in G province and B Prefecture in S prov-
ince in northwest of China. We obtained a list of all 435 primary schools in the two prefectures
from local education bureaus. For logistical reasons we excluded those with fewer than 50 or

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 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

more than 150 students in the fourth and fifth grade combined (19% of sample frame). This is
because screening at the larger schools could not be reliably completed in a day, which would
have interfered with the screening schedule, whereas smaller schools would be expected to
have fewer than 10 children requiring glasses, below our power requirements. We randomly
selected one school from each township in the sample, and within each school we randomly
selected one class in each of the fourth and fifth grades (likely age range 9–12 years). All 19,934
students in these 252 schools completed a detailed questionnaire concerning potential risk fac-
tors for myopia, including weekly time spent in near activities and time spent outdoors.
Institutional Review Boards at Stanford University and Zhongshan Ophthalmic Center
(ZOC, Sun Yat-sen University, Guangzhou, China) approved the research protocol in full, and
the principles of the Declaration of Helsinki were followed throughout. Written informed con-
sent was obtained from at least one parent for all child participants.

Assessment of Visual Acuity (VA), refraction, and myopia

In each school, a nurse and staff assistant, who had been trained by optometrists from ZOC,
carried out visual acuity (VA) screening for all selected students to determine eligibility for the
original trial. In a well-lighted indoor space at each school, children underwent assessment of
uncorrected VA (UCVA) in each eye separately at 4 meters, using an Early Treatment Diabetic
Retinopathy Study (ETDRS) chart (Precision Vision, La Salle, Illinois, USA)[15]. If a child cor-
rectly identified at least 4 of 5 optotypes on the top 6/60 line, s/he was examined on the 6/30
line, the 6/15 line, and then line by line to 6/3. When a student failed a line, the lines above
were tested successively until the child correctly identified 4 of 5 optotypes; the VA for an eye
was defined as the lowest line read. For cases in which a student could not correctly read the
top line, visual acuity was tested as above at 1 meter, with the measured VA divided by 4. VA
was expressed during data analysis using LogMAR, the negative base-10 logarithm of the Mini-
mum Angle of Resolution.
If a child’s VA was �6/12 in either eye, cycloplegia was applied with up to 3 drops of cyclo-
pentolate 1%, and a refractionist, previously trained by ZOC’s experienced pediatric optome-
trists, carried out automated refraction (Topcon KR 8900; Tokyo, Japan) with subjective
refinement in each eye separately. Refractive error was recorded as the spherical equivalent
(SE, spherical power + ½ the cylindrical power). Myopia was defined as VA �6/12 and SE
�-0.5D in at least one eye.
A total of 19934 students underwent vision screening, 4939 (24.8%) of them failed. Among
these who failed the vision screening, 280 (5.7%) students did not complete refraction due to
absence on the day our optometrists team conducted cycloplegic refraction in school. The
remaining 4659 (94.3%) completed cycloplegic refraction. Students that did not complete
cycloplegic refraction and those who did were not significantly different with regard to indi-
vidual characteristics or time allocation variables.
Among the 4659 (94.3%) students that completed cycloplegic refraction, 1052 (22.6%) of
them were not diagnosed with myopia due to visual acuity not being correctable to 6/12 in at
least one eye with refraction, due to amblyopia or other non-refractive disorders. These chil-
dren were referred to higher level hospitals (prefectural and municipal facilities) for treatment.
In our analysis, we define these children as not myopic, because their vision could not be
improved with refraction. Fig 1 presents a flowchart of the participants in this study.

Assessment of self-reported time spent in near activities and outdoors

Students completed a previously-reported, self-administered questionnaire concerning mean
times spent throughout the day on near activities (including computer and smartphone use,

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 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

Fig 1. Flowchart of the participants. (A)19934 students underwent vision screening,4939 (24.8%) of them failed, (B) 4659 (94.3%)
students that completed cycloplegic refraction, (C) 3607 (77.4%) were diagnosed with myopia.
https://doi.org/10.1371/journal.pone.0215827.g001

television viewing and after-school study), and time spent outdoors before school, around
midday and after school. Reports of time spent on various activities at different periods in the
day were categorized as follows: 0 minutes, 1 to 30 minutes, 31 to 60 minutes and > 60
minutes.

Covariates
Our questionnaire also collected additional information potentially related to myopia, includ-
ing grade (4th or 5th), age, gender, family wealth, migration status of each parent, parental edu-
cation, child’s main residence (home, school dormitory, relative’s home, etc.), and province.
Family wealth was calculated based on a parental questionnaire that asked about ownership of
13 selected items, summing their values as listed in the China Rural Household Survey Year-
book (Department of Rural Surveys, National Bureau of Statistics of China, 2013). S province’s
GDP per capita of USD 6108 ranked 14th among China’s administrative regions in 2012, and
was very similar to that for the country as a whole (USD 6091) in the same year, while G prov-
ince was the second-poorest province in the country (per capita GDP of USD 3100)[17].

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 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

Higher income is a risk factor for myopia[13], and thus province of residence was explored as
a potential determinant of VA and refractive error.

Statistical methods
Mean UCVA (LogMAR averaged over two eyes, with its 95% Confidence Interval) and myopia
prevalence are reported for children stratified by various demographic and behavioral factors.
The impact of time allocation and other variables on UCVA in cross-sectional fashion were
assessed using Generalized Estimating Equation (GEE) regression models, which adjusted for
the correlation between the two eyes of each child and between children in the same school. In
the multivariate model (full regression model), to improve the efficiency of the estimation, we
included variables which were statistically significant associated with visual acuity and myopia
(age, sex, family wealth, parental migrant status, parental education, child’s residence). Missing
observations, which constituted a small proportion of the data, were excluded. For statistical
interference, robust standard errors adjusted for clustering at the school level were used. Simi-
larly, we constructed logistic models with outcome defined as the presence of myopia and
included the same potential explanatory variables. All analyses were performed using Stata
14.0 (StataCorp, College Station, Texas, USA).

Results
Among the 19,934 children in our study (mean age 10.6 +/-1.15 years, 48% girls), the mean
LogMAR UCVA was -0.172, equivalent to a Snellen fraction of approximately 6/9. A total of
4939 children (24.8%) failed vision testing, and among them, 4659 (94.3%) underwent refrac-
tion. The prevalence of myopia was 18.1% (3607 of 19,934 students).
Students in grade 5, girls, those from wealthier families, those with at least one parent at
home, those with better-educated patents and S province residents had significantly (P < .001)
worse UCVA and greater myopia. Greater computer use (P < .001), smartphone use, televi-
sion viewing, and after-school study as well as less midday outdoor time, were also associated
with lower UCVA and greater myopia prevalence (P < .001). Before- and after-school outdoor
time were unassociated with UCVA and myopia (Table 1).
In multivariate regression models (Table 2), we found that using a computer 1 to 30 minutes
and 31 to 60 minutes per day were associated with worse UCVA (0.019 LogMAR units, P = .001
and 0.024 LogMAR units, P = .008), while greater than 60 minutes of daily use was associated
with a greater reduction in UCVA (0.040 LogMAR units, P< 0.001). Using smartphones for 1 to
30 minutes and 31 to 60 minutes per day was uncorrelated with visual acuity, but use for greater
than 60 minutes was associated with reduced UCVA (0.043 LogMAR units, P< 0.001). Neither
television viewing nor after-school study were significantly associated with declines in UCVA.
Our logistic regression models with myopia as the outcome (Table 2) showed consistent
results for computer use and increased myopia risk, though smartphone usage was not signifi-
cantly associated with visual acuity. As with the model using UCVA, television viewing and
after-school study were unassociated with myopia risk. Such findings are consistent with those
found in the literature[18–22].
Regarding time spent outdoors, statistically significant associations between outdoor time
and reduced myopia were found only for the midday interval. Myopia prevalence was highest
in students who spent the least time outdoors around noon (Table 1), though no such trend
was present for before or after school. In the linear regression model for UCVA, children
spending 30 to 60 minutes outdoors at noon per day (0.016 LogMAR units, P = .014) and
those with greater than 60 minutes outdoors at noon per day (0.016 LogMAR units, P = .042)
had significantly better vision compared to children who rarely went outdoors at this time;

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 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

Table 1. Visual acuity by student characteristics.

CHARACTERISTICS n Mean Uncorrected Visual Acuity 95% Confidence Myopia (VA �6/12 and SE �-0.5D in at Missing Data
(LogMARa) intervalb least one eye) (%)
All Students 19,934 -0.172 -0.175 - -0.169 18.1% 0(0)
Grade 0(0)
4 9,842 -0.156 -0.160 - -0.152 14.8%
5 10,092 -0.188 -0.193 - -0.183 21.3%
Age 18(0.09%)
< = 10 10,304 -0.174 -0.178 - -0.169 18.4%
>10 9,630 -0.170 -0.175 - -0.165 17.7%
Sex
Girls 9,548 -0.189 -0.194 - -0.184 19.6% 0(0)
Boys 10,386 -0.157 -0.161 - -0.152 16.7%
Family Wealth 924(4.6%)
Bottom tercile 5,573 -0.153 -0.159 - -0.147 14.3%
Middle tercile 7,024 -0.173 -0.179 - -0.167 17.7%
Top tercile 6,413 -0.188 -0.194 - -0.182 21.8%
Parental Migration Status 187(0.9%)
Both out-migrated 2,474 -0.148 -0.157 - -0.139 18.6%
One or more present 17,273 -0.175 -0.179 - -0.172 14.6%
Maternal Education 127(0.6%)
Less than junior high 4,304 -0.162 -0.169 - -0.155 17.0%
school
Junior high school 13,784 -0.171 -0.175 - -0.167 18.0%
At least high school 1,719 -0.205 -0.217 - -0.192 21.3%
Paternal Education 72(0.4%)
Less than junior high 1,363 -0.163 -0.175 - -0.151 17.1%
school
Junior high school 15,861 -0.170 -0.173 - -0.166 17.8%
At least high school 2,638 -0.190 -0.200 - -0.181 20.2%
Child’s main residence 852(4.3%)
Home 11,019 -0.162 -0.167 - -0.158 16.2%
School dormitory 4,081 -0.182 -0.190 - -0.175 21.9%
Relative’s home 323 -0.166 -0.190 - -0.142 16.1%
Rental home 3,641 -0.191 -0.199 - -0.183 19.8%
Other 18 -0.230 -0.386 - -0.074 16.7%
Province 0(0)
S province 9,625 -0.206 -0.211 - -0.201 24.3%
G province 10,309 -0.141 -0.145 - -0.136 12.3%
Daily Computer Use 0(0)
0 minutes 15,493 -0.160 -0.164 - -0.157 16.5%
1–30 minutes 2,634 -0.207 -0.217 - -0.297 22.4%
31–60 minutes 998 -0.217 -0.233 - -0.201 27.2%
> 60 minutes 809 -0.232 -0.250 - -0.213 23.5%
Daily Smartphone Use 0(0)
0 minutes 13,161 -0.166 -0.170 - -0.162 17.5%
1–30 minutes 5,360 -0.180 -0.186 - -0.173 19.4%
31–60 minutes 829 -0.184 -0.201 - -0.167 18.0%
> 60 minutes 584 -0.216 -0.237 - -0.194 20%
Television Viewing Time 0(0)
(Continued )

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 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

Table 1. (Continued)

CHARACTERISTICS n Mean Uncorrected Visual Acuity 95% Confidence Myopia (VA �6/12 and SE �-0.5D in at Missing Data
(LogMARa) intervalb least one eye) (%)
0 minutes 2,205 -0.170 -0.180 - -0.160 16.4%
1–30 minutes 8,765 -0.161 -0.166 - -0.156 16.8%
31–60 minutes 4,765 -0.179 -0.186 - -0.172 19.9%
> 60 minutes 4,199 -0.189 -0.196 - -0.181 19.8%
After-School Study Time 0(0)
0 minutes 665 -0.159 -0.176 - -0.141 14.0%
1–30 minutes 6,227 -0.162 -0.168 - -0.156 16.1%
31–60 minutes 7,259 -0.169 -0.175 - -0.164 18.0%
> 60 minutes 5,783 -0.189 -0.195 - -0.182 20.8%
Before-School Outdoor Time 0(0%)
0 minutes 1,427 -0.171 -0.183 - -0.159 17.2%
1–30 minutes 15,782 -0.174 -0.178 - -0.170 18.3%
31–60 minutes 1,710 -0.154 -0.165 - -0.143 17.1%
> 60 minutes 1,015 -0.173 -0.188 - -0.158 18.0%
Mid-Day Outdoor Time 855(4.3%)
0 minutes 6,200 -0.184 -0.190 - -0.178 19.5%
1–30 minutes 9,770 -0.170 -0.174 - -0.165 18.0%
31–60 minutes 1,958 -0.155 -0.165 - -0.145 15.3%
> 60 minutes 1,151 -0.157 -0.171 - -0.144 16.0%
After-School Outdoor Time 860(4.3%)
0 minutes 4,701 -0.173 -0.180 - -0.167 17.3%
1–30 minutes 9,356 -0.170 -0.175 - -0.166 17.9%
31–60 minutes 2,915 -0.174 -0.183 - -0.165 19.3%
> 60 minutes 2,102 -0.175 -0.185 - -0.164 19.3%

a LogMAR = negative of the base-10 log of the Minimum Angle of Resolution

b Based on LogMAR averaged over two eyes for each individual

https://doi.org/10.1371/journal.pone.0215827.t001

these differences for other times of day were not statistically significant (Table 2). Results for
myopia risk were consistent: only spending more time outdoors at midday had a significant
protective effect (Table 3).
Regarding other potential correlates of uncorrected vision and myopia, after controlling for
school grade, there was no association between older age and UCVA. Girls had greater risk of
worse vision and myopia than boys (Tables 2 and 3). Having both parents out-migrated for
work significantly increased the risk of myopia in multivariate models (OR -0.155, P = .017)
(Table 3), but was not significantly associated with UCVA (Table 2).
None of the parental education indicators were associated with UCVA or myopia risk in
multivariate models, though either parent finishing high school was significantly associated
with worse UCVA in the univariate analysis. Compared with living at home, living in a school
dormitory was negatively correlated with UCVA and myopia, while other residence types were
unassociated with vision or refractive error (Tables 2 and 3).

Discussion
Our models of UCVA and myopia both point to consistent conclusions that time using com-
puters and smartphones is associated with more myopic refractive error, while television

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 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

Table 2. Effects of various factors on visual acuity (LogMAR of both eyesa) in Generalized Estimating Equations (GEE)b regression models.
Univariate Model Multivariate Model
(n = 35,828)
VARIABLES beta 95% CI p-valuec beta 95% CI p-value
Grade -0.030 -0.039 - -0.022 (< 0.001) -0.033 -0.044 - -0.023 (< 0.001)
Age -0.003 -0.006–0.000 (0.070) 0.002 -0.002–0.006 (0.356)
Male sex 0.037 0.030–0.043 (< 0.001) 0.038 0.030–0.045 (< 0.001)
Family Wealth -0.000 -0.000 - -0.000 (0.004) -0.000 -0.000–0.000 (0.179)
Both Parents Out-Migrated for Work 0.009 -0.001–0.018 (0.066) 0.010 -0.000–0.019 (0.053)
Maternal Education (Less than junior high school as reference)
Junior high school -0.002 -0.010–0.007 (0.681) -0.000 -0.010–0.010 (0.991)
At least high school -0.023 -0.037 - -0.008 (0.002) -0.013 -0.029–0.004 (0.131)
Paternal Education (Less than junior high school as reference)
Junior high school -0.002 -0.015–0.011 (0.777) -0.001 -0.014–0.013 (0.939)
At least high school -0.016 -0.032 - -0.000 (0.050) -0.004 -0.021–0.013 (0.619)
Child’s Main Residence (Home as reference)
School dormitory 0.013 0.001–0.024 (0.029) 0.020 0.008–0.032 (0.001)
Relative’s home -0.004 -0.028–0.019 (0.726) -0.005 -0.030–0.019 (0.667)
Rental home 0.009 -0.003–0.021 (0.146) 0.010 -0.002–0.023 (0.106)
Other -0.086 -0.216–0.044 (0.196) -0.122 -0.273–0.028 (0.110)
G province 0.064 0.047–0.080 (< 0.001) 0.065 0.048–0.083 (< 0.001)
Time Using Computers (0 minutes as reference)
1–30 minutes -0.019 -0.030 - -0.008 (0.001) -0.013 -0.026 - -0.001 (0.041)
31–60 minutes -0.024 -0.042 - -0.006 (0.008) -0.019 -0.038–0.001 (0.058)
>60 minutes -0.040 -0.058 - -0.022 (< 0.001) -0.025 -0.045 - -0.006 (0.011)
Time Using Smartphones (0 minutes as reference)
1–30 minutes -0.004 -0.012–0.003 (0.264) -0.005 -0.013–0.003 (0.217)
31–60 minutes -0.007 -0.026–0.012 (0.483) -0.011 -0.031–0.008 (0.260)
>60 minutes -0.043 -0.066 - -0.020 (< 0.001) -0.041 -0.066 - -0.016 (0.001)
Television Viewing Time (0 minutes as reference)
1–30 minutes 0.010 -0.001–0.021 (0.084) 0.011 -0.001–0.023 (0.080)
31–60 minutes -0.001 -0.014–0.012 (0.838) 0.003 -0.011–0.017 (0.666)
>60 minutes -0.007 -0.020–0.006 (0.290) -0.003 -0.016–0.011 (0.680)
After-School Study Time (0 minutes as reference)
1–30 minutes 0.004 -0.015–0.023 (0.700) -0.003 -0.022–0.016 (0.738)
31–60 minutes -0.000 -0.019–0.019 (0.994) -0.002 -0.020–0.017 (0.866)
>60 minutes -0.017 -0.036–0.002 (0.082) -0.016 -0.036–0.003 (0.099)
Before-School Outdoor Time (0 minutes as reference)
1–30 minutes 0.077 -0.066–0.220 (0.292) -0.009 -0.026–0.007 (0.268)
31–60 minutes -0.007 -0.193–0.180 (0.945) -0.001 -0.020–0.017 (0.880)
>60 minutes 0.059 -0.152–0.270 (0.581) -0.013 -0.054–0.028 (0.523)
Mid-Day Outdoor Time (0 minutes as reference)
1–30 minutes 0.007 -0.001–0.016 (0.079) 0.008 -0.001–0.016 (0.089)
31–60 minutes 0.016 0.003–0.028 (0.012) 0.016 0.003–0.029 (0.014)
>60 minutes 0.019 0.004–0.034 (0.011) 0.016 0.001–0.032 (0.042)
After-School Outdoor Time (0 minutes as reference)
1–30 minutes 0.006 -0.001–0.014 (0.111) 0.004 -0.004–0.012 (0.361)
31–60 minutes 0.007 -0.004–0.018 (0.195) 0.004 -0.008–0.016 (0.517)
(Continued )

PLOS ONE | https://doi.org/10.1371/journal.pone.0215827 April 26, 2019 8 / 13

 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

Table 2. (Continued)

Univariate Model Multivariate Model

(n = 35,828)
VARIABLES beta 95% CI p-valuec beta 95% CI p-value
>60 minutes 0.009 -0.003–0.021 (0.136) 0.005 -0.007–0.018 (0.410)
a
LogMAR = negative of the base-10 log of the Minimum Angle of Resolution
b
GEE adjusts for the correlation between eyes of a child.
c
Confidence intervals and p-values are based on robust standard errors adjusted for clustering at the school level.

https://doi.org/10.1371/journal.pone.0215827.t002

viewing and after-school study are not. Evidence about the impact of smart phone and com-
puter use on myopia has been inconsistent[13,14,23,24]. It may be that, as prevalence of use of
these devices among school-age children continues to rise in China and elsewhere, the associa-
tion is becoming clearer. It should be noted, though, that in the present cohort, daily use of
both smartphones and computers was less common than the frequency of television watching
and after-school study. Given the greater expected viewing distance for television, our failure
to find an association with visual acuity or myopia, as opposed to computers and smartphones
(generally used at closer distances), is consistent with the prevailing hypothesis that viewing
distance plays a role in the influence of near work on myopia, due perhaps to greater periph-
eral defocus at closer working distance[25]. We speculate that the lack of an observed associa-
tion between after-school study time and myopia or decline in vision might be due to
afternoon study and tutorial classes, which some children may not have considered as “reading
after school”. Once again, evidence on the impact of reading and studying on myopia inci-
dence and progression has been somewhat inconsistent[13,14].
Recent trial evidence strongly suggests a causal association between increased time out-
doors and decreased incidence of myopia[8,9]. However, the mechanism for the protective
effect of outdoor time is still not definitively understood, with both increased exposure to
bright light and decreased time focusing at near targets having been proposed. Our finding
that statistically significant associations between outdoor time and reduced myopia were
found only for the midday interval, and not before or after school, may support the hypothesis
that light intensity plays a crucial role in the protective effects of outdoor time, corroborating
results from animal experiments[26,27]. Time of day has not previously been identified as an
important aspect when considering outdoor time as a mediator of myopia risk. Parts of China,
including the current settings in our sample province, are at similar latitudes to areas with
high sun exposure[28]; however, it may be that other factors such as weather and air pollution
[29] partially block sunlight in China, requiring exposure at the brightest midday hours for
protection against UCVA and leading in part to China’s high myopia prevalence[30,31].
Our findings suggest a potential anti-myopia intervention of encouraging more outdoor
activities during the noon hours when illumination is greatest. Such strategies would be practi-
cal in China, due to the widespread practice of offering a noon break of up to 2 hours in Chi-
nese schools, during which many children return home to rest. Strategies to exploit this noon
break in order to increase sunlight exposure would have to contend with the cultural norm of
sleeping during this time, and would also need to address issues such as protecting children’s
skin from bright sunlight. Further evidence of the greater effectiveness of noon-time outdoor
activity in protecting against myopia is needed in other settings.
Strengths of this study include its population-based nature and large size. The “diary”
approach explored the effects of time spent on a variety of activities, including various types
of near work and outdoor activities at different times of day. This alleviates concerns of

PLOS ONE | https://doi.org/10.1371/journal.pone.0215827 April 26, 2019 9 / 13

 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

Table 3. Effects of various factors on myopia (VA � 6/12 and SE < = -0.5D in at least one eye) in logistic regression models.
Univariate Model Multivariate Model
(n = 17,914) (n = 17,914)
VARIABLES beta 95% CI p-value a beta 95% CI p-value
Grade 0.448 0.374–0.521 (< 0.001) 0.509 0.419–0.598 (< 0.001)
Age -0.031 -0.062 - -0.000 (0.049) -0.063 -0.105 - -0.020 (0.004)
Male sex -0.198 -0.270 - -0.126 (< 0.001) -0.218 -0.298 - -0.137 (< 0.001)
Family Wealth 0.000 0.000–0.000 (< 0.001) 0.000 0.000–0.000 (< 0.001)
Both Parents Out-Migrated for Work -0.294 -0.413 - -0.176 (< 0.001) -0.155 -0.283–0.027 (0.017)
Maternal Education (Less than junior high school as reference)
Junior high school 0.069 -0.021–0.160 (0.681) 0.003 -0.097–0.102 (0.959)
At least high school 0.281 0.141–0.421 (0.002) 0.041 -0.122–0.204 (0.619)
Paternal Education (Less than junior high school as reference)
Junior high school 0.051 -0.096–0.197 (0.777) 0.033 -0.128–0.193 (0.689)
At least high school 0.205 0.035–0.375 (0.050) 0.089 -0.103–0.282 (0.363)
Child’s Main Residence (Home as reference)
School dormitory 0.373 0.283–0.463 (0.029) -0.184 -0.294 - -0.074 (0.001)
Relative’s home -0.008 -0.309–0.293 (0.726) -0.030 -0.351–0.291 (0.854)
Rental home 0.244 0.148–0.340 (0.146) -0.069 -0.176–0.038 (0.208)
Other 0.033 -1.207–1.274 (0.196) 0.335 -1.152–1.823 (0.659)
G province -0.827 -0.902 - -0.752 (< 0.001) -0.785 -0.886 - -0.683 (< 0.001)
Time Using Computers (0 minutes as reference)
1–30 minutes 0.379 0.278–0.480 (< 0.001) 0.017 -0.097–0.131 (0.776)
31–60 minutes 0.635 0.489–0.780 (< 0.001) 0.305 0.141–0.468 (< 0.001)
>60 minutes 0.440 0.272–0.608 (< 0.001) 0.032 -0.161–0.226 (0.744)
Time Using Smartphones (0 minutes as reference)
1–30 minutes 0.131 0.049–0.212 (0.002) 0.025 -0.065–0.115 (0.591)
31–60 minutes 0.034 -0.149–0.217 (0.714) -0.015 -0.215–0.185 (0.885)
>60 minutes 0.168 -0.039–0.376 (0.112) 0.161 -0.068–0.391 (0.168)
Television Viewing Time (0 minutes as reference)
1–30 minutes 0.032 -0.094–0.158 (0.619) -0.040 -0.177–0.097 (0.565)
31–60 minutes 0.239 0.106–0.373 (< 0.001) 0.031 -0.117–0.178 (0.685)
>60 minutes 0.235 0.099–0.372 (0.001) 0.067 -0.086–0.220 (0.394)
After-School Study Time (0 minutes as reference)
1–30 minutes 0.170 -0.060–0.400 (0.148) 0.101 -0.148–0.349 (0.428)
31–60 minutes 0.305 0.077–0.534 (0.009) 0.176 -0.071–0.423 (0.163)
>60 minutes 0.483 0.254–0.713 (< 0.001) 0.318 0.070–0.566 (0.012)
Before-School Outdoor Time (0 minutes as reference)
1–30 minutes 0.077 -0.066–0.220 (0.292) -0.011 -0.162–0.139 (0.883)
31–60 minutes -0.007 -0.193–0.180 (0.945) -0.101 -0.299–0.097 (0.318)
>60 minutes 0.059 -0.152–0.270 (0.581) -0.046 -0.478–0.387 (0.836)
Mid-Day Outdoor Time (0 minutes as reference)
1–30 minutes -0.147 -0.231 - -0.063 (0.001) -0.091 -0.182 - -0.000 (0.049)
31–60 minutes -0.354 -0.490 - -0.217 (< 0.001) -0.254 -0.402 - -0.106 (0.001)
>60 minutes -0.317 -0.484 - -0.149 (< 0.001) -0.196 -0.377 - -0.014 (0.035)
After-School Outdoor Time (0 minutes as reference)
1–30 minutes 0.041 -0.051–0.133 (0.381) -0.002 -0.101–0.097 (0.965)
31–60 minutes 0.131 0.012–0.251 (0.030) -0.004 -0.135–0.127 (0.949)
(Continued )

PLOS ONE | https://doi.org/10.1371/journal.pone.0215827 April 26, 2019 10 / 13

 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

Table 3. (Continued)

Univariate Model Multivariate Model

(n = 17,914) (n = 17,914)
VARIABLES beta 95% CI p-value a beta 95% CI p-value
>60 minutes 0.131 -0.001–0.263 (0.053) -0.013 -0.159–0.134 (0.866)

a Confidence intervals and p-values are based on robust standard errors adjusted for clustering at the school level.

https://doi.org/10.1371/journal.pone.0215827.t003

substitution effects as confounding factors. There is also a clear distinction between noon-time
outdoor activity and other outdoor activities in our data.
Some caution is necessary in interpreting our results. Self-reported recall data, as adopted
by most studies of visual acuity[3], depend on the reliability of informants[32–34]; this issue
may be greater when younger children are involved, as in the present case. Given the resource
limitations to researchers following a large cohort of young children for long periods of time,
the self-reported recall was determined to be the best method for our study’s visual acuity data
collection. Parents’ refraction data are often included to explain the incidence and progression
of myopia[1], while resources to refract parents are rarely available in studies of children’s
refractive error, glasses wear can be used as surrogate for myopia in high resource settings, but
this is less useful in rural China, where few who need glasses wear them. Though a large num-
ber of schools (over 250) were involved, all were selected from two adjacent areas in western
China, and application of our findings to other settings must be made with caution.
Even with these limitations, our findings offer a novel evidence supporting the protective
effects of increased outdoor time is protective against myopia. If further work confirms our
assumption that higher levels of exposure to brighter lights during noontime is an effective
method of myopia prevention, this may open the door to entirely-new myopia prevention
strategies, such as the use of bright artificial lights in classrooms and various architectural
accommodations to increase children’s exposure to higher levels of natural light.

Supporting information
S1 File. Related survey questions.
(ZIP)

Author Contributions
Conceptualization: Ning Neil Yu, Nathan Congdon.
Formal analysis: Hongyu Guan, Ning Neil Yu.
Funding acquisition: Matthew Boswell, Yaojiang Shi, Scott Rozelle, Nathan Congdon.
Investigation: Ning Neil Yu, Matthew Boswell, Yaojiang Shi, Scott Rozelle, Nathan Congdon.
Methodology: Huan Wang, Scott Rozelle, Nathan Congdon.
Project administration: Hongyu Guan, Huan Wang, Matthew Boswell.
Resources: Matthew Boswell, Scott Rozelle.
Supervision: Yaojiang Shi.
Writing – original draft: Hongyu Guan, Ning Neil Yu.
Writing – review & editing: Hongyu Guan, Huan Wang, Matthew Boswell, Nathan Congdon.

PLOS ONE | https://doi.org/10.1371/journal.pone.0215827 April 26, 2019 11 / 13

 Impact of near work, time spent outdoors on visual acuity among Chinese school-going children

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 www.nature.com/scientificreports

OPEN Ocular effects of virtual reality

headset wear in young adults
1 1,2
Philip R. K. Turnbull & John R. Phillips

Virtual Reality (VR) headsets create immersion by displaying images on screens placed very close to
Received: 5 July 2017
the eyes, which are viewed through high powered lenses. Here we investigate whether this viewing
Accepted: 2 November 2017 arrangement alters the binocular status of the eyes, and whether it is likely to provide a stimulus for
Published: xx xx xxxx myopia development. We compared binocular status after 40-minute trials in indoor and outdoor
environments, in both real and virtual worlds. We also measured the change in thickness of the ocular
choroid, to assess the likely presence of signals for ocular growth and myopia development. We found
that changes in binocular posture at distance and near, gaze stability, amplitude of accommodation and
stereopsis were not different after exposure to each of the 4 environments. Thus, we found no evidence
that the VR optical arrangement had an adverse effect on the binocular status of the eyes in the short
term. Choroidal thickness did not change after either real world trial, but there was a significant
thickening (≈10 microns) after each VR trial (p < 0.001). The choroidal thickening which we observed
suggest that a VR headset may not be a myopiagenic stimulus, despite the very close viewing distances
involved.

The era of consumer-grade virtual reality (VR) is upon us, with a variety of inexpensive VR head mounted dis-
plays (HMDs) currently available. However, there is a perception that using VR HMDs may be associated with
risks, possibly to the eyes, and several suppliers include non-specific warnings associated with HMD use. Here
we investigate whether use of a VR HMD affects binocular vision status and whether there is evidence that it
may be a stimulus for development of myopia (short-sightedness). VR HMDs create a sense of presence in a
computer-generated world by presenting dichoptic images whose perspective shifts with the user’s head move-
ments in the real world. Inside the HMD helmet, images are displayed on screens fixed very close in front of each
eye. The screens are viewed through powerful convex lenses, so that the images appear to be located at a distance.
A sense of depth is created by imposing a relative lateral offset of objects within the images presented to each eye,
which in turn creates image disparity on the retina. The higher the lateral offset, the nearer the object appears.
To prevent double vision as gaze shifts between objects, users make both version and vergence eye movements,
which minimise retinal disparity of the object between eyes, and permit the object of interest to be perceived
binocularly. In real world viewing of objects, vergence eye movements are associated with changes in accommo-
dation to focus the eyes at the depth of the object. However, in VR, the focal distance of all objects on the screen is
constant, and the eyes must converge without changing accommodation to maintain a clear retinal image. Thus,
wearing a VR HMD creates a dissociation between convergence and accommodative demands1, which may con-
tribute to visual discomfort2–4. Whether the effects of this disassociation persist after HMD use is unknown5, but
a study investigating the effect of an early VR HMD system found a shift towards esophoria, and an extended near
point of binocular convergence shortly after a 10-minute VR exposure6. Although VR technology has improved
in recent years, the same optical limitations are present in current VR HMDs7.
Another potential issue which has received little attention is whether VR HMDs are likely to induce myopia
(short-sightedness) with long term use. An association between near work and the development and progression
of childhood myopia has long been recognised8,9. More specifically, the occupational use of microscopes has been
linked with the development and progression of myopia in adults10,11, and microscopes and HMDs have similar
near proximity cues: both involve binocular viewing of a close target through high powered lenses. In addition,
recent research has implicated several other factors as important in myopia development and progression, includ-
ing ambient light levels12, binocular status13, and refractive status of the peripheral retina14, all of which may be
altered while using a VR HMD. Since the abnormal eye enlargement which underlies myopia development occurs
over months and years, assessing the likely ‘myopiagenicity’ of a stimulus such as a VR HMD is challenging.

1
School of Optometry and Vision Science, The University of Auckland, Auckland, New Zealand. 2Department of
Optometry, Asia University, Taichung, Taiwan. Correspondence and requests for materials should be addressed to
P.R.K.T. (email: p.turnbull@auckland.ac.nz)

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However, the results of animal studies indicate that imposing hyperopic defocus on the retina with lenses (image
plane located posterior to the retina), causes rapid thinning of the choroid, followed later by eye elongation and
the development of myopia15. Conversely, creating myopic defocus (image plane located anterior to the retina),
causes rapid thickening of the choroid, followed by a slowing of eye growth and the development of hyperopia16.
Because of the predictable order in which these events occur, it is thought that the choroid is an intermediary
between the retina and sclera, with changes in choroidal thickness reflecting changes in the eye-growth sig-
nalling pathway between retina and sclera17. Thus, an observed increase in choroidal thickness would indicate
signals for reduced eye growth, whereas a decrease in thickness would indicate signals for accelerated growth and
myopia development. Similar changes in the thickness of the human choroid have also been reported following
short-term imposition of retinal defocus18–21 and it has been proposed that changes in choroidal thickness may
serve as an indicator of pending changes to refractive status18,22. Therefore, monitoring the sign of any changes in
choroidal thickness following a visual stimulus could provide a timely prediction of whether the stimulus would
lead to myopia.
This study aimed to investigate whether wearing a current generation VR HMD altered binocular vision sta-
tus, and whether it was likely to provide a myopiagenic stimulus if used over extended periods.

Methods
Study design. This was a prospective, multiple crossover study with four different environments in a two by
two design: real and virtual indoor environments, and real and virtual outdoor environments. Each participant
(n = 19) experienced all four environments in a randomised order. Participants were aged between 18–35 years
(mean age 24.7 SD 4.0 years, 10 females), and were either emmetropic (n = 9) or wore their habitual contact lenses
(n = 10, mean refractive error −1.66 SD 2.20D) to eliminate spectacle lens prism and difficulties fitting glasses
under the HMD.
Participants gave written informed consent, were free to withdraw at any stage without giving reason, and
their data has been deidentified and presented as pooled distributions. Exclusion criteria included stereopsis
worse than 200 minarc, treatment for the progression of myopia (e.g. atropine, orthokeratology, or myopia con-
trol contact lenses), corrected visual acuity poorer than 6/7.5 in either eye, or a history of motion-sickness.
All participants took a battery of tests immediately prior, then immediately after exposure to each environ-
ment. The tests were completed in the following order: fixation stability, binocular vision tests and then choroi-
dal thickness measures. After 40 minutes inside each environment, the battery of tests was repeated in reverse
order: choroidal thickness, binocular vision, and then fixation stability, so that choroidal thickness was measured
immediately before and after the trial. All tests were completed within five minutes of completing each environ-
ment, with equipment located to minimise participant movement. The optics of the HMD had a fixed lens centre
separation of 65 mm, and light emitted from the headset focussed at approximately 1-meter from the headset.
Participants were instructed to align the HMD on their head so that the screen appeared clear. A default stereo-
scopic camera setup was used with an in-game height of 180 cm, and a lateral camera separation of 65 mm. The
participants were instructed on the use of the gamepad to move and interact in the virtual world, and they could
converse with the experimenters if required. To normalise the conditions before each trial, participants worked at
a computer for at least 45 minutes prior to baseline measurements. Each participant began each trial on different
days, but at a similar time of day, to minimise any diurnal effects. The study was approved by The University of
Auckland Human Participants Ethics Committee (Ref: 015908), and adhered to the Tenets of the Declaration of
Helsinki.

Environments. Each environment had a different combination of perceived viewing distance, accommoda-
tive demand, proximity cues for accommodation, convergence demand and luminance, to assist in determining
which environmental parameters or combinations, might be important in any observed effects of HMD wear.
The virtual environments were created in the Unity game engine (Version 5, Unity Technologies, USA), com-
piled for the Oculus DK2 (SDK version 0.8.0) and executed on a virtual reality capable machine (Intel Xeon
E5–1650, nVidia GTX970, 16 Gb RAM). Audio was 3-D positional, and delivered through over-the-ear head-
phones to maximise immersion. The VR outdoor (VRO) environment consisted of a 1 km2 island, with features
designed to mimic the real-world environment (Fig. 1). The island was bounded by water (to expose a distant
horizon) on one side, and by mountain ranges along the three other sides. The island was scattered with ruins,
hills, trees, and other items which rewarded exploration, and there were several treasure chests around the island
which the participants were encouraged to find. Maximum speed of movement was restricted to a fast walking
pace (1.5 meters per second) to minimise nausea, and to mimic the instructions given for the real-world environ-
ments. The VR indoor (VRI) environment was a small (~9 m2) cabin with dim lighting, and a large virtual tele-
vision on the wall. The television played a documentary on the future of virtual reality (https://goo.gl/aDSGNr),
and there were many objects on shelves and a table inside the cabin which provided a range of vergence demands.
The maximum viewing distance in this environment was 3.5 m, designed to match the indoor environment in
the real world. Light levels, measured by placing a lux meter sensor (LT300, Extech, USA) inside the headset in a
darkened room, was approximately 180 lux in VRO, and 130 lux in VRI.
The real world outdoor (RWO) environment was the Auckland Domain, a large city park across the road from
the University campus. Participants were instructed to walk around the park, avoiding near activities such as
reading or using cell phones. Light level was measured during the middle of each RWO trial, and ranged from 150
to 90000 lux (median: 45000, interquartile range: 24500–81500 lux). The real world indoor (RWI) environment
was a small office (approximately 9 m2), with no window. As was the case for the virtual indoor room, there was a
range of items at various distances, and the maximum viewing distance was 3.5 m. Participants remained seated,
and were encouraged to either work on or watch videos on an LCD computer monitor at approximately 1 m view-
ing distance. Overhead florescent tubes provided a 210-lux illumination in the vertical plane at eye height. Heart

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Figure 1. Examples of the four environments and comparison of their features. The outdoor real (RWO)
and virtual (VRO) environments were brighter and spacious, while the indoor real (RWI) and virtual (VRI)
environments were dimmer and more contained. Both virtual environments had constant accommodative
demands, while accommodative demands in the real world varied by fixation distance. All environments
except RWO had proximal near cues – in the virtual reality environments proximal cues were also due to the
perception of wearing a headset.

rate was measured with a wrist worn optical tracker (Charge HR, Fitbit, USA) throughout each trial, and served
as a timer for the 40-minute trial duration.

Binocular Vision Tests. Fixation disparity and stability at 50 cm was assessed on a binocular infrared
eyetracker (Eyelink 1000, SR Research, Canada), which was calibrated before every measurement. The eyetracker
camera was set to ‘remote’ mode, which uses a calibrated sticker positioned on the forehead to allow more nat-
uralistic free-space measures, rather than requiring a chin/headrest. Participants were instructed to look at a
maximum contrast target consisting of a combined bullseye and crosshair, which has been shown to be the
most stable fixation target23. The target subtended 0.85 degrees, and was in the centre of an LCD monitor (Asus
VG278H, 1920 × 1080 resolution, 120 Hz refresh, 28 pixels per degree). After a one second stabilisation period,
the eyetracker recorded binocular eye position at 500 Hz for 4 seconds. If monitoring of the pupil or corneal
reflex was disturbed during this period (e.g. due to a blink), the data was discarded and the test restarted. The
pixel co-ordinates of the gaze position of each eye were captured directly into Matlab (2016a, Mathworks, USA).
Fixation disparities were calculated as right eye minus left eye, with the result that a positive horizontal value
represented an uncrossed ocular misalignment, and a positive vertical value a right eye hypo- misalignment.
Fixation stability was computed as 95% confidence interval bivariate contour ellipse areas (BCEA, minarc2)
of binocular eye movements within the four second measurement interval24,25, as:

95%BCEA = 2.291 × π × σx × σy × 1 − p2

where σx = horizontal SD, σy = vertical SD, 2.291 is the χ2 value corresponding to two standard deviations, and p
is the Pearson product moment correlation coefficient between X and Y data.
Accommodation, phorias and stereopsis Tests of binocular vision status included amplitude of accommoda-
tion (mean of three binocular push-up RAF rule measures), dissociated phorias at both distance (6 m) and near
(0.4 m) using modified Thorington26, and stereopsis (Wirt Stereo fly test, Stereo Optical Co Inc). Inter-pupillary
distance at both distance and near (0.4 m) were measured (Digital pupillometer, Essilor, France), so the effect of
HMD lens decentration could be computed and correlated with changes in binocular vision, fixation stability,
and fixation disparity.
Choroidal thickness, used as a proxy for the risk of myopia progression22,27, was measured using swept-source
optical coherence tomography (SS-OCT, Atlantis DRI, Topcon, Japan) in horizontal and vertical cross scan mode
(6 mm length, 1024 intervals, 96 averaged samples per line). The SS-OCT uses 1050 nm light to better penetrate
the choroid, and minimises visibility of the scan line, which reduces patient tracking. The scan takes less than
two seconds, and captures 100,000 A-scans per second at a resolution of 20 μm across the surface, and 8μm in
depth, and is interpolated to give 1024 measures across the full 6 mm length. As the environmental exposure

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Fixation
MT – 6 m MT – 0.4 m Stereopsis AoA Disparity
VRI 0.14EsoP ± 1.59∆ 1.66ExoP ± 3.06∆ 25 ± 8″ 10.0 ± 1.6D 0.49∆ ± 0.35
∆ ∆
VRO 0.20EsoP ± 1.55 1.79ExoP ± 2.46 23 ± 7″ 9.7 ± 1.5D 0.45∆ ± 0.20
∆ ∆
RWI 0.14EsoP ± 1.63 1.42ExoP ± 2.30 23 ± 4″ 10.4 ± 1.7D 0.37∆ ± 30
∆ ∆
RWO 0.33EsoP ± 1.89 1.29ExoP ± 2.10 27 ± 9″ 9.8 ± 1.9D 0.37∆ ± 0.24
p 0.983 0.927 0.612 0.912 0.49

Table 1. Distribution of Binocular Vision (BV) parameters at baseline. There were no significant differences in
any of the BV parameters prior to starting the trials: virtual reality indoor (VRI), virtual reality outdoor (VRO),
real world indoor (RWI) and real world outdoor (RWO). MT = Modified Thorington, AoA = Amplitude of
Accommodation, EsoP = Esophoria, ExoP = Exophoria.

was binocular, but the eyes are not independent, one eye was randomly determined by coin-toss per-participant
at the first visit, and this eye was used for all subsequent visits (42% left eyes). Automatic segmentation of the
choroidal-scleral interface was performed by the machine software (Topcon FastMap, Version 9, Topcon Medical
Systems, USA), and the segmented data was exported into Matlab for analysis. In rare cases where any of the 96
scan lines were missed (e.g. due to blink), or if the software reported poor choroidal image quality (<30), or if
the choroidal-scleral boundary was incorrectly identified by the automated software, the scan was immediately
repeated. Mean choroidal thickness measures, centred on the middle of the scan, were calculated for across the
subfoveal (central 1 mm), parafoveal (3 mm) and perifoveal regions (6 mm)28.

Statistical analysis. Baseline comparisons were normally distributed and compared with 1-way ANOVA
with environment as the factor. Some of the changes (post-pre) in the measures of binocular vision, fixation sta-
bility, and choroidal thickness were not normally distributed, and were compared with non-parametric Kruskal
Wallis tests, with post-hoc, paired-wise testing using Wilcoxon–Mann–Whitney using Šidák p-value correction
for multiple comparisons, as required. As all 19 participants completed four environments, this gave 3 degrees of
freedom between factors and 75 total degrees of freedom for all measures. Correlations were made with Pearson’s
R. Heart rate was monitored throughout each trial, rather than pre and post, so the mean heart rate measure of
the 40-minute period was compared between conditions. The datasets analysed during the current study are
available from the corresponding author on reasonable request. Differences were treated as significant at p < 0.05.

Results
Binocular Vision. There were no significant differences in distance or near phoria, stereopsis, amplitude of
accommodation, or fixation disparity between conditions at baseline (i.e. pre-trial, Table 1).
The change in binocular status (post-pre) was compared between each environment (Fig. 2). There was no
significant difference in the change of the distance phoria (VRI: −0.22 SD 0.618∆, VRO: 0.23 SD 0.843∆, RWI:
−0.30 SD 0.495∆, RWO: −0.13 SD 0.968∆, χΔ = 7.235, p = 0.065), near phoria (VRI: 0.66 SD 1.334∆, VRO:
0.53 SD 0.920∆, RWI: 0.55 SD 1.563∆, RWO: 0.26 SD 1.418∆, χ2 = 0.711, p = 0.871), maximum amplitude of
accommodation (VRI: −0.54 SD 0.639D, VRO: −0.29 SD 0.505D, RWI: −0.44 SD 0.653D, RWO: −0.45 SD
0.694D, χ2 = 0.866, p = 0.834), or stereopsis (VRI: −2.5 SD 8.6″, VRO: −1.3 SD 4.7″, RWI: +1.6 SD 8.3″, RWO:
−2.7 SD 7.1″, χ2 = 2.194, p = 0.533).
Participants had a mean inter-pupillary separation of 63.1 SD 2.58 mm (range 59–68 mm) at distance, and
58.8 SD 2.53 mm (range 55–64 mm) at near (0.4 m). The difference between an individual’s pupillary distance
and the fixed lens separation distance in the HMD (65 mm) was computed as the lens centre offset: higher abso-
lute values invoke greater horizontal prism as a factor of lens power. For participants with pupillary distances
equal or less than the lens separation, this prism creates additional convergence demand during near viewing.
Participants whose pupillary distance is greater than the lens separation, the induced prism depends on viewing
distance: a greater divergence demand during distance viewing, and greater convergence during near viewing.
Lens decentration was not correlated with individual changes in distance phoria (VRO: r = 0.412, p = 0.064, VRI:
r = −0.142, p = 0.540) near phoria (VRO: r = 0.040, p = 0.864, VRI: r = −0.354, p = 0.115), or the change in max-
imum accommodation (VRO: r = −0.157, p = 0.497, VRI: r = −0.158, p = 0.494).

Fixation Disparity. When the eyes converged on near objects in VR, the off-axis viewing through the pow-
erful HMD lenses would have induced opposing prism to each eye. For the lens powers, viewing distances, and
mean pupillary distances involved in this study, we calculate that this prismatic effect would approximately dou-
ble the convergence demand compared to real world viewing of near objects, and thus could have doubled the
potential stress to the visual system. In contrast, vertical off-axis viewing through the HMD lenses would have
induced yoked prism, which would effectively cancel, and therefore require no adaptation. We separated fixation
disparity into horizontal and vertical components to investigate the effect of any prismatic adaptation. There were
no differences in baseline measures prior to environmental exposures for either horizontal (F = 0.43, p = 0.733),
nor vertical fixation disparity (F = 0.4, p = 0.757). Also, there were no differences between the magnitudes of
horizontal and vertical disparities (t(75) = −1.051, p = 0.297).
Post-trial, there was no difference in the change of fixation disparity between environments in either the
challenged horizontal direction (VRI: 0.08 SD 0.56∆, VRO: −0.13 SD 1.22∆, RWI: −0.01 SD 0.60∆, RWO:
0.06 SD 0.60∆, χ2 = 0.874, p = 0.832), nor the unchallenged vertical direction (VRI: 0.22 SD 0.54∆, VRO: 0.00 SD
0.63∆, RWI: −0.14 SD 0.51∆, RWO: 0.20 SD 0.38∆, χ2 = 6.709, p = 0.082). Although participants with smaller

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Figure 2. Change in binocular status after 40-minute exposure to virtual reality indoor (VRI), virtual reality
outdoor (VRO), real world indoor (RWI) and real world outdoor (RWO) environments. There was no
significant difference in the effect of environment on any of the four measures of binocular vision (Kruskal
Wallis, all p > 0.05). Black crosses indicate group medians.

inter-pupillary distances would have experienced higher prismatic effects, there was no correlation between an
individual’s change in horizontal fixation disparity and their lens decentration in either VRI (r = 0.224, p = 0.330)
or VRO (r = −0.28, p = 0.219) groups.

Fixation Stability. Fixation stability, defined as a bivariate contour ellipsoid area (BCEA) containing 95% of
individual fixation samples, was consistent at baseline (VRI: 0.53 SD 0.406 minarc2, VRO: 0.80 SD 0.513 minarc2,
RWI: 0.68 SD 0.392 minarc2, RWO: 0.67 SD 0.520 minarc2, F = 1.13, p = 0.343). BCEA did not change post-trial,
and there was no obvious effect of virtual reality (VRI: 0.15 SD 0.450 minarc2, VRO: −0.06 SD 0.981 minarc2,
RWI: 0.55 SD 1.229 minarc2, RWO: 0.61 SD 1.511 minarc2, χ2 = 1.57, p = 0.203).

Choroidal Thickness. There was no difference in the baseline choroidal thickness values before starting
each environment (subfoveal: Mean: 294.1 SD 95.4 µm, F = 0.08, p = 0.971, parafoveal: Mean: 292.8 SD 89.3 µm,
F = 0.06, p = 0.981, perifoveal: Mean: 280.0 SD 78.2 µm, F = 0.08, p = 0.973). After the exposure, there was a sig-
nificant difference in the choroidal thickness change between the different environments in the subfoveal (VRI:
+13.9 SD 3.3 µm, VRO: + 9.3 SD 3.2 µm, RWI: + 0.5 SD 4.4 µm, RWO: +2.1 SD 4.3 µm, χ2 = 51.7, p < 0.001,
Fig. 3) and parafoveal zones (+14.6 SD 4.5 µm, VRO: +9.0 SD 8.3 µm, RWI: −1.2 SD 4.4 µm, RWO: +1.2 SD
5.1 µm, χ2 = 42.6, p < 0.001). The subfoveal increase after VRI was larger than both RW groups (both p < 0.001),
as was the change following VRO (vs RWI p < 0.001, RWO p = 0.003). The subfoveal change in choroidal thick-
ness was not different between VRO and VRI (p = 0.174). In the parafoveal area, the change in VRI was greater
than both real-world groups (both p < 0.001), as was the change in choroidal thickness after VRO (vs RWI:
p < 0.001, RWO: p = 0.025). Over the full 6 mm perifoveal scan length, despite higher variance, there was still
a difference between groups (VRI: 12.5 SD 7.8 µm, VRO: 7.2 SD 10.4 µm, RWI: −0.4 SD 6.8 µm, RWO: −1.0 SD
7.6 µm, χ2 = 25.5, p < 0.001). VRI was significantly higher than both RWI and RWO (both p < 0.001) groups, but
VRO was not (RWI, p = 0.085, RWO, p = 0.059). Individual changes in choroidal thickness were not correlated
with light levels (r − 0.02, p = 0.149), nor lens decentration (r = 0.04 p = 0.563).

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Figure 3. There was significant increase in the choroidal thickness after 40 minutes in both VR groups at
subfoveal (1 mm) and parafoveal (3 mm) regions of the choroid (Kruskal Wallis, <0.001). In contrast, there
was no significant increase in choroidal thickness in either of the real-world groups. Black crosses mark group
medians.

Figure 4. Heart rate was significantly higher in RWO (left figure, Kruskal Wallis, p < 0.001), but was not
different between RWI, VRI, or VRO. However, the mean heart rate during each trial was not correlated in the
change in choroidal thickness (right, Pearson’s r = 0.03, p = 0.800).

Heart Rate. Heart rate was continuously measured during each 40-minute environmental exposure.
Participants had a higher heart rate (beats per minute, bpm) during the VRO condition than all other conditions
(VRI: 77.3 SD 11.1 bpm, VRO: 77.3 SD 10.9 bpm, RWI: 71.2 SD 7.4 bpm, RWO: 94.2 SD 14.0, χ2 = 24.7, p < 0.001,
Fig. 4 left). There was no difference in heart rate between the two VR conditions (p = 0.999), nor VRI and RWI
(p = 0.545), nor VRO and RWI (p = 0.600). There was also no correlation between heart rate and the change in
choroidal thickness (n = 76, r = 0.03, p = 0.800, Fig. 4 right).

Discussion
A 40-minute exposure to a VR HMD environment appears to have minimal effects on the binocular vision sys-
tem. The fixed accommodative demand in a HMD, and the associated accommodative-convergence disconnect
did not affect the dissociated position of the eyes either at distance or near, nor the maximum amplitude of
accommodation, suggesting no accommodative fatigue. As the HMD lenses are relatively high powered (approx-
imately 25D), eye movements away from the lens centres will have induced large prismatic effects. While this is of
little importance for version eye movements where the prism will be yoked, vergence eye movements, stimulated
by viewing objects at different depths, will create asymmetric prism and potentially binocular vision stress. If
eye position (and therefore induced prism) remains relatively constant, such as the case in the indoor VR scene,
the constant prism could cause the binocular vision system to adapt (which would then be expected to re-adapt
once out of VR). However, we found no difference in binocular fixation disparity between horizontal and vertical
directions, and all measures fell well within reported normal population ranges25. This indicates that if there is
any binocular stress, it is likely minimal or able to dissipate by the time second measurement was taken (between
five and ten minutes post exposure), and is not significantly higher after VR use compared to an equivalent task
conducted in the real world.
While there were no overall effects of VR on the binocular vision system in our adult cohort, there may still
be of concerns if children use VR headsets which have been calibrated for adults, as children tend to have smaller

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pupillary distances29. Some VR headsets allow the lens centration distance to be adjusted, but many phone-based
headsets, which may be more available to children, do not currently allow lens separation adjustment.
Somewhat unexpectedly, we found a significant increase in choroidal thickness after VR HMD wear. As
briefly discussed in the introduction, the opposite effect (choroidal thinning) would be expected to occur when
the eye is exposed to a myopiagenic stimulus17. This effect may be caused by the unique optical arrangement
inside HMDs causing a lead of accommodation. Choroidal thickening occurs after myopic defocus20,30, which
is associated with a slowing of eye growth16, and a reduction in myopia progression31. We hypothesise that the
choroidal thickening effect seen after VR HMD wear is a consequence of the fixed viewing distance, combined
with convergence-induced accommodation in the virtual environment. Viewing objects at near in the real world
involves both accommodation and convergence, but is typically associated with a lag of accommodation32, and
therefore hyperopic retinal defocus, which has been proposed as a stimulus for myopia development and progres-
sion33. However, the lenses in a HMD are generally set for distance viewing (i.e. primary position of gaze), and
convergence of the eyes results in off-centre viewing through the lenses. This induces a base-out prismatic effect,
and an image displacement which increases the amount of convergence required compared to an equivalent
real-world task. Thus, participants would need to execute exaggerated convergent eye movements to view virtual
near objects. This would be expected to strengthen any associated convergence-induced accommodation34, and
as the HMD screen distance is fixed, this would create a lead of accommodation and a myopically defocussed
retinal image, which causes choroidal thickening20. Relatively small amounts of image defocus are not necessarily
associated with a decrease in visual acuity35, and the limited spatial resolution of the screens would help mask
any subtle defocus. The more contained indoor virtual environment would have been associated with higher
convergence, thus more convergence-induced accommodation and greater myopic retinal defocus compared to
the distant virtual outdoor environment. Therefore, the virtual indoor environment would be expected to show
greater choroidal thickening than the virtual outdoor environment, which our results support. However, further
longitudinal research is required to determine whether this change in choroidal thickness could influence myopia
progression. Future improvements to VR displays, such as the move towards light field displays, will address the
convergence-accommodative conflict, but would also remove this potential lead of accommodation as a pro-
tective mechanism against myopia. Another factor which may have caused the eyes to accommodate and thus
created myopic retinal defocus while wearing the HMD, is the effect of proximal accommodation36, and this may
help account for the observation that both VR environments were associated with choroidal thickening, while
the real environments were not. An alternative explanation for choroidal thickness could be changes in ambient
air temperature within the HMD, or modification of blood flow directly related to the wearing of the headset
itself. The choroid is a dense vascular layer at the back of the eye, which also has a function to draw heat energy
away from the eye37,38, and changes in peripheral blood vessel dilation can be detected through changes in ocular
surface temperature39. While wearing the VR headset, the air temperature inside the headset quickly approached
skin temperature, and this may have influenced the thermal gradient from the anterior to posterior eye by modi-
fying tear film evaporation40 or aqueous humour dynamics41, ultimately causing a change in choroidal blood flow.
It should be noted that our preconditioning — in which all participants used a computer for at least 45 minutes
— would have likely caused thinning of the choroid at baseline, and this is supported by the minimal change in
choroidal thickness after the real world indoor condition. Therefore, we can only say that VR caused choroidal
thickening relative to an equivalent real world task.
While the study was well powered using a repeated measures cross-over design, there are limitations to the
extrapolation of our results. The 40 minute trial length was a compromise between convenience for participants,
while being sufficiently long enough to detect changes in choroidal thickness20, and much longer than previous
studies6 which were able to show an effect on binocular vision. However, in practice, some users are likely to
spend much longer periods than 40 minutes in VR. Also, our study only investigated the effect on a cohort with
normal binocular vision. As converging on near objects in VR creates a higher convergence demand than in real
world viewing, those with abnormal binocular vision, such as myopic children who may already excessively con-
verge at near42, could still become symptomatic. Further, while the choroidal thickness change is promising, we
used an adult cohort who are unlikely to be at significant risk of myopic progression. Additional work is needed
to investigate the cause of this change, and whether the mechanism of choroidal thickening could help protect
against myopia progression in children.

Conclusions
Using a VR headset for 40-minutes did not appear to affect the binocular vision status, compared to a real world
equivalent task. However, an unexpected finding was that choroidal thickness markedly increased when using a
VR headset. We hypothesise that this was due to convergence-induced accommodation when viewing near vir-
tual objects which would have created a myopically defocussed retinal image of the virtual environment because
of the fixed viewing distance. Myopic retinal defocus causes choroidal thickening in humans: it is also associated
with a reduced myopia progression rate in children. On this basis, our results suggest that VR HMD wear may not
provide a myopia-inducing stimulus despite the close viewing distances involved.

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Acknowledgements
Thanks to Amy Young, Gunnika Gill, James Lee, and Anh-Dao Le for helping with the collection of data.

Author Contributions
Both authors wrote and reviewed the manuscript. P.T. designed the experiment, and prepared the figures.

Additional Information
Competing Interests: The authors declare that they have no competing interests.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and
institutional affiliations.

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A comparative study of near lateral phoria of young adults using the von
graefe technique and maddox wing

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 International Journal of Research and Review
Vol. 9; Issue: 5; May 2022
Website: www.ijrrjournal.com
Research Paper E-ISSN: 2349-9788; P-ISSN: 2454-2237

A Comparative Study of Near Lateral Phoria of

Young Adults Using the Von Graefe Technique and
Maddox Wing
Azuamah Y. C.1, Ukonu S. C.2, Esenwah E. C.3, Ikoro N. C.4, Megwas A. U.5
1-5
Department of Optometry, Federal University of Technology, Owerri, Nigeria
Corresponding Author: Azuamah Y. C.

DOI: https://doi.org/10.52403/ijrr.20220543

ABSTRACT INTRODUCTION
Heterophoria is a latent deviation in
Near lateral phoria is common among which the eyes have a constant tendency to
individuals and can lead to visual problems. It deviate and this occurs when one eye is
could be in the form of exophoria or esophoria.
covered. This deviation is overcome by
This study was a clinical study carried out at the
Department of Optometry Teaching Clinic,
muscular effort because of the strong desire
Federal University of Technology, Owerri, Imo to maintain binocular single vision. [1] A
state, Nigeria. An informed consent was gotten person with two normal eyes has single
from all the subjects who were part of the study. vision because of the combined use of the
The near lateral phoria was measured using the sensory and motor systems. The motor
von Graefe technique and was repeated using system acts to point both eyes at the target
the Maddox wing. A total of 60 subjects and any offset is dictated visually.
between the ages of 18 and 30 years were used Heterophoria only occurs during
for this study. Out of this number, 49 (81.67%) dissociation of the left and right eye, when
has exophoria, 6 (10.00%) had esophoria, and 5 fusion is absent. This tendency is identified
(8.33%) had orthophoria. Results showed that
by occurrence of an actual deviation in the
using the von Graefe technique, the mean
exophoria value was 5.67±2.77 and 4.37±2.89
absence of adequate stimulus to fusion,
with the Maddox wing. Also, the mean occurring in various designated forms
esophoria value was 1.33±0.82 with the von according to relative direction or orientation
Graefe technique and 1.00±0.00 with the of deviation. [2] When the eye deviates
Maddox wing. Data analysis with the SPSS outward, it is termed exophoria and when
version 23 data output using the Paired sample the eye deviates inward, it is termed
T-test at 0.05 level of significance and 95% esophoria. When no deviation exists, it is
confidence interval showed that there was a termed orthophoria. Exophoria is a latent
significant difference in near lateral phoria deviation outward from the line of sight
values (P< 0.05) between the von Graefe while esophoria is an inward deviation from
technique and the Maddox wing method of the line of sight. The expected value of near
measuring near lateral phorias among young
phoria is about 4 to 6∆ (prism diopters) of
adults. Further research among other age groups
was recommended. exophoria [1], but it is not unusual for a
patient’s near phoria finding to be as high as
Keywords: Exophoria, Esophoria, Orthophoria, 10 to 12∆ of exophoria or 4 to 5∆ of
Heterophoria, Near lateral phoria. esophoria. At distance, the expected near
phoria is 0 to ½∆ exophoria. The tendency
for asymptomatic individuals to have this
moderate amount of exophoria at distance

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Azuamah Y. C. et.al. A comparative study of near lateral phoria of young adults using the von Graefe technique
and Maddox wing

and near is termed physiological exophoria. vertigo. [7] Symptoms are persistent and not
[3]
relieved in spite of correction of refractive
errors which may be associated with it.
The etiology of heterophoria has been
Eyestrain, asthenopia, diplopia, headaches
attributed to three main categories [4];
are some of the symptoms experienced by
(1) Anatomical factors which include;
people with lateral phorias. When the
interpupillary abnormalities, abnormal
condition persists, it can lead to strabismus
strength or structure of the extraocular
which is manifest deviation resulting from
muscles, variation to the optical axis of
the paralysis. [6]
the eye;
Von Graefe technique is the most
(2) Physiological factors which include:
commonly used technique for measurement
Age; esophoria is more common in
of lateral and vertical phorias. It employs a
young age groups as compared to
measuring prism over one eye and
exophoria; Accommodation; increased
dissociating prism over the other eye. The
accommodation leads to esophoria as
tests are done under bright illumination at
seen in hypermetropia;
both far and near. The Maddox wing is an
(3) Neurogenic causes; lower motor neuron
instrument utilized in the measurement of
disease leads to incomitant heterophoria
near phoria. It is a quantitative and
and upper motor neuron disease leads to
subjective method of measuring the size of
comitant heterophoria.
phoria deviation by dissociation of the eyes
Heterophoria and heterotropia have brought about by two septa which are
the same etiological factors. They are placed in such a way as to present fields to
differentiated solely by patient’s ability to either eye separated by a diaphragm at the
overcome the deviation. Slight phoria is center. The Maddox wing measures the size
present in most normal individuals and is of the heterophoria and some of heterotropia
overcome by the fusion reflex. Heterophoria at near when normal retinal correspondence
condition can be esophoria, where the eye is present. It is especially helpful when the
tends to cross inward in the absence of patient present with symptoms of diplopia
fusion; exophoria in which they diverge, with no apparent cause. [8]
hyperphoria, in which one eye points up or
down relative to the other. [2] Heterophoria MATERIALS AND METHODS
is usually asymptomatic. In severe cases, This study was a clinical study
when the heterophoria is not overcome by carried out at the Department of Optometry
fusion vergence, signs and symptoms Teaching Clinic, Federal University of
appear, this is called decompensated Technology, Owerri, Imo state, Nigeria. An
heterophoria. [5] When fusion is insufficient informed consent was gotten from all the
to control the imbalance, the phoria is subjects who were part of the study. Ethical
described as decompensating and is often approval for this study was obtained from
associated with symptoms of binocular the Ethics Committee of the School of
discomfort, asthenopia (eyestrain), Health Technology, Federal University of
headache, photophobia (increased Technology Owerri, Nigeria. The case
sensitivity to light), difficulty in changing history, ophthalmoscopy and external eye
focus from near to distance and vice versa, examination of the subjects were carried out
and double vision (diplopia). [6] to rule out people with ocular pathological
Near lateral phoria is common conditions. The near lateral phoria was
among individuals, the most common being measured using the von Graefe technique
lateral deviation. It could be in the form of and was repeated using the Maddox wing.
exophoria or esophoria and this can lead to
visual problems. Patient usually experience
occasional diplopia, severe headache with

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Vol. 9; Issue: 5; May 2022
Azuamah Y. C. et.al. A comparative study of near lateral phoria of young adults using the von Graefe technique
and Maddox wing

Procedure for Von Graefe Technique Statistical Methods

The rotary prism on the phoropter The data obtained from the study
was placed before the right eye and turned was uploaded into the Statistical Package
to 12ΔBI (Base In) serving as the measuring for Social Sciences (SPSS) version 23
prism. The prism on the left eye was turned software. The Paired-Sample T test was
to 6ΔBU (Base Up) serving as the used to test the hypothesis at 0.05 level of
dissociating prism. To measure the lateral significance and 95% confidence interval.
phoria at near, the patient focused at the best
VA line on the reduced Snellen chart placed RESULTS
at 40cm. The prisms double the images of A total of 60 subjects between the
the chart, one above the other. The upper ages of 18 and 30 years were used for this
target was seen with the right eye and the study. Figure 1 showed the distribution of
lower with the left eye. The upper target phoria status of subjects. Exophoria had a
was seen to the right of the lower target. On frequency of 49 subjects and percentage of
confirmation that the patient saw the two 81.67%, esophoria was 6 and a percentage
targets, the BI prism before the right eye of 10.00% while orthophoria had a
was reduced gradually until the patient frequency of 5 and a percentage 8.33%.
indicated vertical alignment of the upper Table 1 showed that using the von Graefe
and lower target. The amount of prism technique, exophoria of 1-4Δ had a
diopters remaining in the measuring prism frequency of 18 and a percentage of
was noted. Any BI prism remaining was 30.00%; 5-8Δ exophoria was 24 and a
recorded as prism diopter of exophoria. If percentage of 40.00%; 9-12Δ exophoria was
the prism has crossed the zero mark into 7 and a percentage 11.67%. The table 1 also
BO, it was recorded as prism diopters of showed that 1-4Δ esophoria had a frequency
esophoria. If it was at the zero mark, it was of 6 subjects and a percentage of 10.00%, 5-
orthophoria. 8Δ and 9-12Δ had a frequency of 0 and a
percentage of 0.00%. Table 2 showed that
Procedure for Maddox Wing using the Maddox wing, exophoria of 1-4Δ
The Maddox wing test was had a frequency of 30 and a percentage of
performed at near with the instrument held 50.00%; 5-8Δ exophoria was 15 and a
in the reading position, slightly inferior percentage of 25.00%, 9-12Δ exophoria was
(approximately 15-degree depression and 2 and a percentage of 3.33%; 13-15Δ
33cm away). The patient was instructed to exophoria was also 2 and a percentage of
hold the Maddox wing and identify the 3.33%. The table also showed that 1-4Δ
number that the white and red arrows point esophoria had a frequency of 6 subjects and
to on their respective scales. The patient a percentage of 10.00%; 5-8Δ, 9-12Δ, and13-
held the Maddox wing up to his eyes and 15Δ had a frequency of 0 and a percentage
looked into the eye piece as if reading a of 0.00%.
book. The patient was asked if the arrow Table 3 showed that the frequency of
was pointing at a number or in-between two subjects with orthophoria using von graefe
numbers. The subject’s answer was noted technique and Maddox wing was 5 and a
and recorded. Only odd numbers show on percentage of 8.33%. Table 4 showed that
one side of the scale and even numbers on using the von Graefe technique, the number
the other. The white arrow on the X-Axis of subjects with exophoria was 49, the
measured for lateral deviations in which, maximum value was 12.00Δ, the minimum
odd numbers represent an “eso” deviation value 1.00Δ, the mean was 5.67Δ and the
and even numbers represent an “exo” standard deviation is 2.77Δ. Using the
deviation. In the absence of any deviation, Maddox wing, the number of subjects were
both red and white arrows point to zero, 49, the maximum value was 13.00Δ, the
indicating orthophoria. minimum value was 1.00 Δ, the mean was

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Vol. 9; Issue: 5; May 2022
Azuamah Y. C. et.al. A comparative study of near lateral phoria of young adults using the von Graefe technique
and Maddox wing

4.37Δ and the standard deviation was 2.89 Δ. Table 4: Descriptive statistics of near exophoria values
Phoria Method n Range Max Min Mean S.D
Table 5 showed that using von Graefe Von Graefe 49 11.00 12.00 1.00 5.67 2.77
technique, the number of subjects with Maddox Wing 49 12.00 13.00 1.00 4.37 2.89
n = number; Min = Minimum; Max = Maximum; S.D =
esophoria was 6, the minimum value was Standard Deviation
1.00Δ, the maximum value was 3.00Δ, the
mean was 1.33Δ and the standard deviation
Table 5: Descriptive statistics of near esophoria values
Phoria Method n Range Max Min Mean S.D
was 0.82Δ. Using the Maddox wing, the Von Graefe 6 2.00 3.00 1.00 1.33 .0.82
Maddox Wing 6 0.00 1.00 1.00 1.00 0.00
number of subjects was 6, the minimum n = number; Min = Minimum; Max = Maximum; S.D =
value 1.00Δ, the maximum value was 1.00Δ, Standard Deviation
the mean was 1.00 and the standard
deviation was 0.00. Data analysis with the DISCUSSION
SPSS version 23 data output using the Lateral phoria is an error of
Paired sample T-test at 0.05 level of binocular alignment that manifest only
significance and 95% confidence interval during monocular viewing conditions or
showed that there was a significant conditions that disrupt binocular vision.
difference in near lateral phoria values There are several methods in which near
[P(0.01) < 0.05] between the von Graefe lateral phoria can be measured. This study
technique and the Maddox Wing method of compared the near lateral phoria using Von
measuring near lateral phorias. Graefe technique and Maddox Wing.
Exophoria was found to be the most
common with a percentage of 81.67%
followed by esophoria with 10.00%. From
this study, there was a significant difference
in the lateral phoria values obtained from
Von Graefe technique and Maddox wing
with mean exophoria values being
5.67±2.77 and 4.37±2.89 respectively. The
mean esophoria values were 1.33±0.82 and
1.00±0.00 respectively. Thus, phoria values
with von graefe technique and Maddox
Figure 1: Distribution of near lateral phoria
wing were not the same. This is similar to
Table 1: Distribution of near exophoria and esophoria values the work carried out by Justin et al. [9] who
using Von Graefe Technique
Phoria (Δ) Exophoria Esophoria
compared the von Graefe technique with the
n % n % Maddox rod for distance phoria and also
1-4 18 30.00 6 10.00 found a significant difference. Oman et
5-8 24 40.00 0 0.00
9-12 7 11.67 0 0.00 al.[10] studied heterophoria in children and
TOTAL 49 81.67 6 10.00 reported a mean of 2.50±3.97 exophoria for
Table 2: Distribution of near exophoria and esophoria values lateral phoria at near using the von Graefe
using Maddox Wing technique. However, another study by Jody
Phoria (Δ)
n
Exophoria
% n
Esophoria
%
et al.[11] found the near horizontal phoria
1-4 30 50.00 6 10.00 values to be 3.84±4.80. Many studies [12-14]
5-8 15 25.00 0 0.00 have established that the von Graefe
9-12 2 3.33 0 0.00
13-15 2 3.33 0 0.00 technique gave a more reliable result of
TOTAL 49 81.67 6 10.00 phoria values. Phoria values can be
Table 3: Distribution of near orthophoria values with Von
influenced by many factors such as near
Graefe Technique and Maddox Wing work, refractive errors, accommodation-
Phoria Method n % vergence anomalies and amplitude of
Von Graefe Technique 5 8.33 accommodation. All these factors impact
Maddox Wing 5 8.33
more when using the Maddox wing as stated
by Dweyer[15] who did extensive studies

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Azuamah Y. C. et.al. A comparative study of near lateral phoria of young adults using the von Graefe technique
and Maddox wing

with the Maddox Wing. Jonathan et al.[16] using the Von Graefe technique was
conducted a study to attempt to improve the significantly different from that of the
reliability of the Maddox wing test for the Maddox wing test. This study showed a
measurement of horizontal heterophoria at significant difference in near lateral phoria
near. values using the Von Graefe and Maddox
Low amplitude of accommodation wing. Further research was advocated
serves as stimulus in exerting among other age groups.
accommodation effort. This effort will
influence convergence resulting in excessive Acknowledgement: None
esophoria. Changes in the phoria could be Conflict of Interest: None
potentially due to the amount of near work Source of Funding: None
the person is subjected to. Therefore, phoria
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Yoon et al. BMC Ophthalmology (2020) 20:200
https://doi.org/10.1186/s12886-020-01471-4

RESEARCH ARTICLE Open Access

Influence of virtual reality on visual

parameters: immersive versus non-
immersive mode
Hyeon Jeong Yoon, Jonghwa Kim, Sang Woo Park and Hwan Heo*

Abstract: Background: To investigate the differences in refraction, accommodative factors, visual parameters, and
subjective symptoms after using two types of virtual reality (VR) content with different depths of perception.
Methods: Twenty-three volunteers, who played VR games in two modes (immersive and non-immersive) for 30
min, were enrolled. Visual parameters were examined before and after using VR. Accommodative factors were
measured using static and dynamic methods. Subjective symptoms were assessed using a questionnaire.
Differences according to VR content were compared, and correlations between each visual parameter were
analyzed.
Results: There were no changes in refraction or accommodative factors after use of the VR. However, there was a
significant increase in the near point of accommodation (NPA), the near point of convergence (NPC), and subjective
symptom scores after using the immersive mode. Correlation analysis revealed a positive correlation between
baseline values of near exophoria and mean accommodative lag of the dominant eye, and also revealed a negative
correlation between NPA and mean accommodative lag in the non-dominant eye.
Conclusions: The use of VR for 30 min increased NPA and NPC, especially after the immersive mode was used. In
addition, higher exophoria and smaller NPA is associated with increased accommodative lag after using VR.
Keywords: Accommodation, Asthenopia, Convergence, Virtual reality

Background min is discouraged, although these warnings are based

A virtual reality (VR) device is an immersive medium on weak evidence [1]. Many studies have reported that a
that uses a head-mounted display (HMD). VR creates significant proportion of VR device users experience a
the sensation of being entirely transported into a virtual highly aversive sense of discomfort, disorientation, nau-
three-dimensional (3D) world, which can provide a far sea, and motion sickness, and these reports suggest that
more visceral experience compared to other video for- viewing stereoscopic images on 3D devices may induce
mats. Recently, VR devices have become available for visual asthenopia, such as visual discomfort and fatigue
purchase on the Internet from a variety of manufac- [2–6]. It is essential to study the effects of VR devices on
turers, and have been widely used for gaming. the eye, as HMD images are presented to users at a
However, the proper use of VR has not yet been estab- short distance with a powerful convex lens to simulate
lished. In health and safety warnings provided by manu- 3D reality. Compared with older types of VR, many im-
facturers, VR is not recommended for use by children < provements have been made to reduce user discomfort
13 years of age. Additionally, continuous use for > 30 including image resolution and corresponding processes
for head movement. However, there are few ophthalmo-
* Correspondence: opheye@hanmail.net logical studies investigating the effects of recent itera-
Department of Ophthalmology, Chonnam National University Medical School
and Hospital, 42 Jebong-ro, Dong-gu, Gwangju 61469, Republic of Korea
tions of VR devices [7–11].

© The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if
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data made available in this article, unless otherwise stated in a credit line to the data.
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 2 of 8

Ha et al. [7] investigated the clinical effects of the HMD covered the field of view. The headset included separate
on visual function, including the oculomotor system; they displays for each eye, each with 960 × 1080 resolution,
found no significant clinical changes, except for transient yielding a 100-degree-horizontal field of view. A fixed-
refractive error or binocular alignment. However, the study degree convex lens was located in front of each display
adopted the method of watching movies using VR rather rendered display content at optical infinity. Inter-
than immersive content such as games. In that case, the pupillary distance was adjusted via a user-enabled key
perceived depth was fixed at one distant point; therefore, it that was located on the right side of the VR device.
is possible that visual parameters change and the Participants used the Oculus Rift device while seated
accommodation-convergence conflict is not fully induced, on a freely rotating chair. They were asked to perform
compared with immersive content, which has variable per- 30 min of gameplay (Minecraft, Mojang AB, Sweden) in
ceived depth. In addition, accommodative change was not two different modes (immersive and non-immersive).
evaluated. It is necessary to investigate accommodative There was a 1-week interval between playing in immer-
change, as it can be related to a user’s transient or perman- sive mode and non-immersive mode. In the immersive
ent myopia, in addition to subjective symptoms [12–14]. mode, the stereo head-tracking head-mounted display
Turnbull et al. [15] reported that refraction and bin- presentation brought the player inside the 360-degree
ocular status (e.g. gaze stability, stereopsis, and ampli- virtual reality environment, which allowed the user to
tude of accommodation) did not change after VR trials. feel as if they were physically present in the game. The
However, the choroid, which is a pigmented vascular tis- viewpoint moves in accordance with the player’s head
sue located outside of the eye, was thickened. In that movements. In the non-immersive mode, the player is
study, investigators used virtual indoor and outdoor en- placed in a static environment (e.g., a living room) while
vironmental content. They also examined accommoda- watching the VR environment on a desktop screen that
tive amplitudes, but they did not analyze their was approximately 2 m away (a desktop view). The
correlation with other visual parameters. players could look around the room; however, the area
In the present study, we examined changes in objective of gameplay was fixed on a virtual screen in front of the
visual parameters and subjective symptoms after playing player (Additional file 1).
two modes of VR content – immersive and non-
immersive – each with a different perception depth. The Measurements of accommodation
change in accommodation was evaluated using static and Refraction and accommodation were measured using a
dynamic methods. In addition, correlations between visual binocular open-field refractor (Auto Ref/Keratometer
parameters after playing VR content were evaluated. WAM-5500, Grand Seiko Co Ltd., Hiroshima, Japan).
The spherical equivalent (sphere + 1/2 of cylinder) was
Methods used for calculation. For static measurement, the accom-
Thirty-five healthy volunteers who had better than 16/20 modative amplitude was calculated by subtracting the
uncorrected visual acuity with above 20/20 best- refractions obtained under monocular condition while
corrected visual acuity were recruited. The subjects had viewing a 1 cm × 1 cm E-shape target at 33 cm from
no ophthalmologic diseases, including strabismus, am- those obtained from viewing the target at 5 m in the
blyopia, corneal or retinal disease, or a history of ocular same manner.
surgery, except for refractive surgery. The number of Software verified by the manufacturer was installed on
subjects was calculated using G-power version 3.1 a computer to allow dynamic mode function. To initiate
(Heinrich Heine University, Dusseldorf, Germany) and measurements, the instrument was aligned with the
considered a drop-out rate of 20%. Twelve subjects with pupil of each eye, and the joystick button was pushed
exophoric deviation > 10 prism diopters (PD) and/or and then released once; the instrument then commenced
esophoric deviation > 5 PD were excluded. Informed recording dynamic measurements at approximately 5
consent was obtained from all 23 volunteers who were samples/s. The observer ensured that the instrument
eligible and ultimately enrolled in this study. Ethics com- remained carefully aligned with the subject’s right eye
mittee approval was obtained from the Chonnam Na- while undergoing dynamic measurements by observing
tional University Hospital Institutional Review Board the alignment target imaged within the pupillary center
(Gwangju, Korea). The study protocol adhered to the in the LCD monitor for the entire duration of testing.
guidelines of the Declaration of Helsinki. The instrument wrote the data to a spreadsheet file
(Excel, Microsoft Corporation, Redmond, WA, USA)
Display that recorded the time of measurement, eye measured,
The Oculus Rift VR device (Oculus VR, LLC., Irvine, spherical equivalent refraction, and pupil diameter ap-
California, USA) was used in this study. The device com- proximately every 0.2 s and converted it to a sine graph
prised a lightweight (0.44 kg) headset that completely form [16, 17]. The velocity of accommodation, mean
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 3 of 8

accommodative lag, and dynamic accommodative re- from the subject during the test. Threshold stereopsis
sponse was investigated to measure dynamic level was recorded in seconds of arc.
accommodation. Ocular dominance was determined by the hole-in-the-
The velocity of accommodation was obtained by card test. The participant was asked to hold a card with
calculating the difference between the maximum and a hole at arm’s length and focus on a target 3 m away
minimum refraction divided by the time taken. Mean with both eyes. The examiner alternately occluded eyes
accommodative lag was calculated by averaging the to determine which eye was viewing the target through
value of the participant’s actual refraction that dif- the hole and that eye was determined to be the domin-
fered from the target refraction. The dynamic accom- ant eye [18].
modative response was calculated according to the The presence and magnitude of far (5 m) and near
dispersion between actual refraction and target refrac- (33 cm) phoria were verified using the cover test and
tion. A higher correlation coefficient was associated alternating cover test with prism. A standard set of
with better dynamic accommodative response (Fig. 1). loose plastic prisms was used, which individual prisms
increased in power from 1 to 10 PD in 1-PD
Other visual parameters increments, and from 10 to 20 PD in 2-PD
Monocular near point of accommodation (NPA) was ob- increments. All measurements were repeated three
tained using Donder’s push-up method. A 20/30 single times for each test, and results reported as the mean
letter about 50 cm from the subject on a fixation stick value [2].
served as the target, and was moved gradually closer to All measurements were performed before and im-
the participant at a rate of about 5.0 cm/s until the par- mediately after playing the VR game in the order
ticipant noticed the target starting to blur. The near listed above. If visual parameters were changed, it was
point of convergence (NPC) was also obtained using the measured repeatedly every 15 min until the initial
same method as previously described for the NPA meas- value was obtained again. The criteria for re-
urement. The first point at which the corneal reflex of examination were > 2-cm changes in NPA and NPC,
the subjects began to extend outward was considered to over 20 s of arc change of stereopsis, and over 0.5 D
be the endpoint [2]. change of refraction. Ocular phoric deviation was
Near stereopsis was measured using a near stereopsis evaluated by the same pediatric ophthalmologist
vision test (Stereo Fly SO-001 test; Stereo Optical Co., (H.H.). The other visual parameters were examined
Chicago, IL, USA). The test stereogram was held 40 cm by a single examiner (H.J.Y.).

Fig. 1 Dynamic accommodative response was calculated according to the dispersion between actual refraction and target refraction (a
demonstrates a better dynamic accommodative response than b)
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 4 of 8

Evaluation of subjective symptoms after using VR. Near stereopsis and phoria were not sig-
Thirteen symptoms were included in the questionnaire. nificantly different in either mode (Table 1).
The questionnaire was based on a computer vision syn- Table 2 summarizes the comparisons between subject-
drome questionnaire previously described by Seguí del ive symptoms according to the immersive and non-
M et al. [19]. The symptom sensation questionnaire con- immersive VR modes. Tearing, blurred vision, double vi-
tained six identical analog scales (0 = none, 6 = too se- sion, difficulty focusing for near vision, and neurological
vere to tolerate) through which the subject recorded the symptoms, including headache, dizziness and nausea
magnitude of each of the symptoms compared with were more severe in the immersive mode than the non-
baseline. After playing two modes of the VR game, the immersive mode of VR (all p < 0.05). Table 3 summa-
subjects completed the questionnaire. rizes the changes in accommodation using static and dy-
namic measurements with the WAM-5500 binocular
refractor after using VR. There was no significant change
Statistical analysis
in accommodative amplitude, velocity of accommoda-
Statistical analysis was performed using SPSS version
tion, mean accommodative lag, or dynamic accommoda-
18.0 (IBM Corporation, Armonk, NY, USA) for Win-
tive response in both modes.
dows (Microsoft Corporation, Redmond, WA, USA).
Table 4 summarizes the correlations between the base-
The normal distribution for all variables was assessed
line data of visual parameters (exophoria at far/near,
using the Kolmogorov-Smirnov test. All variables were
NPA in the dominant/non-dominant eye and NPC),
not normally distributed. Data are presented as the me-
changes in ocular and accommodative parameters, and
dian (interquartile range). A Wilcoxon signed-rank test
the sum of subjective symptom scores after using VR.
was used to compare changes in variables before and
There was a positive correlation between baseline values
after performing VR. Differences in subjective symptoms
of near exophoria and mean accommodative lag of the
according to the contents were also compared using the
dominant eye (r = 0.372, p = 0.014). The NPA in the
Wilcoxon signed-rank test. Spearman’s rho correlation
non-dominant eye exhibited a negative correlation with
test between each of the visual parameters was used for
changes in mean accommodative lag of the dominant
correlation analysis. The variables for a single eye, in-
eye (r = − 0.328, p = 0.032). Two correlation measure-
cluding NPA and accommodative parameters, were
ments were excluded due to a false positive rate above
solely correlated with the corresponding eye. For all
0.3: exophoria versus changes of the NPA and the NPC
tests, statistical significance was determined to be p <
versus changes of mean accommodative lag.
0.05. with differences corrected by the Benjamini-
No visual parameters were found that directly corre-
Hochberg procedure using false discovery rates of 0.25.
lated with symptoms. There was no correlation between
total symptom score and baseline value of visual param-
Results eters (all p > 0.05; Table 4). This result was the same as
Among the 23 participants, 11 were men and 12 were the analysis using logistic regression (all p > 0.05). Add-
women, with a mean age of 23.9 ± 3.7 years (range, 20– itionally, we examined the correlations between visual
35 years). The mean uncorrected visual acuity (logarithm parameters and each of the subjective symptoms, and
of the minimum angle of resolution [logMAR]) was there were no statistically significant correlations (all p >
0.03 ± 0.04 logMAR. Fifteen of the participants had pre- 0.05).
vious experience with VR within 1 h, which was outside
of this experiment. Eleven of the participants had a his- Discussion
tory of refractive surgery for myopia. One participant VR is a technology that renders a proximal display to be
was excluded from the analysis after discontinuing the perceived as a real-world experience using powerful con-
immersive mode of VR due to severe headache and vex lenses. In a VR environment, accommodation is
nausea. fixed to a single depth of field at a distant point. How-
In the immersive mode, the mean refractive error of ever, convergence is constantly induced. The resulting
both eyes did not change significantly (p = 0.935 in the accommodation-convergence conflict and sustained eye-
dominant eye; p = 0.654 in the non-dominant eye). How- ball movement are known to cause fatigue and 3D asthe-
ever, the NPA was increased in both eyes (p = 0.005 in nopia [8, 20–22].
the dominant eye, p = 0.002 in the non-dominant eye). In this study, the NPA and NPC increased in the im-
The NPC was also increased in the immersive mode mersive mode. In the immersive mode, more image dis-
(p = 0.001). In the non-immersive mode, the mean re- parity can occur than in the non-immersive mode.
fractive error did not change significantly for either eye Image disparity activates an accommodative response
(p = 0.261 in the dominant eye; p = 0.881 in the non- and convergence-accommodation to a change in accom-
dominant eye). Only the NPC (p = 0.002) was increased modation [23–25]. However, the actual accommodative
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 5 of 8

Table 1 Comparison of changes in visual parameters after playing a virtual reality game with different depths perception
Variable Pre Post p-value
Immersive mode
Refraction (dominant eye), Diopter −0.38 (0.63) − 0.25 (0.50) 0.935
Refraction (non-dominant eye), Diopter −0.19 (0.81) −0.13 (0.81) 0.654
NPA (dominant eye), cm 8.50 (3.00) 10.0 (3.50) 0.005*
NPA (non-dominant eye), cm 8.50 (3.00) 10.0 (3.50) 0.002*
NPC, cm 7.00 (3.00) 9.00 (3.25) 0.001*
Near stereopsis, sec 40.0 (12.50) 40.0 (12.50) 0.180
Phoria, PD (near) 2.00 (8.00) 2.00 (8.00) 0.086
Non-immersive mode
Refraction (dominant eye), diopter −0.38 (0.63) −0.25 (0.63) 0.261
Refraction (non-dominant eye), diopter −0.25 (0.88) −0.25 (0.63) 0.881
NPA (dominant eye), cm 9.00 (3.00) 9.50 (4.25) 0.058
NPA (non-dominant eye), cm 9.50 (3.00) 9.00 (4.00) 0.120
NPC, cm 8.00 (3.00) 9.00 (4.00) 0.002*
Near stereopsis, sec 40.0 (10.0) 40.0 (20.0) 0.234
Phoria, PD (near) 5.00 (8.00) 5.00 (8.00) 0.257
Data presented as median (interquartile range). NPA, near point of accommodation; NPC near point of convergence; PD prism diopter *statistically significant
value using the Benjamini-Hochberg procedure

target was fixed; thus, there was a possibility of fatigue vision, double vision, and defocusing symptoms were
due to the excessive activation of accommodative adap- worse in the immersive mode. These results suggest that
tation [24]. As shown in Table 3, other accommodative ocular fatigue due to excessive accommodation-
factors did not change. It appears that ocular fatigue in- convergence response is more severe in the immersive
duced an increase in the NPA and NPC, which is more mode.
of a subjective factor than actual accommodation. As shown in Table 4, correlation analysis revealed that
The data presented in Table 2 demonstrate that in the higher exophoria is associated with increased accommo-
immersive mode, neurological symptoms, such as head- dative lag. Vergence adaptation modulates fast response
ache, dizziness and nausea, were more severe than dis- and reduces error and fatigue by maintaining vergence
comfort in the eyes such as dryness. In addition, blurred stimulus [23–25]. Exophoric subjects may require more
Table 2 Comparison of subjective symptoms after playing a virtual reality game
Symptom Immersive mode Non-immersive mode p-value
Burning 0.0 (0.0) 0.0 (0.0) 0.564
Feeling of a foreign body 0.0 (0.0) 0.0 (0.0) 0.705
Excessive blinking 1.0 (3.0) 1.0 (2.0) 0.554
Tearing 0.0 (1.0) 0.0 (0.0) 0.014*
Dryness 0.0 (3.0) 1.0 (2.0) 0.942
Tingling 0.0 (2.0) 0.0 (1.0) 0.084
Blurred vision 1.0 (2.0) 0.0 (1.0) 0.005*
Double vision 0.0 (0.0) 0.0 (0.0) 0.046*
Difficulty focusing for near vision 0.0 (1.0) 0.0 (0.0) 0.018*
Increased sensitivity to light 0.0 (1.0) 0.0 (0.0) 0.623
Headache 2.0 (3.0) 0.0 (1.0) 0.012*
Dizziness 3.0 (3.0) 1.0 (2.0) 0.012*
Nausea 3.0 (3.0) 0.0 (1.0) 0.004*
Total 12.0 (9.0) 5.0 (10.5) 0.002*
Data presented as median (interquartile range). *statistically significant value using the Benjamini-Hochberg procedure
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 6 of 8

Table 3 Changes in accommodation using static and dynamic measurement after playing a virtual reality game
Variable Immersive mode Non-immersive mode
Pre Post p-value Pre Post p-value
Static measurement – Accommodative amplitude, diopter
Dominant eye 2.063 (1.188) 1.938 (0.688) 0.722 2.000 (0.500) 1.875 (0.875) 0.571
Non-dominant eye 2.063 (1.375) 2.125 (1.313) 0.442 2.000 (0.875) 2.000 (1.000) 0.572
Dynamic measurement
Velocity of accommodation, diopter/s
Dominant eye 0.334 (0.126) 0.395 (0.144) 0.095 0.382 (0.105) 0.361 (0.154) 0.910
Non-dominant eye 0.416 (0.104) 0.383 (0.119) 0.664 0.364 (0.133) 0.358 (0.133) 0.362
Mean accommodative lag, diopter
Dominant eye 0.660 (0.628) 0.602 (0.483) 0.108 0.575 (0.520) 0.591 (0.528) 0.322
Non-dominant eye 0.707 (0.430) 0.836 (0.590) 0.274 0.734 (0.357) 0.680 (0.489) 0.548
Dynamic accommodative response
Dominant eye 0.912 (0.078) 0.901 (0.101) 0.778 0.884 (0.105) 0.904 (0.126) 0.664
Non-dominant eye 0.881 (0.112) 0.900 (0.150) 0.821 0.914 (0.057) 0.886 (0.101) 0.099
Data presented as median (interquartile range).

effort in accommodation and convergence due to re- in mean accommodation lag. Although not included in
duced adaptation, which can cause more fatigue than in the table, the baseline NPA and NPC demonstrated a
normal individuals [4, 26–29]. high positive correlation with baseline values of mean
In addition, subjects who had a smaller NPA were accommodative lag (dominant eye, p = 0.010, p = 0.008;
more likely to exhibit an increase in mean accommoda- non-dominant eye, p = 0.026, p = 0.017, respectively).
tive lag after using VR. The baseline NPC was removed Sreenivasan et al. [30] reported that retinal image
due to a high false discovery rate; however, the baseline quality is better when accommodative lag is greater, be-
NPC showed a relatively high correlation with changes cause, paradoxically, the depth of field is more

Table 4 Correlations between the value of baseline and changes of ocular parameters
Changes in value (post - pre) Exophoria (far) Exophoria (near) NPA(dominant eye) NPA (non-dominant eye) NPC
r P r P r P r P r P
Ocular parameters
Exophoria (far) −0.100 0.514 0.127 0.410 −0.032 0.841 − 0.064 0.685 −0.087 c
Exophoria (near) − 0.166 0.276 − 0.149 0.335 − 0.054 0.731 −0.066 0.676 −0.152 0.324
NPA (dominant eye) 0.296 0.048 −0.007 0.963 −0.255 0.099 0.007 0.966
NPA (non-dominant eye) 0.202 0.184 −0.028 0.859 −0.204 0.189 0.105 0.498
NPC 0.046 0.764 0.046 0.764 0.051 0.747 0.051 0.747 −0.188 0.222
Accommodative parameter (dominant eye)
Accommodative amplitude −0.110 0.507 0.030 0.860 −0.220 0.191 −0.003 0.987
Velocity of accommodation 0.059 0.703 −0.034 0.834 0.123 0.438 0.001 0.994
Mean accommodative lag 0.165 0.285 0.372 0.014* −0.175 0.266 −0.159 0.309
Dynamic accommodative response 0.032 0.845 0.139 0.393 0.085 0.607 0.004 0.979
Accommodative parameter (non-dominant eye)
Accommodative amplitude −0.141 0.373 −0.034 0.834 −0.128 0.430 −0.008 0.959
Velocity of accommodation −0.017 0.913 0.151 0.327 −0.005 0.976 0.207 0.187
Mean accommodative lag 0.111 0.468 0.076 0.624 −0.328 0.032* −0.333 0.027
Dynamic accommodative response −0.110 0.507 0.019 0.903 0.003 0.983 −0.069 0.659
Sum of symptom scores −0.110 0.471 0.173 0.261 0.198 0.202 0.222 0.152 0.084 0.587
NPA near point of accommodation, NPC near point of convergence *statistically significant value using the Benjamini-Hochberg procedure
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 7 of 8

structurally or functionally wider in individuals with content, instead of non-immersive VR content. Addi-
higher accommodative lag. In individuals with a large tionary, accommodation was analyzed using dynamic
baseline NPA and NPC, retinal image quality was para- techniques. Our results may form the basis for recom-
doxically better, as with high accommodative lag. This mendations for users who may need to be more careful
may reduce the ocular fatigue that is induced by the regarding VR use.
rapid accommodation response.
However, there is controversy regarding whether ac- Supplementary information
commodative lag enhances accommodative stress by Supplementary information accompanies this paper at https://doi.org/10.
1186/s12886-020-01471-4.
promoting a blurred retinal image [31]. Further research
is needed to support this hypothesis. Shiomi et al. [32] Additional file 1: Immersive vs. non-immersive mode (AVI). Video clip
reported that actual accommodation could change de- showing the difference between immersive and non-immersive modes.
pending on the perception depth of the 3D content, even In the non-immersive mode, the viewing angle is fixed parallel to the
ground, while in the immersive mode, the view is free to rotate.
when the 3D display is fixed. Unlike previous reports,
the use of VR appears to affect accommodation, and fur-
Abbreviations
ther studies are needed to resolve these issues. VR: Virtual reality; NPA: near point of accommodation; NPC: near point of
Turnbull et al. [15] suggested that increased choroidal convergence; HMD: head-mounted display; PD: prism diopters
thickness caused by myopic retinal defocus could be as-
Acknowledgements
sociated with reduced myopia progression. In our study,
The statistical analysis in this research was supported by J.R. Park (Master of
myopic shift and hyperopic shift > 0.5 D was evident in Statistics).
each of the 3 cases in the immersive mode. In the non-
immersive mode, myopic shift and hyperopic shift were Author’s contributors
Design of the study (H.H); Conduct of the study (H.J.Y, H.H); Collection and
observed in 8 cases and 4 cases, respectively. However, management of data (H.J.Y, S.W.P); Analysis and interpretation of data (H.J.Y,
these changes fully recovered within 1 h. Our study J. K, H.H); Preparation, review, or approval of the manuscript (H.J.Y, J. K, S.W.P,
showed that myopic and hyperopic shifts presented atyp- H.H).
All authors read and approved the final manuscript.
ically and did not correlate with other ocular factors or
VR mode. In addition, a study by Ha et al. [7] reported Funding
that transient myopia could occur after using VR. The This research was supported by Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of
hypothesis of increased or reduced myopic progression Education (NRF- 2017R1D1A3B03032579) and this research was supported by
using VR appears to require a more cautious approach. a grant (CRI 17031–1) Chonnam National University Hospital Biomedical
One limitation of this study was the inclusion of a spe- Research Institute. Funding body was not involved in the design of the
study and collection, analysis, and interpretation of data and in writing the
cific population (i.e., 23 subjects 20 to 35 years of age, manuscript.
with an uncorrected visual acuity of 0.8 or higher). Thus,
it is difficult to judge the effect on users of different con- Availability of data and materials
Data supporting our findings are contained in the manuscript. However, the
ditions. Notably, the actual refraction data were not ana- raw data set on which the conclusion was made is available on request from
lyzed, including those of subjects with a history of Professor Hwan Heo (contact email:opheye@hanmail.net).
refractive surgery. It is possible that the use of VR for
Ethics approval and consent to participate
only 30 min was insufficient to produce changes in visual This study received ethical approval from the Institutional Review Board of
parameters. Furthermore, there were no control groups the Chonnam National University Hospital. A written informed consent was
that did not use VR. Additional larger-scale studies are obtained from all patients before study initiation.
needed to resolve these limitations. However, we Consent for publication
propose that it may be meaningful that the results of this Not applicable.
study demonstrate the difference in visual parameters
depending on the contents, despite using the same VR Competing interests
The authors report no conflicts of interest.
device.
Received: 19 March 2019 Accepted: 13 May 2020
Conclusion
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1040-5488/03/8003-0214/0 VOL. 80, NO. 3, PP. 214–225
OPTOMETRY AND VISION SCIENCE
Copyright © 2003 American Academy of Optometry

ORIGINAL ARTICLE

Nearpoint of Convergence: Test Procedure,

Target Selection, and Normative Data
MITCHELL SCHEIMAN, OD, FAAO, MICHAEL GALLAWAY, OD, FAAO,
KELLY A. FRANTZ, OD, FAAO, ROBERT J. PETERS, OD, FAAO, STANLEY HATCH, OD, FAAO,
MADALYN CUFF, OD, and G. LYNN MITCHELL, MAS, FAAO
Pennsylvania College of Optometry, Philadelphia, Pennsylvania (MS, MG), Illinois College of Optometry, Chicago, Illinois (KAF), Private
Practice, Grand Rapids, Michigan (RJP), New England College of Optometry, Boston, Massachusetts (SH), Private Practice, (MC), College
of Optometry, The Ohio State University, Columbus, Ohio (GLM)

ABSTRACT: Background. The purpose of this study was to help determine the most appropriate target to be used for
the assessment of the nearpoint of convergence, normative data for the break and recovery in adults, and the diagnostic
value of the red-glass modification and repetition of the nearpoint of convergence. Methods. A total of 175 subjects
with normal binocular vision and 38 subjects with convergence insufficiency were evaluated. The nearpoint of
convergence was measured three ways, with an accommodative target, a penlight, and a penlight with red and green
glasses. The nearpoint of convergence was also measured using a penlight for 10 repetitions. Results. Results suggest
a clinical cutoff value of 5 cm for the nearpoint of convergence break and 7 cm for the nearpoint of convergence
recovery with either an accommodative target or a penlight with red and green glasses. Conclusion. This study
establishes normative data for the nearpoint of convergence break and recovery in the adult population and supports
the value of various test modifications when other testing is equivocal. (Optom Vis Sci 2003;80:214–225)

Key Words: nearpoint of convergence, convergence insufficiency, binocular vision testing

T
he assessment of the nearpoint of convergence (NPC) is review of the literature found only one recent study that was de-
widely used by eye care practitioners in the routine primary signed to determine the normative data for this test. In this study,
care examination1, 2 and is often included as a test procedure Hayes et al.33 suggested expected values for the NPC for school-
for vision screenings.3–7 The NPC is also considered an important aged children. We were unable to retrieve any study that system-
diagnostic finding in the assessment of convergence insufficien- atically investigated the expected values for the NPC in adults.
cy.8 –13 For example, Daum10 reviewed 58 studies of convergence Table 1 is a compilation of recommendations in current textbooks
insufficiency and found that 36% of the studies specified a receded and articles.1, 2, 7, 17–33 As the table illustrates, there are a variety of
nearpoint of convergence as an important criterion for diagnosis of recommendations for target selection, and the recommended ex-
convergence insufficiency. A survey conducted by Rouse et al.14 pected findings for the break range from 5 to 17.5 cm. More than
determined that the NPC was used in making the diagnosis of half of the authors did not include an expected finding for the
convergence insufficiency by 93.8% of optometrists surveyed. recovery measurement. The expected findings for those who did
Thirty-five percent of the doctors indicated that one criterion was report a recovery finding ranged from 8 to 11 cm. Other than the
sufficient to diagnose convergence insufficiency, and the most fre- study by Hayes et al.,33 not one of the other authors provided
quently used single diagnostic characteristic was the NPC. Given supportive data or a reference for their suggestions for either the
its widespread use and diagnostic importance, it is surprising that recommended target or expected findings.
the NPC test procedure, target selection, and normative data have We were able to find one article that reported the NPC in adults
received limited investigation since its introduction as an impor- with normal binocularity and adults with convergence insufficien-
tant routine test procedure in the late 19th and early 20th cy.25 This study was not designed specifically to investigate nor-
centuries.15, 16 mative data for NPC. However, in the course of studying the
Although most authors describe the NPC as part of the mini- relationship between the NPC and vergence amplitudes in conver-
mum database for a routine vision examination,1, 2, 7, 17–33 our gence insufficiency patients, the authors reported an average break

Optometry and Vision Science, Vol. 80, No. 3, March 2003

 TABLE 1.
Previous suggestions for NPC target selection and expected findings.a
Source Expected Break Expected Recovery Target/Method Based on
17
Davies 7 cm NR Target not specified, suggests repeating test 8–12 times Not stated
Capobianco18 6–10 cm NR Penlight, stresses importance of red glass test Not stated
Carter19 5 cm 9 cm Pencil, pen point, or other small object Not stated
Duke-Elder20 10 cm NR Target and method not specified Not stated
Burian and von Noorden21 8–10 cm NR Target not specified, stresses importance of red glass modification Not stated
Hoffman and Rouse22 5 cm 8 cm Target and method not indicated Not stated
Mohindra and Molinari23 6 in Within 2 in of break Red glass, repetition suggested Not stated
Pickwell and Hampshire24 10 cm NR Black vertical line on a white card Not stated
Cohen et al.7 10 cm NR A bell Not stated
Shippman et al.25 5 cm NR Target and method not indicated Clinical study
London26 5 cm 8 cm Penlight, important to repeat to check for fatigue effects, talks of Not stated
diagnostic potential of NPC with red glass (suggests that a difference of
3 cm has diagnostic value)
Helveston et al.27 11 cm NR Accommodative target Not stated
Wick28 NR NR Target not indicated, repeat test five or more times Not stated
Pickwell29 10–15 cm NR Any suitable target Not stated
Carlson et al.2 !17.5 cm Within 7.5 cm of Penlight for screening, use accommodative target and red glass/penlight Not stated
break if NPC is greater than 4 in/7 in.
London30 6–10 cm NR Accommodative target or penlight, repeat five times Not stated

Optometry and Vision Science, Vol. 80, No. 3, March 2003

Eggers31 10 cm NR Target not indicated Not stated
Griffin and Grisham32 8 cm 11 cm Small detailed target, perform five times Class notes
Grosvenor1 8 cm NR Penlight, suggests repeated testing; if NPC is greater than 12–15 cm with Not stated
repeated testing, it is evidence of a convergence insufficiency.
Hayes et al.33 6 cm 10 cm Astron International Accommodative Rule and a 20/30 single column of Well-designed
letters. Examiner moved target at a rate of approximately 1–2 cm/s. study
a
NPC, nearpoint of convergence; NR, not reported.
Nearpoint of Convergence—Scheiman et al.
215
216 Nearpoint of Convergence—Scheiman et al.

finding of 5 cm (range, 1 to 15 cm) for a group of 46 adult clinical METHODS

patients with normal binocular vision. The average break for adults
diagnosed with convergence insufficiency was 7.9 cm (range, 1 to Two groups of subjects were evaluated. The first group con-
20 cm). The authors did not report the target used for the testing. sisted of optometry students (N " 175; age range, 22 to 37 years;
The recent study by Hayes et al.33 was designed to establish average age, 24.9). These subjects received a full eye examination.
normative values for the NPC using a standardized and reliable We orally reviewed the study with each subject and outlined the
protocol. They studied 297 schoolchildren in kindergarten, third risks and benefits. After receiving consent, we enrolled subjects in
grade, and sixth grade who had passed a school-based Modified the study. All subjects had 20/20 visual acuity in both eyes (dis-
Clinical Technique vision screening. Based on their results, they tance and near) with best refraction. We excluded all subjects with
suggested a clinical cutoff value of 6 cm for the NPC break for a strabismic or nonstrabismic binocular vision problem or an ac-
schoolchildren using an accommodative target. commodative disorder. We used traditional normative values to
There has also been speculation about modification of the stan- determine whether a subject had an accommodative or nonstrabis-
dard procedure to make the test more sensitive and of greater mic binocular vision disorder.34, 35
diagnostic and prognostic value. In an entirely anecdotal report in Group two consisted of clinic patients at The Eye Institute of
1952, Capobianco18 recommended that the NPC be performed The Pennsylvania College of Optometry and the Illinois Eye In-
twice, once with a penlight and again with a penlight and a red glass stitute of the Illinois College of Optometry (N " 38; age range, 9
in front of one eye. Capobianco refers to this second method as the to 52 years; average age, 20.2). We orally reviewed the study with
subjective NPC. She suggests that this modification might poten- each subject and outlined the risks and benefits. After receiving
tially yield useful diagnostic and prognostic information. Accord- consent, we enrolled subjects in the study. We excluded subjects
ing to Capobianco, in cases in which a convergence insufficiency is with #20/20 visual acuity in each eye with best correction or
suspected but the NPC with a penlight alone is normal, the sub- constant strabismus. All subjects in this group were diagnosed with
jective NPC may be more remote and demonstrate a better corre- convergence insufficiency using criteria of exophoria greater at
lation with fusional amplitudes. Thus, the red-glass NPC may be a near than at far, receded NPC (we selected a value of 5 cm for the
more sensitive diagnostic test. She claims that the test also provides NPC based on the previous work of Shippman et al.25), and re-
information about progress in treatment because the NPC tested duced positive fusional vergence amplitudes (!1 SD from Mor-
with the red-glass modification improves more rapidly than the gan’s expected findings). The NPC was considered abnormal if the
NPC tested with the penlight alone. Although several authors rec- measurement was receded with any of the three targets described
ommend that this procedure be incorporated as part of the stan- below.
dard assessment of convergence amplitude,2, 21, 23, 26 no research We administered an eight-item symptom questionnaire (Table
data have been produced to support its use or any of Capobianco’s 2) to all subjects. This questionnaire was adapted from a question-
assertions about the value of the test. naire developed and used by Cooper et al.36 Each item was scored
Another modification that has been suggested is repetition of on a scale of one to five. The lower the score, the less the symptoms.
the NPC, with the assumption that symptomatic patients will The highest possible symptom score was 40, indicating a very
show a greater recession in the NPC with repeated testing com- symptomatic subject, and the lowest possible score was eight, in-
pared with normals. Davies,17 in 1946, appears to be the original dicating no symptoms. This survey has not been tested for reliabil-
source for this recommendation. He advocated repeating the NPC ity and repeatability. A symptom survey (Convergence Insuffi-
eight to 12 times and suggested that a breakdown would occur at ciency and Reading Study symptom survey37) is now available that
about five to six repetitions and the break would recede to 25 to 30 has been shown to be a valid instrument for differentiating conver-
cm. This would indicate poor convergence reserve. Subsequent gence insufficiency (CI) children from those with normal binocu-
authors have suggested that this might be a worthwhile part of the lar vision. However, at the time we performed this study, this
NPC evaluation.1, 2, 23, 26, 28, 30, 32 Although this may make intu- symptom survey was not available.
itive sense, again no supporting data exist. Even if a clinician adopts For all subjects, the NPC was assessed by one of the authors
this approach, the literature is unclear about how much of a change using a standard push-up technique with a Bernell Accommo-
in the NPC with repetition should be considered significant. dative Rule. We used an instructional set similar to the one first
Thus, we have a clinical test that is considered part of the min- described by Hayes et al.33 The Accommodative Rule was
imum database for a primary eye care examination, is often used as placed just above the nose at the brow between the two eyes.
a screening procedure for binocular vision problems, and is a key The target was moved toward the subjects at a rate of about 1 to
criterion for the diagnosis of convergence insufficiency, yet there is 2 cm/s. Subjects were encouraged to try to keep the target
a lack of supportive, clinical research. There is a lack of agreement single. The subjective break and recovery values were measured
about the most appropriate target and the expected finding for a and recorded in centimeters. If there was no subjective report of
normal or abnormal break in adults. In addition, although clini- diplopia, the points at which the patient objectively lost and
cians have developed several potentially valuable modifications of regained ocular alignment were recorded as the break and re-
the NPC test, there are no data to validate the use of these covery. The NPC was measured once with each of the follow-
modifications. ing: an accommodative target (AT) (single 20/30 letter), a pen-
The purpose of this study is to investigate these issues and help light (PL), and with penlight while the subject wore red/green
determine the most appropriate target to be used for the NPC, glasses (PLRG). Red and green glasses were used instead of a red
normative data for the break and recovery in adults, and the clinical glass to free both hands of the examiner so that he could hold
value of modifications of the test using filters and repetitions. the Accommodative Rule with both hands. The order of testing

Optometry and Vision Science, Vol. 80, No. 3, March 2003

 Nearpoint of Convergence—Scheiman et al. 217

TABLE 2.
Symptom Questionnaire
1. How long can you do nearwork (i.e., reading, writing, computer work, sewing, etc.) without discomfort, headaches, eye ache,
burning, stinging, watering, blurriness, double vision, loss of vision, or tiredness?
1. at least 3 hours
2. up to 2 hours
3. up to 1 hour
4. up to 30 minutes
5. up to 15 minutes
2. How often do you get headaches when you do nearwork?
1. never (0% of the time)
2. occasionally (approximately 25% of the time)
3. often (approximately 50% of the time)
4. very often (approximately 75% of the time)
5. every time I do nearwork (100% of the time)
3. If you experience headaches during nearwork how bothersome are these headaches (i.e., to what degree do they interfere with
your normal functioning)?
1. minimally bothersome
2. mildly bothersome
3. moderately bothersome
4. very bothersome
5. extremely bothersome
4. Do your eyes pull, ache, or water when you do nearwork?
1. never (0% of the time)
2. occasionally (approximately 25% of the time)
3. often (approximately 50% of the time)
4. very often (approximately 75% of the time)
5. every time I do nearwork (100% of the time)
5. Does the reading material ever become blurry, run together, or jump when you do nearwork?
1. never (0% of the time)
2. occasionally (approximately 25% of the time)
3. often (approximately 50% of the time)
4. very often (approximately 75% of the time)
5. every time I do nearwork (100% of the time)
6. Does reading material ever become double when you do nearwork?
1. never (0% of the time)
2. occasionally (approximately 25% of the time)
3. often (approximately 50% of the time)
4. very often (approximately 75% of the time)
5. every time I do nearwork (100% of the time)
7. Immediately following prolonged nearwork do objects at distance appear blurry for a short period of time?
1. never (0% of the time)
2. occasionally (approximately 25% of the time)
3. often (approximately 50% of the time)
4. very often (approximately 75% of the time)
5. every time I do nearwork (100% of the time)
8. Do your eyes feel tired or do you lose your concentration when doing nearwork?
1. never (0% of the time)
2. occasionally (approximately 25% of the time)
3. often (approximately 50% of the time)
4. very often (approximately 75% of the time)
5. every time I do nearwork (100% of the time)

with these three targets was randomized. After a 30-s break, the were generated for both break and recovery measurements ob-
NPC with a penlight was then performed an additional 10 tained using each of the three target types. These calculations were
times. Finally, each subject filled out the eight-item symptom performed separately for normal and convergence insufficiency
questionnaire to assess the presence and severity of asthenopic subjects. Within each group, a repeated-measures analysis of vari-
complaints during nearpoint activities. All testing was per- ance was used to compare the mean break and recovery values
formed with full room illumination. obtained with each of the targets. Post hoc testing was performed
Descriptive statistics (means, standard deviation, medians, etc.) using Scheffe’s method of multiple comparison. Given the non-

Optometry and Vision Science, Vol. 80, No. 3, March 2003

218 Nearpoint of Convergence—Scheiman et al.
normal distribution of NPC values for both groups, natural log- tically significant, were #0.7 cm and therefore not clinically mean-
transformed values were used when performing these analyses. ingful. For clinical testing purposes, the break and recovery data for
Mixed model analysis was used to model the repeated measure- normal subjects are essentially identical for each of the three target
ments obtained for both NPC break and recovery as a function of types.
measurement number. In addition, indicator variables were in- The distribution of break and recovery values for each of the
cluded in the model to determine whether the slope of the line three targets is shown in Figs. 1 and 2. Cumulative distributions
relating break or recovery to measurement number was consistent are shown in Fig. 3. Break values for each of the three targets are
across the entire range of repeats. Akiake’s Information Criterion unimodal and right skewed with a concentration of measurements
values were used to judge model fit. Due to the non-normal dis- in the 0.5- to 3.0-cm range (Fig. 1). In fact, slightly more than 70%
tribution of break and recovery values, natural log transformed of the subjects had break values of "3 cm with AT (Fig. 3). The
data was used in all analyses. These analyses were performed sepa- percentage of subjects with values of "3 cm increases to 75% using
rately for CI and normal subjects. PL and exceeds 80% using PLRG. More than 90% of subjects had
break values in the range of 0.5 to 5.0 cm regardless of the target.
RESULTS As with break, the distributions of NPC recovery values are
Normative Data and Target Selection unimodal and skewed to the right (Fig. 2). As indicated by the
median values in Table 3, more than 50% of the recovery measure-
Descriptive statistics for both break and recovery values ob- ments were "4.0 cm for each of the three targets. Recovery mea-
tained from the normal subjects are summarized in Table 3. The surements of "10 cm were observed in over 90% of the subjects
mean break for both the AT and PLRG targets is about 2.5 cm. using each target.
The mean break value for the PL target is slightly lower at 2.0 cm. Also included in Table 3 are descriptive statistics for the NPC
The mean recovery for the AT and PLRG targets was also similar at break and recovery measurements obtained from subjects with
4.35 cm and larger than the mean for PL at 3.74 cm. Repeated- convergence insufficiency. The mean break for AT is lowest at just
measures analysis of variance (ANOVA) indicated a significant over 9 cm, increasing to almost 12 cm with PL and almost 15 cm
difference in the mean break measurements obtained via the three with PLRG. AT also produced the lowest mean recovery at 12.5
targets (p # 0.0001). Post hoc testing indicated a significant differ- cm, followed by PL with a mean of just over 17.5 cm. As with
ence in AT and PL (p " 0.0001), AT and PLRG (p " 0.0033), break, the mean recovery for PLRG was largest at slightly more
and PL and PLRG (p " 0.0018). The actual differences, however, than 20.5 cm. The repeated-measures ANOVA indicated a signif-
were #0.5 cm and are not considered clinically meaningful. A icant difference in the mean break measurements obtained via the
significant difference in mean NPC recovery for the three targets three targets (p # 0.0001). Post hoc testing indicated a significant
was also found (p # 0.0001) using the repeated-measures difference in AT and PL (p " 0.0104) and AT and PLRG (p #
ANOVA. The mean recovery for AT and PL were significant dif- 0.0001). Unlike normal subjects, the statistically significant differ-
ferent (p " 0.0001) along with the means for PL and PLRG (p " ences in break values obtained with the three targets were also
0.0001). As with the break values, the differences, although statis- clinically meaningful. A significant difference in mean NPC recov-

TABLE 3.
Descriptive statistics for NPC break and recovery by method obtained and study group.a
Method Obtained Mean SD Minimum Median Maximum
CI subjects (N " 38)
Break
Accommodative target 9.32 6.74 0.5 7.75 31.0
Penlight 11.86 8.40 2.0 10.0 41.0
Penlight with R/G glasses 14.75 10.0 2.0 11.0 41.0
Recovery
Accommodative target 12.47 7.89 1.0 10.5 36.0
Penlight 17.68 11.24 4.0 14.25 51.0
Penlight with R/G glasses 20.59 12.32 5.0 15.5 56.0

Normal binocular vision subjects (N " 175)

Break
Accommodative target 2.49 1.74 0.5 2.0 7.0
Penlight 2.06 1.85 0.5 1.5 10.0
Penlight with R/G glasses 2.38 2.11 0.5 2.0 11.0
Recovery
Accommodative target 4.35 2.74 1.0 4.0 11.0
Penlight 3.74 2.87 0.5 3.0 14.0
Penlight with R/G glasses 4.35 3.26 1.0 4.0 17.0
a
NPC, nearpoint of convergence; CI, convergence insufficiency; R/G, red and green.

Optometry and Vision Science, Vol. 80, No. 3, March 2003

 Nearpoint of Convergence—Scheiman et al. 219

FIGURE 2.
FIGURE 1.
Distribution of nearpoint of convergence (NPC) recovery values for nor-
Distribution of nearpoint of convergence (NPC) break values for normal mal subjects by measurement method.
subjects by measurement method.

poses in patients with convergence insufficiency, it would appear

ery for the three targets was also found (p # 0.0001) using the that testing with the AT target results in break and recovery data
repeated-measures ANOVA. The mean recovery for AT and PL that are significantly lower than what would be obtained using
were significant different (p " 0.0008), along with the means for either PL or PLRG targets.
AT and PLRG (p # 0.0001). These differences are both statisti- The distribution of break and recovery values for convergence
cally significant and clinically meaningful. For clinical testing pur- insufficiency subjects using each of the three targets are shown in

Optometry and Vision Science, Vol. 80, No. 3, March 2003

220 Nearpoint of Convergence—Scheiman et al.

FIGURE 3.
Cumulative distribution of nearpoint of convergence (NPC) break and
recovery for normal subjects by method of measurements.

Figs. 4 and 5. Cumulative distributions are plotted in Fig. 6. Both

AT and PLRG have unimodal NPC break distributions that are
skewed to the right. A high proportion (70%) of the break mea-
surements with AT are in the range of 0.5 to 10 cm. PLRG values,
although skewed, are not quite as concentrated with #50% in the
range of 0.5 to 10 cm. The distribution of break values for PL has
three modal values at 8, 10, and 12 cm and appears less skewed
than the distribution for the other two targets. In fact, the distri-
bution appears almost uniform in the range of 0.5 to 15 cm.
NPC recovery measurements obtained using the AT target also
follow a unimodal distribution with a large concentration in the
range of 0.5 to 15 cm. In fact, nearly 80% of the subjects have
measurements in this range. The distribution of values for the PL FIGURE 4.
target is also unimodal but appears less skewed than the AT distri- Distribution of nearpoint of convergence (NPC) break values for conver-
bution. Only slightly more than one half of the subjects have values gence insufficiency subjects by measurement method.
of "15 cm. As with NPC break, the distribution of recovery values
for the PLRG target is more evenly spread across the range of values 7a shows the change in mean NPC break over the 10 repeated
compared with the other two target types. Slightly #50% of the measurements. According to the mixed model analysis, the mean
subjects have recovery values of "15 cm using the PLRG target. NPC break increases from measurement one to measurement five
(p value for slope # 0.0001) but then changes little from measure-
Modification of Test Procedure: Ten Repetitions ments six to 10 (p value for slope " 0.0865). The mean NPC
recovery increased from 17.7 cm at measurement one to 21.9 at
The average NPC break for CI subjects was 12.1 cm at the first measurement 10 (Fig. 7b. The change, although not remarkable,
measurement, but it increased to 15.9 cm at measurement 10. Fig. was more dramatic from measurement one to three (p value for

Optometry and Vision Science, Vol. 80, No. 3, March 2003

 Nearpoint of Convergence—Scheiman et al. 221

break value for CI subjects, the break values do not change more
dramatically at early repeated measurements and then remain rel-
atively constant. In contrast, there does appear to be an initial jump
in the NPC recovery values for normal subject from measurement
one to measurement two (p value for slope " 0.004), after which
the values remain relatively constant (p value for slope " 0.4737).
At measurement one the mean NPC recovery is 3.99 cm, but it
increases to 4.49 cm at repeat two and is 5.3 cm at the last mea-
surement (Fig. 8b).

Symptoms
On the eight-item symptom questionnaire, the lowest possible
score was eight with a maximum of 40. The mean value for normal
subjects was 13.06 (range, 8 to 32), and the mean for convergence
insufficiency subjects was 22.03 (range, 9 to 32). This difference
was statistically significant (t " 11.80, p # 0.0001). Correlations
between symptoms and the NPC findings did vary with target
selection. The highest correlation between symptom score and
NPC finding (r " 0.37) was observed for NPC recovery performed
with the PLRG (Table 4).

DISCUSSION
This study was designed to determine normative data for the
NPC break and recovery in adult subjects, the most appropriate
target(s) to be used for the assessment of the NPC, and the diag-
nostic value of commonly used modifications of the NPC. The
distribution of NPC break values showed a concentration for all
targets in the 0.5- to 5-cm range. The maximum break value ob-
served with the AT was 7 cm, compared with 10 cm for PL and 11
cm for PLRG. Eighty-five percent of subjects had a break of "4.5
cm with all targets. In a previous study designed to determine
normative values for children, Hayes et al.33 suggested a clinical
cutoff value of 6 cm. In their study, 85% of their subjects had a
break of "6 cm. In our study, 98% of the subjects had a break of
"6 cm with the AT, and 96% had a break of #6 cm with the
PLRG target. Using a similar criterion of 85%, we recommend a
value of 4.5 cm. Because clinicians generally do measure the NPC
break to the half-centimeter, we suggest rounding the clinical cut-
off value for the NPC break to 5 cm.
For the NPC recovery, about 85% of subjects had a recovery of
"7 cm with all targets. Thus, we recommend a clinical cutoff value
of 7 cm for the NPC recovery. Others studies have used values
ranging from 5 to 11 cm for the break and 8 to 11 cm for the
recovery.1, 2, 7, 17–33 However, our finding of "5 cm as the ex-
FIGURE 5. pected break value for normal subjects compares favorably with the
Distribution of nearpoint of convergence (NPC) recovery values for con-
expected break value of "6 cm for children found by Hayes et al.33
vergence insufficiency subjects by measurement method. One of the questions we wanted to address was target selection.
The results of this study suggest that clinical diagnosis can be made
with any of the three targets, although the accommodative target
slope " 0.0298) than from measurement four to 10 (p value for appears to provide the best precision. When evaluating the NPC,
slope " 0.2543). we are trying to determine the patient’s ability to converge using all
There was little change in the NPC break values for normal aspects of convergence including fusional convergence, proximal
subjects across the 10 measurements. The mean break was 2.2 at convergence, and accommodative convergence. Because the use of
measurement one and 2.9 at measurement 10 (Fig. 8a). The mixed an AT maximizes the accommodative demand and accommoda-
model analysis indicated that the small increase in mean break was tive convergence, the NPC should, theoretically, be maximized
consistent across the 10 repeated measurements. That is, unlike the with this type of target. Ciuffreda38 recommended the use of an

Optometry and Vision Science, Vol. 80, No. 3, March 2003

222 Nearpoint of Convergence—Scheiman et al.

FIGURE 6.
Cumulative distribution of nearpoint of convergence (NPC) break and recovery for convergence insufficiency subjects by method of measurements. R/G,
red and green.

AT for another reason. He found less variability in the NPC when significant but very small (#1 cm). Although they were statistically
measured with an AT vs. a PL. It is important to note, however, significant, due to a large sample size and the repeated-measures
that our results suggest that to best discriminate the symptomatic design, such differences are not clinically significant. The conver-
CI, the PLRG break and recovery is most accurate. gence insufficiency group showed very different results. For the
In contrast to Ciuffreda’s study, we used traditional, clinical break values in the convergence insufficiency group, statistically
evaluation tools, and in our normal group, and the differences and clinically significant differences were found between the AT
among NPC measurements with various targets were statistically and the PLRG. For the recovery values, the differences between AT

Optometry and Vision Science, Vol. 80, No. 3, March 2003

 Nearpoint of Convergence—Scheiman et al. 223

FIGURE 8.
FIGURE 7. Mean nearpoint of convergence (NPC) break and recovery for normal
Mean nearpoint of convergence (NPC) break and recovery for conver- subjects across repeated measurements. Each bar represents 1 SD.
gence insufficiency subjects across repeated measurements. Each bar
represents 1 SD.
insufficiency had a break that was !5 cm more receded with the
PLRG and a recovery !8 cm more receded with the PLRG com-
and PL as well as between AT and PLRG were clinically significant. pared with the AT. Differences approaching these values should
There was also a statistically significant difference between the PL alert a clinician to the possibility of a subtle convergence insuffi-
and PLRG, although the mean difference of 2.9 cm between the ciency. Our data tend to support Capobianco’s suggestion.
break with a PLRG and a PL may be too small to be clinically Of our 38 subjects with convergence insufficiency, 13 had an
useful. The greatest differences between any two tests were 5.43 cm NPC that was within the expected range for both break and recov-
between the AT break and the PLRG break and 8.17 cm between ery using the AT alone. When the NPC was repeated with a PLRG,
the AT recovery and the PLRG recovery in the convergence insuf- all 13 of these subjects were found to have a receded NPC. In 11 of
ficiency group. the 13, the NPC was receded with the PL target, and all 13 of these
Theoretically, the PLRG target not only minimizes the accom- subjects were found to have a receded NPC with the 10-repetition
modative-convergence component, but also makes binocularity
more difficult because of the dissociative factor created by the red TABLE 4.
and green glasses. Capobianco18 suggested that the use of two Correlations between symptoms and NPC variables for the
targets (AT followed by a PLRG) might allow clinicians to detect convergence insufficiency group.a
more subtle convergence insufficiency problems. Most clinicians
have encountered situations in which diagnostic testing is equivo- AT Brk AT Rec PL-Brk PL-Rec PLRG-Brk PLRG-Rec
cal, although the history suggests a clinical hypothesis of a binoc- Symptom r " 0.19 r " 0.21 r " 0.27 r " 0.35b r " 0.33b r " 0.37b
ular problem. The use of the PLRG target may be particularly score
useful in this situation. Our results suggest that in a patient with a
NPC, nearpoint of convergence; AT, accommodative target;
normal binocularity, there should be virtually no difference be- Brk, break; Rec, recovery; PL, penlight; PLRG, penlight while
tween the break and recovery findings when the NPC is performed wearing red and green glasses.
with an AT or a PLRG. In this study, patients with convergence b
p # 0.05.

Optometry and Vision Science, Vol. 80, No. 3, March 2003

224 Nearpoint of Convergence—Scheiman et al.
procedure. Thus, the use of the PLRG target or repetition of the 12. Daum KM. Convergence insufficiency. Am J Optom Physiol Opt
NPC appears to be important for the diagnosis of subtle cases of 1984;61:16–22.
convergence insufficiency. Also, the highest correlation between 13. Rouse MW, Borsting E, Hyman L, Hussein ME, Cotter SA, Flynn
symptoms and the type of target in this study was with the PLRG M, Scheiman M, Gallaway M, and the Convergence Insufficiency
(Table 4). and Reading Study Group (CIRS). Convergence insufficiency and
reading ability among fifth and sixth graders. Invest Ophthalmol Vis
The repetition modification of the NPC suggested by Davies17
Sci 1997;38:S975.
also appears to be useful in the diagnosis of more subtle cases. A
14. Rouse MW, Hyman L, Hussein ME, and the Convergence Insuffi-
difference of more than 4 cm between the first and 10th repetition ciency and Reading Study Group (CIRS). How do you make the
suggests a problem. Our data suggest that most of the change diagnosis of convergence insufficiency? survey results. J Optom Vis
occurs from the first to the fifth repetition. However, because of Devel 1997;28:91–97.
the considerable amount of time required to perform the NPC 15. Landolt E. Examination of the eyes. Huntington, NY: Robert E.
even five times, this may not be the most practical method for Krieger, 1979.
detecting a subtle CI. The PLRG procedure requires less time and 16. Duane A. A standard test-object for determining the near point and
also enables the clinician to detect a subtle CI. range of accommodation. Ophthalmol Rec 1909;18:358–60.
Based on these results, we suggest that the NPC should be rou- 17. Davies CE. Orthoptic treatment in convergence insufficiency. Can
tinely evaluated with an AT. If the NPC is normal, but there are Med Assoc J 1946;55:47–9.
other signs or symptoms of convergence insufficiency, or if the 18. Capobianco NM. The subjective measurement of the near point of
NPC is borderline (reduced break or recovery or a large difference convergence and its significance in the diagnosis of convergence in-
between the two), the NPC should be repeated with a PLRG. sufficiency. Am Orthop J 1952;2:40–2.
This study does have a number of limitations. First, the age 19. Carter DB. Optometric Examination: Multimedia Center, School of
Optometry, University of California, 2nd Ed. Berkeley, CA: Univer-
range of adult subjects used to determine expected clinical values
sity of California, 1973.
was limited to 22 to 37 years of age. Further investigation is nec-
20. Duke-Elder, S. System of Ophthalmology, Vol. VI. Ocular Motility
essary to determine whether our conclusions about normative data and Strabismus. London: Henry Kimpton, 1973.
and test modifications can be applied to the presbyopic popula- 21. Burian HM, von Noorden GK. Binocular Vision and Ocular
tion. In addition, all of our adult subjects used to determine ex- Motility: Theory and Management of Strabismus. St. Louis: Mosby,
pected values were optometry students, and this may limit the 1974.
applicability of these data to the general population. 22. Hoffman LG, Rouse M. Referral recommendations for binocular
function and/or developmental perceptual deficiencies. J Am Optom
Assoc 1980;51:119–26.
ACKNOWLEDGMENT 23. Mohindra I, Molinari J. Convergence insufficiency: its diagnosis and
management - part 1. Optom Monthly 1980;71:155–60.
Received January 12, 2001; revision received October 2, 2002. 24. Pickwell LD, Hampshire R. The significance of inadequate conver-
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25. Shippman S, Infantino J, Cimbol D, Cohen KR, Weseley AC. Con-
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 Journal of Engineering and Science Research 4 (1): 35-39, 2020
e-ISSN: 2289-7127
© RMP Publications, 2020
DOI: 10.26666/rmp.jesr.2020.1.6

Short Term Effect of Virtual Reality on Eye Accommodative Ability

Mohd Syarifuddin Mukhtar1, Mohd Zulfaezal Che Azemin1, Mohd Hafidz Ithnin1 and
Mohd Izzuddin Mohd Tamrin2
1Kulliyyah of Allied Health Sciences, International Islamic University Malaysia,

Bandar Indera Mahkota, 25200 Kuantan, Malaysia.

2
Kulliyyah of ICT, International Islamic University Malaysia, Gombak, 53100 Kuala Lumpur,
Malaysia.

Abstract: Virtual reality (VR) is a fast-growing technology in the world today. Many countries use virtual reality
for many purposes such as education, military and entertainment. Despite the benefits of VR, harmful effects of
VR on the users are still inconclusive. With only a few reliable studies that investigate the effect of virtual reality
on the users especially on the eyes, yet still there are a lot more things we do not know about the effects of VR. The
purpose of this study was to compare the amplitude of accommodation before and after watching 3-dimesional
(3D) movie utilizing VR and notebook (control group). Thirty-two participants volunteered in this study and all
participants underwent amplitude of accommodation (AA) test using Royal Army Force (RAF) rule before and
after watching three-dimensional (3D) movie for 30 minutes using VR and two-dimensional (2D) movie by laptop.
The amplitude of accommodation between pre- and post-watching 3D movie on VR was insignificantly changed
(p= >0.05). The similar trend was also found after 30 minutes watching movie using laptop (p= >0.05). The
utilization of VR and laptop for 30 minutes did insignificantly alter the eye accommodation.

Key words: Virtual reality, Accommodation, 3-dimensional, Technology, Effect

INTRODUCTION to develop better virtual reality devices and systems to

allow users to interact more naturally with the virtual
Virtual Reality (VR) has already existed since the end reality environment. The companies that make the
of 1960s and it is not an entirely new concept. virtual reality devices must improve their devices for
Previously it was given many names such as synthetic better ergonomics and less health issues when using
environment, cyberspace, artificial reality, and their devices.
simulator technology before VR was eventually
adopted. The latest concept of VR is desktop VR. This paper [3] stated that the main idea of VR is to
Desktop VR is also known by other names such as make the users feel and see through their eyes exactly
Window on World (WoW) or non-immersive VR [1]. like in the real situation even though it is only virtual
Virtual reality has been used in various purposes such condition. Angular movement, relative size and
as education, engineering, military and gaming brightness are the visual cues that drive the perception
platform. of virtual reality. The basis of binocular vision is the
fusion of different images on right and the left eyes into
Besides, this paper [2] found that post-traumatic stress one 3D perception and it is very effective for near
disorder and phobias like fear of spiders and heights objects like in VR compared to distance objects. The
can be treated using virtual reality and it shows equipment comprises of a goggle-like display one for
effective result. Virtual reality is also used for training each eye. The display gives different perspective
and it improves performance and decision-making of images between the two eyes to create 3D effect to
the users [2]. The challenge of virtual reality world is enable the user feels he is in real situation. When the

Corresponding Author: Mohd Zulfaezal Che Azemin, Kulliyyah of Allied Health Sciences, International Islamic University
Malaysia, Bandar Indera Mahkota, 25200 Kuantan, Malaysia, email: zulfaezal@iium.edu.my.

35
 Mohd Zulfaezal Che Azemin et. al./ Journal of Engineering and Science Research, 4(1) 2020, Pages:35-39

user moves his head, the image will be rapidly updated this experiment. The setup was attached to Samsung
to imitate changes of head movement, thus the user will Galaxy Note 8 to provide the display for the Samsung
sense that the action is because of his head movement Gear VR. It has 6.3-inch screen with Active-Matrix
not because of the change in the image. Organic Light-Emitting Diode (AMOLED) Quad high
definition (HD) and a resolution of 2960 x 1440 pixels
Accommodation is stimulated when the eye changes with 552ppi. 13-inch Toshiba Satellite L745 notebook
focus from distant to near images. It happens when the with 184 lux and 1366 x 768 pixels was used to watch
crystalline lens changes its shape because of the action 30 minutes movie for the control group. Royal Air
of the ciliary muscle on the zonular fibres. The Force (RAF) rule was employed to measure the pre-
crystalline lens loses its ability to changes shape with and post-monocular amplitude of accommodation
increasing age and most flexible during childhood and (AA). N5 target on the RAF rule was used as the target
young adult. The lens lost its nucleus rigidity thus for the participants and the measurement was taken 3
reduces the ability to accommodate after 40 years. It is times on each eye. Occluder was used to close subject’s
because the nucleus cannot bulge anteriorly to change one eye during the AA test.
its anterior curvature as in the young age. Change of
the lens shape occurs at the central anterior of the lens Thirty-two participants among students in International
surface based on von Helmholtz’s theory. Islamic University Malaysia (IIUM) Kuantan, Pahang
were volunteered to become our subjects. Participants
There is lack of research about the effect of virtual were explained about consent form and ethical
reality because it is a new technology that is always consideration before being selected in the study. After
developing and improving. However, there are some participant selection, preliminary assessment was
previous research [4, 5] about the effect of virtual performed on the participants to screen for inclusion
reality on the accommodation of the eyes which are and exclusion criteria. The inclusion criteria were
related to our study. Although previous study stated individuals with age from 18 – 25 years, best corrected
that there is no significant effect of virtual reality on visual acuity; Distance at least 6/9, Near N5@40 cm,
the eyes, but there are some differences such as field of no chronic diseases, ocular trauma, ocular diseases, no
view, screen resolution and distance from eyes to the accommodation insufficiency, accommodation excess,
target and the duration of VR use compared to our accommodation infacility, accommodation paralysis,
current study. convergence insufficiency, convergence excess,
divergence insufficiency and divergence excess. The
This study investigated the short-term impact and exclusion criteria were visual acuity worse than 6/9 and
effect of virtual reality on accommodation of the eyes. N5 at distance and near respectively, accommodation
Some research suggested possible harmful effect of and vergences problems.
virtual reality devices especially with head mounted
display [6]. It also studied the effect of virtual reality The assessments include dry refraction, auto refractor,
on convergence of the eyes because accommodation best-corrected visual acuity (BCVA), amplitude of
acts together with convergence during the process. The accommodation, vergence tests (near point of
results of this study would benefit the healthcare convergence), and slit lamp bio microscopy (SLB).
providers to ensure the accommodation data taken The BCVA was measured using Snellen Chart, the AA
during the eye check-up is not due to virtual reality use and near point of convergence was tested using RAF
prior the eye examination. This study will give rule. After the participants pass the preliminary
important information to them. examination, they underwent pre-assessment of
monocular amplitude of accommodation (AA) using
METHODS AND MATERIALS RAF rule and the result was recorded.

This study has been approved by Kulliyyah of Allied Next, the virtual reality group was asked to watch 3-
Health Sciences (KAHS) Ethics Committee (KAHS dimensional movie with 1080p resolution entitled
73/18). Samsung Gear Virtual Reality (VR) 2017 “How to Train Your Dragon 2” on Samsung Gear
weighs 345 grams with 101º field of view was used in Virtual Reality (VR) for 30 minutes. The participants
36
 Mohd Zulfaezal Che Azemin et. al./ Journal of Engineering and Science Research, 4(1) 2020, Pages:35-39

watch the same movie for 30 minutes on laptop for the accommodation, we also test the short-term effect of
next visit in one week. The distance between virtual reality usage on non-invasive tear break-up time
participants and laptop is 1 meter. Immediately after 30 (NIKBUT), tear meniscus height, inter-blinking rate
minutes, post assessment of monocular AA was tested and convergence (phoria and fusional vergence).
on the participants using RAF rule before they were
discharged. In this study, besides amplitude of

Table 1 Comparison of amplitude of accommodation between pre- and post-watching 3D movie on VR using each eye.
No. of Measured eye Type of Time of Median (IQR) p-value
participants measurement measurement
Participants Right Eye Break AA pre 9.09 (2.78) 0.243
watching movie post 8.71 (1.67)
on virtual
reality Recovery AA pre 8.33 (2.72) 0.614
(n=32) post 7.69 (1.95)
Left Eye Break AA pre 9.09 (2.50) 0.743
post 9.09 (1.67)
Recovery AA pre 8.33 (2.08) 0.631
post 8.33 (1.81)

Table 2 Comparison of amplitude of accommodation between post-watching movie on VR and on laptop using each
eye.
No. of Measured eye Type of Type of devices Median (IQR) p-value
participants measurement
Right Eye Break AA VR 8.71 (1.67) 0.640
Participants Laptop 9.09 (1.67)
watching movie
on VR & laptop Recovery AA VR 7.69 (1.95) 0.570
(n=32) Laptop 8.33 (1.95)
Left Eye Break AA VR 9.09 (1.67) 0.985
Laptop 9.09 (1.67)
Recovery AA VR 8.33 (1.81) 0.911
Laptop 8.33 (1.40)

RESULTS DISCUSSION

Statistical analysis was conducted with IBM SPSS Our results are consistent with the previous study
(Version 24.0, SPSS Inc., Chicago, Illinois, USA). The which showed that there was no significant effect of
non-parametric test which was Wilcoxon signed-rank virtual reality headset on the accommodation [4]. The
test was used for data analysis. A value of p<0.05 was study used negative relative accommodation (NRA)
considered significant. There were no significant and positive relative accommodation to test for
differences (p>0.05) in terms of amplitude of accommodation of the eye. The study also compared
accommodation of the thirty-two participants between the effect of playing games on head-mounted display
pre- and post-watching 3D movie on virtual reality for (HMD) virtual reality and on cathode ray tube (CRT)
30 minutes on both eyes (Table 1). There were also no display, equipped with i-glasses head-mounted display
significant differences (p>0.05) of amplitude of made by Virtual I/O (Seattle, WA) as the main
accommodation between post-watching movie for 30 instrument of virtual reality. The i-glasses has display
minutes on virtual reality and notebook (Table 2). of 789 x 230 pixels and field of view of 25 x 20º. The
37
 Mohd Zulfaezal Che Azemin et. al./ Journal of Engineering and Science Research, 4(1) 2020, Pages:35-39

interpupillary distance of the device is fixed at 61 mm. Our test was 30 minutes of usage while their study was
The research work used computer game named Ascent only tested for 10 minutes of using the smartphone. The
(Gravity, San Francisco, CA) as a stimulus on the resolution of iPhone 4S and Samsung Galaxy Note 8
display on 37 subjects and the time exposed on HMD also different whereby iPhone 4S has only 960 x 640
and CRT was 30 minutes. The result showed no small pixels with 326 ppi and the Samsung Galaxy Note 8
changes occurred on the accommodation when using has 2960 x 1440 pixels with 552 ppi. The distance
the device. Another study also measured the amplitude between patient’s eye and the focusing point is also
of accommodation using RAF rule after playing game different; in our study we used 10 mm distance, while
for 40 minutes on virtual reality headset and the result [9] used distance of 0.25 meter from the subjects’ eyes
was also not significant [5]. It can be concluded that to the screen of the smartphone. Therefore, the duration
even though using different type of virtual reality of using VR for 30 minutes with display’s resolution of
headset for 30 minutes, the result on the 552 ppi and 10 mm distance from display to the eyes
accommodation has no significant effect. did not give any significant change to accommodation
of the eyes.
Another drawback of this study was we were not able
to ensure the participants to really watch the movie on CONCLUSION
the virtual reality as the current setting did not allow
for monitoring the eye fixation. They might close their Our findings support the previous study that there is no
eyes while watching the movie which resulted in no short-term effect on the accommodation of the eyes
change in amplitude of accommodation because the when using virtual reality headset for a short period of
eyes were not focus on anything. According to [7], time. However, further study must be conducted to test
accommodation is the change in optical dioptric power the effect of virtual reality headset for longer period
of the eyes that allows the point of focus of the eyes and younger population. Our study also proved that
changed from distance to near targets. If the there is no significant difference on the
participants did not open the eyes and focus on the near accommodation system between watching movie on
screen in virtual reality headset, the accommodation virtual reality headset and laptop.
cannot take place. So, to overcome this drawback, we
observe the behavior of our participants and talk to ACKNOWLEDGEMENT
them when we noticed no response from them through
their behavior to ensure they were awake. We would like to thank and appreciate to the co-
researchers, Mohd Syarifuddin Bin Sidik Ahmad, Anas
Our participants aged from eighteen to twenty-five Bin Ibrahim and Hafizah binti Zaini that were very
years which were categorized as young adults helpful in data collection process in this study. We also
according to [8]. We hope that in the future there will want to thank to all participants for involving in this
be a study to investigate the effect of VR on children study. Their contributions are really appreciated and
because their interpupillary distance is smaller and greatly acknowledged. This work is supported by the
their amplitude of accommodation is high, so they need International Islamic University Malaysia (IIUM),
more concern than adult [5]. No statistically significant under the Research Initiative Grant (P-RIGS18-035-
differences were found in the accommodation 0035).
responses when reading on smartphones and tablets for
10 minutes according to [9]. The study used iPhone 4S REFERENCES
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resolution of the smartphone. Scientific and Engineering Research, 4, 4, 304–309.
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integrating virtual reality into an introductory design
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Cancer Medicine, 6th edition, Hamilton.
[9] Moulakaki, A.I., Recchioni, A., Aguila-Carrasco,
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39
 ISSN: 2641-6360 DOI: 10.33552/WJOVR.2018.01.000509

World Journal of
Ophthalmology & Vision Research

Research Article Copyright © All rights are reserved by Akujobi AU

Assessment of amplitude of accommodation (AA) in

Owerri Municipal Council, Southeast, Nigeria
Akujobi AU1*, Vincent CL2, Ekenze CJ1, Obioma-Elemba JE1 and Vincent CC3
1
Department of Optometry, Imo State University, Owerri, Nigeria
2
Cypress Eye Centre, Abuja, Nigeria
3
Department of Nursing Science, Imo State University, Owerri, Nigeria

*Corresponding author: Akujobi AU, Department of Optometry, Imo State University, Received Date: November 28, 2018
Owerri, Nigeria. Published Date: December 10, 2018

Abstract
Amplitude of accommodation (AA) was measured in 399 consecutive participants, aged 5 to ≥55 years, in Owerri Municipal
Council, Southeast, Nigeria using Donders’ Push-up-to-blur (PUB) technique; and compared with the average AA derived from
Hofstetter’s equation. Each participant wore the subjective prescription and the reduced Snellen chart was gradually moved towards
the participant until the first sustained blur on the 6/6 (20/20) line was reported. The distance between the test chart and the
spectacle plane was recorded as the near point of accommodation (NPA) in centimeters. Dividing 100 by the NPA gave the AA value.
The results showed that age and AA were inversely but significantly (p<0.05) correlated, with the 5-14 and ≥55 years age categories
having the highest (17.49D) and least (1.60D) AA values respectively. Males had insignificantly (p>0.05) higher mean AA (46.74D)
than the age-matched females (45.52D), while students were found to have significantly (p<0.05) higher mean AA (11.23D) than
other occupation groups. Among all age groups, the total measured mean AA was observed to be significantly (p<0.05) lower than
the calculated average AA derived from Hofstetter’s equation. Therefore, Hofstetter’s equation did not accurately predict the real
amplitude of accommodation among various age groups in Owerri Municipal Council, Nigeria.
Keywords: Amplitude of accommodation; Push-up method; Hofstetter’s equation; Nigeria

Introduction
cases, may result in errors in the assessment of accommodative
When fixation changes from far to near, the crystalline lens
function and prescription of reading addition.
varies its focal length (refractive power) in response to changes
in the vergence of the incident light. This process is known Amplitude of accommodation diminishes progressively from
as accommodation. Amplitude of accommodation (AA) is the childhood to old age [4] and is affected by a propensity for near
maximum refractive power the eyes can exert. It refers to a certain work and also influenced by the general health of the patient,
range of object distances for which the retinal image is as sharply previous use of the ciliary muscle, vascular and glandular defects
focused as possible [1]. It remains a very important parameter in the [5]. The age-related decline in accommodative amplitude occurs
diagnosis and treatment of accommodative anomalies, as well as the almost universally, to less than 2 diopters (D) by age 45-50 years,
determination of reading addition of presbyopes. Accommodative and within this age group, most persons begin to experience a
anomalies are among the most common visual disorders, and they decline in the ability to focus on close objects, hence, require glasses
may be associated with symptoms, such as headache, asthenopia, or bifocal lenses for reading [6].
pain in the eye, occasional diplopia, intermittent blurred vision,
Furthermore, a study [7] had reported a mean AA of 14.5D
and swimming of print while reading [2]. Although a study [3] put
among the 6-10 years age group which subsequently diminished
the prevalence of accommodative anomalies of patients who come
more or less regularly, so that at 61-65 years, it came down to 0.8D,
to optometry clinics at 20%, AA is rarely incorporated into routine
with the age of onset of presbyopia in some, being between the ages
clinical examinations by eye-care practitioners, which in many

This work is licensed under Creative Commons Attribution 4.0 License WJOVR.MS.ID.000509. Page 1 of 5
 World Journal of Ophthalmology & Vision Research Volume 1-Issue 2

of 31-40 years. However, the average amplitude of accommodation approximately 58Km2 and has an estimated population of 127,
of persons of various ages was calculated using Hofstetter’s 213 people (males = 62,990; females = 64, 223) based on the 2006
equation and has been invaluable in predicting the reading addition National Census Exercise, with about 17, 000 households, including
of presbyopes. According to another study [8], Hofstetter’s equation shops and offices. It has the highest number of schools, hospitals,
predicted the average amplitude of accommodation as 18.5 - (0.30 x educational institutions, business outfits, government ministries
patient’s age in years), the minimum amplitude of accommodation and parastatals in Imo state.
as 15 - (0.25 x patient’s age in years) and the maximum amplitude
It is bounded on the North by Amakohia, on the Northeast
of accommodation as 25- (0.40 x patient’s age in years). However,
by Uratta, on the East by Egbu, on the Southeast by Naze, on the
the accuracy of this formula in predicting the average amplitude
South by Nekede and on the Northwest by Irete. The municipal
of accommodation has been queried for certain age groups and
council is made up of five main villages; Umuoyima, Umuororonjo,
populations [9].
Umuonyeche, Amawom and Umuodu. It also has the layout of
In addition, a study [10] compared the amplitude of Ikenegbu, Aladinma, Trans-Egbu, New Owerri, Works Layout
accommodation in different vertical viewing angles among 31 and Uratta road housing estates. Owerri Municipal Council has 2
Malaysians aged 18-26 years using the minus-lens method. The geological regions; the coastal plain and a plateau portion.
amplitude of accommodation was found significantly changed in
The vegetation is typically rainforest, although some parts
four sets of viewing angles (p<0.001), with the mean difference of
consist of Guinea savanna due to poor environmental management
2.52D from 20 Degree up-gaze to 40 Degree down-gaze. However,
and pollution. It has a mean annual rainfall of about 2,250-
the association of amplitude of accommodation in different viewing
2,500mm with a mean temperature of 25-270c and a relative
angles among genders and age groups was not significant (p>0.05).
humidity of 80%. It has 72 health institutions, 28 primary schools,
Furthermore, the amplitude of accommodation evaluated by
8 secondary schools, 2 universities and one college of education
dynamic retinoscopy, push-down and minus lens techniques was
with the inhabitants being mostly civil servants, students, traders
measured and compared among 1,298 subjects between 5 and
and artisans.
60 years of age [11]. For the dynamic retinoscopy findings, no
significant change was observed between 5 and 19 years of age Study-population and sample size
(mean AA = 8.3 D) and between 45 and 60 years of age (mean The study-population comprised of the 127, 213 persons
AA = 0.6 D). Similarly, the amplitude of accommodation among estimated by the 2006 National Census Exercise. A sample size of
genders and race for different reading postures was shown to be 399 consecutive participants was drawn using the Taro Yamane’s
insignificant (p>0.05) [12]. formula for determination of minimum sample size.
In many instances, accommodative anomalies and non- Ethical considerations
strabismic binocular dysfunctions are associated with depletion
Ethical protocols established by the Helsinki Declaration
of accommodative amplitude, especially among occupation groups
on Human Experiments were adopted. Ethical approval for the
and populations predisposed to rigorous reading and other near
study was obtained from the Imo State Ministry of Health, while
tasks [13]. In urban populations, near tasks are predominant, life
oral informed consent of the participants was obtained prior to
expectancy is high, hence, many more people attain older ages and
examinations.
consequently experience depletions in accommodative amplitude.
As a result, there is need to generate territory-specific data, which Procedure for data collection
could provide normative AA values for urban populations and The Donders’ push-up-to-blur (PUB) method was adopted for
also enhance precision in the prescription of reading addition for the measurement of amplitude of accommodation. Each participant
presbyopes in such populations. was clerked and subjected to a comprehensive eye examination
Furthermore, previous investigations into amplitude of which included; distant and near visual acuity, ophthalmoscopy
accommodation (AA) merely compared the measured AA with and static retinoscopy. Subjective refraction and refinement
the calculated AA derived from Hofstetter’s equation [9,10,14], techniques were used to determine the prescription. Participants
without reporting its distribution among various age-matched were excluded if they had amblyopia, accommodative anomaly,
demographic groups. The study, therefore, investigated the recent cycloplegic refraction, ocular surgery, cataract and ocular
amplitude of accommodation among diverse population groups and pathologies that could alter the accommodative status.
also compared the measured mean AA with the calculated average Each participant wore the subjective prescription. The reduced
AA of Hofstetter’s formula to ascertain the accuracy of Hofstetter’s Snellen chart was gradually brought closer to the participant until
equation in predicting real amplitude of accommodation in the the first sustained blur on the 6/6 (20/20) line was reported. The
study-area. distance from the chart to the spectacle plane was measured as the
Materials and Methods near point of accommodation (NPA) in centimeters. One hundred
(100) was divided by the NPA to obtain the AA.
Study-area
Statistical analysis
Owerri is the capital of Imo State, Nigeria and has geographical
coordinates of 50 29” 0” N and 70 2” 0’’ E. It occupies an area of Data were analyzed using Chi-square (X2) statistical method.

Citation: Akujobi AU, Vincent CL, Ekenze CJ, Obioma-Elemba JE, Vincent CC. Assessment of amplitude of Accommodation (AA) in Owerri Page 2 of 5
Municipal Council, Southeast, Nigeria. W J Opthalmol & Vision Res. 1(2): 2018. WJOVR.MS.ID.000509. DOI: 10.33552/WJOVR.2018.01.000509.
 World Journal of Ophthalmology & Vision Research Volume 1-Issue 2

Result Table 3: Demographic distribution of amplitude of accommodation (AA)

in Owerri Municipal Council.
Out of the 399 consecutive participants examined for amplitude
Demographic variables Mean AA (Diopters) X2cal. p-value
of accommodation (AA), males (151) constituted 37.84% and
females (248) 62.16% of the study population. The 15-24 years Age (Years)
age-group had the highest number of participants among both 5-14 17.49 21.65 0.00061
males (61) and females (150) while the ≥ 55 years category had
15-24 10.7
the least male (3) and female (2) participants as shown in (Table 1).
25-34 7.7
Table 1: Age and gender distribution of sample population in Owerri
Municipal Council. 35-44 5.4

Age (Years) Males (%) Females (%) Total (%) 45-54 3.3

≥55 1.6
5-14 22 (14.6) 30 (12.1) 52 (13.0)
Gender

15-24 61 (40.4) 150 (60.5) 211 (52.9) Males 46.74 0.016 0.899

Females 45.52
25-34 34 (22.5) 39 (15.7) 73 (18.3)
Occupation

35-44 27 (17.9) 20 (8.1) 47 (11.8) Students 11.23 8.08 0.044

Traders 5.61
45-54 4 (2.6) 7 (2.8) 11 (2.8)
Civil servants 4.71

≥55 3 (2.0) 2 (0.8) 5 (1.3) Farmers 1.75

Discussion
Total 151 (100) 248 (100) 399 (100)
The 5-14 years age group recorded the highest mean amplitude
The 5-14 years age category recorded the highest mean AA of accommodation (AA) (17.49D), followed by the 15-24 years
(17.49D) followed by the 15-24 years group with a mean AA of category, with a mean AA of 10.70D. The least mean AA was
10.70D, while the ≥55 years group had the least AA of 1.60D. observed among the ≥55 years group, where a mean AA of 1.60D
Across gender, males (46.74D) had insignificantly (p>0.05) higher was recorded. The mean AA values were found to be significantly
amplitudes than their age-matched female counterparts (45.52D). (p<0.05) and inversely correlated with age. The result corroborates
those of previous studies [4,6,7,15,16,17] and therefore
Four occupation groups; students, civil servants, traders and
strengthens corroborative evidences for the declination of AA with
farmers were identified and evaluated. Among these, more students
age but disagrees with the findings of Majumder & Ying [11]. The
(326) were sampled than other groups, while the least number
discrepancy between the findings of the present study and those of
of participants was found among farmers (2) (Table 2). Students
a previous study [11] may be predicated on variance in the study
had the highest mean AA of 11.23D among the occupation groups,
design of both studies. While the present study investigated the
followed by traders (5.6D), while farmers recorded the least mean
amplitude of accommodation at the primary position of gaze, the
AA of 1.75D.
previous study [11] examined the AA from four sets of viewing
Table 2: Occupation distribution of sample population in Owerri Municipal angles. AA values obtained from these angles, undoubtedly, should
Council.
differ from those at the primary position of gaze as a result of the
Occupation Frequency Frequency(%) altered tonicity of the relevant extra ocular muscles involved in
ocular motility.
Students 326 81.7
The gender distribution of AA showed that males had a higher
Trading 24 6 mean AA of 46.74D than their age-matched female counterparts
(45.52D), although the difference was not statistically significant
Civil service 47 11.8 (p>0.05). The result compares very closely with the findings of a
recent study [13] which reported that mean AA distributions were
Farming 2 0.5
not significantly correlated with gender. Although the higher mean
Total 399 100
AA observed among the male participants does not conform to any
known explanation, it is not unlikely that males may have stronger
The 5-14 years group had a mean AA of 17.49D with a iris sphincter muscles which are responsible for pupil constriction
calculated average AA of 15.65D using Hofstetter’s formula, while in accommodation. On the other hand, the result disagrees with
the ≥55 years age group recorded the least mean AA of 1.60D with the submissions of another study [18] where females exhibited
a corresponding average AA of 2.00D calculated with Hofstetter’s significantly higher AA than the males. The younger age distribution
equation (Table 3). of the females could have been accountable for the discrepancy in

Citation: Akujobi AU, Vincent CL, Ekenze CJ, Obioma-Elemba JE, Vincent CC. Assessment of amplitude of Accommodation (AA) in Owerri Page 3 of 5
Municipal Council, Southeast, Nigeria. W J Opthalmol & Vision Res. 1(2): 2018. WJOVR.MS.ID.000509. DOI: 10.33552/WJOVR.2018.01.000509.
 World Journal of Ophthalmology & Vision Research Volume 1-Issue 2

the findings of the previous study and the present one where both while the present study reported lower values, both queried the
genders were age-matched. accuracy of Hofstetter’s equation for calculating the amplitude of
accommodation in all age groups and populations. Furthermore,
Among the occupation groups, students were found to have the
the findings of the present study corroborate the evidence of other
highest mean AA (11.23D), followed by traders (5.61D), civil servants
studies [14,15,20] against the accuracy of Hofstetter’s equation.
(4.71D) and farmers who had the least mean AA of 1.75D. Moreover,
Although the studies sampled 6-12, 6-18 and 6-10-year-old school
the observed differences in mean amplitude of accommodation
children respectively, the measured mean AA was found to be lower
within the occupation groups were statistically significant
than that calculated with Hofstetter’s equation in the 6-12- and
(p<0.05). The findings do not agree with those of a previous study
6-10-years age categories. Both studies [14,20] therefore suggested
[19] where depletion of AA was established among occupation
that Hofstetter’s equation did not accurately predict the amplitude
groups predisposed to rigorous reading and other near works. On
of accommodation of children within these age groups.
the contrary, students examined in the present study did not record
a lower mean AA despite rigorous academic indulgences; however, Conclusion
the higher AA observed among students is most likely attributed to
The depletion of AA with advancing age, opined by previous
the age of the students, who were younger and therefore, exhibited
studies, was re-established in the present study. Normative data
higher amplitudes. Furthermore, it is likely that if the students’
for mean AA were generated for various age and population groups
amplitudes were compared with an age-matched non-student
in Owerri Municipal Council, Nigeria and will therefore provide an
population, the findings may have been in good agreement with
empirical baseline for future comparative studies. Males recorded
those of the previous study [19]. The observed distribution of mean
higher AA than females, while students had higher AA than other
AA among occupation groups conforms to logical explanations; the
occupation groups. Furthermore, the present study corroborated
mean AA value of traders (5.61D) may suggest that most traders
previous evidences against the accuracy of Hofstetter’s equation
within the study-area were relatively young; moreover, trading is
in predicting real amplitude of accommodation among various
not a near-specific task and therefore, does not impose a significant
age groups, hence, clinical assessment of AA is recommended over
demand on the accommodative mechanism. The mean amplitude
Hofstetter’s equation.
of accommodation of civil servants (4.71D) was found to be lower
than that of traders because civil servants are predisposed to Acknowledgement
lengthy hours of near tasks, which could result in the depletion The authors are grateful to the Department of Optometry, Imo
of AA. The least mean AA value of 1.75D recorded by farmers may State University, Owerri, Nigeria for granting the permission to use
imply that most of the farmer were within the ≥55 years age group the Optometry Clinic throughout the period of the study.
and had amplitudes which were already depleted as a result of age
and other geriatric-related conditions (Table 4). Conflict of Interest
Table 4: Age distribution of measured mean AA and Hofstetter’s average The authors declare that there were no personal or financial
AA in Owerri Municipal Council. interests which may have inappropriately affected the conduct or
Average AA (D) by the results of the study.
Measured Mean AA
Age (Years) Hofstetter (18.5-
(D)
(0.3XAge) References
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Citation: Akujobi AU, Vincent CL, Ekenze CJ, Obioma-Elemba JE, Vincent CC. Assessment of amplitude of Accommodation (AA) in Owerri Page 4 of 5
Municipal Council, Southeast, Nigeria. W J Opthalmol & Vision Res. 1(2): 2018. WJOVR.MS.ID.000509. DOI: 10.33552/WJOVR.2018.01.000509.
 World Journal of Ophthalmology & Vision Research Volume 1-Issue 2

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Yoon et al. BMC Ophthalmology (2020) 20:200
https://doi.org/10.1186/s12886-020-01471-4

RESEARCH ARTICLE Open Access

Influence of virtual reality on visual

parameters: immersive versus non-
immersive mode
Hyeon Jeong Yoon, Jonghwa Kim, Sang Woo Park and Hwan Heo*

Abstract: Background: To investigate the differences in refraction, accommodative factors, visual parameters, and
subjective symptoms after using two types of virtual reality (VR) content with different depths of perception.
Methods: Twenty-three volunteers, who played VR games in two modes (immersive and non-immersive) for 30
min, were enrolled. Visual parameters were examined before and after using VR. Accommodative factors were
measured using static and dynamic methods. Subjective symptoms were assessed using a questionnaire.
Differences according to VR content were compared, and correlations between each visual parameter were
analyzed.
Results: There were no changes in refraction or accommodative factors after use of the VR. However, there was a
significant increase in the near point of accommodation (NPA), the near point of convergence (NPC), and subjective
symptom scores after using the immersive mode. Correlation analysis revealed a positive correlation between
baseline values of near exophoria and mean accommodative lag of the dominant eye, and also revealed a negative
correlation between NPA and mean accommodative lag in the non-dominant eye.
Conclusions: The use of VR for 30 min increased NPA and NPC, especially after the immersive mode was used. In
addition, higher exophoria and smaller NPA is associated with increased accommodative lag after using VR.
Keywords: Accommodation, Asthenopia, Convergence, Virtual reality

Background min is discouraged, although these warnings are based

A virtual reality (VR) device is an immersive medium on weak evidence [1]. Many studies have reported that a
that uses a head-mounted display (HMD). VR creates significant proportion of VR device users experience a
the sensation of being entirely transported into a virtual highly aversive sense of discomfort, disorientation, nau-
three-dimensional (3D) world, which can provide a far sea, and motion sickness, and these reports suggest that
more visceral experience compared to other video for- viewing stereoscopic images on 3D devices may induce
mats. Recently, VR devices have become available for visual asthenopia, such as visual discomfort and fatigue
purchase on the Internet from a variety of manufac- [2–6]. It is essential to study the effects of VR devices on
turers, and have been widely used for gaming. the eye, as HMD images are presented to users at a
However, the proper use of VR has not yet been estab- short distance with a powerful convex lens to simulate
lished. In health and safety warnings provided by manu- 3D reality. Compared with older types of VR, many im-
facturers, VR is not recommended for use by children < provements have been made to reduce user discomfort
13 years of age. Additionally, continuous use for > 30 including image resolution and corresponding processes
for head movement. However, there are few ophthalmo-
* Correspondence: opheye@hanmail.net logical studies investigating the effects of recent itera-
Department of Ophthalmology, Chonnam National University Medical School
and Hospital, 42 Jebong-ro, Dong-gu, Gwangju 61469, Republic of Korea
tions of VR devices [7–11].

© The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if
changes were made. The images or other third party material in this article are included in the article's Creative Commons
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licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain
permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the
data made available in this article, unless otherwise stated in a credit line to the data.
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 2 of 8

Ha et al. [7] investigated the clinical effects of the HMD covered the field of view. The headset included separate
on visual function, including the oculomotor system; they displays for each eye, each with 960 × 1080 resolution,
found no significant clinical changes, except for transient yielding a 100-degree-horizontal field of view. A fixed-
refractive error or binocular alignment. However, the study degree convex lens was located in front of each display
adopted the method of watching movies using VR rather rendered display content at optical infinity. Inter-
than immersive content such as games. In that case, the pupillary distance was adjusted via a user-enabled key
perceived depth was fixed at one distant point; therefore, it that was located on the right side of the VR device.
is possible that visual parameters change and the Participants used the Oculus Rift device while seated
accommodation-convergence conflict is not fully induced, on a freely rotating chair. They were asked to perform
compared with immersive content, which has variable per- 30 min of gameplay (Minecraft, Mojang AB, Sweden) in
ceived depth. In addition, accommodative change was not two different modes (immersive and non-immersive).
evaluated. It is necessary to investigate accommodative There was a 1-week interval between playing in immer-
change, as it can be related to a user’s transient or perman- sive mode and non-immersive mode. In the immersive
ent myopia, in addition to subjective symptoms [12–14]. mode, the stereo head-tracking head-mounted display
Turnbull et al. [15] reported that refraction and bin- presentation brought the player inside the 360-degree
ocular status (e.g. gaze stability, stereopsis, and ampli- virtual reality environment, which allowed the user to
tude of accommodation) did not change after VR trials. feel as if they were physically present in the game. The
However, the choroid, which is a pigmented vascular tis- viewpoint moves in accordance with the player’s head
sue located outside of the eye, was thickened. In that movements. In the non-immersive mode, the player is
study, investigators used virtual indoor and outdoor en- placed in a static environment (e.g., a living room) while
vironmental content. They also examined accommoda- watching the VR environment on a desktop screen that
tive amplitudes, but they did not analyze their was approximately 2 m away (a desktop view). The
correlation with other visual parameters. players could look around the room; however, the area
In the present study, we examined changes in objective of gameplay was fixed on a virtual screen in front of the
visual parameters and subjective symptoms after playing player (Additional file 1).
two modes of VR content – immersive and non-
immersive – each with a different perception depth. The Measurements of accommodation
change in accommodation was evaluated using static and Refraction and accommodation were measured using a
dynamic methods. In addition, correlations between visual binocular open-field refractor (Auto Ref/Keratometer
parameters after playing VR content were evaluated. WAM-5500, Grand Seiko Co Ltd., Hiroshima, Japan).
The spherical equivalent (sphere + 1/2 of cylinder) was
Methods used for calculation. For static measurement, the accom-
Thirty-five healthy volunteers who had better than 16/20 modative amplitude was calculated by subtracting the
uncorrected visual acuity with above 20/20 best- refractions obtained under monocular condition while
corrected visual acuity were recruited. The subjects had viewing a 1 cm × 1 cm E-shape target at 33 cm from
no ophthalmologic diseases, including strabismus, am- those obtained from viewing the target at 5 m in the
blyopia, corneal or retinal disease, or a history of ocular same manner.
surgery, except for refractive surgery. The number of Software verified by the manufacturer was installed on
subjects was calculated using G-power version 3.1 a computer to allow dynamic mode function. To initiate
(Heinrich Heine University, Dusseldorf, Germany) and measurements, the instrument was aligned with the
considered a drop-out rate of 20%. Twelve subjects with pupil of each eye, and the joystick button was pushed
exophoric deviation > 10 prism diopters (PD) and/or and then released once; the instrument then commenced
esophoric deviation > 5 PD were excluded. Informed recording dynamic measurements at approximately 5
consent was obtained from all 23 volunteers who were samples/s. The observer ensured that the instrument
eligible and ultimately enrolled in this study. Ethics com- remained carefully aligned with the subject’s right eye
mittee approval was obtained from the Chonnam Na- while undergoing dynamic measurements by observing
tional University Hospital Institutional Review Board the alignment target imaged within the pupillary center
(Gwangju, Korea). The study protocol adhered to the in the LCD monitor for the entire duration of testing.
guidelines of the Declaration of Helsinki. The instrument wrote the data to a spreadsheet file
(Excel, Microsoft Corporation, Redmond, WA, USA)
Display that recorded the time of measurement, eye measured,
The Oculus Rift VR device (Oculus VR, LLC., Irvine, spherical equivalent refraction, and pupil diameter ap-
California, USA) was used in this study. The device com- proximately every 0.2 s and converted it to a sine graph
prised a lightweight (0.44 kg) headset that completely form [16, 17]. The velocity of accommodation, mean
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 3 of 8

accommodative lag, and dynamic accommodative re- from the subject during the test. Threshold stereopsis
sponse was investigated to measure dynamic level was recorded in seconds of arc.
accommodation. Ocular dominance was determined by the hole-in-the-
The velocity of accommodation was obtained by card test. The participant was asked to hold a card with
calculating the difference between the maximum and a hole at arm’s length and focus on a target 3 m away
minimum refraction divided by the time taken. Mean with both eyes. The examiner alternately occluded eyes
accommodative lag was calculated by averaging the to determine which eye was viewing the target through
value of the participant’s actual refraction that dif- the hole and that eye was determined to be the domin-
fered from the target refraction. The dynamic accom- ant eye [18].
modative response was calculated according to the The presence and magnitude of far (5 m) and near
dispersion between actual refraction and target refrac- (33 cm) phoria were verified using the cover test and
tion. A higher correlation coefficient was associated alternating cover test with prism. A standard set of
with better dynamic accommodative response (Fig. 1). loose plastic prisms was used, which individual prisms
increased in power from 1 to 10 PD in 1-PD
Other visual parameters increments, and from 10 to 20 PD in 2-PD
Monocular near point of accommodation (NPA) was ob- increments. All measurements were repeated three
tained using Donder’s push-up method. A 20/30 single times for each test, and results reported as the mean
letter about 50 cm from the subject on a fixation stick value [2].
served as the target, and was moved gradually closer to All measurements were performed before and im-
the participant at a rate of about 5.0 cm/s until the par- mediately after playing the VR game in the order
ticipant noticed the target starting to blur. The near listed above. If visual parameters were changed, it was
point of convergence (NPC) was also obtained using the measured repeatedly every 15 min until the initial
same method as previously described for the NPA meas- value was obtained again. The criteria for re-
urement. The first point at which the corneal reflex of examination were > 2-cm changes in NPA and NPC,
the subjects began to extend outward was considered to over 20 s of arc change of stereopsis, and over 0.5 D
be the endpoint [2]. change of refraction. Ocular phoric deviation was
Near stereopsis was measured using a near stereopsis evaluated by the same pediatric ophthalmologist
vision test (Stereo Fly SO-001 test; Stereo Optical Co., (H.H.). The other visual parameters were examined
Chicago, IL, USA). The test stereogram was held 40 cm by a single examiner (H.J.Y.).

Fig. 1 Dynamic accommodative response was calculated according to the dispersion between actual refraction and target refraction (a
demonstrates a better dynamic accommodative response than b)
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 4 of 8

Evaluation of subjective symptoms after using VR. Near stereopsis and phoria were not sig-
Thirteen symptoms were included in the questionnaire. nificantly different in either mode (Table 1).
The questionnaire was based on a computer vision syn- Table 2 summarizes the comparisons between subject-
drome questionnaire previously described by Seguí del ive symptoms according to the immersive and non-
M et al. [19]. The symptom sensation questionnaire con- immersive VR modes. Tearing, blurred vision, double vi-
tained six identical analog scales (0 = none, 6 = too se- sion, difficulty focusing for near vision, and neurological
vere to tolerate) through which the subject recorded the symptoms, including headache, dizziness and nausea
magnitude of each of the symptoms compared with were more severe in the immersive mode than the non-
baseline. After playing two modes of the VR game, the immersive mode of VR (all p < 0.05). Table 3 summa-
subjects completed the questionnaire. rizes the changes in accommodation using static and dy-
namic measurements with the WAM-5500 binocular
refractor after using VR. There was no significant change
Statistical analysis
in accommodative amplitude, velocity of accommoda-
Statistical analysis was performed using SPSS version
tion, mean accommodative lag, or dynamic accommoda-
18.0 (IBM Corporation, Armonk, NY, USA) for Win-
tive response in both modes.
dows (Microsoft Corporation, Redmond, WA, USA).
Table 4 summarizes the correlations between the base-
The normal distribution for all variables was assessed
line data of visual parameters (exophoria at far/near,
using the Kolmogorov-Smirnov test. All variables were
NPA in the dominant/non-dominant eye and NPC),
not normally distributed. Data are presented as the me-
changes in ocular and accommodative parameters, and
dian (interquartile range). A Wilcoxon signed-rank test
the sum of subjective symptom scores after using VR.
was used to compare changes in variables before and
There was a positive correlation between baseline values
after performing VR. Differences in subjective symptoms
of near exophoria and mean accommodative lag of the
according to the contents were also compared using the
dominant eye (r = 0.372, p = 0.014). The NPA in the
Wilcoxon signed-rank test. Spearman’s rho correlation
non-dominant eye exhibited a negative correlation with
test between each of the visual parameters was used for
changes in mean accommodative lag of the dominant
correlation analysis. The variables for a single eye, in-
eye (r = − 0.328, p = 0.032). Two correlation measure-
cluding NPA and accommodative parameters, were
ments were excluded due to a false positive rate above
solely correlated with the corresponding eye. For all
0.3: exophoria versus changes of the NPA and the NPC
tests, statistical significance was determined to be p <
versus changes of mean accommodative lag.
0.05. with differences corrected by the Benjamini-
No visual parameters were found that directly corre-
Hochberg procedure using false discovery rates of 0.25.
lated with symptoms. There was no correlation between
total symptom score and baseline value of visual param-
Results eters (all p > 0.05; Table 4). This result was the same as
Among the 23 participants, 11 were men and 12 were the analysis using logistic regression (all p > 0.05). Add-
women, with a mean age of 23.9 ± 3.7 years (range, 20– itionally, we examined the correlations between visual
35 years). The mean uncorrected visual acuity (logarithm parameters and each of the subjective symptoms, and
of the minimum angle of resolution [logMAR]) was there were no statistically significant correlations (all p >
0.03 ± 0.04 logMAR. Fifteen of the participants had pre- 0.05).
vious experience with VR within 1 h, which was outside
of this experiment. Eleven of the participants had a his- Discussion
tory of refractive surgery for myopia. One participant VR is a technology that renders a proximal display to be
was excluded from the analysis after discontinuing the perceived as a real-world experience using powerful con-
immersive mode of VR due to severe headache and vex lenses. In a VR environment, accommodation is
nausea. fixed to a single depth of field at a distant point. How-
In the immersive mode, the mean refractive error of ever, convergence is constantly induced. The resulting
both eyes did not change significantly (p = 0.935 in the accommodation-convergence conflict and sustained eye-
dominant eye; p = 0.654 in the non-dominant eye). How- ball movement are known to cause fatigue and 3D asthe-
ever, the NPA was increased in both eyes (p = 0.005 in nopia [8, 20–22].
the dominant eye, p = 0.002 in the non-dominant eye). In this study, the NPA and NPC increased in the im-
The NPC was also increased in the immersive mode mersive mode. In the immersive mode, more image dis-
(p = 0.001). In the non-immersive mode, the mean re- parity can occur than in the non-immersive mode.
fractive error did not change significantly for either eye Image disparity activates an accommodative response
(p = 0.261 in the dominant eye; p = 0.881 in the non- and convergence-accommodation to a change in accom-
dominant eye). Only the NPC (p = 0.002) was increased modation [23–25]. However, the actual accommodative
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 5 of 8

Table 1 Comparison of changes in visual parameters after playing a virtual reality game with different depths perception
Variable Pre Post p-value
Immersive mode
Refraction (dominant eye), Diopter −0.38 (0.63) − 0.25 (0.50) 0.935
Refraction (non-dominant eye), Diopter −0.19 (0.81) −0.13 (0.81) 0.654
NPA (dominant eye), cm 8.50 (3.00) 10.0 (3.50) 0.005*
NPA (non-dominant eye), cm 8.50 (3.00) 10.0 (3.50) 0.002*
NPC, cm 7.00 (3.00) 9.00 (3.25) 0.001*
Near stereopsis, sec 40.0 (12.50) 40.0 (12.50) 0.180
Phoria, PD (near) 2.00 (8.00) 2.00 (8.00) 0.086
Non-immersive mode
Refraction (dominant eye), diopter −0.38 (0.63) −0.25 (0.63) 0.261
Refraction (non-dominant eye), diopter −0.25 (0.88) −0.25 (0.63) 0.881
NPA (dominant eye), cm 9.00 (3.00) 9.50 (4.25) 0.058
NPA (non-dominant eye), cm 9.50 (3.00) 9.00 (4.00) 0.120
NPC, cm 8.00 (3.00) 9.00 (4.00) 0.002*
Near stereopsis, sec 40.0 (10.0) 40.0 (20.0) 0.234
Phoria, PD (near) 5.00 (8.00) 5.00 (8.00) 0.257
Data presented as median (interquartile range). NPA, near point of accommodation; NPC near point of convergence; PD prism diopter *statistically significant
value using the Benjamini-Hochberg procedure

target was fixed; thus, there was a possibility of fatigue vision, double vision, and defocusing symptoms were
due to the excessive activation of accommodative adap- worse in the immersive mode. These results suggest that
tation [24]. As shown in Table 3, other accommodative ocular fatigue due to excessive accommodation-
factors did not change. It appears that ocular fatigue in- convergence response is more severe in the immersive
duced an increase in the NPA and NPC, which is more mode.
of a subjective factor than actual accommodation. As shown in Table 4, correlation analysis revealed that
The data presented in Table 2 demonstrate that in the higher exophoria is associated with increased accommo-
immersive mode, neurological symptoms, such as head- dative lag. Vergence adaptation modulates fast response
ache, dizziness and nausea, were more severe than dis- and reduces error and fatigue by maintaining vergence
comfort in the eyes such as dryness. In addition, blurred stimulus [23–25]. Exophoric subjects may require more
Table 2 Comparison of subjective symptoms after playing a virtual reality game
Symptom Immersive mode Non-immersive mode p-value
Burning 0.0 (0.0) 0.0 (0.0) 0.564
Feeling of a foreign body 0.0 (0.0) 0.0 (0.0) 0.705
Excessive blinking 1.0 (3.0) 1.0 (2.0) 0.554
Tearing 0.0 (1.0) 0.0 (0.0) 0.014*
Dryness 0.0 (3.0) 1.0 (2.0) 0.942
Tingling 0.0 (2.0) 0.0 (1.0) 0.084
Blurred vision 1.0 (2.0) 0.0 (1.0) 0.005*
Double vision 0.0 (0.0) 0.0 (0.0) 0.046*
Difficulty focusing for near vision 0.0 (1.0) 0.0 (0.0) 0.018*
Increased sensitivity to light 0.0 (1.0) 0.0 (0.0) 0.623
Headache 2.0 (3.0) 0.0 (1.0) 0.012*
Dizziness 3.0 (3.0) 1.0 (2.0) 0.012*
Nausea 3.0 (3.0) 0.0 (1.0) 0.004*
Total 12.0 (9.0) 5.0 (10.5) 0.002*
Data presented as median (interquartile range). *statistically significant value using the Benjamini-Hochberg procedure
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 6 of 8

Table 3 Changes in accommodation using static and dynamic measurement after playing a virtual reality game
Variable Immersive mode Non-immersive mode
Pre Post p-value Pre Post p-value
Static measurement – Accommodative amplitude, diopter
Dominant eye 2.063 (1.188) 1.938 (0.688) 0.722 2.000 (0.500) 1.875 (0.875) 0.571
Non-dominant eye 2.063 (1.375) 2.125 (1.313) 0.442 2.000 (0.875) 2.000 (1.000) 0.572
Dynamic measurement
Velocity of accommodation, diopter/s
Dominant eye 0.334 (0.126) 0.395 (0.144) 0.095 0.382 (0.105) 0.361 (0.154) 0.910
Non-dominant eye 0.416 (0.104) 0.383 (0.119) 0.664 0.364 (0.133) 0.358 (0.133) 0.362
Mean accommodative lag, diopter
Dominant eye 0.660 (0.628) 0.602 (0.483) 0.108 0.575 (0.520) 0.591 (0.528) 0.322
Non-dominant eye 0.707 (0.430) 0.836 (0.590) 0.274 0.734 (0.357) 0.680 (0.489) 0.548
Dynamic accommodative response
Dominant eye 0.912 (0.078) 0.901 (0.101) 0.778 0.884 (0.105) 0.904 (0.126) 0.664
Non-dominant eye 0.881 (0.112) 0.900 (0.150) 0.821 0.914 (0.057) 0.886 (0.101) 0.099
Data presented as median (interquartile range).

effort in accommodation and convergence due to re- in mean accommodation lag. Although not included in
duced adaptation, which can cause more fatigue than in the table, the baseline NPA and NPC demonstrated a
normal individuals [4, 26–29]. high positive correlation with baseline values of mean
In addition, subjects who had a smaller NPA were accommodative lag (dominant eye, p = 0.010, p = 0.008;
more likely to exhibit an increase in mean accommoda- non-dominant eye, p = 0.026, p = 0.017, respectively).
tive lag after using VR. The baseline NPC was removed Sreenivasan et al. [30] reported that retinal image
due to a high false discovery rate; however, the baseline quality is better when accommodative lag is greater, be-
NPC showed a relatively high correlation with changes cause, paradoxically, the depth of field is more

Table 4 Correlations between the value of baseline and changes of ocular parameters
Changes in value (post - pre) Exophoria (far) Exophoria (near) NPA(dominant eye) NPA (non-dominant eye) NPC
r P r P r P r P r P
Ocular parameters
Exophoria (far) −0.100 0.514 0.127 0.410 −0.032 0.841 − 0.064 0.685 −0.087 c
Exophoria (near) − 0.166 0.276 − 0.149 0.335 − 0.054 0.731 −0.066 0.676 −0.152 0.324
NPA (dominant eye) 0.296 0.048 −0.007 0.963 −0.255 0.099 0.007 0.966
NPA (non-dominant eye) 0.202 0.184 −0.028 0.859 −0.204 0.189 0.105 0.498
NPC 0.046 0.764 0.046 0.764 0.051 0.747 0.051 0.747 −0.188 0.222
Accommodative parameter (dominant eye)
Accommodative amplitude −0.110 0.507 0.030 0.860 −0.220 0.191 −0.003 0.987
Velocity of accommodation 0.059 0.703 −0.034 0.834 0.123 0.438 0.001 0.994
Mean accommodative lag 0.165 0.285 0.372 0.014* −0.175 0.266 −0.159 0.309
Dynamic accommodative response 0.032 0.845 0.139 0.393 0.085 0.607 0.004 0.979
Accommodative parameter (non-dominant eye)
Accommodative amplitude −0.141 0.373 −0.034 0.834 −0.128 0.430 −0.008 0.959
Velocity of accommodation −0.017 0.913 0.151 0.327 −0.005 0.976 0.207 0.187
Mean accommodative lag 0.111 0.468 0.076 0.624 −0.328 0.032* −0.333 0.027
Dynamic accommodative response −0.110 0.507 0.019 0.903 0.003 0.983 −0.069 0.659
Sum of symptom scores −0.110 0.471 0.173 0.261 0.198 0.202 0.222 0.152 0.084 0.587
NPA near point of accommodation, NPC near point of convergence *statistically significant value using the Benjamini-Hochberg procedure
Yoon et al. BMC Ophthalmology (2020) 20:200 Page 7 of 8

structurally or functionally wider in individuals with content, instead of non-immersive VR content. Addi-
higher accommodative lag. In individuals with a large tionary, accommodation was analyzed using dynamic
baseline NPA and NPC, retinal image quality was para- techniques. Our results may form the basis for recom-
doxically better, as with high accommodative lag. This mendations for users who may need to be more careful
may reduce the ocular fatigue that is induced by the regarding VR use.
rapid accommodation response.
However, there is controversy regarding whether ac- Supplementary information
commodative lag enhances accommodative stress by Supplementary information accompanies this paper at https://doi.org/10.
1186/s12886-020-01471-4.
promoting a blurred retinal image [31]. Further research
is needed to support this hypothesis. Shiomi et al. [32] Additional file 1: Immersive vs. non-immersive mode (AVI). Video clip
reported that actual accommodation could change de- showing the difference between immersive and non-immersive modes.
pending on the perception depth of the 3D content, even In the non-immersive mode, the viewing angle is fixed parallel to the
ground, while in the immersive mode, the view is free to rotate.
when the 3D display is fixed. Unlike previous reports,
the use of VR appears to affect accommodation, and fur-
Abbreviations
ther studies are needed to resolve these issues. VR: Virtual reality; NPA: near point of accommodation; NPC: near point of
Turnbull et al. [15] suggested that increased choroidal convergence; HMD: head-mounted display; PD: prism diopters
thickness caused by myopic retinal defocus could be as-
Acknowledgements
sociated with reduced myopia progression. In our study,
The statistical analysis in this research was supported by J.R. Park (Master of
myopic shift and hyperopic shift > 0.5 D was evident in Statistics).
each of the 3 cases in the immersive mode. In the non-
immersive mode, myopic shift and hyperopic shift were Author’s contributors
Design of the study (H.H); Conduct of the study (H.J.Y, H.H); Collection and
observed in 8 cases and 4 cases, respectively. However, management of data (H.J.Y, S.W.P); Analysis and interpretation of data (H.J.Y,
these changes fully recovered within 1 h. Our study J. K, H.H); Preparation, review, or approval of the manuscript (H.J.Y, J. K, S.W.P,
showed that myopic and hyperopic shifts presented atyp- H.H).
All authors read and approved the final manuscript.
ically and did not correlate with other ocular factors or
VR mode. In addition, a study by Ha et al. [7] reported Funding
that transient myopia could occur after using VR. The This research was supported by Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of
hypothesis of increased or reduced myopic progression Education (NRF- 2017R1D1A3B03032579) and this research was supported by
using VR appears to require a more cautious approach. a grant (CRI 17031–1) Chonnam National University Hospital Biomedical
One limitation of this study was the inclusion of a spe- Research Institute. Funding body was not involved in the design of the
study and collection, analysis, and interpretation of data and in writing the
cific population (i.e., 23 subjects 20 to 35 years of age, manuscript.
with an uncorrected visual acuity of 0.8 or higher). Thus,
it is difficult to judge the effect on users of different con- Availability of data and materials
Data supporting our findings are contained in the manuscript. However, the
ditions. Notably, the actual refraction data were not ana- raw data set on which the conclusion was made is available on request from
lyzed, including those of subjects with a history of Professor Hwan Heo (contact email:opheye@hanmail.net).
refractive surgery. It is possible that the use of VR for
Ethics approval and consent to participate
only 30 min was insufficient to produce changes in visual This study received ethical approval from the Institutional Review Board of
parameters. Furthermore, there were no control groups the Chonnam National University Hospital. A written informed consent was
that did not use VR. Additional larger-scale studies are obtained from all patients before study initiation.
needed to resolve these limitations. However, we Consent for publication
propose that it may be meaningful that the results of this Not applicable.
study demonstrate the difference in visual parameters
depending on the contents, despite using the same VR Competing interests
The authors report no conflicts of interest.
device.
Received: 19 March 2019 Accepted: 13 May 2020
Conclusion
In conclusion, the use of VR for approximately 30 min References
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 PHILIPPINE JOURNAL OF

VOL. 35 • NO. 2 Ophthalmology JULY – DECEMBER 2010

ORIGINAL ARTICLE

Karen B. Reyes, MD1

Harvey S. Uy, MD.1,2 Refractive errors in Filipino
1
Asian Eye Institute
Makati City, Philippines eyes in a single-center
2
Department of Ophthalmology
and Visual Sciences
study population
Sentro Oftalmologico Jose Rizal
University of the Philippines–Philippine
General Hospital
Manila, Philippines

ABSTRACT
Objective
We determined the frequency of refractive errors among Filipino patients
undergoing refractive or cataract screening at an ambulatory eye-care center,
correlated them with demographic and ophthalmic variables, and compared
refractive and keratometric astigmatism.

Methods
Clinical records of 666 consecutive patients who had optical interferometry
were reviewed and the demographic and clinical parameters obtained were
subjected to statistical analyses.

Results
The mean axial length (AL) was 24.43 ± 1.56 mm, significantly shorter
among females (p < 0.001). Axial length (AL) was also inversely correlated
with age (r = –0.50). Mean sphere was –1.3 ± 3.4D, and correlated with age
(r = 0.57); younger age groups had higher negative values. Myopia was the
major refractive-error type (90 to 100%) in the 17- to 30-year-old age bracket.
Hyperopia was the predominant refractive error (50 to 75%) among the elderly.
The mean keratometry reading was 44 ± 1.5D. Females had steeper K readings
than males (p = 0.000). Refractive astigmatism was observed in 47.88% and
keratometric astigmatism in 71.7%. Against-the-rule (ATR) was the prevalent
type (46.1%) in refractive astigmatism and with-the-rule (WTR) (59.1%) in
keratometric astigmatism.

Conclusion
In the older population, AL was shorter and hyperopia was more prevalent;
Correspondence to
Harvey S. Uy, MD
in the younger population, myopia was more prevalent. Astigmatism did not
Asian Eye Institute differ between gender groups but differed among age groups. WTR
9F Phinma Plaza Building
Rockwell Center, Makati City astigmatism was more prevalent in the younger age group and ATR in the
Philippines 1200 elderly.
Telephone : +632-8982020
Fax : +632-8982002
E-mail : harveyuy@yahoo.com

No financial assistance was received for this study. Keywords: Refractive error, Astigmatism, Myopia, Hyperopia
The authors have no propriety or financial interest in any
product used or cited in this study. PHILIPP J OPHTHALMOL 2010; 35(2): 50-55 © PHILIPPINE ACADEMY OF OPHTHALMOLOGY

50 PHILIPP J OPHTHALMOL VOL 35 NO. 2 JULY - DECEMBER 2010 PHILIPPINE ACADEMY OF OPHTHALMOLOGY
 ACCORDING to the World Health Organization’s the axis was at 0 ± 150, against-the-rule (ATR) when the
(WHO) “Global Initiative for the Elimination of Avoidable axis was at 90 ± 150, oblique astigmatism (OA) when the
Blindness 2020” (Vision 2020), errors of refraction axis was within 200 to 700 and 110 to 1600.
accounted for 18% of visual disability, next to cataract Data were stored in Excel (Microsoft Corp, Redmond
(39%), and that “reversing or treating these refractive WA, USA) and transferred to Statistical Package for the
errors greatly decreased the number of subjects with visual Social Sciences (SPSS) version 16 for statistical analyses.
disability”.1 Awareness of the frequency of refractive errors Descriptive statistics, bivariate correlation study
is, therefore, a crucial step in the Vision 2020’s goal of (Spearman’s), parametric and nonparametric tests for
eliminating refractive-error-related visual disability. independent samples were used to compare values
Locally, there are few published reports on the depending on whether the distribution was Gaussian
prevalence of refractive errors. We, therefore, determined (t-test) or non-Gaussian (Mann-Whitney). The statistical
the frequency of refractive errors in an ambulatory-care level of significance was p < 0.05.
center and assessed the refractive status of patients using
axial length measurements by optical interferometry2 and RESULTS
corneal power by automated keratometry. These were A total of 1,332 eyes of 666 patients (276 males and 390
compared with the manifest refraction (retinoscopy) of females) were included in the study. The mean age was
the patients. We also correlated them with demographic 52 ± 18 years. Of the 666 pairs of eyes, 13% had
and other ophthalmic variables, and compared refractive anisometropia with a mean diopteric difference of 3.13D
and keratometric astigmatism. between each pair of anisometropic eyes (Table 1).
Anisometropia was observed in 12% of males and 15% of
METHODOLOGY females. However, this gender difference was not
This is a retrospective study involving 1,332 eyes of 666 statistically significant.
consecutive patients who underwent refractive or cataract
screening and optical biometry (IOL Master, Carl Zeiss Mean Axial Length
Meditec, Jena, Germany) measurements from March to The mean axial length was 24.43 ± 1.56 mm for the
October 2008. Patients who could not physically undergo entire study population. The axial length between males
optical biometry or manifest refraction were excluded. (24.67 mm) and females (24.46 mm)was statistically
The manifest refraction of each patient using the different (p < 0.001) (Table 2), with the male population
retinoscopy technique for best-corrected visual acuity was
retrieved from the clinical record. Table 1. Prevalence of anisometropia by gender.
The following data were obtained: age and gender, axial Male Female Total p
length, manifest refraction, type of refractive error, corneal Prevalence (%) 12 15 27
power, refractive (by manifest refraction) and kerato- Standard Deviation 1.86 1.86
metric astigmatism (by IOL Master), and axis of astig- Confidence 0.63 0.82
Diopteric difference
matism (refractive and keratometric).
Mean 3.13 ± 1.86D 3.13 ± 3.16D 0.38a
Manifest refraction was measured using retinoscopy
Range 1.5 to 8.75 1.5 to 8.75 1.5 to 8.75
where sphere, cylinder (measured in negative cylinders),
Mann-Whitney test for independent samples between men and women
a

and axis were noted. Spherical equivalent (SE) was

calculated as sphere plus half the cylinder power. Presence
of anisometropia, defined as SE of >1D between the right
and left eyes, was noted. The types of refractive errors were
26 ------------------------------------------------------------------------------------------------
classified as follows: myopia with SE ≥ –0.50D, hyperopia
with SE ≥ + 0.50D, and emmetropia with SE ± 0.25D. 25 ------------------------------------------------------------------------------------------------
Axial Length (mm)

Eyes with and without astigmatism were noted.

24 ------------------------------------------------------------------------------------------------
Astigmatism was defined as >0.50D measured in two ways: male
(1)Refractive – by the cylinder obtained in the retinoscopic 23 ------------------------------------------------------------------------------------------------
female
manifest refraction (MR-astigmatism), and (2)
22 ------------------------------------------------------------------------------------------------
Keratometric – by calculating the difference between the r = -0.50
steepest and flattest keratometric readings obtained by 21 ------------------------------------------------------------------------------------------------
the IOL Master (IM-astigmatism). The axis of astigmatism
20 ------------------------------------------------------------------------------------------------
was determined by manifest refraction (MR-axis) and by 17-20 21-30 31-40 41-50 51-60 60-70 71-80 81-90 >90
the axis of the flattest K as measured by the IOL Master
(IM-axis). With-the-rule (WTR) astigmatism occured when Figure 1. Axial-length measurements by gender and age groups.

PHILIPPINE ACADEMY OF OPHTHALMOLOGY PHILIPP J OPHTHALMOL VOL 35 NO. 2 JULY - DECEMBER 2010 51
having a longer axial length. A Table 2. Clinical parameters and correlation of age with different parameters by gender.
statistically significant negative
Parameter Mean Range p Correlation
correlation (r = –0.50) between axial
length and age was present; as the age Axial Length (mm) 0.000a,c
increased, there was a decrease in Male 24.67 ± 1.9 20.47 to 31.95 –0.45b,d
Female 24.46 ± 7.2 20.81 to 31.33 –0.53b,d
measured axial length (Figure 1). This
Total 24.43 ± 1.57 20.4 to 34.83 –0.50b,d
correlation was similar for each Manifest Refraction
gender group. Sphere (diopters) 0.56
Male –1.45 ± 3.36 –19.5 to 9.25 0.62b,d
Manifest Refraction Female –1.36 ± 3.5 –19.5 to 6.50 0.54b,d
Mean Sphere Total –1.3 ± 3.4 –19.5 to 9.25 0.57b,d
The mean sphere was –1.3D for the Cylinder (diopters) 0.56a
Male –0.79 ± 0.75 –5.00 to 0 -0.22b
entire population(Table 2). There was
Female –0.81 ± 0.89 –6.75 to 0 -0.16b,d
a positive correlation between Total –0.81 ± 0.83 –6.75 to 0 -0.17b,d
spherical diopter and age(r = 0.57, Axis (degrees) 0.48a
p < 0.001); more plus spheres were Male 86.70 ± 63.5 0 to 180 -0.03b
frequent in the middle-aged and the Female 79.89 ± 62.1 0 to 180 -0.04 b
elderly (Figure 2). A similar Total 79.93 ± 60.1 0 to 180 -0.02b
correlation when matched for gender Spherical Equivalent (diopters) 0.52a
and age was present. Male –1.88 ± 3.41 –20.13 to 8.88 0.61b,d
Female –1.78 ± 3.62 –20.88 to 6.00 0.52b,d
Myopia was more frequent in the
Total –1.81 ± 3.49 –20.88 to 8.88 0.55b,d
younger age group while hyperopia Keratometry
increased with age and became the Flattest K (diopters) 0.000a,c
predominant refractive error (50 to Male 43.37 ± 1.5 34.83 to 47.67 0.08b
75%) in the elderly group. Female 43.67 ± 1.5 36.41 to 51.53 0.12b,d
Total 43.50 ± 1.5 34.83 to 51.53 0.10b,d
Mean Cylinder Steepest K (diopters) 0.000a,c
Male 44.35 ± 1.6 38.01 to 51.84 0.03b
The mean cylinder power was
Female 44.80 ± 1.6 37.84 to 51.84 0.01b
–0.81D for the entire population; Total 44.60 ± 1.6 37.84 to 51.84 0.01b
–0.79D for males and –0.81D for Mean K (diopters) 0.000a,c
females. The difference between the 2 Male 43.84 ± 1.5 36.49 to 49.76 0.02b
groups was not significant(p = 0.56). Female 44.20 ± 1.5 37.13 to 51.69 0.03b
However, there was a negative corre- Total 44.00 ± 1.5 36.49 to 51.69 0.05b
lation between cylindrical power and Axis of flattest K(degrees) 0.79a
age (r = –0.17; p < 0.001; Table 2) in Male 86.70 ± 63.5 0 to 179 0.04b
Female 88.40 ± 67.5 0 to 179 0.04b
the entire population; the cylinder
Total 87.70 ± 65.0 0 to 179 0.04b
lessened among the middle aged a
Mann-Whitney test for independent samples between men and women
group and then increased in the b
Spearman rho correlation between parameter and age
c
Significant if p <0.05
elderly group (Figure 3). This corre- d
p<0.001
lation remained when gender-specific-
age match was done (Table 2).
only in the female group (Table 2). males and 44.2D for females. The
Corneal Power/Keratometry (K) female group showed higher mean
Flattest K Steepest K keratometric readings (p < 0.001).
The mean flattest K was 43.5D for The mean steepest K was 44.6D for Cor relation with age was not
the entire population, 43.37D for the entire population; 44.35D for significant for the entire population
males, and 43.67D for females. The males and 44.8D for females. Females (r = 0.05, p = 0.10) (Table 2; Figure
difference between the 2 groups was had higher corneal curvature radii 4).
significant (p < 0.001), with females than the males (p = 0.001) (Table 2).
having a flatter keratometry reading. Refractive Astigmatism (MR-Astigmatism)
Age also showed a positive correlation Mean K Eyes with astigmatism (> –0.50D)
with K measurements higher with age The mean K reading was 44.0D for accounted for 47.88% of the total
(r = 0.10, p < 0.001) and significant the entire population; 43.84D for population with a mean cylinder of

52 PHILIPP J OPHTHALMOL VOL 35 NO. 2 JULY - DECEMBER 2010 PHILIPPINE ACADEMY OF OPHTHALMOLOGY
 flattest K was 1.10 ± 0.80D for the entire population, 1.02D
0.5 ----------------------------------------------------------------------------------------------
for males and 1.15D for females. The difference was
significantly higher (p < 0.001) in females. The keratometric
0 ----------------------------------------------------------------------------------------------
difference was negatively correlated with age; this difference
-0.5 ----------------------------------------------------------------------------------------------
was less in the older population (r = –0.23, p < 0.001) and
Mean Sphere (Diopters)

-1 ----------------------------------------------------------------------------------------------
was observed in both males (r = –0.24, p < 0.001) and females
-1.5 ---------------------------------------------------------------------------------------------
(r = –0.23, p < 0.001; Table 3). 71.70% of the total population
male
-2 ----------------------------------------------------------------------------------------------
had astigmatism (Kdiff > 0.05D) with a mean of 1.33 ± 0.79D.
-2.5 ---------------------------------------------------------------------------------------------- female
65.94% of males and 75.77% of females had mean of 1.27D
-3 ----------------------------------------------------------------------------------------------
r = -0.57 and 1.37D respectively (Table 3).
-3.5 ----------------------------------------------------------------------------------------------
-4 ---------------------------------------------------------------------------------------------- Refractive (MR-Astigmatism) vs. Keratometric (IM-Astigmatism)
-4.5 ----------------------------------------------------------------------------------------------
17-20 21-30 31-40 41-50 51-60 60-70 71-80 81-90 >90
There was a significant difference between the
astigmatism measured by manifest refraction and the IOL
Figure 2. Mean sphere measurements by gender and age groups. Master, where the keratometic astigmatism showed a
higher astigmatism rate than the refractive astigmatism
(p < 0.001) (Table 3).
0 ------------------------------------------------------------------------------------------------
Axis of Astigmatism
-0.4 ------------------------------------------------------------------------------------------------
Refractive Axis (MR-Axis)
Mean Cylinder (Diopters)

-0.6 ------------------------------------------------------------------------------------------------ The mean MR-Axis was 97.970 ± 51.50 for the entire
population; 94 0 for males and 101 0 for females. No
-0.8 ------------------------------------------------------------------------------------------------
difference was noted between the two groups (p = 0.10).
-1 ------------------------------------------------------------------------------------------------ Male There was a significant correlation with age in the entire
Female population (r = –0.13, p < 0.001) (Table 3).
-1.2 ------------------------------------------------------------------------------------------------
r = -0.17
-1.4 ------------------------------------------------------------------------------------------------ Keratometric Axis (IM-Axis)
The mean IM-axis was 87.960 ± 68.60 for the population,
-1.0 ------------------------------------------------------------------------------------------------
17-20 21-30 31-40 41-50 51-60 60-70 71-80 81-90 >90 87 for males, 890 for females. No difference between the
0

Figure 3. Mean cylindrical measurements by gender and age group.

male and female groups was seen(p = 0.89). No correlation
was obtained with age (r = 0.03, p = 0.34) (Table 3).

Mean K MR-Axis and IM-Axis

46 ---------------------------------------------------------------------------------------------- The refractive axis and the keratometric axis showed a
45.5 ---------------------------------------------------------------------------------------------- significant difference between the two groups (p < 0.001)
(Table 3).
45 ----------------------------------------------------------------------------------------------
Keratometry (D)

44.5 ---------------------------------------------------------------------------------------------- Types of Astigmatism

44 ---------------------------------------------------------------------------------------------- Male Refractive
43.5 ---------------------------------------------------------------------------------------------- Female Against-the-rule (ATR) (46.1%) was the prevalent type
of astigmatism, followed by with-the-rule-astigmatism
43 ----------------------------------------------------------------------------------------------
r = 0.05
(WTR) (32.8%), and oblique astigmatism (OA) (21.1%).
42.5 ---------------------------------------------------------------------------------------------- WTR was more prevalent in the younger age groups (20
42 ----------------------------------------------------------------------------------------------
to 40 years old), while ATR was more prevalent in the
17-20 21-30 31-40 41-50 51-60 60-70 71-80 81-90 >90 middle and elderly age groups (Figure 5).
Figure 4. Mean keratometric measurements by gender and age groups.
Keratometric
–1.35 ± 81D. 48.55% of males and 47.44% of females The prevalent type of astigmatism measured by the IOL
had a mean cylinder of –1.27D and –1.40D respectively Master was WTR (59.1%), followed by ATR (20%), and
(Table 3). OA (20%). WTR astigmatism was the predominant type
in the younger and middle-aged groups (20 to 60 years
Keratometric Astigmatism (IM-Astigmatism) old), while ATR was more prevalent in the older age group
The mean difference between the steepest K and the (Figure 6).

PHILIPPINE ACADEMY OF OPHTHALMOLOGY PHILIPP J OPHTHALMOL VOL 35 NO. 2 JULY - DECEMBER 2010 53
Table 3. Correlation of age with different astigmatic parameters by gender. predominant refractive error type in
this study. Similar to other East Asian
Parameter Mean Range p Correlation population-based reports, which were
Refractive astigmatism (MR-Astigmatism) e
0.26 a
done on a larger scale, myopia was also
(diopter) (47.88% of total population) the leading type of refractive error,
Male -1.27 ± 0.69 -5.00 to 0 -0.16c,d
with rates at 31 to 40%.5-6, 9 Our smaller
Female -1.40 ± 0.89 -6.75 to 0 -0.02c
Total -1.35 ± 0.81 -6.75 to 0 -0.09c,d
single- center population had a higher
Refractive axis (MR-Axis) (degrees)f 0.10a rate of myopia at 59% because
Male 94.4 ± 49.8 0 to 180 -0.03c patients specifically consulted for
Female 100.5 ± 52.5 0 to 180 -0.20c,d refractive or cataract screening, which
Total 97.7 ± 51.4 0 to 180 -0.13c,d created a bias toward higher degree
Keratometric (IM-Astigmatism)e (diopter) 0.02a,b of refractive errors.
(71.70% of total population)
The crude frequency of hyperopia
Male 1.27 ± 0.82 0 to 5.83 -0.24c,d
in this study was 30%, which was
Female 1.37 ± 0.76 0 to 4.66 -0.23c,d
Total 1.33 ± 0.79 0 to 5.83 -0.23c,d similar to those reported in Singapore
Keratometric axis (IM- Axis) (degrees)f 0.89a (35.9%),5 Mongolia (32.9%),7 and
Male 86.80 ± 66.73 0 to 179 0.01c Japan (27.9%).3
Female 88.68 ± 69.80 0 to 179 0.05c Age is one of the factors known to
Total 87.96 ± 68.63 0 to 179 0.04c correlate with refractive errors. The
a
Mann-Whitney test for independent samples between men and women age-related changes in the human
b
Significant if p<0.05
c
Spearman rho correlation between parameter and age
optical system shift the refraction over
d
Significant if p<0.001 time: a trend towards myopia during
e
Tested for significant difference
(MR-measured vs IM-measured astigmatism t-test: p = 0.000, SD = 0.89, CI = 0.24 to 0.34) high school to college years, followed
f
Tested for significant difference
(MR-measured vs IM-measured axis of astigmatism t-test: p = 0.000, SD = 78.2, CI = 4.4 to 13.2) by a hyperopic shift in the elder years.
Other changes are a shift from with-
the-rule astigmatism to against-the-
100 rule astigmatism and steepening of
90 the corneal curvature with age.12 In
80
this study, the younger population was
more myopic, and as the age group
70
WTR increased the frequency of myopia
(0±15
60
degrees)
decreased, illustrating a hyperopic
50
ATR
shift with age. The reasons for this
40 (90±15 hyperopic shift are unknown.13 We
30
degrees) also obser ved that shorter axial
OA lengths were correlated with older age
20
(20-70 groups. Others reported that corneal
and
10
110-160 curvature flattens with age.3,12 In this
0 degrees) study, only the flattest K (measured by
Female

Female

Female
Male

Female

Female

Female
Male

Female

Male
Female

the IOL Master) was correlated with

Male
Male

Male

Male
Male

age in the female group.

16-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 Refractive astigmatism was
Age group (years) correlated with age; increased
Figure 5. Prevalence (in percent) of refractive astigmatism (MR-measured) in 638 astigmatic eyes by age
negative cylinders were seen in the
and gender groups. older age group consistent with other
reports. 3, 4, 6-8,10 However, keratometric
DISCUSSION Our rates were slightly lower than astigmatism showed an inverse
Anisometropia was obser ved in those reported in Singapore (20%)5 correlation with age in this study, in
13% of the patient population, 12% and Taiwan (19.9),6 while it was higher contrast to a previous report3 where
among males and 15% among compared to the rates reported in astigmatic K was not correlated with
females. This finding was similar to Mongolia (10.7%)7 and Bangladesh age.
population-based reports done in (11.9%).8 The shift from with-the-rule
Japan (15.1%)3 and Australia (14%).4 Myopia was noted to be the astigmatism to against-the-rule

54 PHILIPP J OPHTHALMOL VOL 35 NO. 2 JULY - DECEMBER 2010 PHILIPPINE ACADEMY OF OPHTHALMOLOGY
 6. Cheng CY, Hsu WM, Liu JH, et al. Refractive errors
100 in an elderly Chinese population in Taiwan: the
Shihpai Eye Study. Invest Ophthalmol Vis Sci 2003;
90 44: 4630-4638.
7. Wickremasinghe S, Foster PJ, Uranchimeg D, et al.
80 Ocular biometry and refraction in Mongolian adults.
Invest Ophthalmol Vis Sci 2004; 45: 776-783.
70 8. Bourne RR, Dineen BP, Ali SM, et al. Prevalence of
WTR
refractive error in Bangladesh adults. Results of the
60 (0±15
National Blindness and Low-Vision Survey of
degrees) Bangladesh. Ophthalmology 2004; 111: 1150-1160.
50
ATR 9. Saw SM, Katz J, Schein OD, et al. Epidemiology of
40 myopia. Epidemiol Rev 1996; 18: 175-187.
(90±15 10. Saw SM, Gazzard G, Koh D, et al. Prevalence rates
degrees) of refractive errors in Sumatra, Indonesia. Invest
30
Ophthalmol Vis Sci 2006; 47: 1845-1852.
OA
20 11. Dandona R, Dandona L, Naduvilath TJ, et al.
(20-70
Refractive errors in an urban population in southern
and
10 India: the Andhra Pradesh Eye Disease Study. Invest
110-160
Ophthalmol Vis Sci 1999; 40: 2810-2818.
0 degrees)
12. Ferrer-Blasco T, Gonzalez-Meijome JM, Montes-Mico
Female

Female

Female
Male

Male

Female

Female

Female
Male
Female

Female

R. Age-related changes in the human visual system

Male
Male

Male

Male
Male

and prevalence of refractive conditions in patients

attending an eye clinic. J Cataract Refract Surg,
2008;34: 424-432.
16-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 13. Fotedar R, Mitchell P, Burlutsky G, Wang JJ.
Age group (years) Relationship of 10-year change in refraction to nuclear
cataract and axial length findings from an older
Figure 6. Prevalence (in percent) of keratometric astigmatism (MR-measured) in 955 astigmatic eyes by population. Ophthalmology 2008; 115: 1273-1278.
14. Gudmundsdottir E, Jonasson F, Jonasson V, et al.
age and gender groups. “With the rule” astigmatism is not the rule in the elderly.
Reykjavik Eye Study: a population-based study of
refraction and visual acuity in citizens of Reykjavik
50 years and older. Acta Ophthalmol Scand 2000;
astigmatism with increasing age as survey is needed, however, to obtain 78: 642-646.
demonstrated in this study is well a more representative sample of the 15. Sperduto RD, Seigel D, Roberts J, Rowland M.
Prevalence of myopia in the United States. Arch
corroborated in other studies. 3,12,14 refractive errors in the general Ophthalmol 1983; 101: 405-407.
Correlation of age with axis values was population. 16. Wang Q, Klein BE, Klein R, Moss SE. Refractive status
in the Beaver Dam Eye Study. Invest Ophthalmol Vis
not statistically significant in this study In summary, the older population Sci 1994; 35: 4344-4347.
in either the refractive or keratometric was characterized by shorter axial
methods. length and more prevalent hyperopia.
There was no significant difference Differences between the refractive
between gender groups in the and keratometric astigmatism indicate
manifest refraction measurements. that other factors, such as lens
Some studies15,16 reported that white changes or higher-order aberrations,
females were significantly more may play a role in the astigmatic
myopic than white males. But other changes in the eye. The type of
studies did not show any significant astigmatism changes with age where
differences between the gender against-the-rule was more prevalent in
groups.3, 6 In this report, axial length the older age group.
and keratometric measurements were
found to be significantly different
between gender groups, similar to that
reported in Japan.3
This study had several limitations: References
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