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196 Theory and Practice of Squint and Orthoptics
deprivation amblyopia and 8 years for anisometropic amblyopia. Patients in anisometropic
amblyopia respond to treatment even in the
teenage, while the strabismic amblyopes do not
respond after 12 years. However, recent reports
show that all types of amblyopes respond to
dichoptic stimulation and computerized vision
therapy. If the disruption in visual input occurs
during this period, the brain's ability to develop
normal visual pathways can be compromised.
iv. Neurotransmitter imbalance: Studies suggest
that amblyopia might involve alterations in the
balance of neurotransmitters and their receptors
in the visual cortex.
CLINICAL CHARACTERISTICS AND
LABORATORY FINDINGS IN AMBLYOPIA
1. Visual acuity. Amblyopia, by definition,
refers to a partial loss of sight in one or both
eyes in the absence of ophthalmoscopic and
other marked objective signs. It has been
recommended that a difference of two lines on
a visual acuity chart should be there to diagnose
amblyopia. However, strictly speaking, any
difference between the two eyes especially in
strabismic amblyopia should be considered
significant.
Certain clinical characteristics associated with
visual acuity in patients with amblyopia are as
follows:
i. Recognition acuity (Snellen's or similar
charts) is more affected than the resolution acuity
(Teller's chart or VER) and the detection acuity
(Catford drum test or Bailey-Hall cereal test).
ii. Snellen's acuity and grating acuity are
affected equally in anisometropic amblyopia
whereas in strabismic amblyopia, the grating
acuity is affected to half the extent of Snellen's
acuity. Thus, strabismic amblyopia is underestimated on grating test.
iii. Effect of neutral density filter. It has been
reported that when visual acuity is tested with
a neutral density filter placed in front of the
affected eye, the visual acuity improves by one
or two lines in patients with developmental
amblyopia; while in patients with organic
amblyopia, the visual acuity decreases by two
to three lines. Therefore, the neutral density filter
test has been recommended to differentiate
between developmental amblyopia and organic
amblyopia.
The neutral density filter test is based on the
fact that under photopic conditions, visual
acuity of amblyopic eye is less than that under
scotopic conditions. Since the neutral density
filter, when placed in front of an eye, produces
a state of scotopic conditions, the vision of
amblyopic eye improves.
iv. Crowding phenomenon. Crowding pheno-
menon, also known as separation difficulty, refers
to the inability of an amblyopic eye to
distinguish letters (or other symbols) crowded
together. Therefore, the vision in an amblyopic
eye is better, when tested with isolated
optotypes than when tested with line or
Snellen's acuity charts having rows of letters. In
other words, single optotype visual acuity is
better than linear visual acuity. The larger the
discrepancy between the linear and single letter
acuity, the poorer the prognosis.
Crowding phenomenon is the result of
contour-interaction between the neighbouring
test targets because of decreased lateral
inhibition in amblyopia.
2. Fixation pattern. Amblyopia may be associated
with central fixation, eccentric viewing or
eccentric fixation. In a normal eye, three
characteristics of foveolar area, which appear to
be responsible for maintaining the fixation reflex
central, are:
a. Peak visual acuity in the foveolar region,
b. A principal oculocentric direction of straight
ahead, and
c. A retinomotor value of zero.
Amblyopia with central fixation. In amblyopia
with foveolar fixation, the foveola has preserved
the principal visual direction and its zero
retinomotor value. Amblyopia is secondary to
a central suppression scotoma.
Amblyopia with eccentric viewing. In
amblyopia with eccentric viewing, patients
prefer to view with an extrafoveal point because
of the deep suppression scotoma, but the fovea
has still not lost its principal visual direction. In
eccentric viewing, patients look past the object
they have been asked to fix. This can be
197Adaptations to Strabismus and Amblyopia
demonstrated during visuscopic examination of
a co-operative patient, who will tell the examiner
that he/she is aware of the fact that he/she has
to look over to one side to see the star clearly
and when he/she looks straight ahead the
fixation target appears blurred. The examiner
can also observe that in eccentric viewing,
patient will place the image of fixation target
first on the fovea and then immediately from
the fovea on to the paramacular retinal elements.
Amblyopia with eccentric fixation. In amblyopia
with eccentric fixation, the fovea has lost its
principal visual direction, its retinal motor value
is no longer zero and an extrafoveal point is now
the bearer of these properties. Patients report
that they are looking straight at an object
stimulating non-foveolar retinal area. If the
image of an object is placed on the patient's fovea
(by means of an instrument), this object is sensed
as being in some other direction than straight
ahead.
Types of eccentric fixation. Depending upon the
retinal area with which the eyes appear to fixate,
the eccentric fixation may be of following types
(Fig. 8.11):
• Parafoveolar—just outside the foveal reflex.
• Parafoveal—outside but close to foveal wall.
• Paramacular—on or just outside the rim of the
macula. Many workers have now abandoned
the use of this term because of vague ophthalmoscopic definition of the macula.
• Peripheral—outside the macula, anywhere
between the macula and extreme retinal
periphery.
Fig. 8.11 Types of fixation pattern.
Steady versus wandering fixation. Central as
well as eccentric fixation may be steady or
wandering. Wandering fixation, which occurs
only upon covering the sound eye, must be
distinguished from the monocular, spontaneous, pendular and vertical oscillations that are
occasionally found in deeply amblyopic eyes.
This condition has been designated as the
Heimann-Bielschowsky phenomenon. It is clinically
similar to other forms of monocular nystagmus
that may occur in connection with posterior
fossa or brainstem disorders.
Paradoxical eccentric fixations. Ordinarily,
there develop nasal eccentricity in esotropic and
temporal eccentricity in exotropic patients.
However, sometimes the eccentric fixation may
be paradoxical, i.e. reverse of the expected
situation. In other words, there may be nasal
eccentricity in exotropic and temporal eccentricity in esotropic patients. Such a situation can
occur under following circumstances:
• Following surgical overcorrection of the
deviation.
• In patients with spontaneous reversal of the
deviation.
• Following prolonged occlusion of the sound
eye in amblyopia.
• With no obvious cause (rarely).
3. Absolute central scotoma. Monocular scoto-
metry on visual field charting may plot an
absolute central scotoma. Visual field charting
for this purpose should never be done
binocularly, otherwise, a facultative binocular
suppression scotoma (present only with
binocular viewing) may be mistaken as the
absolute central scotoma. The scotometry may
not be possible in amblyopic, patients with
unsteady fixation.
4. Localization of an object of regard. Locali-
zation of an object of regard is normal in patients
having amblyopia with central as well as
eccentric fixation. However, in patients having
amblyopia with eccentric viewing, localization
of an object of regard is faulty.
5. Colour vision. Colour vision anomalies may
occur in patients with amblyopia only if visual
acuity is markedly reduced below 6/36. Anomalous colour vision in such cases has been
related to peripheral eccentric fixation, i.e. a
198 Theory and Practice of Squint and Orthoptics
peripheral retinal area is being used for fixation
rather than foveola.
6. Light perception. There occurs a dissociation
of the form vision and light perception in
amblyopia, since form vision is abnormal
(especially under photopic condition) while
absolute light threshold is found to be normal.
However, differential threshold (i.e. how much
brighter the test field must be than its
surrounding so that a difference is perceived) is
elevated in amblyopia.
7. Pupillary light reflexes. Generally speaking,
pupillary light reflex is normal in amblyopes.
However, rarely in patients with a deep
amblyopia, an afferent pupillary defect may
occur. It has been suggested that perhaps the
afferent pupillary defect may result from
synaptic inhibition in the retina, since this
pathway does not reach the geniculate body.
8. Light and dark adaptation. Usually, dark
adaptation is not abnormal in amblyopes,
though a significant difference in adaptation
between amblyopic and normal eyes has been
found in the region of Kohlrausch's bend (the
kink or bend in the adaptation curve normally
produced by increased sensitivity of the rods).
9. Critical flicker frequency (CFF). It has been
reported that in amblyopia, central CFF tends
to approach the CFF of peripheral retina or of
rod mechanism. It has also been reported that
CFF is significantly faster in amblyopic eyes that
fixated eccentrically than in those with foveal
fixation. Some workers have reported that
examination of CFF with a simple apparatus is
a useful tool to distinguish reduced visual acuity
in maculopathies from amblyopia, since in the
former the thresholds are below normal.
10. Electroretinography (ERG) and electrooculography (EOG). An enormous data is
available on ERG studies in amblyopia, however, till date it has not been definitely answered
whether ERG is normal or abnormal in amblyopia. Many studies report that ERG is essentially
normal and EOG shows unsteadiness of fixation
in amblyopia.
EVALUATION AND DIAGNOSIS
Diagnosis of amblyopia is made by a reduced
best corrected visual acuity that cannot be
entirely explained on the basis of physical ocular
abnormalities. Clinical evaluation of a suspected
case of amblyopia should include the following:
1. Evaluation of visual acuity.
2. Neutral density filter test.
3. Test for crowding phenomenon.
4. Thorough ocular examination including
fundus examination.
5. Refraction.
6. Evaluation for central versus eccentric
fixation.
7. Tests for other sensory anomalies.
1. Evaluation for visual acuity. As mentioned
above, clinical evaluation of visual acuity is most
important for the diagnosis of amblyopia.
Generally speaking, a difference of two lines
between the best corrected visual acuity of the
two eyes (e.g. OD 6/6, OS 6/12 or OD 6/5, OS
6/9) is considered diagnostic for amblyopia. For
practical purposes, particularly after amblyopia
treatment has started, any acuity difference is
considered amblyopia.
Severity of amblyopia. In the "Amblyopia
treatment study (ATS) group trials, amblyopia
has been graded as below:
• Mild to moderate amblyopia is defined as visual
acuity in the amblyopic eye of 20/80 or betta.
• Severe amblyopia is defined as visual acuity in
the amblyopic eye of 20/100 to 20/400.
Methods employed to evaluate visual acuity
depend upon the age of the patient and have
been described in detail on pages 41 to 53.
However, for a ready reference, important
points for different age groups are mentioned
as follows.
Methods for evaluating visual potential in
infants and very young children (up to 2½ years
of age). Infancy and early childhood is probably
the most important age to be bothered, since it
is the most sensitive period to develop
amblyopia. At the same time, testing of vision
during this period is also not so easy. However,
untiring efforts should be made to detect
unequality of vision in two eyes. Certain useful
methods are as follows:
Fixation behaviour test. Fixation behaviour test
is a reliable and useful test in infancy to obtain
a rough estimate of visual acuity. Each eye is
199Adaptations to Strabismus and Amblyopia
covered alternately and behaviour of the infant
is noticed. If vision is equal or nearly equal in
both eyes, an infant or very young child will not
object to having either eye covered. However,
if the visual acuity is reduced in one eye, the
child will show objection (in the form of a cry or
pushing the occluder away) when the normal
eye is covered. In such cases, one should suspect
any ocular disease, high refractive error or
amblyopia.
A rough estimate of visual acuity can be made
by testing with brightly coloured toys of varying
size while occluding one eye. It is noticed
whether the child can fix and follow the toy.
Binocular fixation pattern (BFP). The binocular
fixation pattern, indicating strength of
preference for one eye or the other under
binocular viewing conditions, is generally relied
upon for estimating the relative level of vision
in two eyes for very young children with
strabismus. It is important to note that, when
the infant's binocular fixation pattern is tested,
an accommodative target such as small toy
should be used. A child with extremely unequal
vision will show great preference for the good
eye. A child with nearly equal vision will have
only mild preference for one eye. Five grades of
binocular fixation pattern described while
making the patient fix with the deviated eye
(Table 8.2).
Binocular fixation pattern test is quite
sensitive for detecting amblyopia but is
sometimes false positive (showing a strong
preference, when vision is equal or nearly equal
in the two eyes), particularly with small-angle
strabismic deviations.
Prism-induced tropia test. Prisms can be used in a
variety of ways to induce a tropia, thus allowing
the binocular fixation pattern to be assessed in
children with small angle strabismus:
• 25 dioptre base-in prism test (Cassin, 1982). A
25D base-in prism is introduced over one eye
and the child's eye preference is noticed. The
prism is then placed over the other eye and
preference is noted. This prism induces a large
esotropia that cannot be overcome by most
children and results in diplopia. Therefore, a
child with equal vision will ordinarily use the
eye without the prism to fixate regardless of
Table 8.2 Grading of binocular fixation pattern in
strabismic patients
Grade Description of response
Grade 0 : Spontaneous alternation (no pre-
ference for one eye).
Grade 1 : Holds fixation through blink (simply
prefers one eye but can use the other
eye with nearly equal frequency).
Grade 2 : Holds fixation until blink, i.e.
habitually fixing eye resumes fixation
with the next blink (moderate fixation
preference).
Grade 3 : Holds fixation for 1–2 seconds but
switches before blinks (strong
fixation preference but the other
is used briefly for fixation.)
Grade 4 : Immediately switches fixation on
removal of cover from nondeviating eye (strong fixation
pattern, and patient uses only one
eye for fixation.)
which eye is viewing through the prism. If a
child shows preference for one eye to fixate
through the prism, the nonpreferred eye is
considered amblyopic.
• Vertical prism test (induced tropia test). Ten to
fifteen dioptre vertical prism test has also been
recommended to assess eye fixation preference
by producing tropia with diplopia, similar to
25D base-inprism test.
Note that the vertical prism test rectifies the
high rate of misdiagnosis of amblyopia by
standard fixation preference testing in patients
with small-angle strabismus and monofixation
syndrome. This is because the vertical prism
breaks up the peripheral fusion and central
scotoma complex, thus allowing the patient to
fixate with either eye.
CSM method of rating monocular fixation. CSM
method has been used to describe the fixation
pattern of a too young patient for visual acuity
measurement by some workers after examination
with a handlight as follows:
• C: Stands for 'central' which refers to the fact
that angle kappa appeared equal in direction
and magnitude.
• S: Stands for 'steady' which means that fixation
is not aimless or wandering as in amblyopia
and also that nystagmus is absent.
200 Theory and Practice of Squint and Orthoptics
• M: Stands for 'maintained', meaning thereby
that there is no shift on the cover test, i.e. a
manifest squint is not present.
It has been reported that rating of monocular
fixation pattern as central, steady and maintained
provides limited information. An eye with
extremely poor visual acuity may also have
central, steady and maintained fixation. Therefore, use of CSM should be avoided, particularly,
if it replaces a visual acuity notation. Similarly
the 'maintained' is no alternative to cover and
cover-uncover test to detect manifest deviation.
Preferential looking test, optokinetic nystagmus
and visually evoked potential. These tests are
also used to measure visual acuity in infants and
very young children (for details see page 42 to
45).
Methods of estimating visual acuity in
preschool children (2½ to 4 years). Commonly
employed tests are listed below (for details see
pages 46 to 49):
• Marble game test
• Hand chart test
• Illiterate E-game test
• Allen’s preschool vision test
• Sheridan Gardiner test
• Stycar matching test
Methods of estimating visual acuity in school
children and adults (age 5 and older). Most
commonly used tests are as follows (for details
see page 50 to 52):
• Snellen's test types
• E-chart for illiterate
• Landolt's broken-C chart
2. Thorough ocular examination including
fundus examination. A thorough ocular
examination including a detailed fundus
examination is very important to rule out any
cause, other than amblyopia, of reduced visual
acuity.
3.Neutral density filter test. Whenever
possible, it is imperative to illucidate this
important characteristic of amblyopic eye—that
the amblyopic eye sees better under mesopic
conditions (between scotopic and photopic
condition). This can be tested with neutral
density filter test. For details see page 196.
4. Test for crowding phenomenon should be
performed to establish the separation
difficulties—another important feature—
exhibited by amblyopic eyes. For the detail see
page 196.
5. Refraction. The importance of a meticulous
refraction cannot be overemphasized in the
clinical evaluation of squint and amblyopia (see
page 109).
6.Evaluation for central versus eccentric
fixation. About one-half of all amblyopic eyes
are associated with eccentric fixation. The
fixation pattern can be evaluated by following
methods:
i. Angle kappa method. An idea about eccentric
fixation can be made by comparing the angle
kappa in each eye. Though it is commonly used
but comparatively less accurate method of
detecting eccentric fixation. Angle kappa can be
estimated by following methods:
• Hand light method. After occluding the non-
fixing eye, patient is made to fix a hand light
held directly below the examiner's eye to avoid
an inaccuracy due to parallax. The location of
corneal reflex is noted. The same procedure is
repeated on the other eye. The angle is positive,
when the corneal reflex is displaced nasally and
negative, when it is displaced temporally
(Fig. 8.12). A positive angle kappa of up to 5° is
physiologic in emmetropic eyes.
– In central fixation, the corneal reflex is located
in a similar position in each eye.
– In eccentric fixation, a significant difference in
the location of corneal reflex in fixing and
nonfixing eye will be noted.
It is not an accurate method. Small degree of
eccentric fixation is often missed. However, it is
the only available method of testing eccentricity
in infants.
• Arc perimeter method. In this technique, after
occluding one eye, patient is asked to fix a
centre mark on the perimeter, and a very fine
light is moved along the perimeter arc until
the light reflex is centred on the cornea.
Location of light on the perimeter arc tells the
angle kappa in degrees.
• Major amblyoscope method. Angle kappa is
measured using special slides with synoptophore (see page 146).
201Adaptations to Strabismus and Amblyopia
iv. Maxwell's spot method. Maxwell spot is a
round, dark, purplish spot of about 3 arc degrees
in diameter. It is perceived entoptically, when
the eye is exposed to a homogenous blue or
purple field. In central fixation, this spot is
centred over the fixation target. In eccentric
fixation, the Maxwell spot is displaced to the side
of fixation target by an angular amount
equivalent to the degree of eccentricity. Like
Haidinger's brushes method, this is also
sparingly used in common clinical practice.
7. Tests for other sensory anomalies. Amblyopia
may be associated with ARC. Therefore, the tests
employed for suppression (see page 183) and
ARC (see page 137) may also be required for a
thorough clinical evaluation of amblyopia.
PREVENTION AND EARLY DETECTION OF
AMBLYOPIA
Early detection as well as early intervention is
most essential for the effective treatment of
amblyopia. The best way for prevention and
early diagnosis of amblyopia is adoption of some
screening programme.
Fig. 8.12 (A) Angle kappa (OPA) is formed between the
visual axis (OF) and central pupillary line (AP). However,
clinically angle kappa (OXA) is measured at a point on the
cornea (X) that lies in the central pupillary line. (B) Angle
kappa is labelled positive when the corneal light is
displaced nasally and negative when it is displaced
temporally.
ii. Visuscope method. In most clinical practice,
visuscope or its ophthalmoscopic alternative is
the most commonly employed method for
testing eccentric fixation (see page 148). However, this technique requires patient's
cooperation and thus can be used in patients
above 4–5 years of age.
iii. Haidinger's brushes method. Patient is made
to perceive the entoptic pattern of the Haidinger
brushes and then asked to touch its centre with
a pointer. In the presence of central fixation,
patient will easily do so. However, if fixation is
eccentric, a gross error will be made and patient
will be unable, despite repeated attempts to
correct the error. Being cumbersome, this
method is not used routinely.
Vision screening examinations should start at
birth and continue as part of routine check ups
by primary care physicians.
• Acronym I-ARM (Inspection—Acuity, Red
reflex, and Motility) can be a helpful reminder
of the essential parts of a paediatric screening
examination. Table 8.3 summarizes the I-ARM
screening eye examination for neonates,
babies, and children.
• Most important test for the newborn is the red
reflex test. If an abnormal red reflex is present,
then an immediate referral to an ophthalmologist is required.
• Infant screening examination takes less than a
minute, but this brief examination is quite
powerful. If performed properly, it can detect
the vast majority of eye pathologies.
Children with risk factor for amblyopia should
have a comprehensive ophthalmic examination.
Some risk factors include:
• Family history of amblyopia or strabismus
• Childhood cataract or glaucoma
• Premature birth of lens than 30 weeks
gestation and/or less than 1500 gm weight
202 Theory and Practice of Squint and Orthoptics
Table 8.3 Screening eye examination: I-ARM
Step Neonate Babies Children
(Birth–2 months) (3 months–2 years) (3 years and older)
Inspection Symmetry face and eyes Face turn or head tilt Face turn or head tilt
Acuity Poor fixation, pupillary Good fixation and Visual acuity: Allen cards,
response smooth pursuit E-game
Red reflex Red reflex test Binocular red reflex Bilateral red reflex test
(Brückner) (Brückner)
Motility Gross alignment (70% Good alignment, light Good alignment, light reflex
small exotropia but reflex and Brückner and Brückner (any misalignesotropia probably (esotropia is abnormal ment is abnormal)
abnormal) after 2 months of age).
Brückner reflex test
The red reflex test is the single best vision
screening exam for infants and young children.
It is best performed using the Brückner
modification, which is simply a simultaneous
bilateral red reflex. Use the direct ophthalmoscope and view the patient’s eyes at a
distance of approximately 2 feet from the
patient. Use a broad beam so that both eyes are
illuminated at the same time. Dim the room
lights and have the child look directly into the
ophthalmoscope light. Start with the
ophthalmoscope on low illumination then
slowly increase the illumination until a red
reflex is seen. The examiner will observe a red
reflex that fills the pupil and a small
(approximately 1 mm) white light reflex that
appears to reflect off the cornea. The white light
reflex is actually a reflex coming from just
behind the pupil and is called the “corneal light
reflex” or the “Hirschberg reflex.” Thus, the
Brückner test gives both a red reflex and the
corneal light reflex simultaneously. Blockage of
the retinal image or large retinal pathology will
result in an abnormal red reflex.
• Cataract can either block the red reflex or
reflect light to give a white reflex.
• Retinoblastoma has a yellowish-white colour
and will produce a yellow reflex.
• Anisometropia (difference in refractive error)
will result in an unequal red reflex.
• Strabismus will cause a brighter red reflex in
the deviated eye, and the corneal light reflex
will be decentered.
Note. The key sign of a normal exam is symmetry.
TREATMENT OF AMBLYOPIA
Goal of amblyopia treatment is to maximise and
potentially normalise visual acuity.
Strategies to treat amblyopia include:
A. Elimination of amblyogenic factor
B. Correction of ocular dominance
C. Adjunct therapy
A. ELIMINATION OF AMBLYOGENIC FACTOR
Treatment of the cause of visual deprivation and
provision of clear retinal image should be done
first.
1. CORRECTION OF REFRACTIVE ERROR AND
SPECTACLE ADAPTATION
Refractive error, if any, should be fully corrected
as determined with cycloplegic refraction before
starting the amblyopia therapy.
Spectacle adoption for 3 to 4 weeks should be
tried in anisometropic amblyopia before starting
occlusion therapy. Refractive correction alone
may improve vision in many cases.
2. OCULAR MEDIA CLEARANCE
Media clearance, whenever required, is the first
step of amblyopia management.
Childhood cataract, when present should be
operated as early as possible with appropriate
aphakic correction depending upon the
circumstances. Correction of amblyopia in
congenital cataract is a challenge full of
frustration. Important guidelines for treatment of
childhood cataract are as follows:
• Significant congenital cataract should be
removed during the first 2–3 months of life.
203Adaptations to Strabismus and Amblyopia
• In symmetric bilateral cases, the interval
between operations on the first and second
eyes should not be more than 1–2 weeks.
• Acutely developing severe traumatic cataracts
in children under 8–10 years of age should be
removed within a few weeks of injury, if
possible.
• Refractive correction for aphakia following
cataract surgery in childhood must be
provided promptly with no further delay.
Severe congenital ptosis should be corrected
at the earliest.
Corneal opacity should be treated by penetrating keratoplasty.
B. CORRECTION OF OCULAR DOMINANCE
Correction of ocular dominance can be done by
stimulating the amblyopic eye with; the use of
following modalities:
I. Monocular treatment
II. Binocular or dichoptic treatment
III.Orthoptek treatment
I. MONOCULAR TREATMENT
Monocular treatment to address ocular
dominance include:
1. Occlusion therapy
Occlusion of sound eye is the most powerful
means of treating amblyopia by forcing the
patient to use amblyopic eye. Occlusion therapy
has been the mainstay of treatment since 18th
century.
Methods of occlusion
Occlusion can be accomplished by an adhesive
patch on skin, gauze pad and tape, use of
Doynes rubber occluder which can be stuck to
spectacle lens, opaque contact lenses, adhesive
tape on glasses or any method that excludes the
use of occluded eye (Fig. 8.13). Adhesive skin
patch is the best method. However, problem
may arise in children with sensitive skin. If
application of tincture of benzoin before the
patch is applied on the skin also does not help,
Fig. 8.13 Methods of occlusion: (A) Elastoplast orthoptic patch; (B) Slip on nose pad; (C) Spectacle mount orthoptic eye
patch; (D) Doyne's rubber occluder; (E) Ground glass occluder; and (F) Smart glasses for occlusion therapy.
204 Theory and Practice of Squint and Orthoptics
then other methods may be tried as a substitute
for a patch.
Programmable electronic glasses are liquid
crystal display (LCD), which can be programmed
to turn opaque, occluding vision in the left or
right eye for different time intervals, acting like
a digital patch that flickers on and off. Amblyz™
occlusion glasses were used for 4 hours daily in
a study, where the lens over the eye with better
vision switched from clear to opaque every 30
seconds, with good results.
Direct versus inverse occlusion
Direct occlusion refers to occlusion of the sound
eye and inverse or indirect occlusion refers to
occlusion of the amblyopic eye. Previously,
many workers recommended that in the
presence of eccentric fixation, first one should
occlude the amblyopic eye for some time, so that
the eccentric fixation becomes less fixed.
However, after long observations, now only
direct occlusion is recommended even in the
presence of eccentric fixation as discussed above.
Full-time versus intermittent (part-time) occlusion
Full-time occlusion involves placing the
occluder over the eye as soon as the child gets
up in the morning and removing only after the
child goes to bed at night. Earlier, constant and
total occlusion was considered the choice for
initial treatment of amblyopia. Standard
teaching has been that children need to be
observed at intervals of 1 week per year of age,
if undergoing full-time occlusion to avoid
occlusion amblyopia in the sound eye. A
simplified schedule for initial treatment of
amblyopia previously recommended with fulltime occlusion is shown in Table 8.4.
Part-time (intermittent) occlusion involves use
of the occluder for a short time each day.
Amblyopia treatment studies (ATS) have
demonstrated:
• In children aged 3–7 years with severe amblyopia
(visual acuity between 20/100 and 20/400),
full-time patching produced a similar effect
to that of 6 hours of patching per day.
• In children aged 3–7 years with moderate
amblyopia (visual acuity better than 20/100),
2 hours of daily patching produced an
improvement in visual acuity similar to that
of 6 hours. In this study, patching was
prescribed in combination with 1 hour of near
visual activities.
• In children aged from 7 years to younger than 13 years,
prescribing 2–6 hours a day of patching can
improve visual acuity, even if the amblyopia
has been previously treated.
• In patients aged from 13 years to younger than
18 years, prescribing 2–6 hours a day of
patching might improve visual acuity, when
amblyopia has not been previously treated;
however, this is likely to be of little benefit, if
amblyopia was previously treated with
patching. Long-term results from these studies
are still pending.
How to go about occlusion
Compliance is the keyword of success in
occlusion therapy and should be ensured by
motivating the child and parents. The initial
phase is the uphill route; once the near vision
and then the distance vision start improving, the
task is easier.
How long to continue occlusion is decided as
below:
• In patients with improvement in vision, assessed
at monthly follow-up visits, the occlusion
should be continued till the amblyopic eye has
not only developed equal vision but also equal
Table 8.4 A simplified schedule for initial occlusion therapy for amblyopia
Age of the patient (in years) Period of occlusion (days) Follow-up after every
Direct : Inverse
Up to 2 2 : 1 15 days
3 3 : 1 15 days
4 4 : 1 1 month
5 5 : 1 1 month
6 and older 6 : 1 1 month
205Adaptations to Strabismus and Amblyopia
preference of fixation compared to the normal
eye. On an average, it may take 3–6 months,
depending upon the age of the patient and
initial level of vision. The younger the patient
the better is improvement in the visual acuity,
when the occlusion is started, and the shorter
is the duration of occlusion required.
• In patients with no improvement with occlusion
on three consecutive monthly follow-up visits,
further occlusion is unlikely to be fruitful.
However, it is essential to once again rule out
any organic disease and carefully recheck the
refraction. Incomplete response to occlusion
tends to be associated with anisohypermetropia and anisoastigmatism.
Maintenance occlusion treatment
• Once the vision has been equalized, the
maintenance occlusion should be continued
till the amblyogenic age, i.e. up to at least
9 years of age and sometimes even till the child
has reached early teens.
• Maintenance occlusion is accomplished by a
part-time occlusion for 2–3 hours in a day with
active vision exercises at home.
2. Penalization
Penalization can be used as an alternative when
occlusion is not possible.
Principle. The word penalization literally means
to punish or to inhibit. The principle is to force
the amblyopic eye to a greater use for distance,
near or both by penalizing the sound eye for
distance, near or both with the help of glasses
and a cycloplegic drug.
Prerequisite to penalization is that the eyes
should be straight and hence is best used in
anisometropic amblyopia without deviation or
with deviation after it has been corrected or
surgically with prisms.
Indications. In the past, penalization therapy
was reserved for children who would not wear
a patch or in whom compliance was an issue.
The amblyopia treatment studies, however,
have demonstrated that atropine penalization
in patients with moderate amblyopia (defined
by the study as visual acuity better than 20/100)
is as effective as patching. The amblyopia
treatment studies were performed in children
aged 3–7 years. Further, penalization may also
be used for maintaining vision obtained through
previous occlusion therapy.
Methods. Penalization can be done by two
methods: Atropine penalization and optical
penalization. (Note. Atropine penalization is a
stronger method.)
i. Atropine penalization of non-amblyopic eye
a. Near penalization. It is most commonly used.
For near penalization, the fixing eye is
atropinized and fully corrected for distance
vision, while amblyopic eye is overcorrected
with +2.0 to +3.0D. This forces the amblyopic
eye to be used in near vision and stimulates
alteration of two eyes for near and distance
fixation. Alternately, sound eye is atropinized
and 1% pilocarpine is used in amblyopic eye.
Pilocarpine gives pinhole effect due to
constriction of pupil and improves near fixation
due to facilitation of accommodation.
b. Distance penalization. Fixing eye is
atropinized and overcorrected by +3.0D lens
while the amblyopic eye is fully corrected. In
this way, sound eye is penalized for distance and
is used for near only, while amblyopic eye is
used for distance.
c. Total penalization. Fixing eye is atropinized
and undercorrected by 4.0 to 5.0D, while
amblyopic eye is fully corrected. This prevents
the fixing eye being used for near as well as
distance.
ii. Optical penalization
Optical penalization is based on over-plussing
(prescribing more plus sphere than needed) the
sound eye to force fixation to the amblyopic eye
for distance targets; the patient will usually use
the sound eye for near targets. Optical
penalization works well for mild amblyopia;
however, some children will look over the tops
of their glasses to use their sound eye.
II. DICHOPTIC STIMULATION THERAPY (BINOCULAR
APPROACH FOR AMBLYOPIA TREATMENT)
Amblyopia is a binocular problem caused by
active suppression that converts structurally
intact binocular system into a functionally
monocular system. So, treating unilateral