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206 Theory and Practice of Squint and Orthoptics
amblyopia with a binocular approach using
dichoptic stimuli is considered by many
workers. The dichoptic treatment presents a
stimulus to each eye separately and the brain is
forced to integrate the images into a single
perception. In these treatment modalities; under
binocular conditions, the signal strength coming
into the patient’s good eye is reduced enough
so that it cannot suppress the amblyopic eye.
The result is binocular perception in a patient
with otherwise deep suppression of the
amblyopic eye. Over time, the viewing
conditions are changed and the image seen by
the good eye is suppressed less until both eyes
see approximately the same image. There are
many popular softwares that are developing
nowadays promoting binocular vision therapy
with 3D gaming and at the same time
stimulating perceptual learning. These are
believed to treat amblyopia not only in children
but also in adults.
Examples of dichoptic stimulation therapy
1. ‘Falling Blocks game’ on an iPad. Hess, et al
have developed this antisuppression therapy as
dichoptic stimuli.
2. Red-green glasses with reduced contrast for
the non-amblyopic eye. Li, et al treated
amblyopia with this dichoptic stimuli,
technique.
3. AmblyGo: Dichoptic amblyopia therapy by
BYNOCS AmblyGo (Bynocs India) is a vision
therapy software programme to treat amblyopia
in the form of tailored computer games. The
patient plays the games with both eyes open
while wearing special 3D anaglyph glasses (with
different colour lenses) for 30 minutes over 30
sessions.
Indications, inclusion and exclusion criteria
• Indications includes Anisometropic, Iso-
ametropic and Strabismic amblyopia (only
after strabismus correction)
• Inclusion criteria. Patient’s age, 5 years and
above without upper age limit, must have
foveal fixation in both eyes, must wear the full
refractive correction, should not receive
occlusion treatment at the same time
(monocular treatment), and must have the
cognitive and motor skills capable of
understanding and carrying out the training
and be able to use a computer.
Guarded prognosis: In patients with microtropia/monofixation syndrome.
• Exclusion criteria: Patients with eccentric
fixation and those with any ocular comorbidity
like nystagmus, retinal pathology, glaucoma
Mechanism of working of AmblyGo This vision
therapy software, presents the contrast-adjusted
images to the two eyes, with the dominant eye
receiving an image of lesser contrast than the
amblyopic eye. Possibly, for the first time ever,
the brain receives an equal image from the two
eyes during the therapy, encouraging it to use
both eyes at the same time.
This contrast differentiation removes the
interocular suppression in the ocular dominance
columns of the amblyopic eye, resulting in a
sustained visual gain. The neuroadaptive
treatment is non-invasive, safe, and very
effective.
Results/outcome of AmblyGo therapy The
results show dichoptic therapy is very effective
in improving visual acuity and stereopsis even
in adult amblyopia, especially when caused by
anisometropia.
III. ORTHOPTEK FOR AMBLYOPIA TREATMENT
Principle: Orthoptek is a new device to treat
amblyopia by correcting ocular dominance. It
is hypothesized by the inventor that orthoptek
treatment is based on the stimulation of
peripheral retinal M cells. These M cells, when
stimulated, activate the attention area 7A in the
parietal cortex and the frontal eye field (FEF)
area in the motor cortex. This activation sets off
a cascade of top-down impulses originating
from the parietal and frontal cortex, which can
be detected as beta oscillations on an EEG. For
the treatment to be effective, these top down
impulses need to reach the occipital cortex layer
4C beta within 150 milliseconds of the arrival of
the retinal impulses.
Orthoptek device. It is like a TV screen, 20 × 20
inch in size, which is mounted on the wall for
use (Fig. 8.14). It has 3 rows of 3 LED lights each.
Central light is green in colour and the
Fig. 8.14: Hand-eye coordination exercises for perceptual
learning.
remaining 8 are red in colour. Each LED glows
for 150 millisecs and the next light comes on in
a random sequence.
Procedure: Patient is made to sit in front of
device at a distance of 1 meter with the eyes in
primary position at a level of central green light.
Patient is given amblyopia treatment as per
programs. The orthoptek device has two
programs:
• Program 1: In it the centeral green light
alternates with other 8 red lights every 150
milliseconds. The patient uses a green laser
pointer in the dominant hand and points at the
light that glows on the board, with the
amblyopic eye, normal eye being covered. This
is done 800 to 900 times. Then the same is
repeated, with both eyes open for 600 times.
• Program 2: In it the central green light keeps
blinking and red light in the periphery keeps
glowing in a random sequence. The patient has
to count the peripheral red light 900 times first
with amblyopic eye only, normal being covered,
and then 600 times with both the eyes open.
Total duration of training, is about 45 minutes
for both the programmes.
Results: This method of treatment is reported
to improve monocular vision, stereopsis,
207Adaptations to Strabismus and Amblyopia
binocular cooperation and hand eye coordination in 90–95% cases. It is also reported to be
effective in treating some cases of squint and
nystagmus.
C. ADJUNCT THERAPY
Adjunct therapy which may enhance visual
recovery include:
1. ACTIVE VISION THERAPY (NEAR VISUAL ACTIVITIES)
Active vision therapy, in the form of near visual
activities, using amblyopic eye has been
suggested as an important supplement to
occlusion therapy based on the assumption that
these activities stimulate the visual system and
thus cause easy recovery.
Active vision exercises by the amblyopic eye
during occlusion therapy, which may enhance
the success of occlusion include:
• Simple tasks like dotting the o’s and encircling
the e’s in the newsprint, joining dots to make
drawing, tracing, colouring threading beads,
watching television, reading comics and story
books may be quite useful and enhance the
recovery.
• Computer based vision therapy for amblyopia are
especially designed to treat amblyopia have
become very popular nowadays. Computerized
Home Vision Therapy (CHVT) is commonly
used software. For details of others see pages
156–159.
2. PHARMACOLOGICAL MANIPULATION
Role of levodopa. Studies indicate that plasticity
of visual system during the sensitive period is
dependent on input from non-adrenergic
neurons and thus can be subjected to
pharmacological manipulation. This aspect has
been tried with the use of levodopa as a
pharmacologic manipulator. Levodopa/
carbidopa has been traditionally used to treat
Parkinson’s disease. Levodopa is a precursor for
the catecholamine dopamine, a neurotransmitter/neuromodulator known to influence
receptive fields. Levodopa/carbidopa has been
studied as an adjunct to patching for the
treatment of amblyopia. However, the role of
levodopa remains limited, as the visual acuity
improvement has been relatively small, not
208 Theory and Practice of Squint and Orthoptics
clearly better than with patching alone and there
are questions regarding long-term stability of
vision.
Role of citicoline. See page 425
Role of omega fatty acid is also reported in
development of vision and cognitive development.
Role of donepezil in residual amblyopia. Results
of some study suggest that amblyopic eye visual
acuity can improve over time with donepezil
treatment (1.2 lines on average and 3.0 lines
maximum). The dosage of donepezil is 2.5 mg
OD for age 8–17 years, 5 mg OD for >18 years.
Treatment with donepezil ± patching for 2 hours,
showed promising results, this supports the
concept that the critical period of visual cortical
plasticity can be pharmacologically manipulated
in visually mature humans to treat amblyopia.
Role of docosahexaenoic acid (DHA) supplementation in amblyopia Docosahexaenoic acid
(DHA) is a long chain polyunsaturated fatty acid
(LCPUFA) that is considered essential for the
maturation of the developing brain and retina.
DHA, a commercially available nutritional
supplement, added to infant formula, has
shown improvement in visual development in
premature infants.
3.ROLE OF PERCEPTUAL LEARNING IN
AMBLYOPIA TREATMENT
Gibson (1963) defined perceptual learning as
“Any relatively permanent and consistent
change in the perception of stimulus array
following practice or experience with this array”
(Fig. 8.15).
Over the last 15 years, number of studies
suggest that ‘perceptual learning (PL) may
provide an important new method for treating
amblyopia. Perceptual learning is reported to
operate via a reduction of internal neural noise
and/or through more efficient use of stimulus
information.
Perceptual learning employes repeatedly
practicing a visual discrimination task, e.g.:
• Positional acuity,
• Contrast sensitivity,
• Stereoacuity, etc.
Fig. 8.15: Orthoptek device mounted on a wall.
Recommended period for preceptual learning
is 2 hours/day, 5 days/week, for a period of
9 months.
Role of perceptual learning is still limited, but
utility is reported in adult amblyops.
4. ROLE OF ACUPUNCTURE IN AMBLYOPIA
Acupuncture is a potentially useful complementary treatment modality that may provide
sustainable adjunctive effect to refractive
correction for anisometropic amblyopia in
young children. Although the treatment effect
of acupuncture appears promising, the
mechanism underlying its success as a treatment
for amblyopia remains unclear. Acupuncture at
vision-related acupoints may modulate the
activity of the visual cortex. Moreover,
acupuncture has been shown to be effective in
increasing blood flow to the cerebral and ocular
vasculatures (including the choroid),
stimulating the expression of retinal nerve
growth factors and leading to metabolic changes
in the central nervous system.
209Adaptations to Strabismus and Amblyopia
5.TRANSCRANIAL MAGNETIC BRAIN STIMULATION
IN AMBLYOPIA
Transcranial magnetic stimulation (TMS) is a
non-invasive method for stimulating parts of the
brain by use of weak electric current that are
induced into the tissue by use of rapidly
changing magnetic currents. TMS produces
effects that last slightly longer than the actual
time of stimulation.
AMBLYOPIA TREATMENT STUDY (ATS)
REPORTS
Reports of various ‘amblyopia treatment studies
(ATS)’ carried out under the 'Paediatric Eye
Disease Investigation Group (PEDIG)' are
summarised below.
ATS-1. Random control trial (RCT) to compare
atropine Vs patching for treatment of moderate
amblyopia. (Arch ophthalmol.2002; 120(3):268–78)
Age and number of patients. 3–<7-year-old 419
patients
Conclusion: Atropine and patching both are
appropriate treatment modalities in initial
treatment of moderate amblyopia (20/40 to 20/
100) in children 3–7 years old.
ATS 1 EXT. ATS 1 subjects followed up to age
10 yrs. (Arch Ophthalmol 2008: 126(8): 1039–44)
Conclusion: Patching and Atropine eye drops
produce comparable improvement in visual
acuity that is maintained through age 10 yrs.
ATS 1 EXT. ATS 1 subjects followed up to age
15 yrs. (JAMA Opthalmol. Jul 2014;132(7):799–805)
Conclusion: Patching and Atropine eye drops
produce comparable improvement in visual
acuity that is maintained through age 15 yrs.
ATS-2 A. RCT to compare 6 hours Vs full time
Occlusion (FTO) for severe amblyopia.
(Ophthalmology 2003,110(11):2075–2087)
Conclusion: 6 hours patching and FTO are
equally effective in severe amblyopia (20/200
to 20/400) in 3–7 years old.
ATS-2B. To compare 2 hours Vs 6 hrs patching
in moderate amblyopia. (Ophthalmology 2003;121(5):603–611)
Conclusion: When combined with 1 hr of near
visual activity, 2 hrs daily patching is equivalent
to 6 hrs patching in treating moderate
amblyopia (20/40 to 20/80 ) in 3–7 years old.
ATS-2C. To evaluate the risk of amblyopia
recurrence after successful treatment. (JAAPOS
2004,8(5):420–428)
Observation: Approximately 25% experience
around 2 lines dip in 1st year.
Conclusion: Patching regimen should be tapered
instead of abrupt stopping.
ATS-3. To evaluate the effectiveness of optical
correction alone vs 2–6 hr/day of patching
combined with near visual activities plus
atropine treatment of amblyopia in 7–13 years
of age. (Arch ophthalmol. 2005;123:437–447)
Conclusions:
• 25% amblyopes improved with optical
correction alone—may require additional
treatment
• 7–12 years: 2–6 hours patching with near
activity may improve VA—even if treated in
past
• 13–17 years: 2–6 hrs patching with near activity
may improve VA—if not treated in past
• 13–17 years: Little benefit: If treated in past.
ATS-4. RCT to compare daily atropine vs
weekend atropine in moderate amblyopia.
(Ophthalmology 2004;111:2076–2085)
Conclusion: Weekend atropine is as effective as
daily atropine in moderate amblyopia (20/40 to
20/80) in 3–7 years old.
ATS-5A. Prospective non- comparative trial to
evaluate 2 hours of daily patching for amblyopia
(eye glass only phase study). (Ophthalmology 2006;113(6):
895–912)
Conclusions:
• Refractive correction alone improves VA in
many cases (77%) of anisometropic amblyopia
and results in resolution in 27% cases.
• Increase in VA is more, if near activities are
combined with patching in mod/severe
amblyopia (3–7 years ).
ATS 5B. Randomized trial to evaluate 2 hours
daily patching for amblyopia (randomization
phase).
Conclusions: Refractive correction alone
improves visual acuity in many cases and results
in resolution of amblyopia in at least one-third
of 3 to <7-year-old children with untreated
anisometropic amblyopia. Following a period
of treatment with spectacles, two hours of daily
210 Theory and Practice of Squint and Orthoptics
patching combined with one hour of near visual
activities modestly improves moderate to severe
amblyopia in children 3 to 7-year-old.
ATS-6. RCT comparing near Vs distance activity
while patching. (JAAPOS 2005;9:129–136)
Conclusion: Performing near activities while
patching may be beneficial in treating
amblyopia in 3–7 years old.
ATS-7. Bilateral refractive amblyopia treatment
study: Response to treatment of previously
untreated presumed bilateral refractive
amblyopia. (Am J ophthalmology 2007;144:487–496)
Conclusion: Treatment of bilateral refractive
amblyopia with spectacle correction improves
binocular visual acuity in children 3–10 years
old, most improving to 20/25 or better in 1 year.
ATS-8. To compare weekend atropine
augmented by a plano lens with weekend
atropine alone for moderate amblyopia. (PEDIG:
pharmacological plus optical penalization: Arch ophthalmol 2009;127(1):
22–30)
Conclusion: As an initial treatment of moderate
amblyopia, the augmentation of weekend
atropine use with plano lens does not improve
amblyopic eye VA when compared with
weekend atropine use alone.
ATS-9. RCT to compare patching with atropine
eyedrops in the treatment of moderate
amblyopia. Ophthalmol.2008;126(12):1634–1642)
Conclusion: Treatment with atropine or
patching led to similar degree of improvement
in 7–12 years old children with moderate
amblyopia. About 20% achieved 20/25 or better
in amblyopic eye.
ATS-10. RCT comparing Bangarter filters and
patching for the treatment of moderate
amblyopia in children (3–7 yrs). (Ophthalmology.2010
May; 117(5): 998–1004)
Conclusion: Bangerter filter treatment is a
reasonable option to consider for initial
treatment of moderate amblyopia.
• Average difference in VA improvement was
less than ½ line between 2 groups.
• Lower burden of treatment on the child and
family.
ATS-11. Randomized Trial to Evaluate
Combined Patching and Atropine for Residual
Amblyopia. (Arch Ophthalmol. Jul 2011;129(7):960–962)
Conclusion: Amblyopic eye VA improved
similarly in both groups suggestive of no
additional benefit of combined treatment in
residual amblyopia.
ATS-12. Vision Therapy Treatment Pilot Study
(Feasibility of a clinical trial of vision therapy
for treatment of amblyopia. (Feasibility of a clinical trial
of vision therapy for treatment of amblyopia. Optom Vis Sci 2013;90(5):
475–81)
Conclusion: More flexible approach that
customizes vision therapy is recommended.
ATS-13. Spectacles alone for Strabismic and
strabismic anisometropic amblyopia. (Optical
Treatment of Strabismic and Combined Strabismic Anisometropic
Amblyopia. Ophthalmology 2011;119(1):150–8)
Conclusion: Optical treatment alone of strabismic
and combined-mechanism amblyopia results in
clinically meaningful improvement in amblyopic
eye visual acuity for most 3- to <7-year-old
children, resolving in at least one quarter without
the need for additional treatment.
ATS-14. A Pilot Study of levodopa dosage as
treatment for residual amblyopia in children
8 to <18 years old. (Arch Ophthalmol. Sep 2010;128(9):1215–
1217)
Conclusions:
• Levodopa/carbidopa therapy for residual
amblyopia in older children and teenagers
may improve visual acuity.
• Partial regression in VA after treatment was
discontinued.
ATS-15. Increasing patching for amblyopia
Study. (# A randomized trial of increasing patching for amblyopia.
Ophthalmology. 2013 Nov;120(11):2270–7. # JAMA Ophthalmol. May
2015;133(5):606–609)
Conclusions: When amblyopic eye VA stops
improving with 2 hours of daily patching,
increasing the daily patching dosage to 6 hours
results in more improvement in VA after 10
weeks compared with continuing 2 hours daily
ATS-16. Augmenting atropine treatment for
amblyopia-A randomized trial of adding a
plano lens to atropine for amblyopia. ( J AAPOS,
2015 Feb;19(1):42–48)
Conclusions: When amblyopic-eye visual acuity
stops improving with atropine treatment, there
is no statistically significant benefit of
augmenting atropine therapy with a plano lens
over fellow eye.
211Adaptations to Strabismus and Amblyopia
ATS-17. A randomized control trial of levodopa
as treatment for residual amblyopia in older
children. (Ophthalmology. 2015 May;122(5):874–81)
Conclusions: For children (7 to 12 years) with
residual amblyopia after patching therapy, oral
levodopa while continuing to patch 2 hours
daily does not produce a clinically or statistically
meaningful improvement in VA compared with
placebo and patching.
ATS-18 EXT. Effect of a binocular iPad game vs
part-time patching in children aged 5 to 12 years
with amblyopia: A randomized clinical trial.
(JAMA Ophthalmol.2016 Dec 1;134(12):1391–1400)
Conclusions:
• In children (aged 5–12 years), amblyopic-eye
VA improved with binocular game play and
with patching, particularly in younger
children (age 5 to <7 years) without prior
amblyopia treatment.
• Although the primary noninferiority analysis
was indeterminate, a post hoc analysis
suggested that VA improvement with this
particular binocular iPad treatment was not
as good as with 2 hours of prescribed daily
patching.
ATS-18. A randomized trial of a binocular iPad
game versus part-time patching in children 13
to 16 years of age with amblyopia. (Am J Ophthalmol
2018;186:104–15)
Conclusions:
• In teenagers aged 13 to <17 years,
improvement in amblyopic eye VA with the
binocular iPad game used in this study was
not found to be better than patching, and was
possibly worse.
• It remains unclear whether the minimal
treatment response to binocular treatment was
owing to poor treatment adherence or lack of
treatment effect.
ATS-20. A randomized trial of binocular dig rush
game treatment for amblyopia in children aged
7 to 12 years. ( Ophthalmology; 2019 Mar;126(3):456–466)
Conclusions: In children aged 7 to 12 years who
have received previous treatment for amblyopia
other than spectacles, there was from 4 or
8 weeks of treatment with the dichoptic
binocular Dig Rush iPad game.
UPCOMING ATS
ATS 19. Excimer laser surgery for anisometropic
amblyopia.
Trial ID: NCT03342235
ATS 21. Evaluation of accommodative behavior
in children with and without amblyopia (pilot
study).
ATS 22. A randomized trial to evaluate
sequential versus simultaneous spectacles plus
patching (NCT04378790).
PROBLEMS IN AMBLYOPIA TREATMENT
1. Problems in applying the occluder
Though occluding the sound eye may appear
to be a simple procedure, and in many cases it
is nevertheless, in the practical application of
this simple treatment, several problems may
arise. Some of the important difficulties
encountered during this treatment are discussed
as follows.
Occluder attached to spectacle lens is easy to
wear but usually ineffective, since children are
able to look over, around and under the patch.
Further, most children will simply take their
glasses off, when unobserved, and for this
reason, most workers rarely use this method of
occlusion.
Adhesive patch applied directly over the
skin (Fig. 8.13) is the best method for a total and
full-time occlusion. However, problems of skin
reactions may arise in some children. Following
measures have been recommended:
• Tincture benzoins may be applied to the skin
before applying the patch. This forms
a protective layer over the skin and also
increases adhesiveness of the patch so
that child is less likely to remove it.
• Opticlude patch is claimed to be hypo-
allergenic and can be replaced for ordinary
sticking patch.
• If a skin problem does develop, the patch
should be left on the eye until it falls off
rather than removing it every night, since
repeated removal of the patch aggravates
the skin irritation.
Occluding soft contact lenses have been
recommended where the above two methods
212 Theory and Practice of Squint and Orthoptics
prove to be frustrating because of one or the
other reason. However, soft contact lenses have
their inherent problem of difficulty in
application and complications associated with
soft contact lens use.
2. Problem of parental co-operation
A full co-operation of the parents is essential in
keeping the sound eye of the child occluded.
Many parents fail to co-operate for one or the
other reason—may be their inability to devote
time for their child or may be a lack of understanding. Therefore, it is the duty of treating
person to hammer on the parents' mind the
importance of patching for the sake of their
child, so much so, that even the parents should
be warned that they will be held responsible for
a permanent loss of vision of their child.
Instilling a feeling of guilt in the parents is not
wrong keeping in view the results of rightly and
timely performed occlusion therapy.
3. Complications of occlusion
Two complications may accompany occlusion—
occlusion amblyopia and occlusion esotropia.
i. Occlusion amblyopia. In very young children,
occlusion amblyopia in the sound eye may occur
in as short a time as 2 to 3 weeks. Not only
amblyopia but also eccentric fixation may
develop in the previously normal eye when it is
constantly occluded. It is, therefore, essential
that visual acuity of the sound eye should be
carefully monitored during occlusion therapy.
If occlusion amblyopia does occur, it usually can
be eliminated in about the same time it took to
develop by changing the occlusion to the other
eye.
Now, it has been established that chances of
developing amblyopia in sound eye are more
with the penalization technique than with total
occlusion. It is because the blurred diffuse
stimulus (a white noise type of stimulus)
produced by penalization is more amblyogenic
than the total occlusion.
ii. Occlusion esotropia. Sometimes, in anisome-
tropic amblyopia, when no deviation exists and
bifoveal single vision is present, constant
occlusion may so disrupt binocular vision that
an esotropia results. The parents should be
warned of this possibility, but it should be
explained that the risk is worthwhile in the
interest of good visual acuity. The use of
intermittent occlusion or partially transparent
occulsion (which allows binocular fixation) is
preferable to total occlusion as a precaution
against the development of this complication.
If a deviation does develop, it may not spontaneously disappear once occlusion is
discontinued.
4. Recurrence of amblyopia
Once amblyopia has been corrected, chances of
recurrence are always there until child is
visually mature (10 years of age). Therefore, a
careful monitoring every month up to the age
of 1 year, every 2 months up to the age of 2 years
and then every 4–6 months up to the age of
visual maturity is required. Not only this, a
maintenance occlusion therapy (see pages 204–
205) should also be carried out.
PROGNOSIS
Prognosis of amblyopia therapy depends upon
the age of patient, type of amblyopia and type
of treatment as below:
• Younger the child better the prognosis.
• Amblyopia due to deprivation (e.g. due to
congenital cataract) carries the poorest
prognosis.
• Strabismic amblyopia has the best prognosis.
• Anisometropic amblyopia has a prognosis
intermediate between strabismic amblyopia
and deprivation amblyopia.
• Presence of eccentric fixation worsens the
prognosis.
• Amblyopia with unilateral high hyper-
metropia has a poorer prognosis than the
amblyopia with unilateral high myopia.
• Prognosis is better, when treated with total
and full time occlusion than when treated with
penalization or other methods.
ABNORMAL RETINAL CORRESPONDENCE
GENERAL CONSIDERATIONS
As we know, in a state of normal single
binocular vision, there exists a precise physiological relationship between the corresponding
213Adaptations to Strabismus and Amblyopia
points of the two retinae. And that the foveae of
two eyes act as principal corresponding points and
have the same visual direction. This adjustment
is called normal retinal correspondence (NRC).
When squint develops, patient may experience
either diplopia or confusion. To avoid these,
sometimes (especially in children with small
degree of esotropia), there occurs an active
cortical adjustment in the directional values of
the two retinae. In this state, fovea of the normal
eye and an extrafoveal point on the retina of the
squinting eye acquire a common visual direction
(i.e. become corresponding points). This
condition is called abnormal retinal correspondence and the child gets a crude type of
binocular vision. Thus, abnormal retinal
correspondence (ARC) is a binocular sensory defence
mechanism against peripheral diplopia and peripheral
confusion. It is important to note that ARC is
entirely a binocular phenomenon, i.e. when the
eyes are used monocularly, there is no change
in visual direction of any retinal element. While
eccentric fixation is a monocular phenomenon
in which patient takes fixation with an
extrafoveal point. If the fixating eye of a patient
with ARC is covered, he/she will turn the
deviating eye to fixate either with his/her fovea
(if he/she has central fixation) or with a
peripheral area (if he has eccentric fixation).
Only in a minority of patients are ARC and
eccentric fixation related in such a manner that
the point of anomaly during binocular single
vision is also the point of eccentric fixation
during monocular vision.
From the above description, an impression is
created as if ARC is a new point-to-point
correspondence between the two eyes leading to
something like normal fusion, stereopsis, etc.
However, in fact, this is not the case, rather the
ARC is a fragile, variable form of binocular cooperation depending very much on the
momentary conditions of binocular vision.
Further, ARC does not replace NRC. In fact NRC
appears to be suspended since on some tests it can
be demonstrated that NRC co-exists with ARC.
Harmonious versus unharmonious ARC
Before exactly defining harmonious and
unharmonious ARC, we should revise the
definitions of objective and subjective angles of
deviation and that of angle of anomaly.
• Objective angle of deviation is the amount of
deviation measured, when no shift of the eyes
is observed by the examiner on the prism and
alternate cover test. When measured on synoptophore, it is the position of the instrument's arm
when no shift occurs as the lights in the tubes
are alternately turned off and on.
• Subjective angle of deviation when measured
with the synoptophore, is denoted by the
position of the instrument's arms at which the
patient can superimpose the images of
dissimilar test objects.
• Angle of anomaly refers to the difference
between the objective angle of deviation and
subjective angle of deviation. In NRC, the
objective and the subjective angles of deviation
are equal and so the angle of anomaly is zero.
In ARC, the subjective angle is always less than
the objective angle and so the angle of anomaly
is more than zero. Depending upon the value of
angle of anomaly, the ARC is of two types as follows:
1. Harmonious ARC is present, when the angle
of anomaly equals the objective angle of squint.
In other words, in harmonious ARC, subjective
angle of deviation is zero indicating total
compensation for the deviation.
2. Unharmonious ARC is present, when the
angle of anomaly is less than the objective angle
of deviation. In other words, in unharmonious
ARC, the subjective angle of deviation is
between zero and the objective angle,
indicating thereby that the ocular deviation has
not been fully compensated. In fact,
unharmonious ARC is presently considered an
artifact of the more dissociating testing
conditions; since a more physiological test (e.g.
Bagolini's test) reveals harmonious ARC and a
more dissociating test (e.g. synoptophore or
red-green glasses) may reveal unharmonious
ARC in the same patient.
DEVELOPMENT OF ARC
Factors affecting development of ARC
ARC is a sensory adaptation that is brought
about by an inherent desire for some form of
binocular vision and to avoid diplopia and
214 Theory and Practice of Squint and Orthoptics
confusion that would otherwise take place.
However, ARC does not develop in each and
every case with strabismus. The factors that have
been reported to favour and unfavour the
development of ARC are as follows:
1. Age of onset of squint. ARC develops only in
visually immature children who have acquired
binocular single vision. Therefore, chances of
ARC development are more, if the squint occurs
between 1 and 6 years of age. Therefore, patients
in whom binocular single vision is not developed
due to presence of early infantile esotropia do not
develop ARC. On the other hand, visually mature
patients (beyond 6–7 years of age) who acquire
strabismus are also incapable of developing ARC.
In other words, in young children (below
6–7 years), with binocular single vision, the
instability of the binocular reflexes leads to
replacement of normal reflex development by
abnormal binocular reflexes.
2. Patient profile. ARC develops more
frequently in patients where binocular single
vision has previously existed and the patient has
a high degree of general adaptability and
intelligence.
3. Type and amount of strabismus
• ARC develops more commonly in esotropes
than exotropes and is less common with
vertical deviations.
• ARC develops more commonly in patients
with uniocular squint than in patients with
alternating squint.
• Patients with constant angle of squint are
more likely to develop ARC as compared to
those with variable angle of squint.
• ARC seems to develop more frequently, when
the angle of esotropia is between 10D and 20D.
This is because of the fact that the retinal area
of the squinting eye, which receives the same
stimulation as the fovea of the fixing eye, is
close to the fovea and possesses good visual
acuity, thus the false image is not only close to
the true one but is also relatively clear and the
resultant diplopia is troublesome, and in order
to overcome this, ARC develops more rapidly.
Natural course of development of ARC
• During natural course of development of
ARC, the angle of anomaly gradually
increases until it equals the amount of
objective deviation and the ARC becomes
harmonious.
• During development of ARC, the NRC is not
immediately and rarely totally suppressed.
NRC and ARC may both coexist in some
patients, especially in those with intermittent
exotropia. Such patients may show NRC while
fusing and ARC while tropic.
• The actual development of ARC appears to
occur slowly. But once established the shift
from ARC to NRC and back again can occur
very rapidly.
ARC AND SUPPRESSION
Both ARC and suppression are the sensory
adaptations to prevent diplopia and confusion.
They may occur alone or may coexist in the same
patient. Following observations have been made
about occurrence of ARC and suppression:
1. In patients with large deviations suppression
is the rule without associated ARC.
2. In patients with low degrees of strabismus
(30D or less), ARC and/or suppression may
develop.
3. The presence of suppression does not prevent
the development of ARC. Rather some studies
quote that suppression is a prerequisite for
establishment of ARC.
4. Co-existence of ARC and suppression can be
demonstrated, when Bagolini's striated
glasses are used to test an esotropic patient.
As shown in Fig. 8.16, when the foveola of
the non-deviating eye is aligned with an
extrafoveal point in the deviating eye
(Fig. 8.16B), the streak seen by the deviating
eye has a gap in it (Fig. 8.16C). This gap corresponds to a suppression scotoma in the deviating eye. Since the patient sees the diagonal
streaks as a cross and the streak seen by the
deviating eye is in line with the fixation light,
this indicates that in the periphery, ARC
coexists with the central scotoma (Fig. 8.16).
5. Suppression and ARC when co-exist, are
complementary mechanisms, since the extrafoveolar suppression in the deviating eye
protects against central diplopia, while ARC
eliminates peripheral diplopia and peripheral
visual confusion.
Fig. 8.16 Bagolini's striated glass test depicting co-
existence of suppression and ARC (for explanation, see
text).
QUALITY OF BINOCULAR VISION IN ARC
Simultaneous perception of images in the two
eyes occurs in ARC due to correspondence of
fovea of one eye with some extrafoveal area of
the other eye.
Fusion in ARC. Possibly, restoration of some form
of motor fusion occurs with the development of
ARC. Since ARC provides a low-quality type of
peripheral fusion, the occurrence of fusional
movements might be expected in ARC. However,
when a change in the strabismic angle occurs,
patients with ARC should be expected to change
the angle of anomaly to avoid peripheral diplopia.
215Adaptations to Strabismus and Amblyopia
Peripheral fusion is thereby achieved without a
fusional movement. Thus, the fusional movements,
if they do occur in ARC, are probably limited.
Further, some workers believe that the fusional
movements noted in ARC may not be true
fusional movements and instead might be the
manifestations of covariations of the angle of
anomaly. The covariation of angle of anomaly
with the angle of deviation is believed to be
possible because of a point-to-area relationship
in ARC. Point-to-area relationship means that
numerous retinal elements in the deviating eye
can apparently be coupled with a single retinal
element in the non-deviating eye.
Stereopsis and ARC. It has been concluded that
ARC and stereopsis do not coexist. The absence
of stereopsis is indicative of the low quality of
ARC peripheral fusion.
It is important to remember that:
• Normal stereopsis (40 seconds or better),
indicates bifixation, i.e. perfect central and
peripheral fusion.
• Gross stereopsis (67–3000 seconds of arc),
indicates monofixation, i.e. peripheral fusion
without central fusion.
• Stereopsis is absent in peripheral fusion with
arc. However, it has been reported that a gross
stereopsis (usually less than 120 minutes of
ARC), is a common finding in patients with
ARC and small angle esotropia or microtropia
which may occasionally even be demonstrable
with random dot stereograms.
CLINICAL PHENOMENA
ASSOCIATED WITH ARC
Paradoxical diplopia
Paradoxical diplopia is the one which is not
expected in a particular type of deviation.
Obviously, it occurs because of abnormal projection. In ARC, paradoxical diplopia occurs
under following circumstances:
1. It can be elicited in patients with ARC, when
both foveas are simultaneously stimulated with
major amblyoscope. It can also be elicited with
after-image test (see page 140 and Fig. 6.33).
2. Paradoxical diplopia can also be perceived by
a patient in which ARC continues after surgical