Chapter 13

STRABISMUS
Anatomy and Physiology
Extraocular muscles The actions of the extraocular
muscles are summarized in Table 13.1.
The actions of the extraocular muscles are examined in the nine
cardinal positions of gaze. This includes moving from the primary
position (looking straight ahead) into the eight other cardinal
positions, as illustrated in Table 13.2; this also shows the muscle
pairs that move the eyes into these positions. Remember SIN
RAD : Superior muscles (superior rectus, superior oblique) INtort;
Recti (superior rectus, inferior rectus) ADduct.
Binocular vision The ability to use both eyes simultaneously
so that each eye contributes to a common vision perception.
Classified as three grades:
■ Simultaneous perception: the ability to simultaneously perceive
two images, one formed on each retina.
■ Fusion: sensory fusion is the ability to fuse these two images
and perceive them as one. Bagolini glasses and the Worth
four dot test can be used to confirm sensory fusion. Motor
fusion is the ability to maintain sensory fusion through a range
of eye movements. Motor fusion is essential to join diplopia
because it allows similar retinal images to fall on
corresponding retinal points in each eye. The prism fusion
range is used to quantify motor fusion.
■ Stereopsis: the perception of depth based on binocular image
disparity.
Binocular single vision (BSV) The ability to use the
foveae and other corresponding retinal points in both eyes to
perceive a single image with stereopsis. Two types exist:
■ Normal retinal correspondence (NRC): the visual directions of
both foveae are the same. The temporal retina of one eye
corresponds with and has a common visual direction with the
nasal retina of the other eye.
■ Abnormal retinal correspondence (ARC): a ‘second-best’ form
of binocular vision in which, in the presence of a constant
manifest strabismus, the fovea of the fixing eye corresponds 583
 Anatomy and physiology Table 13.1: The actions of the extraocular muscles
Muscle Primary Secondary Tertiary
action action action
Medial Adduction
rectus
Lateral Abduction
rectus
Superior Elevation Intorsion Adduction
rectus
Inferior Depression Extorsion Adduction
rectus
Superior Intorsion Depression Abduction
oblique
Inferior Extorsion Elevation Abduction
oblique

Table 13.2: The actions of the extraocular muscles examined in

the nine cardinal positions of gaze

RSR,RIO
LSR,LIO

RSR,LIO Dextroelevation Direct Laevoelevation LSR,RIO

elevation
RLR,LMR Dextroversion Primary Laevoversion LLR,RMR
position

RIR,LSO Dextrodepression Direct Laevodepression LIR,RSO

depression

RIR,RSO
LIR,LSO
R,right; L,left; S,superior;I,inferior; R,rectus; O,oblique.

with a nonfoveal area of the deviating eye, e.g. in a manifest

left convergent strabismus the fovea of the right eye
corresponds with an area of nasal retina in the left eye; all
retinal areas (nasal and temporal) similarly adapt when both
eyes are open. The angle of anomaly refers to the difference
between the subjective and objective angles of deviation.
Suppression The cortical inhibition of the visual sensation
from one eye, when both eyes are open. Suppression occurs to
avoid diplopia or visual confusion. Suppression areas (scotomata)
584 may vary in position, size, and density.
 Chapter 13 STRABISMUS
History and Examination
Background Eyes that are correctly aligned are described as
orthophoric or ‘straight’. A manifest deviation or heterotropia is
when one or other visual axis is not directed towards the fixation
point – esotropia if convergent, exotropia if divergent, hypertropia
if up, and hypotropia if down. In heterophoria (latent deviation)
both eyes are directed towards the fixation point but deviate on
dissociation – esophoria (latent convegence), exophoria (latent
divergence) or hyperphoria/hypophoria (latent vertical deviations).
A constant deviation typically refers to a deviation that is present
for near and distance, whereas an intermittent deviation is only
present at one distance, e.g. near fixation, or certain conditions,
e.g. without spectacles. A concomitant deviation occurs when the
angle of deviation is the same in all directions of gaze, whichever
eye is fixing, unlike an incomitant deviation where the angle
between the two eyes is different in different gaze positions, or
with asymmetrical accommodative effort.
History Ask about the presenting complaint and its duration. If
diplopia (double vision) is a symptom, ask whether it is monocular
or binocular, constant or intermittent, the position of gaze in which
it occurs, or whether it produces vertical, horizontal or tilted
images. Does it stop the patient from doing anything? Is the
deviation socially embarrassing? Check medications and allergies.
In children, take a birth history, ask about any family history of
amblyopia, strabismus, or refractive error, and make brief
developmental assessment if appropriate. Inquire about any
previous ophthalmic treatment including spectacles, prisms,
occlusion therapy, and surgery. Ask adults about their occupation
and if they drive. If in doubt about a prior eye position or abnormal
head posture, ask to see old photographs.
General examination and cover testing
■ Measure VA and record refraction.
■ Observe any abnormal head posture (AHP). The AHP may
include head tilt, face turn, or chin elevation or depression.
Deafness, torticollis, and cervical spine abnormalities may also
cause an AHP.
Observe ocular posture including an obvious manifest
squint, wide epicanthic folds (pseudosquint), spectacles, and
any spectacle prisms (‘stick-on’ Fresnel or incorporated
prisms).
■ Using a pen torch at 33 cm, observe the corneal reflections
(CRs). Look for symmetry or asymmetry. Small manifest
deviations may not be obvious by CRs. Note: CRs are central 585
 History and examination or slightly nasal in most people. Not all abnormal-appearing
CRs are due to strabismus, e.g. high refractive errors can
produce a pseudosquint – a pseudoesotropia in high myopia.
■ Perform cover test (CT) at near (33 cm) to a light and then
accommodative target (e.g. reduced Snellen letter at VA of
worst eye, or pictures/toys for children). CT involves two
manoeuvres.
1. Cover-uncover test: to detect manifest strabismus. Cover
one eye and look at the noncovered eye. Movement inward
to take up fixation indicates an exotropia. Movement
outward indicates an esotropia. If the eye is slow to fix the
target, this may indicate poor VA or amblyopia. Note any
manifest or manifest latent nystagmus (MLN) – horizontal
jerky, often very fine nystagmus with fast phase towards
the uncovered eye. Note any movement of the covered eye,
particularly dissociated vertical deviation (DVD) associated
with infantile esotropia.
2. Alternating cover test: to detect latent strabismus.
Alternately occlude left and right eye for 1–2 seconds
each. Do not allow both eyes to view in between covering
as fusion may then take place. A latent deviation
sometimes becomes temporarily manifest after fusion is
disrupted by testing. Note how fast the eyes resume
binocular single vision (BSV) once testing is complete
(rapid or slow recovery) indicating how well the deviation
is compensated.
■ Move the target occasionally to check fixation.
■ Perform CT at distance to a suitably sized chart letter or
picture/toy.
■ Repeat CT with/without glasses, and then with/without AHP.
Eye movements (cranial nerves III, IV, VI)
■ If suggested by the history, check for monocular diplopia by
occluding each eye seperately. Note VA. If not already known,
clarify if diplopia is horizontal, vertical, or tilted.
■ Observe any head tilt, proptosis, ptosis, or lid retraction.
■ Ask the patient to follow a light held at 50 cm and report if
they see diplopia. Go from primary position and back again in
the other 8 cardinal positions of gaze (p. 583). Ensure the
CRs are always visible; the patient or examiner may need to
lift the lids, particularly in downgaze. Check for lid-lag on
downward smooth pursuit and any narrowing (Duane’s
586 syndrome) or widening (aberrant 3rd nerve regeneration or
 Brown’s syndrome) of the palpebral aperture on adduction;

Chapter 13 STRABISMUS
observe any pupillary changes, e.g. constriction of the pupil in
adduction or other gaze position may occur with aberrant 3rd
nerve regeneration.
■ If diplopia occurs in any position, check which image
disappears when an eye is covered; the more peripheral
image comes from the eye with the paretic muscle(s).
■ If CT shows a hypertropia/hyperphoria in the primary position
(step 1) then do a CT in right and left gaze to see where the
height is greatest (step 2). If greater in right gaze then repeat
the CT up and down to the right. If the deviation is greater
down to the right, a left 4th nerve palsy is suspected. Finally,
compare CT (eyes in primary position, fixing at 3 metres) with
the head tilted right and left, to see if the height differs (step
3). If the deviation is greater on head tilt left, a left 4th nerve
palsy is likely. However, the head tilt test is not always reliable
and other causes of hypertropia should not be ignored, e.g.
thyroid eye disease. Park’s three-step test refers to steps 1, 2,
and 3 combined. Step 3 is also called the Bielschowsky head-
tilt test. However, the diagnostic importance of testing in all 9
positions cannot be overemphasised. Vertical deviations can
be associated with bilateral (often asymmetrical) muscle
under- or overactions.
■ Examine saccades in suspected supranuclear lesions to help
differentiate newly acquired palsies from mechanical
strabismus (normal in the latter). Position a target to the right
and left of the patient’s eyes, within the visual field; instruct
the patient to look from one target to the other as quickly as
possible, without moving the head. Repeat the test in the
vertical plane. Compare the vertical and horizontal velocity of
the excursion, as well as the velocity in each eye; e.g. a
reduced excursion of the adducting eye on horizontal saccades
may indicate an internuclear ophthalmoplegia (same side).
Cerebellar disease and MS may produce hypermetric
saccades (eyes overshoot the target). Myasthenia gravis and
Parkinson’s disease may produce hypometric saccades (eyes
undershoot the target).
■ Test convergence to a detailed target, and observe normal
pupillary constriction.
■ If required perform Doll’s head manoeuvre to differentiate
supranuclear from nonsupranuclear lesions (e.g. Steele
Richardson Olszewski Syndrome). Ask the patient to fixate a
target in the distance. Inform the patient that you are going to
gently move the head right, left, up and down. Observe the 587
 History and examination extent of ocular rotations. Doll’s head movements may be
absent in supranuclear lesions.
■ Test optokinetic nystagmus with an OKN drum rotated slowly
in front of the patient, both horizontally and vertically, and in
both directions. Horizontal asymmetry may indicate a parietal
lesion. Convergence retraction nystagmus with a downward
moving drum (producing upward re-fixation saccades)
suggests Parinaud’s syndrome.

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 Chapter 13 STRABISMUS
Amblyopia
Background Amblyopia is a condition of reduced visual
function, in one or both eyes, which is not improved by the
correction of any refractive error, or by removal of a pathological
obstacle to vision.
Causes are form deprivation and abnormal binocular interaction
producing degraded retinal images in the sensitive or critical
period of visual system development, particularly in the first 2–3
years of life, decreasing with age, up to 7 years of age. During
this sensitive period, amblyopia can develop as well as respond to
treatment.
Classification
■ Strabismic amblyopia: from a constant, unilateral, manifest
strabismus.
■ Anisometropic amblyopia: >1 dioptre interocular difference.
■ Ametropic amblyopia: bilateral moderate to high refractive
errors.
■ Meridional amblyopia: astigmatism >1.5 dioptres.
■ Stimulus deprivation amblyopia: lack of adequate visual
stimulus in early life, e.g. cataract; ptosis.
History Take a full ophthalmic history noting any previous eye
surgery, squint, refractive error, and relevant birth history such as
premature delivery. Review previous VA data if available and note
poor compliance with glasses or occlusion.
Examination Crowded or logMAR-based VA tests increase
the sensitivity to detect amblyopia. If vision testing is unreliable or
unachievable, compare fix and follow responses. Suspect
amblyopia with constant unilateral strabismus; suspect equal
vision (good or bad) with alternating strabismus. Refract and
undertake a media and fundus examination. Assess eccentric
fixation, which can occur in strabismic amblyopes – under
monocular conditions the amblyopic eye does not fixate centrally,
but instead with a nonfoveolar area of the retina. Classify as
macular, paramacular, peripheral, steady, or unsteady.
In children >6 years old, the presence of motor fusion affects
whether to treat or continue to treat, and the risk of causing
potentially troublesome diplopia. If good motor fusion is present,
certain cases can be occluded after the usual 7–8 year cut-off. If
motor fusion is poor or absent, warn parents to stop occlusion if
diplopia occurs. Also assess with the Sbisa bar. Place the Sbisa
bar over the better eye and record the filter at which diplopia is 589
 Amblyopia appreciated. Readily appreciated diplopia indicates weak fusion or
less dense suppression and occlusion should be avoided to reduce
the risk of subsequent diplopia.
Treatment Prescribe the full spectacle correction to be worn
full-time. If vision is 6/24 (0.7 logMAR) or worse, commence
treatment (usually occlusion) at the same time; otherwise,
reassess VA in spectacles at 6–8 weeks (some practitioners
recommend waiting up to 12 weeks). Then commence therapy if:
■ VA is stable at worse than 6/9.5 (0.2 logMAR).
■ There is more than one logMAR (0.1) line interocular VA
difference.
■ VA cannot be reliably tested and there is fixation preference.
Occlusion normally starts with 2 hours per day part-time total
(light and form deprivation) using an adhesive patch. Increase to 4
hours, and if necessary continue to double hours if VA fails to
improve. If necessary, full-time total occlusion can be used for
dense amblyopia, particularly if there is poor compliance with
occlusion in the past, and/or a VA of 6/24 (0.7 logMAR) or worse.
Penalization is the optical reduction of form vision in the
nonamblyopic eye, usually with G. atropine 1% once daily to the
better eye. It compares well to occlusion, and is acceptable to
carers, but there is a slower improvement.
In children <2.5 years use minimal occlusion because of the
risk of occlusion amblyopia, e.g. 20–30 minutes/day, or fixing eye
3 days/nonfixing eye 1 day. If using optical penalization, reduce
the frequency of installation of drops.
Stimulus deprivation amblyopia (e.g. cataract) requires a more
intensive regimen.
The key to success is compliance. If this cannot be achieved,
consider inpatient admission for treatment.
Follow–up Regular follow-up is required. Continue with the
dose of occlusion that works for that patient, aiming to achieve
6/9.5 or better, or less than one line interocular difference. Once
therapy has ceased, observe for visual regression until 8 years of
age. If VA drops more than one line, re-commence occlusion.

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 Chapter 13 STRABISMUS
Esotropia
Background Esotropia is a manifest convergent squint. The
following relates to concomitant esotropia (angle of deviation the
same in all positions of gaze and regardless of which eye is
fixing). For incomitant strabismus see page 601.
Classification
■ Primary esotropia:
1. Constant.
a. Constant esotropia with an accommodative element:
esotropia at near and distance with hypermetropic
correction. Increases without glasses. Formerly called
partially accommodative esotropia.
b. Early-onset esotropia (infantile esotropia): esotropia at
near and distance caused by failure of cortical motor
fusion. Onset before 6 months of age. Associated with
manifest latent nystagmus (MLN) and dissociated
vertical divergence (DVD) where on cover test either
eye elevates and extorts under cover.
c. Late-onset esotropia: A sudden-onset esotropia for
near and distance usually in a child >5 years, often
with diplopia.
2. Intermittent.
a. Fully accommodative esotropia: binocular single vision
(BSV) with hypermetropic correction for near and
distance. Becomes esotropic for near and distance on
accommodation without correction.
b. Convergence excess esotropia: BSV at distance and
near when fixing a light; esotropia at near with an
accommodative target. Usually associated with
hypermetropia but patients can be emmetropic and
rarely myopic. Associated with a high
accommodative convergence/accommodation (AC/A)
ratio.
c. Near esotropia: BSV at distance; esotropia at near
with both a light and accommodative target. Normal
AC/A ratio.
d. Distance esotropia: BSV at near; esotropia at distance.
No limitation of abduction, unlike a lateral rectus palsy

591
Esotropia which can appear similar. Often associated with
myopia; if not, consider neurological disease.
e. Cyclic esotropia: varies between straight and large-
angle esotropia in rhythmic cycles, usually every 48
hours. Onset usually <5 years of age. Assess patients
on alternate days to confirm the diagnosis. Rarely may
occur in adults with a secondary squint. Can become
a constant esotropia over time.
■ Consecutive esotropia: esotropia in a previously divergent eye.
May be constant or intermittent. Usually follows surgical
overcorrection.
■ Secondary esotropia: constant deviation secondary to visual
loss or impairment occurring usually before 2 years of age.
History Ask about the nature of any double vision (rare in
children), family history of strabismus, febrile illness (may
precede accommodative esotropia), developmental delay, previous
history of occlusion, spectacles, strabismus surgery, and birth
history.
Examination Check VA, cover test with light and
accommodative targets (near and distance, with and without
spectacles), alternate cover test, and eye movements. ‘V’ patterns
are common (p. 603). Obtain refraction (with cycloplegia in
children).
Investigations Stereopsis, fusion, prism cover test, AC/A
ratio, synoptophore, and postoperative diplopia test, as required.
Treatment More than one treatment option often applies.
■ Optical correction: fully correct any hypermetropia (minus
working distance) and treat amblyopia if appropriate. If
myopic, reducing the myopic correction may help maintain
BSV but the patient must retain good VA to prevent amblyopia.
Only in children <3 years might a low myopic correction be
postponed. Executive bifocals (+3 D add) can help
convergence excess esotropia, and the patient can then be
weaned off as the condition stabilizes, e.g. postoperatively.
■ Amblyopia therapy: if <8 years (p. 589).
■ Observe: consecutive esotropia following exotropia surgery
usually settles in a few months; correcting previously
uncorrected low hypermetropia or prisms may help.
Consecutive estropia should not be allowed to persist for more
than 6 weeks without intervention. Consider botulinum toxin or
further surgery if it persists. In children <8 years, monitor for
592 amblyopia risk if suppression occurs.
■ Botulinum toxin: useful in assessing the risk of postoperative

Chapter 13 STRABISMUS
diplopia in ‘cosmetic’ cases not expected to achieve BSV. Also
diagnostically useful in cases with weak potential BSV to see if
this can be restored, and where several previous squint
procedures make surgery unpredictable.
■ Orthoptic exercises: aim to improve the quality of BSV, and are
based on physiological diplopia and using the relationship
between accommodation and convergence (e.g. bar reading,
stereograms) in carefully selected patients with fully
accommodative or convergence excess esotropia, or as an
adjunct to surgery.
■ Prisms: may help small-angle deviations, especially in distance
esotropia.
■ Surgery: performed to improve the appearance of the eyes
and, where possible, to restore binocular vision. In general, if
surgery is aiming to restore binocular function, it is
performed once hypermetropia is satisfactorily corrected and
amblyopia treated. The timing of surgery to improve
appearance is largely a matter of patient (or parent)
preference.
1. Constant esotropia with an accommodative element:
operate if cosmetically unsatisfactory with glasses.
Consider medial rectus recession (MR−) with lateral
rectus resection (LR+) if the deviation is a similar size
near and distance, or bilateral MR (bimedial) recessions if
the deviation is larger at near. Undercorrect, as residual
convergence tends to reduce over time: in the absence of
BSV there is a high risk of consecutive exotropia.
2. Early-onset esotropia: requires early surgery, preferably
before age 1 year, for any chance of binocular vision, but
most patients suppress and there is a risk of consecutive
exotropia. Surgery may involve bimedial recession, or
medial rectus recession/lateral rectus resection. Patients
usually need more than one procedure; however, multiple
procedures mean a higher risk of consecutive exotropia as
many patients continue to suppress. For cosmetically poor
DVD consider bilateral inferior oblique anterior positioning
or bilateral superior recti recessions (with Faden procedure
for worse eye, if asymmetrical).
3. Late-onset esotropia: botulinum toxin or surgery if BSV not
restored by glasses.
4. Convergence excess esotropia: notoriously difficult to
manage. Start with bimedial recessions, with further
593
 Esotropia surgery including Faden procedures or supramaximal
medial rectus recessions.
5. Near esotropia: normally undertake bimedial recessions.
6. Distance esotropia: if not controlled with prisms, usually
requires bilateral lateral rectus resections using adjustable
sutures, although not all clinicians agree with this surgical
approach.
7. Cyclic esotropia: consider medial rectus recession with
lateral rectus resection as the deviation is usually a similar
size near and distance.

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 Chapter 13 STRABISMUS
Exotropia
Background Exotropia is a manifest divergent squint. The
following relates to concomitant exotropia (angle of deviation the
same in all positions of gaze and regardless of which eye is
fixing). For incomitant strabismus, see page 601.
Classification
■ Primary exotropia.
1. Constant.
a. Early-onset exotropia: typically associated with
dissociated vertical deviation and manifest latent
nystagmus. Much less common than early-onset
esotropia and found particularly in Asian or African
populations.
b. Constant primary exotropia: rare. Exclude secondary
exotropia or decompensating intermittent exotropia. Be
suspicious of an associated neurological or
developmental problem.
2. Intermittent.
a. Distance exotropia: binocular single vision (BSV) at
near; intermittent or constant exotropia at distance.
Diplopia is very rare as suppression normally occurs
on divergence. Subdivided into 2 types – true distance
exotropia and simulated distance exotropia (see
differential diagnosis below).
b. Near exotropia: BSV for distance; exotropia for near.
Commoner in adults than children, e.g. existing near
exophoria decompensated by presbyopic correction.
c. Non-specific exotropia: intermittent exotropia can
present in any age group, and for either near or
distance fixation.
■ Consecutive exotropia: usually follows surgery for esotropia after
a variable period of time. Usually constant but can be intermittent.
■ Secondary exotropia: constant, secondary to visual
impairment, usually >2 years of age.
Symptoms Ask about diplopia, although this is rare except in
near and occasionally in consecutive exotropia. Intermittent
exotropia commonly presents in toddlers or infants; ask about
closure of one eye in bright sunlight. Ask about previous eye
treatment or surgery. 595
 Exotropia Signs VA is reduced in secondary esotropia. Amblyopia is
common in consecutive exotropia but rare in intermittent exotropia.
Examination Perform a cover test for near and distance with
and without spectacles. Perform the alternate cover test; a slowly
recovering latent deviation may decompensate (become manifest)
later. Examine eye movements. Intermittent exotropia may have a
‘V’ exo or ‘X’ pattern and can have slight limitations of adduction.
Consecutive exotropias may have limitation of adduction from
previous squint surgery. Reduced convergence can be associated
with near exotropia. If VA is reduced, carefully exclude intraocular
disease. Obtain refraction.
Investigations The accomodative convergence/
accomodation (AC/A) ratio is often high (>5 : 1) in simulated
distance exotropia, normal (3–4 : 1) in true distance exotropia, and
low in near exotropia (<3 : 1). Consider postoperative diplopia
testing if there is no BSV, e.g. consecutive or secondary exotropia.
Differential diagnosis Distinguish true from simulated
distance exotropia. Simulated distance exotropia can be controlled
for near by a high AC/A ratio and/or fusional convergence. Disrupt
fusion by occluding one eye for >45 minutes then measure the
maximum true near angle.
Treatment Treat amblyopia in children <8 years (p. 589). Give
the full myopic correction (minus working distance). Use of minus
lenses or reduced hyperopic correction can be considered in
children to stimulate accommodative convergence to help control
intermittent distance exotropia or to improve the cosmetic
appearance in consecutive exotropia, but should avoid asthenopia
and retain 6/6 VA. The effect of the full hypermetropic correction,
which may increase the angle of squint, must be considered when
planning surgery. Orthoptic exercises have a role in intermittent
deviations <20 prism dioptres, and can be useful postoperatively,
e.g. convergence exercises, stereograms. Treat any convergence
insufficiency with exercises in near exotropia. Small to moderate-
sized distance exotropia often remains stable without deterioration:
consider surgery if control deteriorates or for cosmesis. Surgery is
often the treatment choice to improve appearance in consecutive
and secondary exotropia. Botulinum toxin is useful diagnostically
where the quality of BSV is poor or where there is a risk of
postoperative diplopia.
Warn the patient that surgery aims for an early postoperative
esotropia, as the eyes drifts outward over a few months. Diplopia
may occur during this period. Undertake lateral rectus recession
(LR−) and medial rectus resection (MR+) in simulated distance
exotropia and other exodeviations if the angle is of similar size for
596 near and distance; or bilateral lateral rectus recessions in true
distance exotropia or other exodeviations if the angle of deviation

Chapter 13 STRABISMUS
is much greater at distance. Consecutive exotropia usually requires
exploration with medial rectus advancement and lateral rectus
recession, using adjustable sutures in patients >10 years of age.

597
 Microtropia
Microtropia
Background Optimal binocular single vision (BSV) exists with
bifoveal fusion. A subnormal variation of BSV can exist with foveal
(central) suppression. Microtropia is a small-angle squint (<10
prism dioptres) with foveal suppression in the deviating eye and
subnormal stereopsis. Motor fusion is present but the range may
be reduced. Microtropia is commonly associated with
anisometropia and amblyopia in the deviating eye. Microtropia can
also occur after surgical or optical treatment for a larger-angle
squint and can be present with other strabismus, e.g. a fully
accommodative right microtropia.
Classification
■ Microtropia with identity: no manifest deviation on cover test
because the eccentric point of fixation coincides with the
angle of squint and is used for monocular and binocular
fixation. There is abnormal retinal correspondence (ARC) with
the angle of anomaly (angle of squint) equal to the angle of
eccentricity (angle between abnormal point of retinal fixation
and normal fovea (p. 583).
■ Microtropia without identity: a minimal flick deviation is seen
on cover testing. Usually but not always esotropic. Central or
eccentric fixation and ARC are more common than normal
retinal correspondence. The angle of anomaly is larger than
the angle of eccentricity.
History and examination Take a full history, perform
cover testing, examine eye movements, and refract. Anisometropia
is common.
Investigations Assess fusion, stereopsis, and measure any
deviation with a prism cover test. The simultaneous prism cover
test, rarely used in practice, measures the manifest component of
the deviation where there is an associated heterophoria. The 4-
dioptre prism test (base out for suspected microesotropia) usually
shows no movement when the prism is placed in front of the
deviating eye due to central suppression.
Treatment Fully correct refractive error (minus working
distance). Consider amblyopia therapy, but if motor fusion is
absent or poor there is a risk of intractable diplopia in patients >6
years (p. 589). Treatment is unlikely to produce bifoveal fixation.
Treat decompensated microtropias as for concomitant strabismus.

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 Chapter 13 STRABISMUS
Accommodation and
Convergence Disorders
Classification
■ Isolated primary convergence insufficiency (CI).
■ Primary CI with secondary accommodative insufficiency (AI).
■ CI secondary to vertical deviation or decompensating near
heterophoria (convergence weakness exophoria).
■ Primary AI.
■ Near reflex palsy (variably involving convergence,
accommodation and pupil).
■ Accommodative fatigue.
■ Accommodative inertia.
History Patients can present at any age. Symptoms are
associated with close work and include blurred vision,
difficulty changing focus, horizontal diplopia, headaches,
eyestrain, and nausea. Check medication, as this may contribute
to reduced accommodative responses, particularly
antidepressants, muscle relaxants, antihistamines, and some
antihypertensives.
Examination Assess ocular motility, as decompensating
vertical deviations may be unable to converge. Check the near
pupil response. In patients with near reflex palsy or marked AI or
CI the pupils may fail to constrict, or even dilate, on attempted
near fixation. Consider neurological assessment in convergence
and accommodation paralysis.
Investigations Test the near point of convergence (NPC).
The patient follows a small detailed target as it is slowly moved to
within 8–10 cm of the nose on an RAF rule. Test three times. CI is
diagnosed if this is not achieved, or if only with effort. Check
accommodation under emmetropic conditions using the RAF rule;
bring the target progressively nearer until blurred. Alternatively,
use increasing plus and then minus lenses to determine the
accommodative facility. Reduced accommodation is commonly
associated with CI. Examine the prism fusion range, as this is
often reduced.
Treatment Leave asymptomatic adults without treatment.
Consider treating asymptomatic children if they have any other
binocular imbalance. Treat with orthoptic exercises (pen and jump
convergence, dot cards, stereograms, and voluntary convergence). 599
 Accommodation and convergence disorders Consider prisms if symptoms persist despite exercises. Convex
lenses may be needed in those with AI. Botulinum toxin may
provide some temporary relief in gross CI. Surgery is unhelpful, as
CI recurs.

600
 Chapter 13 STRABISMUS
Incomitant Strabismus
Background Incomitant strabismus occurs when the angle of
deviation differs depending upon the direction of gaze or according
to which eye is fixing. It is associated mostly with defective eye
movements, particularly neurogenic or mechanical lesions such as
3rd nerve palsy or thyroid eye disease, and less commonly with
asymmetrical accommodative effort such as anisometropia.
History Establish when symptoms occurred (acute, chronic,
congenital). Vague symptoms imply that the deviation is
longstanding. Ask about diplopia (monocular or binocular;
intermittent or constant; position of gaze when it occurs; vertical,
horizontal, or tilted images), trauma, hypertension, diabetes,
thyroid conditions, previous eye surgery or treatment. Neck ache
may occur with recently acquired abnormal head posture (AHP).
Longstanding AHP may be asymptomatic.
Examinations Note any AHP, nystagmus, lid malposition, and
pupil reactions. Check spectacles for prisms. Perform cover test
with and without any AHP. In paralytic strabismus the secondary
deviation of the unaffected eye when fixing with the affected eye
will be larger than the primary deviation when fixing with the
normal eye, based on Hering’s law of equal innervation. Examine
ocular movements and perform cover tests in the nine cardinal
positions to establish overactions, underactions, and where the
deviation is greatest. Compare subjective diplopia with objective
findings. Check for bilateral asymmetric underactions. Examine
ductions (movements tested monocularly with fellow eye covered)
to differentiate underactions versus (mechanical) limitations.
Investigations
■ Note the angle of deviation in primary position, fixing right and
left eyes, and relevant positions of gaze with the prism cover
test (PCT). Look for binocular single vision (BSV), stereopsis,
and sensory and motor fusion. Patients with severe head
injuries may lose motor fusion with intractable diplopia.
■ Synoptophore: useful if BSV is not present in free space and
for measuring torsion.
■ Hess chart: based on simultaneous perception. Compare the
patient’s results with the ‘normal’ grid on the chart. Different
size fields show incomitance. Observe the deviation in the
primary position. The smaller field indicates the primarily
affected eye (primary deviation), the larger field shows the
secondary deviation. The greatest inward displacement
compared to the normal field shows the primarily affected 601
 Incomitant strabismus muscle(s), or in mechanical limitations the greatest restriction
of movement. In the larger field the greatest outward
displacement from normal indicates the main overacting
muscle(s) – the contralateral synergist according to Hering’s
law. Longstanding deviations may be more concomitant and
identifying the primarily affected muscle can be difficult. In
mechanical strabismus the outer field of the Hess chart tends
to be compressed. A sloping field indicates an ‘A’ or ‘V’
pattern and not torsion.
■ The field of BSV chart: usually plotted on an arc perimeter –
shows the size, position, and usefulness of the area of BSV.
■ Imaging: and other investigations may be required depending
on the cause. See the appropriate sections for details
(p. 612, 3rd nerve palsy; p. 615, 4th nerve palsy; p. 618 6th
nerve palsy).
Treatment Initial management of acquired incomitance
includes advice about the use of an AHP, stick-on prisms, or
occlusion. The prism is usually placed on one eye: the paralytic
eye or the eye with the worst VA. The prism can be tilted to join
both vertical and horizontal diplopia. It can also be placed on the
top segment (or bifocal segment) of spectacles as required. Aim
to join diplopia in the primary position and depression at least.
Diplopia on extremes of gaze may not be correctable with prisms
as this may lead to overcorrection in another position of gaze.
Monitor patients, reducing prism strength as they recover or
stabilize. Occlusion, (graded frosting, foils, frosted tape, or lenses)
is used for large deviations or large incomitance, unsatisfactorily
controlled with prisms, or where there is a lack of fusion. Long-
term prisms can be incorporated into glasses for small deviations.
Orthoptic exercises have a limited role except for associated
convergence insufficiency. Treat amblyopia if age <8 years (p.
589).
Surgery for acquired incomitance is indicated for symptomatic
deviations such as unsatisfactory control with prism/large AHP,
and when the deviation is stable for at least 6 months. The type
and number of operations depends on clinical findings. For
neurogenic strabismus, weaken the overacting muscle(s). For
mechanical strabismus, weaken the tight muscle(s).

602
 Chapter 13 STRABISMUS
Alphabet Patterns
Background Alphabet pattern refers to a change in the angle
of horizontal strabismus in up/downgaze.
Classification
■ ‘V’: a ‘V’ pattern is a relative divergence of >15 prism dioptres
on upgaze compared to downgaze. The normal physiological
‘V’ pattern measures <15 prism dioptres.
■ ‘A’: a relative divergence of ≥10 prism dioptres or more on
downgaze compared to upgaze.
■ ‘X’: relative increase in elevation and depression, usually in an
exodeviation.
■ ‘Y’: relative exodeviation on upgaze. Downgaze and primary
positions have similar deviations
■ ‘ ’: relative exodeviation on depression. Upgaze and primary
Y
positions have similar deviations.
Aetiology The main causes are bilateral oblique and vertical
rectus muscle dysfunction, e.g. inferior recti overactions producing
adduction on depression (‘V’ pattern). Conversely, inferior recti
underactions produces an ‘A’ pattern. Super oblique underaction
can produce a ‘V’ pattern, and overaction can produce an ‘A’
pattern. High medial recti insertions can produce a ‘V’ Eso pattern
and low lateral recti insertions a ‘V’ Exo pattern. There is a high
incidence of ‘A’/‘V’ patterns in children with craniofacial
abnormalities.
Symptoms Possible diplopia where the deviation is greatest.
Signs Chin elevation or depression to achieve binocular single
vision. May have vertical deviation in primary position if vertical
recti/oblique actions are asymmetrical.
Examination Cover test for near, distance, depression, and
elevation by moving the head up and down. Look for bilateral
asymmetrical muscle dysfunctions during eye movement testing.
Investigations Hess chart: slanting sides indicate an ‘A’ or
‘V’ pattern.
Treatment Usually surgical. If there are oblique overactions
these should be weakened. For a ‘V’ pattern, inferior obliques are
recessed or disinserted. Superior oblique posterior tenotomies are
used for ‘A’ patterns. Weakening the obliques may cause a relative
esodeviation. Where no oblique overactions occur, transposing the
horizontal recti by a half or full muscle width, as shown in Figure
13.1 below, can reduce ‘A’ or ‘V’ patterns. This can be combined
with recession/resection of horizontal recti. 603
 Alphabet patterns ‘V’ pattern ‘A’ pattern
MR MR
LR LR LR LR

MR MR
Fig. 13.1: Transposition of horizontal recti in ‘A’ and ‘V’
patterns. The arrows indicate the direction of muscle
transposition.

604
 Chapter 13 STRABISMUS
Duane’s Syndrome
Background Duane’s syndrome is a spectrum of congenital
motility disorders with anomalous innervation of the lateral rectus
muscle, together with retraction of the globe, and narrowing of the
palpebral fissure on adduction. There is hypoplasia of the 6th
nerve nucleus and/or absence of the 6th nerve, with the lateral
rectus being innervated by a branch of the 3rd nerve. Secondary
myogenic factors are also involved.
The condition usually presents in childhood but mild cases are
sometimes picked up incidentally in adults. It is more common for
the left eye and females to be affected. The condition may also be
bilateral, although very asymmetrical in some cases. Systemic
abnormalities may coexist: hearing defects, ear malformations,
Goldenhar’s syndrome, Klippel-Feil syndrome, or spinal
abnormalities.
Classification
■ The essential underlying pathophysiology is similar in all types,
but they differ in the degrees of abnormality of the lateral and
medial rectus innervations (Table 13.3).
■ Alternatively, Duane’s syndrome is sometimes referred to as
typical (mainly limited abduction), or atypical (mainly limited
adduction).
History and examination Check for anisometropia and
amblyopia. Observe any abnormal head posture (AHP): usually a
face-turn to the affected side in types 1 and 3 and to the opposite
side in type 2. Most patients have fusion and stereopsis, often
maintained by the AHP. Diplopia is rare.
■ Cover test:
1. Types 1 and 3: orthophoria/esophoria/esotropia
2. Types 2 and 3: exophoria/exotropia.
■ Ocular movements: lids show narrowing of the palpebral
fissure on adduction with retraction of the globe on attempted
adduction (Fig. 13.2). The palpebral fissure often widens on
abduction. Upshoot or downshoot of the eye may occur on
Table 13.3: Classification of Duane’s syndrome
Abduction Adduction
Type 1 Limited or absent Normal or slight limitation
Type 2 Normal or slight limitation Limitation or absent
Type 3 Marked limitation or Marked limitation or
absent absent
605
 Duane’s syndrome adduction. Retraction on adduction is often best observed
from the side. Palpebral fissure changes can be subtle.
Treatment Treat associated refractive error and amblyopia if
<8 years (p. 589). Prisms and botulinum toxin have a limited role.
Surgery: has a limited role and patients/carers must understand
that normal eye movements cannot be restored. Indications
include a large primary position deviation, noticeable AHP, marked
globe retraction or up/downshoots, and large decompensating
phorias.
■ Type 1: ipsilateral/bilateral medial rectus recession(s).
a. With large deviation and no abduction beyond midline:
vertical muscle transposition (superior rectus and inferior
rectus moved to lateral rectus).
■ Type 2: ipsilateral/bilateral lateral rectus recessions.
a. With globe retraction: ipsilateral medial and lateral rectus
recessions.
b. With up/downshoots: lateral rectus recession and splitting.

606
 Chapter 13 STRABISMUS
A

C
Fig. 13.2: Bilateral type 1 Duane’s syndrome.

607
 Brown’s syndrome
Brown’s Syndrome
Background A condition with limited elevation in adduction
due to mechanical restriction rather than inferior oblique paresis.
Commonly congenial but can be acquired. May be constant or
intermittent. Congenital cases were previously thought to be due
to a short superior oblique tendon sheath, but are now considered
to be caused by anomalies of the tendon–trochlear complex.
Causes of acquired Brown’s syndrome include trauma of the
trochlear region, sinus surgery, inflammatory conditions, and
following superior oblique surgery.
History and examination
■ In congenital cases, parents may not notice reduced elevation
but instead the overaction of the contralateral eye on
elevation. Acquired cases usually complain of diplopia.
■ Abnormal head posture (AHP): if present, this is usually a chin
elevation but there may be a head tilt to the affected side ±
face turn away from the affected side.
■ Cover test: often straight in the primary position with binocular
single vision (BSV) or less commonly, hypotropia in the
affected eye/hypertropia in the unaffected eye.
■ Ocular movements: features include limited elevation in
adduction, usually good elevation in abduction (Fig. 13.3),
overaction of the contralateral superior rectus, and no/minimal
ipsilateral superior oblique overaction (Fig. 13.4). A ‘V’ pattern

B
608 Fig. 13.3: Left Brown’s syndrome.
 Left eye Right eye

Rect.sup. Obl.inf. Obl.inf. Rect.sup.

Rect.sup. Obl.inf. Obl.inf. Rect.sup.

Rect.ext. Rect.ext. Rect.int. Rect.int. Rect.int. Rect.int. Rect.ext. Rect.ext.

Rect.inf. Obl.sup. Obl.sup. Rect.inf.

Rect.inf. Obl.sup. Obl.sup. Rect.inf.

Green before left eye Green before right eye

Fig. 13.4: Hess chart of a typical left Brown’s syndrome. Note there is no deviation in the primary position; the smaller left
field shows the greatest constriction in the field of action of the left inferior oblique muscle (the primary defect). The
greatest outward displacement in the right field is in the field of action of the superior rectus (the contralateral synergist).

Chapter 13 STRABISMUS

609
Brown’s syndrome is often present. There may be down-drift in adduction and
widening of the palpebral fissure in adduction. There is
resistance to passive elevation of the eye in adduction
(positive forced-duction test). Check for an audible/palpable
click on elevation. Diplopia, if present, is usually on elevation.
Treatment
■ Treat any refractive error and amblyopia, if present (p. 589).
Most patients do not require surgery as BSV is present in
primary position and downgaze. Also, spontaneous
improvement can occur in congenital cases, particularly in
cases with a click/discomfort on elevation in adduction.
■ If associated with an acquired inflammatory condition local,
peritrochlear steroid injections can be beneficial.
■ Surgery is indicated if there is significant hypotropia in the
primary position or marked AHP. Surgical options include
superior oblique tenotomy/tenectomy (be aware of the risk of
palsy requiring further treatment), silicone band, or suture
spacer on the tendon. Patients with Brown’s syndrome
secondary to trauma often have a poor surgical outcome.

610
 Chapter 13 STRABISMUS
Thyroid Eye Disease
Background Patients with thyroid eye disease (TED) may
experience diplopia due to inflammatory change that is then
followed by extraocular muscle fibrosis. For other aspects of TED,
see page 81.
History Diplopia is commonly intermittent, vertical, and in
either upgaze or primary position. It may be worse in the morning
(fusion interrupted overnight) and evening (from decompensation).
Examination Note any abnormal head posture (commonly
chin elevation to maintain binocular single vision because of
limited elevation) perform cover test, and examine eye movements.
A tight inferior rectus is most common (restricted elevation,
maximum in abduction, hypodeviation, and possible excyclotorsion)
followed by medial rectus (restricted abduction, esodeviation) and
then superior rectus (limited depression maximum in abduction).
Hypo- and esodeviations often coexist. Look for bilateral signs that
may be asymmetric. An ‘A’ esodeviation is common, especially
following orbital decompression.
Investigations Compare the Hess chart to ocular motility. Be
aware that patients with moderately limited elevation may guess
the upper points when plotting the Hess chart (using visual fields,
or moving head). Consider testing field of binocular single vision
and monocular eye movements (uniocular fixation).
Management Prisms are often helpful if diplopia is present in
the primary position. Occlusion of one eye may be necessary for
very large incomitant deviations. Await stability for at least 6
months before undertaking corrective surgery. Both horizontal and
vertical surgery may be required. Use recession not resections,
preferably with adjustable sutures. Recess the inferior rectus for
hypotropia or restricted elevation. Recess the medial rectus for
esotropia. Bilateral surgery is common, e.g. bilateral inferior recti
recessions for bilateral limited elevation. Four muscles can be
recessed at one operation (two in each eye); usually one eye has
the medial and inferior recti on adjustable sutures.

611
3rd nerve palsy
3rd Nerve Palsy
Background Signs and symptoms vary depending on the site
of the lesion. A painful, unilateral 3rd nerve palsy suggests a
posterior communicating artery aneurysm, requiring an emergency
neurological referral. Microvascular, pupil-sparing 3rd nerve
palsies are commonly associated with diabetes and hypertension.
Other causes of 3rd nerve palsy include trauma, neoplasm,
infection, migraine, congenital cases, and giant cell arteritis (rare).
Signs A total 3rd nerve palsy affects all extraocular muscles
except the superior oblique and lateral rectus, which act
unopposed to give exotropia and hypotropia (‘down and out’) plus
a dilated pupil with paralysis of accommodation. The extent to
which the 3rd nerve is affected can depend on various factors,
including aetiology, e.g. pupil-sparing 3rd nerve palsies are
commonly ischaemic (microvascular). Superior division involvement
produces ptosis and superior rectus underaction.
There may be aberrant regeneration as the affected nerve
fibres are misdirected during recovery, producing retraction of
the upper lid on attempted adduction ± depression, adduction
on attempted elevation (+ rarely depression), pupil constriction on
attempted adduction ± depression, retraction of the globe on
attempted elevation ± depression. Aberrant regeneration is rare
with microvascular causes.
History Ask about the duration of onset, diabetes,
hypertension, headache, trauma, features of giant cell arteritis,
myasthenia gravis, and MS.
Examination Check BP, other cranial nerves including visual
fields, fundus for retinopathy and optic disc swelling. Examine the
temporal arteries if aged >50 years. Exclude proptosis, lid fatigue
with sustained upgaze, and Cogan’s lid twitch (overshoot of lid
when going from sustained downgaze to primary position). Check
eye movements and cover test.
Investigation
■ Orthoptic testing
■ Pupil sparing: fasting glucose. ESR and CRP if age >50 years.
Request CT or MRI if age <50 years and no diabetes or
hypertension.
■ Pupil or other cranial nerves involved, or optic disc swelling:
urgent CT or MRI via a neurologist.
■ Myasthenia suspected: consider Tensilon test via a
612 neurologist, or ‘ice-pack test’ (an ice pack applied to lid for 10
 minutes improves neuromuscular transmission and improves

Chapter 13 STRABISMUS
function).
■ Congenital cases: work-up for a possible neurological
aetiology.
■ Hess chart: Figure 13.5.
Treatment
■ Treat amblyopia in children <8 years old (p. 589). Prisms may
help in isolated muscle cases.
■ Surgery only achieves a limited area of binocular single vision
at best. The main aim is to improve the appearance. Residual
diplopia can be treated with an occlusive contact lens.
1. Exotropia with hypotropia: large lateral rectus recession,
and resection of medial rectus with supraplacement of the
insertions. Consider recessions of contralateral synergists.
2. Mainly exotropia: large lateral rectus recession and
resection of medial rectus with temporary traction sutures.
Nasal transposition of the vertical rectus muscles.
3. Superior division: Knapp procedure for hypotropia.
4. Inferior division: inverse Knapp for hypertropia on
depression.
Follow–up Review isolated pupil-sparing cases at 1 week,
then monthly. Expect improvement in presumed microvascular
cases and arrange CT or MRI if not evident in 6–12 weeks, or if
any aberrant regeneration occurs.

613
614
3rd nerve palsy

Left eye Right eye

Rect.sup. Obl.inf. Obl.inf. Rect.sup.

Rect.sup. Obl.inf. Obl.inf. Rect.sup.

Rect.ext. Rect.ext. Rect.int. Rect.int. Rect.int. Rect.int. Rect.ext. Rect.ext.

Rect.inf. Obl.sup. Obl.sup. Rect.inf.

Rect.inf. Obl.sup. Obl.sup. Rect.inf.

Green before left eye Green before right eye

Fig. 13.5: Hess chart of a total right third nerve palsy. This shows the primary deviation with the right eye exotropic and
hypotropic (right lateral rectus and superior oblique functioning). The right affected eye has the smaller field and is
markedly constricted in the fields of action of the affected muscles (right superior, medial and inferior recti and inferior
oblique). The large left field shows the secondary deviation and overactions of the contralateral synergists (Sherrington’s
law).
 Chapter 13 STRABISMUS
4th Nerve Palsy
Background May be uni- or bilateral, congenital (abnormal
superior oblique tendon/trochlea) or acquired (head trauma;
trochlear/orbital injury; ENT surgery; sinus infections, or vascular).
May also be idiopathic or associated with hypertension and
diabetes, or more rarely demyelination, giant cell arteritis,
tumours, and aneurysms.
Symptoms Vertical diplopia ± torsional and horizontal
components.
Signs A left superior oblique palsy may have a head tilt ± slight
face turn to right, with chin depression. Cover testing shows left
hyperdeviation, greater for near, with slight esodeviation.
Excyclotorsion may be present. Hyperdeviation and diplopia
increase in dextrodepression but this is less common in congenital
cases. Torsional diplopia is maximum on laevodepression. The
Bielschowsky head tilt test (p. 587) may show a positive result –
an increased hyperdeviation on head tilt left – but not all
practitioners feel it is reliable.
History and examination Diplopia is a presenting sign in
acquired cases – usually vertical, on depression and to one side;
diplopia may also have horizontal and tilted components. In
congenital cases diplopia is less common as a presenting
symptom. Such cases may present at any age because of signs of
decompensating or abnormal head posture (AHP) noticed by
others.
Investigations
■ Full orthoptic testing including vertical fusion range.
■ Synoptophore.
■ Hess chart: Figure 13.6.
■ Blood tests: fasting glucose. ESR and CRP if age > 50 years.
■ CT or MRI : request if age <50 years and no diabetes or
hypertension, or if failure to improve after 6 weeks. Image
urgently via neurologists if there are other signs, including
cranial nerve palsies.
Differential diagnosis
■ Congenital versus acquired: acquired cases are more likely to
experience diplopia, especially with subjective torsion, have
binocular single vision, and normal (6 dioptres) vertical fusion
range. Congenital cases may have contralateral facial
615
616
4th nerve palsy

Left eye Right eye

Rect.sup. Obl.inf. Obl.inf. Rect.sup.

Rect.sup. Obl.inf. Obl.inf. Rect.sup.

Rect.ext. Rect.ext. Rect.int. Rect.int. Rect.int. Rect.int. Rect.ext. Rect.ext.

Rect.inf. Obl.sup. Obl.sup. Rect.inf.

Rect.inf. Obl.sup. Obl.sup. Rect.inf.

Green before left eye Green before right eye

Fig. 13.6: Hess chart showing a left fouth nerve palsy. This shows the primary deviation of a small left hypertropia. The
smaller left field shows greatest constriction in the field of action of the superior oblique. The left fields also shows a small
overaction of the unopposed ipsilateral antagonist – left inferior oblique. The right field shows the larger secondary
deviation – right hypotropia (Hering’s law) and the greatest enlargement is in the field of action of the right inferior rectus
(contralateral synergist – Sherrington’s law), and a slight underaction of the right superior rectus (secondary inhibitional
palsy of contralateral antagonist).
 hypoplasia, a large vertical fusion range (but not always), and

Chapter 13 STRABISMUS
often have a longstanding AHP which can be seen in old
photographs.
■ Unilateral versus bilateral: assume bilateral until proved
otherwise. Bilaterality is suggested by a ‘V’ esodeviation, a
right over left in laevodepression and left over right in
dextrodepression (may be very asymmetrical), and ≥10
degrees of excyclotorsion on the synoptophore in depression.
■ Other diseases: consider myasthenia gravis, skew deviation,
and orbital disease.
Treatment
■ Medical: prisms (and/or AHP) may control small deviations.
Joining the vertical diplopia often allows patients to then fuse
a remaining slight horizontal deviation, if present. Prisms
cannot join torsional diplopia. Treat any amblyopia in children
<8 years (p. 589).
■ Surgery: options for hyperdeviation include:
1. Ipsilateral inferior oblique weakening if <15 prism dioptres
in primary position.
2. Superior oblique tendon tuck if superior oblique
underaction exceeds inferior oblique overaction,
particularly in congenital cases.
3. Ipsilateral superior rectus recession.
4. Contralateral inferior rectus recession.
For bilateral acquired cases perform bilateral Harado-Ito
procedure to correct torsion first.
Follow–up
■ All patients are seen every 1–2 months whilst being monitored
for stability/recovery.
■ Unilateral cases of vascular origin usually recover within 6
months. Congenital cases, as the deviations tend to be larger,
often require surgery as do large unrecovered vascular or
more frequently bilateral traumatic superior oblique palsies.
Small unrecovered deviations with vertical/horizontal diplopia
can be managed long-term by incorporating prisms into
spectacles.

617
6th nerve palsy
6th Nerve Palsy
Background Several possible causes include:
■ Adults: microvascular (particularly if aged >50 years,
hypertensive, or diabetic), MS, neoplasm, head trauma,
infection (bacterial or viral), raised intracranial pressure, and
idiopathic.
■ Children: similar to adults except microvascular causes are
unlikely. Transient 6th nerve palsies may occur in neonates.
‘Benign 6th nerve palsy of childhood’ may occur 1–3 weeks
after a febrile viral illness.
Symptoms Horizontal diplopia greater looking to the affected
side and in the distance. Diplopia may be constant in total 6th
nerve palsy.
Signs Patients may have a head turn to the same side, limited
abduction, and esodeviation most easily detected with cover test
and a distant target, comparing findings in extreme left and right
gaze. Nuclear lesions are accompanied by a gaze palsy to the
same side because of involvement of the conjugate gaze
mechanism. Pontine lesions may be accompanied by a 7th nerve
palsy, and cavernous sinus disease is often accompanied by 3rd ,
4th and trigeminal division of 5th nerve palsies. Look for
bilaterality.
History and examination See page 612.
Investigations
■ Orthoptic testing: a prism cover test (PCT) in right and left
gaze at distance quantifies incomitance and is useful
diagnostically and preoperatively. There is usually motor fusion
and stereopsis at near.
■ Hess chart: Figure 13.7.
■ Blood tests: fasting glucose. ESR and CRP if >50 years old.
■ MRI and CT: request if age <50 years and no diabetes or
hypertension, or if failure to improve after 6 weeks. Image
urgently via a neurologists if there are other signs, including
other cranial nerve palsies.
Management Most microvascular palsies and childhood
postviral palsies resolve spontaneously, so offer prisms or, for
large very incomitant deviations, occlusion. If stable and
symptomatic after more than 6 months, consider surgery. The
selected operation depends on the abduction status (check with
618
 Left eye Right eye

Rect.sup. Obl.inf. Obl.inf. Rect.sup.

Rect.sup. Obl.inf. Obl.inf. Rect.sup.

Rect.ext. Rect.ext. Rect.int. Rect.int. Rect.int. Rect.int. Rect.ext. Rect.ext.

Rect.inf. Obl.sup. Obl.sup. Rect.inf.

Rect.inf. Obl.sup. Obl.sup. Rect.inf.

Green before left eye Green before right eye

Fig. 13.7: Hess chart of a left lateral rectus palsy. The left smaller field in the affected eye shows a small esotropia in the
primary position; there is greatest constriction in the field of action of the left lateral rectus. The right field shows a larger
secondary esotropia in the primary position (Hering’s law); the greatest enlargement is in the field of action of the right
medial rectus (contralateral synergist – Sherrington’s law).

Chapter 13 STRABISMUS

619
 6th nerve palsy diagnostic botulinum toxin to medial rectus). If there is abduction
beyond the midline, the palsy is incomplete, so perform medial
rectus recession and lateral rectus resection. If not, the palsy is
complete, so undertake transposition of the vertical recti to the
lateral rectus border and botulinum toxin to the ispilateral medial
rectus. Treat bilateral palsies like unilateral ones, but these often
need second-stage surgery.
Follow–up Monitor for stability or recovery. Review every 1–2
months. Unilateral cases of vascular origin usually recover within 6
months. If recovery does not take place, smaller unrecovered
deviations can be managed long-term by incorporating prisms into
spectacles. Larger/very incomitant deviations may require surgery.

620
 Chapter 13 STRABISMUS
Optometry and General
Practice Guidelines
General comments
All children under the age of 5 years with suspected strabismus
and amblyopia should be assessed by an orthoptist. However,
those who are older than 5 years may be seen initially by a
community optometrist. The risk of amblyopia and strabismus is
increased in children with a family history of squint, amblyopia or
a high refractive error (particularly hypermetropia), and children
born prematurely or with developmental delay.

Optometrists
Preschool children who are found to have abnormal amounts of
hypermetropia (>+3.5 dioptres), or anisometropia (>1.0 dioptre),
or astigmatism (>1.5 dioptres) are at risk of developing strabismus
and amblyopia. To prevent this, a spectacle correction is required. If
in doubt about management, discuss with a Hospital Eye Service.

General practice
Never delay referral for suspected squint as it is very rare for a
child to ‘grow out of it’. Squint or limited eye movements due to
trauma, even when the eye does not appear inflamed, should be
seen the same day. Childhood refractive errors (without squint) are
usually treated by an optometrist experienced in refracting
children.
The following guide to referral urgency is not prescriptive, as
clinical situations vary.
Same day
■ 3rd nerve palsy with pupil involvement p. 612
■ Orbital blow-out fractures p. 101
■ 6th nerve palsy with papilloedema (refer to neurology) p. 618
Urgent (within 1 week)
■ Sudden onset of squint with diplopia in a child
>5 years p. 585
■ Sudden onset of double vision p. 585
■ Squint with other neurological symptoms or signs p. 585 621
Optometry and general practice guidelines Routine
■ Disorders of accommodation and convergence p. 599
■ Childhood squints with full ocular movements p. 585
■ Adults, without diplopia, requesting squint surgery
for cosmetic improvement p. 585
■ Long-standing squints with increasing angle of
deviation p. 585
■ Amblyopia p. 589
■ Suspected squint p. 585
■ Refractive errors (see comments above) p. 560

622