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Published by pknagar7815, 2021-04-06 05:15:12

BASIC EXAMINATION OF STRABISMUS -mail

BASIC EXAMINATION OF STRABISMUS

ESBxatasrmiacibnaistimonuosf

Dr. Subhash Dadeya
Dr. Savleen Kaur

DOS Office Bearers

Dr. Subhash C Dadeya Dr. Pawan Goyal Dr. Namrata Sharma Dr. Hardeep Singh
President Vice President Secretary Joint Secretary

Dr. Jatinder S Bhalla Dr. Vinod Kumar Dr. Manav Deep Singh
Treasurer Editor Library Officer

Executive Members

Dr. Dewang Angmo Dr. Jatinder Bali Dr. Shantanu Gupta Dr. C. P. Khandelwal

Dr. Rahul Mayor Dr. Vipul Nayar Dr. Rajendra Prasad Dr. Kirti Singh

DOS Representative to AIOS Ex-Officio Members

Dr. Jeewan S. Titiyal Dr. M. Vanathi Dr. Rakesh Mahajan Dr. Arun Baweja

Basic
Examination of

Strabismus

Dr. Subhash Dadeya
Dr. Savleen Kaur

Foreword

Respected Seniors and Dear Friends

“Strabismus” is one of the most challenging

subspecialty in ophthalmology. It’s not very

uncommon to encounter patients with strabismic

disorders in an ophthalmic outpatient department,

prevalence ranging from 0.5-5%. These disorders

besides causing cosmetic blemish, result in

disruption of normal binocular vision, altered

stereopsis and amblyopia. Early detection and Prof. (Dr.) Namrata Sharma
Hony. General Secretary
timely initiation of treatment therefore becomes

very important in order to prevent the occurrence of these sequelae. Ocular

examination should ideally begin in the newborn period and continue up to

all subsequent visits of the child. The understanding of various strabismic

pathologies has tremendously increased over the past two decades both in

terms of their etiopathogenesis as well as their management. There has been

an increased application of imaging modalities due to better understanding

of their role in detection and further classification of these strabismic

pathologies. No investigative modality can however replace or surpass a

good clinical examination. A comprehensive clinical examination is the key

for appropriately managing these disorders. This special issue on “Basic

Examination of Strabismus” focusses on various examination skills starting

from detailed history taking to assessment of visual acuity In pediatric age

group to various tests for evaluation of motor as well as sensory system, in

a sequential manner. Various tests have been described in sufficient detail

to help the newly joined residents gain confidence while examining patients

both in the outpatient as well in patient department.

We look forward for better understanding and learning of strabismic disorders
contributing to creation of good clinicians.

Prof. (Dr.) Namrata Sharma

Hony. General Secretary, DOS

ii | Basic Examination of Strabismus

Basic Examination of
Strabismus

Strabismus work up is an important component of ophthalmological
examination. It includes all aspects of a comprehensive pediatric
vision examination with addition of the sensory, motor, refractive,
and accommodative functions. It needs to be well focused and target
oriented without causing unnecessary fatigability of the patient. The
goals of the strabismus examination are

(1) History taking
(2) Measuring and characterizing the deviation
(3) Assessing the binocular status
(4) Establishing a diagnosis

In addition to the above goals, the strabismus examination is
incomplete without ruling out amblyopia. Adequate management
of the patient depends on amblyopia, sensory status, size of the
deviation, and the possibility of an underlying neurological problem
or systemic disease. The prevalence of amblyopia is estimated to
be 1% to 4% in children. The detailed assessment of a patient with
amblyopia was discussed in the previous CME series by the authors.

Regardless to say; any strabismus examination is incomplete without
a complete anterior and posterior segment examination. The present
CME gives an outline of the broad steps involved in a strabismus
examination. Strabismus examination includes the following
described steps but is not limited to it. In addition to these points,
the pediatric ophthalmologist may use his professional judgment for
the benefit and further testing of the patient.

Basic Examination of Strabismus | 1

HISTORY TAKING

A carefully obtained history is necessary in any strabismus
examination. Information of the patient’s general health as well as
the ocular health should be gathered. Birth history in case of a child
including mother’s pregnancy and factors such as prematurity and
birth weight, unusual length of labor, abnormal position, and use of
instruments should be documented. Then the development history
of the child should be asked along with behavioral problems. Old
photographs should be brought and examined. One should establish
the age at which the position of the patient’s eyes was first noted
to be abnormal. Preceding history in the form of fever, disease, or
trauma should be questioned. Parents should be asked as to point at
the eye that deviates and whether it is always the same eye that is
turned. It is also important to ask whether the deviation was constant
or intermittent at first and whether it becomes worse when the child
is tired or ill, more obvious in distance fixation or in near vision, and
worse or better when the patient is visually attentive or daydreaming.
The existence of a cyclic pattern of strabismus should always be
considered and inquired.

History of diplopia should be interrogated and differentiated from
blurred vision. In case diplopia is elicited, it should be differentiated
into binocular or monocular. The diplopia is maximum in which gaze;
at near or distance; constant or intermittent; more at near or distance
and what the patient does to counteract diplopia (head posture/
closing one eye) should be inquired. Last but very important, one
should ask about any treatment for the strabismus (glasses, patching,
exercises, surgeries) that the patient may have had.

A careful history can often point towards a specific diagnosis or at
least guide us towards the examination. For example, closing one eye
in bright light might indicate an intermittent exodeviation. Hence
quality time should be spent with the parents or the patients to illicit
all relevant history. Past and present medications should be recorded,
along with drug sensitivities and allergic responses. Any history of
thyroid or neurologic problems should be particularly emphasized. It
is also important to document anesthetic methods used for previous

2 | Basic Examination of Strabismus

surgeries and any related problems, and a detailed family history of
strabismus or other eye disorders. One should ask not only about
grandparents, parents, and siblings but also about more distant
relatives with ocular problems or symptoms. The following table
lists the key points of history taking in strabismus.

Table 1: History taking in strabismus
1. Time of onset
2. Onset of strabismus - acute or gradual
3. Frequency of deviation-constant or intermittent
4. Eye dominance-unilateral or alternating
5. Progression/diurnal variation of strabismus or relation of

strabismus to any gaze.
6. Presence of diplopia-onset-sudden/gradual/painful/progress-

ion or resolution, nature-horizontal/vertical/torsional
7. Abnormal head posture
8. Associated developmental, systemic, or neurological features
9. Family history and birth history
10. Treatment history with history of patching/glasses/surgery
11. Use of glasses and its effect on deviation
12. History of trauma

REMEMBER BY THE PNEUMONIC “TOFFEES”

Basic Examination of Strabismus | 3

PEDIATRIC EYE EXAMINATION

Examination of a child is quite different from that of the adult. The
history is largely from the parents or the guardian, and the examination
requires patience and talent. When we are eliciting the history from
the parents; it is an invaluable time to observe the child for any
abnormal head position and eye alignment. The child may become
uncooperative once we pay attention to him/her. Extra time should
be taken to gain the trust of the child. Toys and colorful gadgets often
are used to attract the child and more than anything else we need to
be patient with the child (Figure 1). One can examine the child in
the mother’s lap. During examination, child should be preferably
active. Also, there should be no distraction from other patients or
gross movements like the door opening. Interesting puppets and near
targets are used to allure the child. Making funny noises can help.
There are various protocols defined for the strabismus examination
and the order in which the tests should be performed.1 In children
it is preferable to do the non-touch tests first- reflex tests; fixation;
cover tests; pupillary reflexes and distant direct ophthalmoscopy.
For uncooperative children, also; one must utilize whatever time
possible for examination as strabismus cannot be evaluated under
sedation or anesthesia.

Figure 1: Traditional methods of vision testing might be difficult in children. They
must be distracted by cartoons or toys that allure them so that they can be examined.

4 | Basic Examination of Strabismus

An accurate objective measurement of refractive error is essential
before we begin any examination because it is often an important
etiologic factor in the development of strabismus. The patient’s
refractive condition generally should be evaluated under both
noncycloplegic and cycloplegic conditions. It might be kept in the
end along with fundus examination to make the patient cooperative
for other tests. The table below describes the essentials of a complete
strabismus examination which will be discussed subsequently.

Table 2: Points in strabismus examination

1. Visual acuity assessment, Refraction and detection of amblyopia

2. Inspection

3. Motor Evaluation

4. Sensory Evaluation

5. Supplementary tests

1. VISUAL ACUITY ASSESSMENT IN CHILDREN

Visual acuity assessment should be the first and the foremost step
in evaluating a child with strabismus. The accurate and reliable
assessment of visual function in infants and young children is important
for ensuring optimal management of those at risk of abnormal visual
development.2-3 Timing of examination in a child without complaints
should be a preferable screening at birth, then at 6 months, at three
years and then 5 years. Screening children younger than age 3 years
for visual acuity is more challenging than screening older children and
typically requires testing by specially trained personnel. Traditional
methods of vision screening are often inappropriate for the preschool
population and almost impossible for those children who are preverbal
or nonverbal, developmentally delayed, and/or have chronic illnesses
or disabilities. Waiting area should have toys, games, and puzzles. One
should try to follow child’s daily rhythm and avoid wearing a white
coat. The nontested eye should be covered and the examiner must
ensure that it is not possible to peek with the nontested eye. In infants

Basic Examination of Strabismus | 5

and children visual functions at near distances are more important
than visual functions at far distances. Therefore, visual acuity should
always be first measured at a near distance.

Various tests are used to identify visual defects in children, and the
choice of tests is influenced by the child’s age. We must remember
the various visual milestones that a child follows (Table below).
Amongst all the tests described in literature, one should use the “most
sophisticated test possible and available” in a child. There are some
general principles to be followed. The normal values may also be
different for different tests as well as different age groups.

Table 3 showing normal visual milestones

At birth • Closing eye to bright light
1 month-3 months • Ocular alignment established
Up to 2 months • Recognition of familiar face(mother)
• Well-developed smooth pursuits.
2-5 months • Well-developed fixation
• Saccades well developed
4 months • Accommodation develops.
5-7 months • Central, steady, maintained fixation
• Stereopsis well developed.
7 months • Contrast sensitivity develops.
15-24 months • Co-ordinate hand eye movements
36-48 months • Recognition of letters and optotypes

In preterm babies visual testing can be carried out depending on their
post term age group. The results suggest that visual acuity is more
closely correlated with age from conception than with age from birth
and that visual acuity screening in preterm infants should be carried
out with acuity gratings appropriate for the infant’s post term age
rather than with acuity gratings appropriate for the infant’s postnatal
age. Similarly, grating acuity tests have made it possible to evaluate
visual acuity from an early age. Table 4 outlines the most commonly
used tests for assessment of visual acuity in children.

6 | Basic Examination of Strabismus

Table 4: Visual acuity tests in children
Tests for PREVERBAL CHILDREN

O ABILITY TO FIXATE AND FOLLOW
O BRUCKNER TEST
O COVER TEST
O PREFERENTIAL LOOKING
O VISUAL EVOKED POTENTIALS
Tests for VERBAL CHILDREN
O PICTURE TESTS
O LEA SYMBOLS
O KAYS PICTURE TESTS
O SNELLEN LETTERS
O SNELLEN NUMBERS
O TUMBLING E TEST
O HOTV TEST
The most easily performed test in a preverbal child is the cover test.
Child with significant asymmetry between vision of two eyes, resists
occlusion of better eye. Remember that this test is positive only when
there is marked disparity between vision of two eyes (Figure 2).

Figure 2: Cover Test in a child who is preverbal. The child is crying when the better
eye is occluded.

Good monocular fixation is described as central, steady, and maintained
(CSM). Central fixation means fixation with central fovea; (May rule
out severe amblyopia). Steady fixation means that object of interest is
held by the fovea continuously i.e. there is no nystagmus. Maintained

Basic Examination of Strabismus | 7

means that in strabismic patient the eye holds fixation after the fellow
eye is uncovered beyond a blink. Occlude one eye for 3 minutes
and fixation behavior of uncovered eye is observed. Repeat with the
other eye. Watch for strabismus, abnormal movements or searching
eye movements. Remember that the child can have excellent fixation
with imperfect visual acuity. CSM fixation may not always indicate
good vision as it may be normal in bilateral visual disability. There are
various grades of fixation described for children4,5 and table 5 outlines
one of them.

Table 5: Grades of fixation*

Grade 4 The dominant eye takes back fixation immediately on
removal of cover

Grade 3 The deviating eye can maintain fixation for a few seconds
on removal of cover from the dominant eye, but fixation
goes back to the dominant eye before the next blink

Grade 2 Fixation goes back to the dominant eye with the next
blink

Grade 1 The deviating eye can maintain fixation through several
blinks, but patient still prefers the dominant eye

Grade 0 Freely alternating strabismus

*Different methods are described in literature; the above
table is just Authors preference.

The other tests that are very commonly used are the force choice
preferential looking tests. It is a type of resolution acuity test. Neonates
and infants dislike boring visual stimuli and hence the babies will tend
to look at the pattern when both quite plane and pattern are shown to
them. The correct response occurs when the observer is correct 75%.
Teller Acuity Cards are based on this principle. The testing is done first
binocularly and then monocularly. The stimulus display is presented
on a large card and observer looks through the peep hole. The observer
makes judgment by noting where the baby is looking. The test cards
of progressively finer gratings are used until the child no longer
demonstrates a preference for the test. There are some limitations of
preferential looking tests. They require trained personnel, takes about

8 | Basic Examination of Strabismus

20-30 min time which might be difficult in a busy clinic. Interpretation
is difficult in children with nystagmus or when there is limited
interest shown by the baby. These tests may underestimate strabismic
amblyopia or macular disease in which grating acuity is much less
affected than Snellen acuity. Off course as stated, it is a resolution test
and not recognition test as Snellen’s.

Allen cards consist of flash cards containing schematic figures: a truck,
house, birthday cake, bear, telephone, horse, and tree. We should
perform initial testing with the child having both eyes open, viewing
the cards at 2 to 3 feet away and we continue to move backward. To
calculate an acuity score, the farthest distance at which the child is
able to identify the pictures accurately is the numerator and 30 is the
denominator. Lea symbols are slightly different from the Allen card
test in that it has flash cards held together by a spiral binding. It reveals
what is the smallest symbol that the child can discern when there is no
disturbing visual information around.

Different instruments and panels test a different function of the whole
umbrella of “vision”. Hence one should keep in mind that acuity tests
from one test may not agree with other test results. The frequency of
examination should be based on the presence of visual abnormality,
child’s age, type of symptoms, and associated systemic conditions.

Sometimes customization of the available tests must be done. Patients
with latent nystagmus may show better visual acuity with both eyes
together than with one eye occluded. To assess distance monocular
visual acuity in this situation, it is helpful to fog the eye not being
tested with a lens that is +5 D greater than the refractive error in that
eye. Better vision in mesopic conditions might be seen in conditions
like anisometropic amblyopia hence lighting should also be constant
in subsequent examinations. Measurement of visual acuity is essential
in a child with strabismus to establish the diagnosis of amblyopia. An
alternating fixation or intermittent strabismus and/or a compensatory
head posture may be signs of equal vision in both eyes. The pediatric
eye disease investigator group recommends using the acuity tests
with surround bars. This prevents overestimating visual acuity due to
crowding phenomenon7 seen in amblyopia.

Basic Examination of Strabismus | 9

Figure 3: Different charts can be taught to the child and the same chart used on
subsequent visits

When do we test visual acuity in small children?

The guidelines by All India Ophthalmological Society for timing of
vision testing in children are given in table below.

Table 6: Timings for vision screening8

Neonates At discharge

High risk cases Within 1 month

Birth-3 years Vision screening by an ophthalmologist at
least once

>3-5 years Comprehensive eye examination by an
ophthalmologist at least once

5-8 years Comprehensive eye examination by an
ophthalmologist at least once

10 | Basic Examination of Strabismus

There are some warning signs which indicate that visual acuity should
be tested immediately in children. These include:
1. Abnormality of the eyelids, eyeballs, conjunctiva, cornea, or

pupils
2. Strabismus
3. Nystagmus
4. Torticollis after reviewing old pictures
5. Abnormal behavior suggesting visual impairment like no smile

by two months.
6. Lack of interest in visual stimuli, no blinking at light from the

first few days of life.
7. No fixation reflex after one month.
8. No blinking reflex in response to threat after 3 months.
9. No pursuit reflex (ability to focus on a moving target) after the

age of 4 months
10. Delay in acquiring ability to grasp objects.
11. In addition to the previous signs, in older children bumping into

things, falling over frequently and tripping over the pavement or
steps
12. After the child starts speaking if the child starts complaining of
stinging or burning, has problems with far or near vision/watches
TV closely, complains of diplopia or headaches (suggest vision
disorder when they occur at the end of the day or after staring for
a long time).
13. Prematurity, Low birthweight, Cerebral palsy, neuromotor
disorders, chromosomal abnormalities, craniostenosis or facial
malformations, family history of refractive errors, strabismus,
developmental delay and/or Infections during pregnancy also
necessitate visual acuity measurement.

Fixation

The method of choice for evaluating monocular fixation is visuoscopy,
using an ophthalmoscope with a calibrated fixation target.9 The
practitioner should determine whether eccentric fixation is present and,
if so, assess its characteristics: location, magnitude, and steadiness.
When there is no foveal reflex, entoptic testing, such as Haidinger’s
brushes or Maxwell’s spot, can be useful in the assessment of
monocular fixation in older children.

Basic Examination of Strabismus | 11

Fixation can be checked with the help of a direct opthalmoscope using
the fixation grid or star. We ask the patient to look at a distance and we
direct the ophthalmoscope beam on the macula of the eye to be tested.
We ask the patient to then look in the centre of the star. Depending
on where the foveola of the patient lies on the star, fixation can be
documented as foveal, para or perifoveal

Points we must look and document for fixation
• Alternate or monofixation
• How much preference-central, steady, maintained
• Grossly eccentric
• Unsteady and central
• Steady but not maintained
• Central steady but prefers one eye

CYCLOPLEGIC REFRACTION

One must always strive to obtain as complete and accurate an estimate
of the refractive error as possible. Cycloplegic refraction is a must
for every new strabismus patient. Cycloplegic refraction should be
carried out in every patient with strabismus, but procedures adopted
by different ophthalmologists may vary. One percent atropine
sulphate ointment should be used in children upto 5 years of age
and cyclopentolate or homatropine can be used after five years of
age. Cyclopentolate can cause neurological symptoms and should be
avoided in children with any associated neurological condition. Since
1% cyclopentolate may cause a transient increase in blood pressure in
infants, we use a 0.5% solution for this age group and add a drop of
2.5% phenylephrine hydrochloride (Neo-Synephrine Hydrochloride)
if mydriasis is unsatisfactory. The standard regimen in older children
can be 1 drop of cyclopentolate 1% and phenylephrine-tropicamide
combination followed by cyclopentolate again in each eye, 5 min
apart; then, perform the refraction 30 min after the last drop. Patients
with blue eyes, or patients with ocular albinism, should receive one set
of drops. The mydriatic effect comes on sooner and lasts longer than
the cycloplegic effect. If the patient shows varying refractive error
during retinoscopy, then it is likely that the patient has only partial
cycloplegia and requires more drops. In cases of heavily pigmented
eyes or in patients with variable refractions or esotropia/esophoria, it

12 | Basic Examination of Strabismus

may be advisable to have the patient return for a 1% atropine refraction.
In these patients, ointment atropine should be given to both eyes twice
a day for 3 days before the refraction. It is important to put a little bit
of ointment (half the size of rice grain) to avoid systemic side effects.
Recent studies have shown that cycloplegic effect of 1% atropine and
1% cyclopentolate are equal in hypermetropic, esotropic, and normal
children.10-12 Since cyclopentolate may cause transient increase in
blood pressure in infants, a 0.5% solution should be used for this age
group. But most physicians use atropine for children less than 5 years
(sometimes even up to 8 years especially in cases of esotropia) and
cyclopentolate for children older than 5 years. After instillation of any
drop, the excess should be wiped, and the canaliculi compressed for
30 seconds. Contrary to the popular brief; hypermetropia may increase
upto 7 years. Hence a repeat cycloplegic refraction should be done at
6 months or whenever necessary; whichever is earlier and continued
till about 10-12 years. After that, decision to use cycloplegic can be
tailored based on the refractive error. Broad guidelines for choosing
the cycloplegic agent are enumerated in the table below.

Table 7: Drugs used for cycloplegia

Till 5 Presence of No strabismus
years esotropia
For office screening use cyclopentolate
>5 Atropine 1% 1% followed by phenylephrine-
years eye ointment tropicamide combination followed by
twice a day for cyclopentolate again.
three days

In case of hyperopic refractive
error/dark iris, use atropine 1%ointment

Cyclopentolate For office screening use cyclopentolate
1% eye drop/ 1% followed by phenylephrine-
Homatropine tropicamide combination followed by
2% eye drop. cyclopentolate again.

Otherwise can use Atropine 1% eye
ointment/Cyclopentolate

1% eye drop/Homatropine 2% eye drop
in dark iris/hyperopia

Basic Examination of Strabismus | 13

2. INSPECTION

The physical examination starts as the patient enters the room. There
are some easily identified conditions which should not be missed
in a child. These include microcephaly, albinism, downs syndrome,
cerebral palsy, cranial dysostosis, hydrocephalus etc. While taking
the history, it is important to observe the patient’s visual behavior,
eye alignment, eye movements, fixation, and head posture. Attention
should be given to the facial features, lid fissures, their width, and their
direction.

Figure 4:Some gross findings like craniosynostosis(left) and blepharophimosis(right)
should not be missed on examination

If the two lid fissures are different in width, the possibility of ptosis
or pseudoptosis of the upper lid with the narrow lid fissure must be
considered and the two conditions differentiated. The patient may have
true ptosis of the upper lid if the superior rectus muscle is involved.
Pseudoptosis means the lid may only appear to be ptotic because of
narrowness of the lid fissure caused by the hypotropic position of the
globe. When the patient fixates with the affected eye; pseudoptosis
often disappears. One should also examine whether the width of one
lid fissure changes when the patient moves the eyes to the right or left,
as in retraction syndrome or a case of aberrant regeneration; when the
jaw is moved, or when the patient speaks or chews, as occurs in the
Marcus Gunn jaw winking phenomenon.

14 | Basic Examination of Strabismus

Presence of pseudo strabismus must be ruled out by cover uncover
test. In infants the epicanthus is more pronounced with a semilunar
fold of skin running downward at the side of the nose and its concavity
directed toward the inner canthus. The variability of epicanthal fold
may create the appearance of esotropia when it is not actually present.
Unusually narrow or unusually wide interpupillary distances should
be noted. Narrow ones may create the impression that an esotropia
is present. Facial asymmetries, mongoloid or antimongoloid features
may create the impression of a heterotropia. A negative angle kappa
may simulate an esodeviation and again produce a pseudo strabismus,
may make an existing esotropia look worse than it is, or may mask all
or part of an exodeviation.

Usually patients with comitant Components of head posture
deviations; do not have any 1. Head tilt to the right or left
abnormal head posture. In cases shoulder
of incomitant strabismus, patients 2. Face turn to right or left
often attain a specific head 3. Chin elevation or depression
position to maximize the diplopia
free fields. Abnormal head posture can be in the form of chin up/
down, face turn or head tilt. Head posture can be appreciated after
six months of age when the child starts sitting upright. In patients
with nystagmus, the frequency and amplitude of the nystagmus
may be reduced or there may be no nystagmus when the eyes are
directed to one side. The patient may hence adopt a head posture
to nullify the nystagmus even in the presence or even absence of a
comitant deviation. While an anomalous head posture should alert the
examiner to search for nystagmus; a paralytic horizontal, vertical, or
cyclovertical strabismus; cyclotropia; or an A or V pattern, normalcy
of the head position does not rule out any of these conditions. Often
a paradoxical head posture can be obtained to increase the distance
between the images. Head posture may also be seen in conditions
where there is no ocular cause. We can differentiate these conditions
by occluding one eye which eliminates the head posture due to
incomitant strabismus. It may not be possible to straighten the head
posture in non-ocular causes of abnormal posture. Head posture
should be examined for near as well as distance. Refractive errors

Basic Examination of Strabismus | 15

Figure 5 showing a head tilt to the right side

can also cause an abnormal head posture and old photographs should
be reviewed to diagnose a congenital posture.

There will be significant acquired neurological signs which can be
easily identified in adults. These include associated nerve palsies like
facial palsy, tremors, ataxia, deafness, or signs of thyroid eye disease.
Head posture should be differentiated from head movements like head
nodding or head thrusts as in cases of ocular motor apraxia.

3. MEASUREMENT OF OCULAR DEVIATION AND
OCULAR MOVEMENTS (MOTOR EXAMINATION)

Strabismus is a condition where visual axis deviate from bifoveal
fixation. There can be a manifest deviation (heterotropia) or a latent
strabismus (heterophoria). The examination of strabismus includes
detection, estimation, and measurement of the deviation. The diagnosis
of strabismus including the diagnosis and frequency of deviation
may be established by cover-uncover test at distance and near while
patient looks at an accommodative target. The magnitude can be
determined by using alternate cover test with prisms. After testing in
primary gazes, the measurements should be repeated in all nine gazes.
Observation of head posture as well as additional version and duction
testing, establishes whether deviation is comitant or incomitant.

16 | Basic Examination of Strabismus

The angle of strabismus can be broadly tested by:
1. Light reflex tests
2. Cover tests
3. Prism bar tests.

3.1 Light Reflex Tests

These tests provide an estimate of the size of the strabismus and
are hence not very precise. They are usually used on uncooperative
patients and children.

Prerequisites
1. Patients’ attention is critical.

2. Patient should be fixating on an accommodative target preferably
with both eyes open. An accommodative target is a target which
requires accommodation. A torch light is not an accommodative
target. The best accommodative target is the Snellen’s line close
to the patient’s visual threshold (6/9 or less) or detailed toys/
small pictures for children.

3. Angle kappa is the difference between the pupillary and
visual axis. Angle kappa is larger in hyperopia and smaller in
myopia. Its importance lies in differentiating cases of pseudo
strabismus. Patients with small interpupillary distance may also
appear to be esotropic. A negative angle kappa where corneal
light reflex appears to be on the temporal side of the pupillary
center can simulate an esodeviation. Like pseudoesotropia,
certain morphological features of the face can result in a false
appearance of eyes to be drifted outwards. Most commonly
hypertelorism, which is widely set eyes, can result in pseudo
exotropia. Traction of the retina resulting in pathologic ectopia
of the macula temporally can cause a positive angle kappa
resulting in nasal displacement of the light reflex on the cornea
simulating a true exotropia. Pseudoexotropia from positive angle
kappa is mostly seen in retinopathy of prematurity which results
in temporal dragging of the macula, it can also be seen in ectopic
macula resulting from toxocara retinal scars, high myopia, or
congenital retinal folds.13-15

Basic Examination of Strabismus | 17

Hirschberg test
The Hirschberg or the corneal light reflex test is used to assess the
alignment of the eye by location of the central reflex on the cornea.
This reflex is a misnomer as it is the first purkinje image behind the
pupil. 16
It is performed by holding a light source in front of the patient and
directing the light into his/her eyes. While the patient looks at the light
source, one can assess the location of reflex in each eye. For all practical
purposes, the Hirschberg tests can only be performed for near since the
patient is required to fix on the light source. An accommodative target
is placed next to the light source to provide accommodation.
Prerequisites: Cooperation of the patient; ability to fixate at the target
with one or both eyes and clear corneas.
Interpretation: With an orthotropic alignment; the reflexes in both eyes
are seen symmetrically in each pupil. They may be slightly decentered
nasally in both eyes because of the positive angle kappa. Asymmetric
displacement of the light reflex indicates a possible tropia. However,
an asymmetric angle kappa can also cause asymmetric displacement
which can be differentiated from a true tropia by cover uncover
tests. Temporal displacement of the reflex indicates esotropia; nasal
indicates exotropia; inferior hypertropia and superior displacement
indicates hypotropia. (figure below)

Interpretation of Hirschberg tests. A: symmetric reflex in both eyes implying
orthotropia. B: Reflex in left eye seen superonasal in the pupil suggestive of
exotropia and hypotropia. C: An Inferiorly placed reflex in the right eye suggestive
of hypertropia D: Nasally placed reflex in the left eye suggestive of exotropia

18 | Basic Examination of Strabismus

Examiner can estimate the amount of deviation by the amount
of reflex displacement. The deviation in prism diopter = 15 *
Displacement of light reflex in millimeters. Hence a 2-mm temporal
displacement indicates 30 PD esotropia. These are just estimates and
should not be used for surgical dosage calculations. Another way to
estimate the angle of deviation is given in table below

Table 8: Estimation of the angle of deviation
from corneal reflex

Position of corneal reflection Angle of Deviation

On margin of pupil 12º-15º ~ 30 ∆

Halfway between margin of pupil and limbus 25º ~ 50 ∆

On the limbus 45º ~ 90 ∆

Bruckner reflex
Bruckner reflex is a screening test used in children. It is performed
by using the direct ophthalmoscope and targeting it in both eyes
simultaneously as in distant direct ophthalmoscopy. Patient should
fix on the light source when approximately one meter in a dimly lit
room; red reflexes of both eyes are assessed simultaneously in both
eyes with direct ophthalmoscope. The red reflexes should appear
bright and equal in both eyes. and in cases of strabismus the deviated
eye gives a brighter reflex (Figure 7 below). The brighter reflex can
also be given by anisometropia, retinal pathology or media opacities.

Figure 7. In the figure above, the left eye with the brighter reflex is the
strabismic eye.

Basic Examination of Strabismus | 19

3.2 Cover Uncover Tests

Cover uncover test is used to pick up tropias. Distance measurements
are done at 6 m and near at 33 cm preferably with an accommodative
target held at eye level. In the cover test, the light reflex is shown
in both the eyes and the examiner covers the apparently fixating
eye while the patient fixates at an accommodative target. The cover
preferably should be an opaque occluder but a Spielmann occluder
(less dissociating) can also be used. The strabismic eye is observed
for a tropia shift. After covering for one or two seconds and observe
for a shift in the uncovered eye. In case of no shift, cover the
opposite eye and observe the uncovered eye for tropia. If there is no
movement/shift of either eye after covering uncovering each eye; it
implies orthotropia with no manifest squint. However, it might not
rule out a phoria.
A refixation shift in the uncovered eye indicates a tropia. A nasal
shift indicates an exotropia; temporal shift indicates an esotropia;
downward shift indicates a hypertropia and upward a hypotropia.
Cover uncover test is followed by the alternate cover test. The
alternate cover test disrupts fusion and manifests the phorias hence
is more dissociating. In this test we hold the occluder over one eye
for several seconds and then rapidly move it over the other eye. The
interpretation is highlighted in table below.

20 | Basic Examination of Strabismus

Table 9: Interpretation of cover test

Response Interpretation

Cover test: covering the Binocular fixation present before

apparently normal eye; No applying the cover.

movement of the other eye: A manifest deviation was present

Cover test: Movement of the before applying the cover
other eye

Uncovering the normal eye:

1. Movement of redress of Phoria is present.
the uncovered eye (fusional

movement); no movement

of the other eye:

No movement of either heterotropia is
eye; or the uncovered eye An alternating
deviated; opposite eye present.
continues to fixate:

Uncovered eye Preference for fixation with one
makes movement of redress eye: a unilateral heterotropia is
and assumes fixation; present
opposite eye

deviates

No shift to alternate cover Orthotropia
testing

No shift on cover uncover Phoria
test/but shift on alternate
cover test

Shift on both cover uncover Tropia
and alternate cover test
(same amount)

Small shift on cover uncover Monofixation syndrome
testing and larger shift on
alternate cover testing

Basic Examination of Strabismus | 21

Points to remember:
1. Cover uncover test should not be done too slowly as that

may manifest a phoria due to prolonged occlusion causing
dissociation.
2. Time must be allowed for the other eye to recover when the
cover is removed.
3. Even in the presence of a unilateral strabismus, squinting eye
should be covered to unmask conditions like DVD.
4. Other conditions which can be picked up by cover test – latent
nystagmus; pseudoptosis, pseudo strabismus and eccentric
fixation.
5. The cover test picks up tropias and the uncover test may pick up
phorias.
6. While recording the results of the test, include type of
deviation, estimation of its degree, whether manifest, constant
or intermittent, and speed of recovery (rapid, moderate, slow or
delayed) as well any special conditions like torsion or DVD.
7. A modification of the test was performed by Speilmann. By
using the Spielmann occluder the diagnosis of heterophoria is
simplified as the deviation of the covered eye can be directly
observed by the examiner without having to remove the cover
(Figure 8).

Figure 8; Covering the right eye with Spielmann occluder when the eye
under cover elevates

22 | Basic Examination of Strabismus

3.3 Prism Bar Tests17-28

Prism Diopter: It is unit of measurement of the deflection of light
rays caused by a prism.

1 PD: It is power of a prism that deflects the rays of light at one meter
by one centimeter.

1 PD =1/2degree [4/7th of a degree]

General principles:

1. Prisms are placed with the apex/base oriented to neutralize the
deviation such that the apex is towards the direction of deviation.
Apex in for esotropia; out for exotropia; down for hypotropia
and apex up for hypertropia.

2. Maintain dissociation by occluding one eye always, even while
changing prisms.

3. Prisms should not be stacked over one another if they are the
same orientation. Vertical prisms can be stacked with horizontal.

4. Measurement of deviation should be done with the refractive
correction at near as well as distance and in nine cardinal
positions of gaze.

5. The patients head can be moved, or the target can be made for
the cardinal positions.

6. When measuring a deviation with prisms, remember that the eye
without the prism is the fixing eye, and the eye looking through
the prism is the nonfixing eye, regardless of fixation preference
or the presence of amblyopia

7. Two types of prisms are used for PBCT. The plastic prisms
are placed in the frontal position i.e. post surface parallel to
infraorbital margin. But glass prisms are placed in the prentice
position, i.e. the post surface of the prism is perpendicular to the
line of sight.14

Basic Examination of Strabismus | 23

Figure 9 depicting prism bar cover test in a child

PREREQUISTES:

1. Patient should be able to fixate at the accommodative target
2. Can be done successfully in cooperative patients
3. Refractive correction should be worn by the patient and if

refractive error is newly corrected, two to three weeks should be
allowed for the patient to adapt to his/her glasses.
4. Measurements can be variable and hence examinations at
different times of the day is required.

FALLACIES:

1. All prism reflex tests are affected by high refractive power.
Plus Glasses always measure less than actual deviation [both in
eso & exo] (minus) glasses always measure more than actual
deviation [both in eso and exo]. Effect is significant when
refractive error is more than five.

2. Being measured on a tangent scale the units of prism diopters are
not equal for longer distances.

24 | Basic Examination of Strabismus

LIMITATIONS

• PBCT requires accurate fixation by fovea.
• Test accuracy is limited by the optical qualities of prisms

[Stronger the prism required, greater the error.]
• Cyclodeviation cannot be measured by PBCT
• Subjective angle cannot be measured.
Krimsky test: It is a modification of the Hirschberg test. A loose
prism/prism bar is placed in front of fixing eye with apex pointing in
the direction of deviation and a torch light is shown into both eyes as
the patient fixates on an accommodative target. The prism power is
increased or decreased to make the reflex symmetrical in both eyes.
(Figure below ). Alternatively, prisms can be placed in front of the
deviating eye, but the reflex can sometimes be difficult to see behind
the prism.

Figure 10 depicting krimsky test with prism placed in front of the deviating eye.
Alternate prism bar test could not be performed because of the corneal opacity in the

left eye

Modified krimsky test: In this method the prisms are held before
the fixing eye or split between the two eyes by adjusting the prisms
so as to center the corneal reflection in the deviated eye it is possible
to approximate & quantitate the deviation.

Prism alternate cover test: This test is used for objective
measurement and is the method of choice in the measurement of
horizontal and vertical deviations in most cases. It can be done at 33
cm and at 6 m as well as far distance. Prism is placed over one eye
with proper orientation. Patient is asked to fixate at an accommodative
target preferably. Cover test is performed to estimate the angle of
deviation. A prism of approximate estimated power is then placed

Basic Examination of Strabismus | 25

over either eye (latent deviation) or squinting eye (manifest
deviation). Alternate cover testing is then performed over the prism.
If there is residual deviation; prism power is increased. The point of
neutralization is no movement on alternate cover test. The strength
of the prism gives the deviation in prism diopters. Accuracy of
measurement can be increased by increasing prism strength until the
movement is reversed, and then reducing till there is no movement.
The testing should be repeated and confirmed by placing prism over
the other eye also.

If the deviation is comitant and ductions are full, a prism can be
placed in front of either eye or even split between the eyes to measure
a deviation. However, when measuring patients with an incomitant
deviation secondary to ocular restriction or muscle paresis, one must
consider the primary versus the secondary deviation. Hence prisms
should be placed in front of either eye in turn. In accordance with
Hering’s law, the deviation is larger when the deviated eye is fixing
(secondary deviation) than when the normal eye (primary deviation)
fixes. The target can be moved into the required gaze position or the
target can remain in the position and patients head turned instead to
test extremes of gazes.

Simultaneous prism cover test: It is useful in patients with a small
tropia and a large phoria. A prism of approximate power (from
Hirschberg) is placed in front of the deviating eye and simultaneously
an occluder is placed over the fixing eye. If the eye behind the prism
does not move; it implies that the deviation is neutralized. Keep
increasing prisms in case there is a shift behind the prism till there is
no refixation movement.

Causes of variable measurements on prism testing:

1. A variation up to 10 PD on subsequent examinations/visits is
usually attributed to test-retest variability. High AC/A ratio-
Use accomodative targets. The reason for using fixation objects
rather than a simple penlight is to control accommodation during
measurement of the deviation at near and distance fixation.
One must understand that a patient’s response depends on the

26 | Basic Examination of Strabismus

stimulus presented, not only during subjective tests, where it is
more obvious, but also during objective tests.

2. Maximal dissociation of the eyes must be achieved to make the
correct diagnosis, especially in patients with heterophoria. Such
patients have a strong compensatory innervation that keeps their
eyes aligned and it is not immediately suspended when one eye
is covered. It is necessary to dissociate the eyes for some time to
bring out the full amount of the deviation. In cases of intermittent
exotropia. Binocular tonic fusion should be prevented by
prolonged occlusion of one eye or patch test. Marlow occluded
the nondominant eye for 14 days and no less than 7 days to
accomplish thorough dissociation of the eyes. Later Scobee and
Burian recommended half an hour patching for the diagnosis of a
pseudodivergence excess type of exodeviation. Patch test is used
to uncover the total angle of deviation and determine the target
angle for surgery. It suspends the tonic fusional convergence and
hence differentiates true divergence from pseudo-divergence
excess. Patching for 30 minutes is now considered appropriate.
In clinical practice, full dissociation is obtained when a reversal
of the direction of movement is observed on prism bar test.

3. Incomitant deviation or pattern strabismus.
Other methods to measure deviation include amblyoscope and
arc perimeter

3.4. Ocular Movements

Conjugate movements are movements of both eyes in the same
direction, termed versions. Dysjugate movements are movements
of both eyes in opposite directions. The term ductions is used to
describe monocular eye movements from primary position to one
of the main positions of gaze. Laws related to ocular movement
are Herings law of equal innervation (When a nerve impulse is
sent to a muscle to contract, equal impulse goes to contralateral
synergist) and Sherringtons law of reciprocal innervation (which
states that when a muscle contracts, its direct antagonist relaxes
to an equal extent). An upward movement of an eye is referred
to as supraduction (sursumduction), a downward movement is

Basic Examination of Strabismus | 27

termed infraduction (dorsumduction), a nasal-ward movement is
termed adduction, and a temporal movement is termed abduction.
Torsional rotations are known as cycloductions, with incycloduction
(intorsion) referring to a nasal rotation of the 12 o’clock position of
the cornea and excycloduction (extorsion) referring to a temporal
rotation of the 12 o’clock position. The contracting muscle is called
the agonist muscle and the relaxing muscle the antagonist. Versions
are movements of both eyes together. Versions can be classified
as follows: dextroversion for right gaze, levoversion for left gaze,
supraversion for upgaze, and infraversion for downgaze. Ductions
and versions should be tested without spectacles also.

Muscle actions and recording muscle movements29-33

To analyze the ocular movements, one must remember the muscle
action (Table 10). Moreover, the examiner should be able to elicit the
extent of movement, quality of movement and defective movement.
There are nine diagnostic positions of gaze and 6 cardinal directions
of gaze (right/up; right; right/down; left/up; left; left/down)

Table 10: Actions of extraocular muscles

Muscle Primary action Secondary Tertiary

action action

Medial rectus Adduction

Lateral rectus Abduction Adduction Intorsion
Superior rectus Elevation Adduction Extorsion
Inferior rectus Depression Depression Abduction
Superior oblique Intorsion

Inferior oblique Extorsion Elevation Abduction

Figures below show both normal and limited abduction; on a scale of
0 to -4, with-1 limitation meaning slight limitation and -4 indicating
severe limitation with inability of the eye to move past midline.
This scale can be used to measure horizontal and vertical ductions.
Similarly, abnormal versions can be noted on a scale of +4 to -4 with
0 indicating normal and +4 indicating maximum overaction, whereas
-4 indicates severe under action. Ductions are denoted as a plus sign
and versions in cardinal positions. A gross estimate can be that in

28 | Basic Examination of Strabismus

maximal adduction an imaginary vertical line through the lower
lacrimal punctum should coincide with a boundary line between the
inner third and the outer two thirds of the cornea. If more of the
cornea is hidden, the adduction is excessive. If more of the cornea
is visible on maximal adduction and if some of the sclera remains
visible, adduction is defective. If abduction is normal, the corneal
limbus should touch the outer canthus. If the limbus passes that
point and some of the cornea is hidden, the abduction is excessive.
If some of the sclera remains visible, abduction is defective.

Left lateral rectus underaction-1

Left lateral rectus underaction-2

PRIMARY GAZE Left lateral rectus underaction-4

Figure 11: Grading of limitation in abduction

Graphic representation of the versions and duction is by making a
plus sign and a cross as shown in the figure below.

Figure 12: Grading of inferior oblique overaction
Basic Examination of Strabismus | 29

Ductions Versions

We can also keep a photographic record of the ocular movements
which sometimes shows subtle limitation of movements.

Saccadic system

A saccade is a rapid eye movement between two points in space. The
purpose is to place the image on the fovea as rapidly as possible.
Accuracy of saccades establishes at 2 months of age.

Horizontal saccades are tested by asking the patient to look into
observer’s nose, then to a similar target held at eye level on one side,
back to the observer’s nose and then to an equidistant target on the
other side. Movement should be repeated several times and targets
held within the limits of gaze. Horizontal as well as vertical saccades
should be tested. The speed of the saccades as well as accuracy should
be compared with the other eye. Increase in saccadic velocity may
be seen in recovering cases of palsies often seen clinically before it
becomes apparent on electrooculography.

Smooth pursuit system

Smooth pursuit movement is a slow eye movement which tracks the
path of an object. This reflex usually develops by 6 weeks and is
well established by 3-4 months of age. It is tested by asking the
patient to watch a moving target from primary position to other
eight positions. Patients head should be kept straight, and depression
should be tested first without raising the lids so that lid abnormalities
can be picked up. We can pick up underaction, overactions, quality
of movement, difference in ductions and versions, and nystagmus.
The presence of a cyclodeviation can also be picked up by looking at
the position of conjunctival blood vessel.

30 | Basic Examination of Strabismus

Vestibular testing

There are some conditions where the vestibular system should
also be tested. In ill and uncooperative patients where movements
cannot be tested by other means, we can test indirectly by vestibular
system. For example, the dolls head movement in cases of infantile
esotropia. The patients head is turned fairly briskly to right and left
to elicit horizontal eye movement. Normal response is conjugate
movement opposite to the direction of head movement. It can be
done in uncooperative children also.

Vergence testing

The vergence system generates dysjugate movements hence the
Vergences move the two eyes in opposite directions.

Fusional vergences are motor responses used to eliminate horizontal,
vertical, or torsional image disparity. They can be grouped by the
following functions:

1. Fusional convergence eliminates bitemporal retinal disparity
and controls an exophoria.

2. Fusional divergence eliminates binasal retinal disparity and
controls an esophoria.

3. Vertical fusional vergence controls a hyperphoria or hypophoria.
4. Torsional fusional vergence controls incyclophoria or

excyclophoria.

Fusional vergences can be measured by using a haploscopic device
(major amblyoscope), a rotary prism, or a bar prism, and gradually
increasing the prism power until diplopia occurs34-37. Accommodation
must be controlled during fusional vergence testing.

Convergence testing

Accurate convergence is well developed by 2-3 months of age.
Proximal convergence is tested by nearness of the object and tonic
convergence is generated by resting tonus of the medial rectus.
Comparison of the alignment in the primary position at both distance

Basic Examination of Strabismus | 31

and near fixation helps assess the accommodative convergence
(synkinetic near) reflex. The near point of convergence is determined
by placing a fixation object at 40 cm in the midsagittal plane of
the patient’s head. As the subject fixates on the object, it is moved
toward the subject until one eye loses fixation and turns out. The
point at which this action occurs is the near point of convergence.
The eye that can maintain fixation is considered to be the dominant
eye. The normal near point of convergence is 8-10 cm or less.
This determination does not distinguish between fusional and
accommodative convergence, and any heterophoria that is present
should be considered and corrected algebraically.

Table 11 with normal fusional amplitude values

1. Normal fusional amplitudes

Convergence Fusional amplitude 15-20 PD (D) and 35-40
PD (N)

Divergence fusional amplitude 6-8 PD (D) and 8-10 PD
(N)

2. Near point of convergence 5-10cm
(In adults)

3. Accommodative amplitude

8 y 13.8 D

12 y 12.9 D

16 y 12 D

22 y 11 D

23 y 10

4. SUPPLEMENTARY TESTS

4.1 Measuring AC/A Ratio

Accommodative convergence to accommodation ratio (AC/A ratio)
should be measured in any case with near distance disparity in the
deviation. So, once we have done the PBCT and we find a gross
difference in the near and distance measuements (>10 PD); one
must calculate the AC/A ratio.38-42 Accommodation is the increase

32 | Basic Examination of Strabismus

in lens power to clearly focus at near. Accommodation is measured
in diopters as is the reciprocal of the fixation distance in meters. On
the other hand, convergence is the reciprocal of the fixation distance
in meters times the interpupillary distance in centimeters because
farther apart are the eye; more convergence is needed to fixate at
a point. AC/A ratio is the amount of accommodative convergence
exerted in response to one unit of accommodation.

A high AC/A ratio means the eyes over converge for a given amount
of accommodation (eso-shift at near), whereas a low AC/A ratio
means there is under convergence per diopter of accommodation
(exo-shift at near). An individual’s AC/A ratio is usually constant
throughout life except in presbyopia when an increase might be
noticed.

Normally AC/A ratio for the heterophoria method and lens gradient method is
4:1 or lower and 5:1 and ratios of 6:1 or more are considered high.

Measurement of AC/A ratio

Prerequisites:

1. Use accommodative targets
2. Patient should wear full optical correction.
3. 6 m (20 ft) is used for distance and 1/3 m (14 in.) for near.
4. For calculations of the AC/A ratio, esodeviations are represented

as positive numbers and exodeviations as negative numbers.

It can be measured by

1. Heterophoria method
AC/A ratio=IPD+N-D/DA
IPD is interpupillary distance (cm), D is distance deviation (PD),
N is near deviation (PD), and DA is diopters of accommodation
for near fixation (e.g.:1/3 m = 3 diopters)

Basic Examination of Strabismus | 33

Example: Measurement of AC/A ratio by heterophoria method

Distance = ET 41 Near = ET 50
Interpupillary distance= 52 mm
Nearest target distance 1/3 m = 3 D accommodation

AC/A = 5.2 + (50-41)/3=8.2 (it is a high AC/A ratio)

2. Lens gradient method

This method is based on the principle that a plus lens relaxes
accommodation so that with less accommodation there is less
convergence. A minus lens causes increased accommodation,
increased convergence, and an eso-shift. Hence it is calculated
by measuring the deviation at a set distance, with and without
supplemental spherical lenses. We usually use concave lenses at
6 m and convex lenses at 33 cm.

AC/A ratio=(Deviation without lens - Deviation with lens)/Lens
in diopters

Measuring AC/A ratio by lens gradient method

Deviation without lens = XT 6
Deviation with- 3.00 lens = ET 15

AC/A = (-6-15)/3=7

The lens gradient method is often considered as the method of
choice because it excludes tonic as well as proximal convergence,
measuring only accommodative convergence. It is often useful
to start with a 1D lens and gradually increase it.

3. Distance near method

Clinical distance–near relationship is a simple method for
identifying or screening patients with a high AC/A ratio. A
distance–near difference within 10 PD is considered normal
whereas differences greater than 10 PD are considered high.

4. Other methods include Graphic method used with the
synaptophore and fixation disparity method. Graphic method
provides the speed of the response in addition to the measurement.

34 | Basic Examination of Strabismus

Fixation disparity method has an advantage that the eyes are not
dissociated. But both methods are time consuming.
5. CA/C Ratio: Defective convergence can be measured by the
CA/C Ratio with a reciprocal relationship with AC/A Ratio. It is
measured by calculating the refraction of eyes before and after
convergence is initiated by a base out prism. Normal is 0.5 D to
1-meter angle.39

4.2 Torsion Measurement

Objective torsion by fundus photography43-45
Objective torsion can be recorded by assessment of the fovea disc
relationship. In normal patients, the fovea is located between the
midpoint and the lower border of the optic disc, around 0.3 disc
diameters below a horizontal line drawn through the centre of the
optic disc. Patients with torsion will have a shift in the position
of the fovea relative to the optic disc. With extorsion, the fovea is
shifted below the inferior border of the optic disc (figure below),
whereas intorsion shifts the fovea higher than the midpoint of the
optic nerve. The view is reversed with an indirect ophthalmoscope
so that in extorsion fovea is above the upper pole of the disc, and in
intorsion fovea is below the midpoint of the disc.

Figure 13 illustrating right eye extorsion. Normally a line passing through the fovea
passes through the inferior one third of the disc. Note the position of disc above the

fovea in the right eye.

Basic Examination of Strabismus | 35

Subjective torsion on Maddox rod test
It tests subjective torsion. Patients with retinal intorsion view the
world as being extorted, and retinal extorsion cause objects to be
perceived as being intorted. This test can be done with a single
lens (Single Maddox rod test) or a lens over each eye (Double
Maddox rod test). In a single Maddox rod test, the patient is shown
a light source with a single Maddox rod in front of one eye. The
patient reports the line from the Maddox as a tilt in the presence of
a cyclovertical muscle palsy. In the double Maddox rod test, two

Maddox rod showing objective extorsion

Maddox rods are placed in trial frames, one before each eye, with the
cylinders vertical, resulting in horizontal line images when viewed
at 6 m. Alternatively the rods can be placed vertically or one vertical/
one horizontal so that the images are perpendicular to each other.
With the double Maddox rod test, the patient is asked to make the
two streaks of the Maddox rod parallel. Patients without torsion see
parallel lines, those with intorsion see the 12 o’clock position turned
nasally and those with extorsion see the 12 o’clock position turned
temporally. Although we can determine the subjective torsion quiet
accurately with the double Maddox rod test; it doesn’t localize the
abnormal eye.
Other methods to measure cyclotropia include Bagolini straited
glasses and amblyoscope.

36 | Basic Examination of Strabismus

4.3 Parks three step and Bielschowsky head
tilt test

Cyclovertical muscle paralyses are often responsible for
hyperdeviations. The 3-step test is an algorithm that can be used
to help identify an isolated paretic cyclovertically acting muscle. It
thus simplifies the diagnosis of vertical muscle palsies

Fallacies
1. It is not always diagnostic and can be misleading, especially in

patients in whom more than one muscle is paralyzed,
2. In patients who have undergone strabismus surgery
3. In the presence of a skew deviation
4. In the presence of restrictions or dissociated vertical deviation

Steps
There are 8 cyclovertically acting muscles:

The 2 depressors of each eye are the inferior rectus (fR) and superior
oblique (SO) muscles; the 2 elevators of each eye are the superior
rectus (SR) and the inferior oblique muscles.
1. Identify the hypertropic eye
2. Identify vertical separation greater in right/left gaze
3. Identify vertical separation greater in right/left sided tilt .

Step 1
Determine which eye is hypertropic
by using the cover·uncover test.
Step 1 narrows the number of
possible underacting muscles from
8 to 4. for example, if the right eye
has been found to be hypertropic;
it means that the paralysis will be
found in either the depressors of
the right eye (RIR, RSO) or the
elevators of the left eye (LIO, LSR).
Draw an oval around the 2 muscle
groups found affected.

Basic Examination of Strabismus | 37

Step 2

Determine whether the vertical deviation is greater in right gaze or
in left gaze by alternate cover test in dextroversion and levoversion.
In the example, the deviation is larger in left gaze. This implicates
one of the 4 vertically acting muscles used in left gaze. Draw an oval
around the 4 vertically acting muscles that are used in left gaze. At
the end of step 2, the 2 remaining possible muscles (one in each eye)
are left.

Step 3

Known as the Bielschowsky head· tilt test, the final step involves
tilting the head to the right and then to the left shoulder during
distance fixation. Head tilt to the right stimulates intorsion of the
right eye (RSR, RSO) and extorsion of the left eye (LIR, LIO). Head
tilt to the left stimulates extorsion of the right eye (RIR, RIO) and
intorsion of the left eye (LSR, LSO). In the example, when the head
is tilted to the right, in order to maintain fixation, the right eye must
intort and the left eye must extort. Because the right superior oblique
is weak, the vertical action of the right superior rectus is unopposed.
Contraction of this muscle in an attempt to incycloduct the eye
results in an upward movement of the right eye, thus increasing the
vertical deviation.

One should be careful to ensure patient fixates with the other eye
when one has been covered. The third step provides useful evidence
before deciding surgery in a patient with cyclovertical muscle palsy.46
For example, inferior oblique weakening has been found to produce
a negative response to this test. According to Jampolsky, DVD gives
opposite results to that of a superior oblique palsy on head tilt test.
It produces right hypertropia on left tilt and left hypertropia on right
indicating a bilateral DVD.

5. TESTING BINOCULAR FUNCTION47-52

The term binocular single vision is used to mean the simultaneous
use of two eyes to give a single mental image in normal conditions
of seeing. Binocular vision is the simultaneous perception of two

38 | Basic Examination of Strabismus

images, one from each eye. Worth described three grades of binocular
vision as

1. Simultaneous perception
2. Fusion
3. Stereopsis

Binocular visual acuity is not visual acuity with two eyes alone, but
maximum visual acuity while maintaining binocular single vision.
Hence in the presence of strabismus , the term binocular visual
acuity is not applied.

Tests to determine the sensory status of the patient are usually
subjective. Some useful definitions before we discuss the sensory
system are as follows:

1. Diplopia should be differentiated from confusion. Where
diplopia is perception of one object as two; visual confusion is
the simultaneous perception of two different objects projected
onto corresponding retinal areas. The two foveal areas are
physiologically incapable of simultaneous perception of
dissimilar objects. The closest foveal equivalent is retinal rivalry,
wherein the two perceived images rapidly alternate. Confusion
may be a phenomenon of nonfoveal retinal areas only. Clinically
significant visual confusion is rare.

2. Normal retinal correspondence is when the two foveae have a
common visual direction. Besides patients with no strabismus,
patients with intermittent deviations, and constant large angle
strabismus may also have normal correspondence. Anomalous
retinal correspondence (ARC) can be described as a condition
wherein the fovea of the fixating eye has acquired an anomalous
common visual direction with a peripheral retinal element in
the deviated eye; that is, the two foveae have different visual
directions. ARC is an adaptation that restores some sense of
binocular cooperation. Suppression precedes the development
of ARC. It is seen in esotropia most commonly between 20-25
PD. It is rarely found in exotropia because the angles are never
that small but invariably found in microtropia.

Basic Examination of Strabismus | 39

3. ARC is of two types: a)Harmonious ARC: Angle of anomaly
is equal to the angle of strabismus and therefore the subjective
angle is zero. b) Unharmonious ARC: The subjective angle is not
equal to the objective angle of strabismus. The subjective angle is
greater than zero but less than the objective angle of strabismus.
The fovea of the fixing eye acquires a correspondence with
external stimuli of the deviated eye but not at the exact objective
angle of deviation (as in harmonious ARC). The pseudo fovea is
at a point, less than the objective angle of deviation. The patient
with unharmonious ARC has learned to use their eyes at a less
crossed angle in an attempt toward binocularity (Objective
angle as the amount of deviation measured by the examiner and
neutralized so that the image of the fixation object falls on the
fovea of each eye, such as the alternate cover and prism test
or alternate cover test on the amblyoscope. Subjective angle is
the amount of correction in prism diopters in which the patient
informs the examiner that he can superimpose and fuse the
images of the test object).

4. Paradoxical diplopia can occur when ARC persists after surgery.
When esotropic patients whose eyes have been set straight
or nearly straight report, postoperatively, a crossed diplopic
localization of foveal or parafoveal stimuli, they are experiencing
paradoxical diplopia.

5. Refractive error, visual acuity, fixation pattern and strabismus all
alter the state of binocular vision. Large esotropias will usually
have normal retinal correspondence. Anomalous correspondence
usually develops in small angled esotropia. Intermittent
exotropes may have a dual system of retinal correspondence,
abnormal when the exotropia is manifest and normal when it
is controlled. Abnormal retinal correspondence is rarely seen in
vertical strabismus for reasons unknown.

Simultaneous perception can be checked on the synaptophore.
Bagolini tests and worth four dot test diagnose the presence of
sensory fusion. Tests for stereopsis are discussed separately.

40 | Basic Examination of Strabismus

5.1 After Image Test

This test is based on comparison of the visual directions of the fovea.
Bielschowsky applied this test on a large scale to the examination
of patients, and the afterimage test has become one of the most
widely used tests for assessing retinal correspondence. The test is
easily performed by using a camera flash. The idea is to produce
a vertical afterimage in one eye and a horizontal afterimage in the
other eye. The reflecting surface is covered with black paper to
expose a narrow slit, the center of which is covered with tape and
serves as a fixation mark, thus protecting the fovea from exposure.
The resulting negative afterimage is that of a line with a break in
its middle, which represents the fovea. The patient is required to
fixate steadily the central mark, first with one eye while the slit
is in a horizontal position, and then with the other eye while the
slit is in a vertical position. After exposure patient perceives two
lines with a gap in the middle. These gaps will be superimposed
in a patient with normal retinal correspondence. If the vertical
afterimage with its central hole appears to the left or to the right
of the hole in the horizontal afterimage, this displacement implies
that the two foveae have different visual directions, and there is
anomalous correspondence. Note that this test cannot be used in
eccentric fixators and in uncooperative child especially below 7/8
years.

L LL
R
R R

Figure 14: Results of after image test
Basic Examination of Strabismus | 41

5.2 Bagolini Striated Glasses

These glasses are plano glasses without refractive power that do not
modify the state of accommodation. They have fine parallel linear
striations that do not significantly alter the visual acuity and the
perception of the visual space. The patient fixates at a small light,
through the striated glasses placed before each eye in a trial frame.
They are placed in a trial frame; one is set at 45° and the other at
135° so that the line images are seen obliquely as the patient fixes a
light. A person with single binocular vision will see a cross passing
through the light. The test can be done at 6 m, 33 cm or any desired
gaze position. The test should be done in normal lighting conditions.
Interpretation: 1. Crossing of the lines when a manifest ocular
deviation (cover test) is present indicates anomalous retinal
correspondence (ARC). 2. Only one-line visible means suppression
of the other eye 3. ET with NRC (fixation light above), XT with
NRC (fixation light is seen below). Broken line in the centre implies
Fixation point scotoma (with manifest deviation and ARC) or foveal
scotoma (with orthophoria and normal retinal correspondence) of
the right eye. See figure below.

Figure 15 Interpretation of Bagolini glasses test

42 | Basic Examination of Strabismus

5.3 Worth Four Dot Test

This test is used to assess the fusional
potential of the eye. it is based on
complimentary colours hence is done by
using red green glasses with red in front
of the right eye. it is not intended as a
dissociation test, but some dissociation
does take place. Four circular lights area
presented on a black/grey background.
When viewed through the red and green glasses, the single red and
white lights are seen through the red glass, and two green lights as
well as white light through the green glass. It is made for 6 m, 0.5 m
and 33 cm. Patient is asked how many dots/lights he/she can see. The
interpretation of this test is as follows:

A) The patient sees all the four dots at any distance deviation
indicates fusion is present i.e. Normal binocular response with
no manifest deviation.

B) The patient sees five dots it indicates diplopia. Uncrossed
diplopia with esotropia, red dots appear to the right and crossed
diplopia with exotropia (red dots appear to the left of the green
dots). Prisms can be used to see if four dots become one.

C) The patient sees three green dots, suppression of right eye.

D) The patient sees two red dots, suppression of left eye.

When testing a patient for monofixation syndrome, the Worth 4-dot
test can be used to demonstrate both the presence of peripheral fusion
and the absence of bifixation. The standard Worth 4-dot flashlight
projects on to a central retinal area of 1° or less when viewed at 10
feet, well within the 1°-4° scotoma characteristic of monofixation
syndrome. Therefore, patients with monofixation syndrome will
report 2 or 3 lights when viewing at 10 feet, depending on their ocular
fixation preference. When the Worth 4-dot flashlight is brought closer
to the patient, the dots begin to project on to peripheral retina outside
the central monofixation scotoma until a fusion response (4 lights) is
obtained. This usually occurs between 2 and 3 feet.

Basic Examination of Strabismus | 43

Note that A fusion response (the patient sees all four dots in a
rectangular arrangement) may occur in the presence of heterotropia
with anomalous retinal correspondence and may be misinterpreted.

5.4 The Four-Prism Diopter Base-Out Prism Test

The four-prism diopter base-out prism test is of some value in
determining whether a patient has bifoveal (sensory) fusion or a
small suppression scotoma under binocular conditions. It quickly
determines state of retinal correspondence. In this test; a four-prism
diopter base-out prism (when suspecting microtropia and vice a
versa) is held before one eye while the patient fixates on a penlight
and the observer notes the presence or absence of movement of the
fellow eye. Presence of movement in the fellow eye in the form
of a biphasic movement response in an orthotropic patient usually
indicates bifoveal fusion. Absence of a corrective eye movement
indicates microtropia due to a small central or paracentral scotoma.

5.5 Tests For Stereopsis

Stereopsis occurs when the two retinal images, slightly disparate
because of the normally different views provided by the horizontal
separation of the two eyes, are cortically integrated. There are two
types of stereopsis tests: contour and random dot. Contour stereopsis
tests involve actual horizontal separation of the targets presented to
each eye (with polarized or red-green glasses) such that monocular
clues to depth are present at lower stereoacuity levels. Random-
dot stereopsis tests circumvent the problem of monocular clues
by embedding the stereo figures in a background of random dots.
These stereograms have geometrical shapes, which are identical but
displaced. The random dots are invisible uniocularly. TNO test and
Frisby test use random dot stereograms.

a. Lang two pencil test for qualitative assessment

It is a method to test for gross stereopsis (3000-5000 secs of
arc). It is a simple test that requires two pencils and a cover. The
patient is given a pencil and is asked to place its base on the base
of another pencil held by the examiner while keeping both eyes

44 | Basic Examination of Strabismus

open. The pencils are held vertically in case of Lang’s method
and horizontally in case of Reinecke’s method. The squinting eye
is covered and the response re checked. The response if equally
good with eyes open and squinting eye covered means binocular
single vision is present. If the patient misjudges the distance
of the examiners pencil with the squinting eye covered but no
difficulty with both eyes open, means ARC is present. Retesting
with this test makes the patient better, so earliest responses are
the most significant.
b. Titmus fly test
The targets are presented as vectographs and the patient has to
wear polarized glasses. In this test, a booklet with superimposed
images of a fly is shown to the patient. Ability to detect the
elevation of the fly’s wings above the plane of the card indicates
gross stereopsis (3000 secs of arc). In the absence of gross
stereopsis, the fly appears as a flat object in the book. In other
figures on the card including circles and animals, there is less
separation between the superimposed images and hence detects
finer stereopsis. If gross stereopsis is present, we should do the
animal test. The test consists of three rows of five animals each
testing 400, 200 and 100 secs of arc. The patient is asked which
of the animals stands in the line. In the absence of stereopsis,

Basic Examination of Strabismus | 45

patients point to a heavily painted animal instead of the disparate
one. The third set of images are circle testing 800 to 40 secs of
arc. Lowest level of steroeoacuity is the fifth circle (100 secs of
arc).
If only the fly is seen, the booklet should be rotated by 180
degree and the answers re confirmed. Monocular clues have
some influence so patient can answer even in the absence of gross
stereoacuity. Overall the test is quite simple to use. However, it
is considered inferior to random dot tests.
c. TNO test
This test is done with dissociating red green glasses, has no
monocular cues but it underestimates the stereoacuity. It consists
of random computer-generated dots printed as red and green
anaglyphs. It is carried out at 40 cm. It tests stereopsis between
480 to 15 secs of arc. It consists of a booklet with several plates
with various shapes. The first three pictures test gross stereopsis
and then finer levels are tested with the next four.

d. Frisby Test
This test also uses random shapes without displacement. The
test is three dimensional and hence des not require glasses. The
disparity is created by thickness of three plates which can be
varied by changing distance of viewing. It can test between 600
to 15 secs of arc stereoacuity.

The ability to determine the presence of fusion potential by
sensory testing may be limited by the patient’s age and cognitive

46 | Basic Examination of Strabismus


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