Laryngoscope Investigative Otolaryngology

C 2017 The Authors Laryngoscope Investigative Otolaryngology

V
published by Wiley Periodicals, Inc. on behalf of The Triological Society

Hearing Loss as a Risk Factor for Dementia: A Systematic Review

Rhett S. Thomson, BA; Priscilla Auduong, MD; Alexander T. Miller, BS; Richard K. Gurgel, MD

Objectives: To review evidence of hearing loss as a risk factor for dementia.

Data Sources: PubMed
Review methods: A systematic review was conducted using the PubMed database using the search terms (hearing loss
OR presbycusis) AND (dementia OR cognitive decline). Initially, 488 articles were obtained. Only those studies evaluating an
association between hearing loss and incident dementia or cognitive decline were included in the analysis. This resulted in
17 articles which were thoroughly evaluated with consideration for study design, method for determining hearing loss and
cognitive status, relevant covariates and confounding factors, and key findings.
Results: All of the 17 articles meeting inclusion criteria indicate that hearing loss is associated with dementia or cogni-
tive decline. The methods used among the studies for ascertaining hearing loss and dementia were notably varied. For hear-
ing loss, peripheral auditory function was tested far more than central auditory function. For peripheral audition, pure tone
audiometry was the most commonly reported method for defining hearing loss. Only a few studies measured central auditory
function by using the Synthetic Sentence Identification with Ipsilateral Competing Message test (SSI-ICM) and the Staggered
Spondaic Word Test (SSW). Dementia was most often defined using the Mini Mental State Exam (MMSE). However, many
studies used extensive batteries of tests to define cognitive status, often including a neuropsychologist. Confounding variables
such as cardiovascular risk factors were measured in 17 studies and family history of dementia was only evaluated in 1
study. Overall, the methods used by studies to ascertain hearing loss, cognitive status and other variables are valid, making
their evaluation appear reliable.
Conclusion: While each of the studies included in this study utilized slightly different methods for evaluating partici-
pants, each of them demonstrated that hearing loss is associated with higher incidence of dementia in older adults.
Key Words: agre-related hearing loss, dementia, cognitive decline, Alzheimer’s disease, presbycusis.
Level of Evidence: Level V, systematic review.

INTRODUCTION at the expense of other cognitive processes such as work-

Dementia and hearing loss are both highly preva- ing memory.5,6 Another hypothesis is that hearing loss
lent neurologic conditions in older adults, each having leads to social isolation, which has been shown to contrib-
considerable impact on quality of life.1,2 A growing body ute to dementia.7,8 The third prominent explanation is
of literature suggests that these two conditions are inter- that there is a common cause to both diseases and that
related and that hearing loss may be a risk factor for hearing loss is the early manifestation of the underlying
the development of dementia in older adults.3,4 Though pathology.9–11 It is also possible that these proposed
several epidemiological studies have demonstrated this mechanisms are not mutually exclusive, and decline in
association, the causal link of how hearing loss increases one pathway consequentially affects the others.
the risk of developing dementia is not well understood. Better understanding the etiology behind the con-
Several possible means have been identified. One nection between hearing loss and dementia could help
line of thought is based on the impact of hearing loss on lead to interventions that preserve cognitive function in
cortical processing. Hearing loss increases the cognitive hearing loss patients. In this way, hearing loss could
load, diverting cognitive resources to auditory processing serve as a potential modifiable risk factor. It is suggested
that interventions delaying the onset of dementia by
even one year would decrease the worldwide prevalence
This is an open access article under the terms of the Creative of dementia by 10%.12 Thus, there is compelling motiva-
Commons Attribution-NonCommercial-NoDerivs License, which permits
use and distribution in any medium, provided the original work is tion to pinpoint the role hearing loss has on cognitive
properly cited, the use is non-commercial and no modifications or decline. The object of this study is to further investigate
adaptations are made. this connection. This will be performed through a sys-
From the Department of Surgery, (R.S.T., P.A., A.T.M., R.K.G.),
and the Division of Otolaryngology, University of Utah, Salt Lake City, tematic review of epidemiological studies published on
Utah, U.S.A. the topic. Each study will be evaluated with consider-
Editor’s Note: This Manuscript was accepted for publication 19 ation to design, method for determining hearing loss and
December 2016.
dementia, relevant covariates and confounding factors,
Support/Funding: No sources of support or funding were received
for this work. key findings, and conclusion.
COI: None of the authors has a conflict of interest.
Send correspondence to Richard K. Gurgel, 50 North Medical
Drive, SOM 3C210, Salt Lake City, UT 84132. E-mail: Richard.Gurgel@ METHODS
hsc.utah.edu
The MEDLINE database was used to search for all
DOI: 10.1002/lio2.65 English and non-English studies linking hearing loss to

Laryngoscope Investigative Otolaryngology 2: April 2017 Thomson et al.: Hearing Loss and Dementia Systematic Review
69
cognitive decline up through June 1, 2016 using the following Hz,3,4,13,17,19,23,25 two of which measured thresholds
search term: (hearing loss OR presbycusis) AND (dementia OR beyond this range.17,25 One study did not use thresholds
cognitive decline). The search details are as follows: (“hearing higher than 3,000 Hz;27 another did not measure above
loss”[MeSH terms]) OR (“hearing”[all fields] AND “loss”[all 2,000 Hz.14 One study obtained a single measure at
fields]) OR (“hearing loss”[all fields]) OR (“presbycusis”[MeSH
2,000 Hz and did not obtain an average.16 Even among
terms]) OR (“presbycusis”[all fields]) AND ((“dementia”[MeSH
these 11 studies using audiometry, the classification for
terms] OR “dementia”[all fields]) OR ((“cogn int conf adv cogn
technol appl”[Journal] OR “cognitive”[all fields]) AND
the severity of hearing loss varied slightly. Three studies
“decline”[all fields])). From this criteria, 488 articles were iden- (3/17) used the common system classifying mild as >25
tified. Articles were excluded if they did not examine hearing dB to 40 dB, moderate as 40 dB to 70 dB, and severe as
loss as well as dementia or cognitive decline. This narrowed the >70 dB.4,19,23 One study stratified hearing loss as mild
collection to 131 relevant articles. (20–29 dB), moderate (30-39 dB), and severe (>40 dB).27
The remaining 131 articles were evaluated through their Another study stratified as mild (20–40 dB), moderate/
abstracts, and where necessary, full text. Articles were removed severe (>40 dB).25 Another study stratified as mild (26–
if they did not test the connection of hearing loss to incident 40 dB HL) and moderate/severe (>40 dB HL).13 Five
dementia or cognitive impairment. All news articles, non- studies did not stratify at all and simply measured hear-
academic research, case reports, editorials, commentary and ing loss as present or absent for anything above 25 dB
reviews were removed. Three non-English articles were exclud- (2/17),3,17 35 dB (1/17),24 or 40 dB (2/17),14,16 respective-
ed because they could not be translated or acquired. Of the 131 ly. In addition to using PTA, two studies employed the
articles, 17 fit the inclusion criteria and evaluated the correla-
Synthetic Sentence Identification with Ipsilateral Com-
tion of hearing loss with dementia (see Table I).
peting Message test (SSI-ICM),14,24 one of which also
Each article included was thoroughly evaluated with
used the Staggered Spondaic Word (SSW) test.14 The six
thoughtful consideration given to the many design elements dis-
cussed below. studies not using audiometry used medical codes such as
ICD-9 and ICD-10 (2/17),20,26 or more subjective mea-
sures such as clinician ascertainment of hearing loss
RESULTS during an interview (1/17)15 and self-report of hearing
Twelve of the 17 studies (70.6%) included in this loss (3/17).18,21,22 No studies reported specifically on
analysis were prospective cohort studies measuring word recognition or speech discrimination scores as a
hearing loss as a risk factor and some form of dementia metric of hearing loss. Hearing aids were controlled for
as an outcome.3,4,13–22 Four of the 17 studies (23.5%) 58.8% (10/17),3,4,13,16,17,19,21,23 only one of which did not
were cross-sectional.23–26 Twelve of the studies followed directly test the effects of hearing aids on cognitive func-
participants living in the United States. Five took place tion.13 Six found no reduction in cognitive decline with
in other localities: Italy (1/17),24 Australia (1/17),25 Ger- hearing aid use.13,16,17,23 Only three studies determined
many (1/17),26 Taiwan (1/17),20 and Japan (1/17).22 The that hearing aids decreased cognitive decline.4,19,21
number of participants ranged anywhere from 200 to
1,338,462. All of the prospective studies measured hear-
ing and cognition at baseline except for one, which mea- Covariates
sured it at the end of the period observed. Many studies controlled for potentially confounding
variables such as diabetes, hypertension, smoking status,
high cholesterol, and history of stroke or cerebrovascular
Dementia Status Ascertainment disease.4,13,15–20,22,23,25,26 Some studies controlled for less
To determine cognitive function and level of decline, commonly tested variables such as depression (7/
64.7% (11/17) of the studies used either the Mini-Mental 17)4,17,20–22,27 and a genetic test indicating the presence
State Exam (MMSE),3,14,21,22,24,25,27 or more recent var- of the APOE-e allele which has been identified as being
iations of the MMSE entitled the Modified Mini Mental associated with Alzheimer’s Disease (AD) (1/17).15
State Exam (3MS or 3MS-R).13,15–17 Three studies used
the Digital Symbol Substitution Test (DSST) to assess
cognition,4,17,19 one of which also employed the 3MS.17 Evaluating Hearing Loss as a Risk Factor for
Other instruments to measure cognition included a sys- Dementia
tem of self-report (1/17),18 complete neurological exams All of the studies (17/17) indicated that hearing loss
(2/17),14,23 or extensive batteries of cognitive tests (5/ is independently associated with higher incidence of
17)3,4,13,24 (see Table I). Only one study (1/17) evaluated dementia. The majority of the studies (12/17) quantified
family history of dementia.27 the relationship using standard deviations, hazard ratio,
relative risk, or odds ratio.4,13–18,20–22,24,26,27 Four stud-
ies (4/17) found a dose-response relationship between
Hearing Loss Ascertainment the severity of hearing loss and an increased risk of cog-
Among the studies examined, there was variation nitive decline.13,18,19,23 One study found that for every
regarding criteria for determining baseline hearing acu- 10 dB HL at baseline, there was a 1.27 increased risk
ity. The most consistently reported measure for deter- for all-cause dementia and 1.20 increased risk for devel-
mining hearing loss was pure tone audiometry at 58.8% oping AD.23 Another study found the dose response
(11/17).3,4,13,14,16,17,19,23–25,27 Seven of the 11 studies curve to be a 1.5 point score decrease in the DSST cogni-
using audiometry (7/11) obtained a pure tone average tive test for every 10 dB HL.19 A third study found that
(PTA) using the thresholds 500, 1,000, 2,000, and 4,000 for every 1 point increase in the hearing impairment

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70
 TABLE I.
Table of the 17 identified publications that evaluate hearing loss as a risk factor for cognitive decline.
Database Number of CV Risk Other Dementia Hearing Loss
Author Year Used Participants Factors Hearing Aid use risk factors Criteria Dementia Criteria Key Finding Conclusion

Amieva22 2015 Personnes 3,670 NA Measured and Depression, Self-Report French version Self-reported Self-reported
Prospective Agees QUID attenuates social of Lawton hearing loss hearing loss
Cohort study cognitive isolation Scale. associated associated
decline MMSE. with cognitive with cognitive
(b 5 0.07, decline at 25 decline at 25
P 5 0.08) year follow up year follow
(b 5 20.04, up. Hearing
P 5 0.01). aids attenuate
Hearing aids cognitive
attenuate decline.
cognitive
decline
(b 5 0.07,
P 5 0.08)
Deal4 2015 Atherosclerosis 253 Diabetes, Decreased likeli- Depression, PTA (500, 1000, DSST, 2013 Standard Moderate
Prospective Risk in Com- Hypertension, hood of cog- 2000, 4000 Delayed word Deviations association
Cohort munities Neu- Smoking nitive decline Hz; mild 25- recall test for moderate/ between
rocognitive 40, moderate (DWRT). Inci- severe HL 5 - moderate/
Study 40-70, severe dental Learn- .47 (P50.02), severe HL
>70)) ing Test, no HL 5 -.29 and memory
Logical Mem- (P50.02) performance

Laryngoscope Investigative Otolaryngology 2: April 2017

ory Test I and
II, Word fluen-
cy Test, Ani-
mals Naming
Test, Boston
Naming Test,
and Trail mak-
ing A and B,
digit span
backwards
test
Deal13 2016 Health, Aging, 3,075 Diabetes, HTN, Measured but NA PTA (500, 1000, 3-MS, record of Moderate/ Hearing loss is
Prospective and Body smoking, cere- did not statis- 2000, 4000 prescribed severe hear- associated
Cohort Composition brovascular tically Hz; Mild 26- dementia ing loss asso- with increased
Study disease decrease 40 dBHL, medication, ciated with risk of devel-
cognitive moderate/ Selective increased risk oping demen-
decline with severe > 40 Reminding of incident tia in older
use. dBHL) Test, Boxes dementia over adults
Test, Digit 9 years (Haz-
Copying Test, ard ratio:
Pattern Com- 1.55, 95% CI:
parison Test, 1.10, 2.19).
and Letter For every 10
Comparison dBHL,
Test HR 5 1.14,
95%
CI 5 1.03-
1.25)
Gates14 1996 Framingham 1662 NA NA NA PTA (500, 1000, MMSE (<24 The relative risk Central auditory
Prospective Heart Study 2000 Hz; considered of developing dysfunction
Cohort >40dB), SSI- dementia), dementia with precedes
ICM, SSW neurologist hearing loss senile demen-
and was 6 in sub- tia and may

71
Thomson et al.: Hearing Loss and Dementia Systematic Review
jects with be an early
 TABLE I.

72
(Continued)
Database Number of CV Risk Other Dementia Hearing Loss
Author Year Used Participants Factors Hearing Aid use risk factors Criteria Dementia Criteria Key Finding Conclusion

neuropsychol- very poor marker for

ogist scores senile
evaluation. (<50%) in dementia.
one ear on
the SSI-ICM
(P5.2), Rela-
tive Risk 12.5
with poor
scores in both
ears
(p50.001)
Gurgel15 2014 Cache County 4,463 Diabetes, Hyper- NA APOE-E allele Clinician ascer- 3MS screen, fol- HR 1.27 Elderly individu-
Prospective Study on tension, Smok- tainment lowed by neu- p50.026 als with HL
Cohort Memory, ing, during ropsycholo- (with 95% CI have an
Health, and Hyperlipidemia interview gist, and 1.03-1.56); increased rate
Aging geropsychia- ’-0.26 points/ of developing
trist year faster dementia
evaluation. decline on
3MS-R with
HL
Lin16 2004 Study of Osteo- 6,112 Diabetes, Smok- Did not improve NA Pure tone audi- 3MS In those with Hearing impair-

Laryngoscope Investigative Otolaryngology 2: April 2017

Prospective porotic Frac- ing, Cerebro- cognition ometry hearing ment lead to
Cohort tures (SOF) vascular dis- (>40dB at impairment, greater odds
ease/stroke, 2 kHz consid- there was a of cognitive
ered hearing trend toward impairment
loss; no aver- greater odds
age taken) of cognitive
decline over
time
(OR51.38,
95% CI50.95–
2.00).
Lin3 2011 Baltimore Longi- 639 Diabetes, Hyper- No cognitive NA PTA (500, 1000, complete neuro- The risk of inci- Hearing loss is
Cross- tudinal Study tension, improvement 2000, 4000 logical and dent all-cause independently
Sectional of Aging Smoking Hz; mild 25- neuropsycho- dementia associated
40, moderate logical exam increased log- with incident
40-70, severe (if older than linearly with all-cause
>70)) 65); BOMC (if the severity of dementia
younger than baseline hear-
65) ing loss (1.27
per 10 db
loss, 95% CI:
1.06 - 1.50);
The risk of
incident AD
also increased
with baseline
hearing loss
(1.20 per 10
dB of hearing
loss, CI 0.94-
1.53)

Thomson et al.: Hearing Loss and Dementia Systematic Review

 TABLE I.
(Continued)
Database Number of CV Risk Other Dementia Hearing Loss
Author Year Used Participants Factors Hearing Aid use risk factors Criteria Dementia Criteria Key Finding Conclusion

Lin54Prospective 2011 National Health 605 Diabetes, Hyper- Significantly NA PTA (500, 1000, DSST Greater hearing Hearing loss is
Cohort and Nutrition- tension, Smok- associated 2000, 4000 loss was sig- independently
al Examina- ing, Cerebro- with higher Hz; mild 25- nificantly associated
tion Survey vascular cognitive 40, moderate associated with lower
Disease scores DSST, 40-70, severe with lower scores on the
difference of >70) scores on the DSST
7.4, p50.03). DSST after
adjustment
for demo-
graphic fac-
tors and med-
ical history
(DSST score
difference of
21.5 [95%
confidence
interval: 22.9
to 20.23] per
10 dBof hear-
ing loss).
Lin24 2011 Baltimore Longi- 347 Diabetes, Hyper- Not associated Depression, PTA (500, 1000, MMSE, Free The reduction in Hearing loss is

Laryngoscope Investigative Otolaryngology 2: April 2017

Prospective tudinal Study tension, with any 2000, 4000 Recall, Stroop cognitive per- independently
Cohort of Aging Smoking scores of Hz; >25 dB Mixed, Trail formance assocatiated
cognition) considered Making B, associated with lower
hearing loss) Free and with a 25 dB scores on
Cued Selec- hearing loss tests of mem-
tive Remind- was equiva- ory and exec-
ing Test lent to the utive function
(FCSRT), Trail reduction
Making A, associated
Stroop Color with an age
and Words, difference of
Category and 6.8 years.
Letter Fluen-
cy, American
Version of the
Nelson Adult
Reading Test
(AMNART)
Lin53 2013 Health, Aging, 1,984 Diabetes, Hyper- No cognitive Depression PTA (250, 500, 3MS (<80 con- hazard ratio, Hearing loss is
Prospective and Body tension, Smok- improvement 1000, 2000, sidered 1.24; 95%CI, independently
Cohort Composition ing, Cerebro- 4000, 6000, dementia, or 1.05-1.48. associated
Study vascular dis- 8000 Hz; decline in Hearing with acceler-
ease/stroke >25dB con- score of 5 impairment >- ated cognitive
sidered hear- points), DSST 25 dB/2kHz: decline and
ing loss) (mild 25-40: incident cog-
1.19 with nitive impair-
95% CI 0.99- ment in
1.44), (moder- community-
ate 40-70: dwelling older
1.36 with adults
95% CI 1.08-
1.70), (severe-
> 70: N/A)

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Thomson et al.: Hearing Loss and Dementia Systematic Review
 TABLE I.

74
(Continued)
Database Number of CV Risk Other Dementia Hearing Loss
Author Year Used Participants Factors Hearing Aid use risk factors Criteria Dementia Criteria Key Finding Conclusion

Quaranta25 2014 Great Age 488 NA NA NA PTA (Threxhold MMSE; OR 4.2, p50.05 The use of hear-
Cross- Study; Italy in Hz not Neurological AD associat- ing tests and
Sectional specified; Exam, Clinical ed with early diagno-
>35 dB, pre- Dementia CAPD; OR sis and treat-
sent or Rating Scale, 1.8, p50.31 ment of ARHL
absent); Unified Par- AD with hear- may potential-
SSI-ICM kinson Dis- ing thresh- ly slow cogni-
ease Rating olds; OR 1.6, tive
Scale part III, p50.31 MIC impairment
Epworth with hearing
Sleepiness impairment;
Scale-Italian OR 2.4
Version, Eat- p50.03 Cog-
ing Assess- nitive Impair-
ment Tool, ment (demen-
Frontal tia and MCI)
Assessment and CAPD;
Battery, Digit OR 1.6,
Modalities p50.03cogni-
Test-Oral Ver- tive impair-
sion, Trail ment (Demen-
Making Test, tia and MCI)

Laryngoscope Investigative Otolaryngology 2: April 2017

Controlled with hearing
Oral Word impairment
Association, (CAPD and
Clock- Peripheral
Drawing Test, HL); OR 1.5,
Boston Nam- p50.08 Cog-
ing Test-Short nitive Impair-
form, Token ment and
Test, Rey’s hearing
Verbal Learn- thresholds
ing Test
Tay26 2006 Blue Mountains 3,509 Diabetes, Hyper- NA NA PTA (250, 500, MMSE (<24 Persons with Age-related cor-
Cross- Eye Study, tension, Smok- 1000, 2000, considered moderate to relation
Sectional Australia ing, Hyperlipid- 4000, 6000, dementia) severe hear- between sen-
emia, Cerebro- 8000 Hz; mild ing loss had a sory and cog-
vascular dis- 20-40, mod/ lower mean nitive function
ease/stroke severe >40) MMSE score in a normal
than those ageing
without hear- sample.
ing loss (28.1
v. 28.7,
p<0.001)
Teipel27 2015 Claims data of 1,338,462 Diabetes, Hyper- NA NA ICD-10 codes ICD-10 codes Regional (based Hearing impair-
Cross- Cross the Allge- tension, Hyper- on 2 digit ment is a risk
Sectional sectional meine Ort- lipidemia, Cere- postal code) factor for
skranken- brovascular dis- dementia dementia
kasse, the ease/stroke prevalence
largest public increased by
health insur- 0.23% when
ance compa- HL prevalence
ny in increased by
Germany 1 SD

Thomson et al.: Hearing Loss and Dementia Systematic Review

 TABLE I.
(Continued)
Database Number of CV Risk Other Dementia Hearing Loss
Author Year Used Participants Factors Hearing Aid use risk factors Criteria Dementia Criteria Key Finding Conclusion

Tomioka23 2015 The Fujiwara- 4,427 Diabetes, Hyper- Not associated Depression Self-report Tokyo Metropol- Baseline HL Hearing loss is
Prospective Kyo Study, lipidemia, BMI, with any itan Institute associated associated
Cohort Nara, Japan smoking, HTN, scores of of Gerontolo- with decline in with measur-
cerebrovascular cognition gy Index of intellectual able cognitive
disease Competence activity dysfunction
(TMIG-IC). (OR 5 1.39, within 5
MMSE (<24 95% years.
considered CI 5 1.02-
dementia) 1.76) at 5
year follow up
Uhlmann27 1989 Adult Medicine 200 NA Did not change Depression, PTA (500, 1000, MMSE (<24 Mild HL (1.5 Hearing impair-
Case-Control Clinics at Har- odds of Family History 2000, 3000 considered with 95% CI ment contrib-
borview Medi- dementia, but (OR 3.3, 95% Hz; Mild 20- dementia) 0.4-5.4), Mod- utes to
cal Center did not direct- CI 1.7-6.4) 29, moderate erate HL (2.2 cognitive dys-
and University ly test its 30-39, mod/ with 95% CI function in
Hospital in effect on severe >40) 0.6-7.8), older adults
Seattle, WA cognition Severe HL
(4.1 with 95%
CI 1.1 to 15.8)
Wallhagen18 2008 Alameda County 2,002 Diabetes, Hyper- NA NA Self-report (Self report on Baseline hearing A relatively
Prospective Study tension, Smok- the hearing impairment strong rela-

Laryngoscope Investigative Otolaryngology 2: April 2017

Cohort (California) ing, Crrebro- impairment increases risk tionship
vascular dis- scale using of cognitive between
ease/stroke the following decline 5 baseline hear-
categories: years later ing impair-
dif-ficulty (OR 5 1.22, ment and
remembering 95% CI 5 1.14 subsequent
things, forget- to 1.29). poorer cogni-
ting where Each point tive function
one put increase in was found in
something, hearing both existing
trouble finding impairment and new
the right scale makes cases of cog-
word, and dif- increases like- nitive
ficulty paying lihood of impairment
attention). developing
dementia by
22% 5 years
later.
Wen21 2016 Taiwan’s Nation- 6,546 Diabetes, history Not measured Depression ICD-9 codes ICD-9 codes Hearing loss Hearing loss
Prospective al Health of head injury, associated associated
Cohort Insurance diabetes, vas- with increased with increased
Research cular diseases risk of risk of
Database dementia dementia
(OR 5 1.577)

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Thomson et al.: Hearing Loss and Dementia Systematic Review
 does not rely heavily on auditory function, as the patient
does not receive continual instruction from the clinician
to complete it. However, other cognitive function tests
which rely on a patient using auditory processing—for
example, responding to spoken questions—are signifi-
cantly impacted by an auditory impairment.31–33 If
patients have hearing loss, they may perform worse on
certain cognitive tests, though they may not have any
cognitive dysfunction. This could make the correlation of
hearing loss with dementia appear higher than it really
is. Tests relying on auditory instruction from the clini-
cian include the MMSE and the 3MS, discussed previ-
ously. However, several studies (7/17) used a variety of
Fig. 1. Schematic of PubMed Database Search Resulting in the methods for evaluating cognition that did not rely heavi-
Identification of 17 Publications that Evaluate Hearing Loss as a ly on auditory function, including a variety of tests that
Risk Factor for the Development of Dementia or Cognitive
rely almost entirely on the patient’s ability to use a pen,
Decline.
paper and cognitive faculties.4,13,17–19,23,24
Hearing loss was also measured a variety of ways.
scale, the likelihood of developing dementia five years Some studies used more subjective measures. Gurgel
later increased by 22%.18 Another study found the dose et al. and Wallhagen et al. used clinician assessment of
response to be an increased hazard ratio of 1.14 for HL and patient self-report of HL, respectively, and found
every 10 dB HL.13 Two of the studies (2/17) also a strong correlation between HL and incidence of
employed other measurement systems; for example, a 25 dementia years later.15,18 Although less quantitative,
dB hearing loss is the equivalent cognitive performance studies have shown subjective measures to be a reliable
of an individual 6.8 years older (1/17),3 or moderate to method for ascertaining HL, but may reveal a slightly
severe hearing loss resulted in worse scores on the higher prevalence of hearing loss than objective mea-
MMSE (1/17).25 One study (1/17) quantifying the rela- sures.34–36 Though accurate, from this method it is not
tionship with a hazard ratio also found a 0.26 points per possible to determine how HL severity corresponds to
year faster decline on 3MS-R when HL was present (see dementia risk.
Table I).15 Pure tone audiometry was the most common method
for determining hearing loss (11/17).3,4,13,14,16,17,19,23–25,27
DISCUSSION Pure tone averages (PTA) can quantify the severity of
All of the studies evaluated in this review found hearing loss (HL) and categorize HL as mild (25-40 dB
hearing loss to be associated with dementia or cognitive HL), moderate (40-70 dB HL), and severe (>70 dB HL).
decline. This substantially supports the hypothesis that Stratification of HL severity provides information helpful
hearing loss is a risk factor for dementia. Because of the in elucidating the relationship of HL to dementia. Three
studies showed that as hearing loss severity increases,
considerable variability among the studies regarding
the odds ratio (OR) and hazard ratio (HR) for developing
how dementia, hearing loss, and potential confounding
dementia increases accordingly.17,23,27 Uhlmann et al. not-
variables were determined, careful scrutiny and compar-
ed the most drastic relationship, showing mild HL corre-
ison of these methods is crucial to fully appreciating the
sponded with an OR of 1.5, moderate HL with an OR of
validity of this observed association.
2.2, and severe HL with an OR of 4.1.27 One limitation of
Most of the studies (8/17) utilized the Mini Mental
the studies evaluated was that none reported on word
State Exam (MMSE), or a variation of it, the Modified
recognition scores (WRS) or speech discrimination scores
Mini Mental State Exam (3MS). In the MMSE, the
that are obtained in routine audiometry as predictors for
patient is instructed to perform a series of basic tasks. dementia. The WRS has been recognized as an important
The patient’s performance is quantified numerically and metric in reporting hearing outcomes. Indeed, it may be
is assigned a score from 0 to 30. A score below 24 usual- the most important metric of hearing acuity because it
ly indicates dementia. However, some flaws have been has predictive value on how well an individual will
found in the sensitivity of the MMSE, which led to the respond to aural rehabilitation (i.e., hearing aids). This
development of the 3MS which is similar but slightly should be considered a covariate in future studies.37
longer and extends the scoring system to a range of 0 to In discussing accuracy of hearing loss ascertain-
100.28 Both the 3MS and MSSE are valid for detecting ment, it should be noted there are several studies that
and tracking dementia but have difficulty identifying indicate that pure tone audiometry fails to adequately
lower levels of cognitive impairment.28,29 Three articles assess central auditory processing.38–40 This would indi-
(3/17) utilized the Digit Symbol Substitution Test cate that while PTA is an accurate measure of hearing
(DSST) to assess dementia.4,17,19 It is a sensitive execu- loss, it may not lend itself to proving or disproving dif-
tive function test which has patients code a series of ferent hypotheses of hearing loss and dementia.
numbers with symbols.30 It is thought that the faster Only two studies evaluated central auditory proc-
the patient matches the symbols with the correct num- essing. They utilized the Synthetic Sentence Identifica-
bers, the better their cognition is. This particular test tion with Ipsilateral Competing Message test (SSI-ICM)

Laryngoscope Investigative Otolaryngology 2: April 2017 Thomson et al.: Hearing Loss and Dementia Systematic Review
76
and the Staggered Spondaic Word Test (SSW).14,24 Due provide valuable insight into the etiology behind the
to the fact that central auditory processing, peripheral connection.
auditory, and cognition function are all likely interrelat- Based on the studies reviewed in this article, there
ed, studies are most powerful when all three elements is an overwhelming consensus that hearing loss is a risk
are evaluated.41 Gates et al. showed that central audito- factor for incident dementia. However, due to inherent
ry processing disorder (CAPD) was related significantly limitations in epidemiological studies, these articles do
to performance decline on the MMSE.14 The relative not adequately describe exactly why hearing loss is a
risk for dementia in patients scoring poorly on the SSI- risk factor. There are multiple hypotheses on how hear-
ICM exam in one ear was 6 (p 5 0.2), and was 12.5 ing loss is associated with dementia: the cognitive load
(p 5 0.001) for patients scoring poorly in both ears. Quar- hypothesis, psychosocial hypothesis, and the common-
anta et al. also found that CAPD increased the odds cause or shared neurobiological pathology hypothesis. In
ratio for developing Alzheimer’s disease dementia (OR of an extensive review on the cognitive load hypothesis,
4.2 [p 5 0.03]). Pichora-Fuller explains that as hearing diminishes,
Cardiovascular risk factors, such as hypertension, mental resources are diverted toward auditory percep-
diabetes, hyperlipidemia, and smoking, have been shown tion. It is thought that as cognitive activities are contin-
to increase one’s risk of developing dementia.42,43 Con- ually neglected at the expense of hearing, dementia will
trolling for cardiovascular risk factors is necessary for ensue.50
accurately measuring the relationship of hearing loss to An alternative hypothesis regarding the relation-
dementia. The majority of the studies (13/17) controlled ship of HL and cognitive decline involves various psycho-
for at least one cardiovascular risk factor. Whitmer et al. social factors that likely influence cognitive ability and
found in a group of 8,845 individuals that the hazard function. Individuals with HL have difficulty communi-
ratio (HR) for developing dementia if only one CV risk cating and maintaining interpersonal relationships,
factor was present was 1.27.43 If four risk factors were often resulting in social isolation.51 Mick et al. found in
present the HR rose to 2.37 (95% CI 1.10 to 5.10). There- a nationally representative sample of 860 females
fore, the presence of CV risk factors, could confound the between the ages of 60-69 that hearing loss is associated
with increased odds of social isolation with an odds ratio
association between hearing loss and dementia.
(OR) of 3.49 per 25-dB HL (95% confidence interval).46
In 61.5% (9/17) of the included articles, the impact
Fratiglioni et al. demonstrated that individuals who
of hearing aid use on cognition was evaluat-
were single, living alone, and had few relatives/friends
ed.3,4,16,17,19,21,23 Most of these studies (6/17) found no
were at an increased risk for dementia.52 Thus, it is pos-
correlation between hearing aid use and level of cogni-
sible that as hearing loss sets in, social activity dimin-
tive function. The two studies that did find an associa-
ishes leading to dementia. Seven of the included studies
tion (2/17) found that hearing aid use diminished the
(7/17) measured and controlled for depression.4,17,20–23,27
likelihood of cognitive decline.4,19,21 However, among the
Since depression is a known risk factor for the develop-
studies, hearing aid use was usually determined by a
ment of dementia, controlling for this covariate strength-
yes or no question. Such simplistic measures leave sig-
ens the association that hearing loss is an independent
nificant gaps regarding compliance and proper manage-
risk factor to dementia in older adults.
ment of hearing aids, thereby making their assessment Another notable hypothesis is the common-cause
of hearing aid impact on cognition less conclusive. Other hypothesis. In a comprehensive review of the potential
articles not included in our systematic review indicate causal links of hearing loss leading to dementia, Wayne
that hearing aids may not improve cognition.44,45 Con- et al. explain the possibility that a single factor associat-
versely, other studies have indicated that improving ed with old age leads to both neurologic processes.10
hearing through the use of hearing aids or cochlear Gates et al. suggested that central auditory dysfunction
implantation has positive effects on cognition.46–49 Addi- and executive dysfunction may arise from the same neu-
tional prospective studies with reliable methods for rodegenerative process.11 If there is a single cause for
determining hearing aid use and compliance would be both neurologic processes, it is possible that hearing loss
necessary to fully ascertain hearing aid impact on cogni- is merely an early manifestation of dementia during its
tive decline. pre-clinical phase.9
Family history of dementia was only controlled for One of the notable limitations of this review article
in one of the studies. Uhlmann et al. found the odds is that several non-English articles were not able to be
ratio (OR) of having dementia in the presence of a fami- translated and evaluated. It is unclear how the inclusion
ly history of dementia was 3.3 (confidence interval 1.7- of these potentially informative articles would impact
6.4). However, having a family history of dementia the discussion of this study. In addition to non-English
doesn’t necessarily preclude hearing loss from serving as articles not reviewed, there may have been other studies
a risk factor for dementia, as one proposed mechanism that investigated hearing loss and dementia but found
behind the HL-dementia relationship is that both dis- no connection. It is possible that such studies were not
eases are results of a shared underlying pathology. published on account of their negative findings. The
Therefore, it is unclear what impact controlling for fami- articles that were not published could potentially alter
ly history would have on the conclusion of the remaining the conclusion of the current study. However, we believe
16 articles. Evaluating family history in the context of that the 17 articles evaluated in this study are a strong
studying hearing loss as a risk factor for dementia could and accurate representation of the topic. Another

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