AMBLYOPIA

DEFINITION

• Amblyopia is the unilateral, or rarely bilateral, decrease in best-corrected visual acuity

• caused by form vision deprivation and/or abnormal binocular interaction, for which there is

• no identifiable pathology of the eye or visual pathway.

CLASSIFICATION1. Strabismic amblyopia2. Stimulus deprivation3. Anisometropic amblyopia4. Bilateral ametropic amblyopia5. Meridional amblyopia

Strabismic amblyopia• Amblyopia seen in those patients with unilateral

constant squint who strongly favour one eye for fixation.

• Typical Features : Grating acuity is better than snellen’s acuity Always unilateral More often in esotropes than exotropes Very rare in hypertropia (anomalous head posture) Do not occur in alternate strabismus.

Stimulus deprivation Amblyopia of Disuse Amblyopia ex anopsia Amblyopia resulting from those conditions wherein

one eye is totally excluded from seeing early in life.

Monocular congenital or traumatic cataract, complete ptosis, corneal opacity, prolonged patching of the normal eye for the treatment of amblyopia etc.

• Features :

Most damaging and difficult to treat

Amblyopic visual loss resulting from U/L deprivation is worser than that produced by B/L deprivation of similar degree.

This is because, in U/L deprivation, interocular effects add to image degradation.

Anisometropic amblyopia• Amblyopia caused by a difference in refractive error

between the eyes and may result from a difference of as little as 1.0 D sphere

• More common in anisosohypermetropia than in those with anisomyopia.

• Strabismus is frequently associated with anisometropia and hence both strabismic amblyopia and anisometropic amblyopia can coexist.

Meridional amblyopia• In patients with uncorrected astigmatic refractive

error due to selective visual deprivation at certain special orientaion.

• Even small amount of U/L astigmatism may cause amblyopia

Bilateral ametropic amblyopia• Amblyopia results high symmetrical refractive

errors, usually hypermetropia (+5.0D).• Myopia in excess of -10.0 D also can induce B/L

amblyopia• Astigmatism > 2.5 D

PATHOGENESIS & PATHOPHYSIOLOGY

Still not elucidated fully

Amblyogenic Factors Role of retina Active cortical inhibition

Amblyogenic Factors

• Form vision deprivation – all forms• Light deprivation – strabismic • Abnormal binocular interaction - all monoocular

forms

Neurophysiologic Studies

- Hubel and WieselDeprivation Studies By suturing the eyelids of experimental

animals• Observations : In the LGB, cells in those layers receiving input

from deprived eye showed a profound shrinkage. Cells of primary visual cortex either lost their

ability to respond to stimultion or showed significant functional deficiency.

• Conclusions : Visual deprivation produces amblyopia by

changes in the visual system neurons.

Deprivation during the early part of the critical period of development is more deleterious

Amblyopia produced by binocular deprivation was less severe than that produced by uniocular deprivation.

Role of Retina• There is some evidence that the retina itself is

abnormal in amblyopia.• Decreased sensitivity of foveal cones in amblyopia• Quicker dark adaptation• However V.A is reduced disproportionately to

reduction in cone function.• ERG - Normal

Active Cortical Inhibition• Physiologic Evidence – Perhaps the normal

eye may be responsible for an active cortical inhibition in unilateral amblyopia

• Pharmacologic Evidence - Perhaps in amblyopia active cortical inhibition might be mediated by inhibitory neurotansmitter GABA.

Clinical Charecterisics

1. Visual Acuity – Difference in 2 lines on V.A chart should be there to diagonse amblyopia

Recognition Acuity – (Snellen) is more affected than resolution acuity ( Teller’s or VER)and detection acuity ( Catford Drum test)

Grating Acuity is less affected in strabismic amblyopia

Effec of neutral density filter – when placed infront of affected eye V.A improves by one or two lines.

Crowding Phenomenon - (Separation difficulty) Refers to the inability of an amblyopic eye to

distinguish letters crowded together. Therefore V.A is better when tested with optotype charts.

2. Fixation Pattern - Central fixation – foveolar fixation

Eccentric viewing – Extrafoveal point because of central suppression scotoma.

Fovea still not lost its principal visual direction.

Patient look past the object they have been asked to fix.

Eccentric fixation – Fovea lost its principal visual direction

• If an image is pojected onto the fovea patient report that the object is seen in some other direction than straight ahead.

• The Heimann-Bielschowsky phenomenon – Unusual ocular motility pattern which may

develop years following uniocular visual loss. Strictly monocular coarse, pendular vertical

oscillations occurring only in the amblyopic eye. • Paradoxical eccentric Fixation – Ordinarily, there develop nasal eccentricity in

esotropia and temporal eccentricity in exotropia. Reverse is called paradoxical fixation.

- surgical overcorrection of deviation - spontaneous reversal of deviation

• Absolute central scotoma• Localisation of object of regard - normal

in patients with amblyopia & eccentric fixation but abnormal in eccentric viewing.

• Colour Vision - Impaired only if V.A is below 6/36. Related to eccentric fixation.

• Light Perception & Form vision - Dissociated.

• Pupillary light reflexes – generally normal. RAPD may occur in deep amblyopia.

• Light and Dark Adaptations - Usually normal. Difference in the region of Kohlrausch’s bend (bend in the adaptation curve) has been found.

• Critical Flicker Frequency - Central CFF tends to approach that of periphery or of rod mechanism. Also, CFF is faster in eccentric fixation.

• ERG & EOG - ERG is normal but EOG shows unsteadiness of fixation.

EVALUATION AND DIAGNOSIS• Evaluation of V.A & Refraction• Neutral density filter test• Test for crowding phenomenon• A/S and fundus examination• Evaluation of fixation• Other sensory anomalies

Binocular Fixation Pattern (BFP)

Grade 0 Spontaneous alternation

Grade 1 Simply fixates with one eye but can use the other eye too

Grade 2 Moderate fixation preference

Grade 3 Holds fixation for 1-2 seconds but switches before blink.(Strong fixation preference)

Grade 4 Uses only one eye for fixation

Prism Induced Tropia Test• 25-D Base in Prism Test : It induces large esotropia

creating diplopia. So normaly infant will not attempt to see through he prism but if it shows prefernce still, indicates amblyopia in the uncovered eye.

• Vertical Prism Test : 10 – 15 D vertical prism is used to induce diplopia

• CSM method of Rating : C – Central, S- Steady, M – Maintained (orthotropic)

Evaluation of Central Vs Eccentic Fixation

1. Angle kappa method – • Hand light method – Occlude the non fixing eye,

ask the patient to fix at light held directly below patient’s eye. Same repeated on the other eye.

Corneal reflex is noted. Angle is positive, if reflex is displaced nasally and negative,if displaced temporally.

In eccentric fixation, significant difference in location of corneal reflex will be noted.

• Arc Perimeter Method – Patient is asked to fixate at the central mark on the perimeter. A very fine light is moved along the arc until the light refel is centered on the cornea. Location of light on the perimeter arc tells the angle kappa in degrees.

• Major amblyoscope Method - Using special slides with synoptophore

2. Visuscope Method - In patients above 4-5 years

3. Haidinger’s brushes Method - Patient is made to percieve the entopic pattern of Haidinger brushes and asked to touch is center.

4. Maxwell’s spot Method – Round dark purplish spot of about 3 arc degrees in d.m. It is percieved entopically when the eyes are exposed to homogenous blue or purple field. In eccentric fixation Maxwells spot is displaced to the side of fixation target.

Management of Amblyopia

Prevention and Early Detection Treatment of Amblyopia

Prevention• Best Way – Vision Screening programmes right

from birth : I-ARM Steps Neonates

(Birth- 2m)Babies(3m – 2 years)

Children(>3 years)

Inspection Symmetry of face & eyes

Face or head tilt Face turn or head tilt

Acuity Poor fixation, pupillary response

Good fixation and smooth pursuit

Allen card, E-game

Red Reflex Red reflex test Bruchner red reflex

Bruchner red reflex

Motility Gross alignment Light reflex and bruchner

Any misalignment is abnormal

TREATMENT Elimination of cause of Visual depriation – eg

congenital cataract, congenital ptosis,corneal opacity

Correction of refractive error and spectacle adaptation should be fully tried before starting occlusion therapy.

Correction of ocular dominance : Occlusion therapy, penalization, active stimulation,pleoptics, pharmacological manipulation.

Occlusion therapy• Methods – Patch on skin, gauze pad and tape, use

of Doyne’s rubber occluder, opaque contact lens etc.

• Timing- Amblyopia Treatment Studies (ATS)In children (3-7y) with severe amblyopia full time

patching produced a similar effect to that of patching for 6 hours a day

In children (3-7y) with moderate amblyopia 2 hours of daily patching produced same improvement as to that of 6 hours.

In children (7-13y) prescribing 2-6 hours of patching can improve visual acuity even if amblyopiahas been previously treated

In patients (13-18y) precribing 2-6 hours of patching might improve visual acuity, but not if amblopia Rx has already been tried previously.

Active vision exercises by amblyopic eye during occlusion; simple tasks such as joining dots to make drawing, tracing, threading beads, watching t,v, reading comics, may enhance visual improvement.

• In patients with visual improvement assessed at monthly follow up visits, occlusion should be continued till equal vision and equal fixation preference is achieved

• Younger the patient, better is the visual improvement.

• In patients with no improvement on 3 monthly follow up, futher occlusion is unlikely to be fruitful

• Management Occlusion Treatment – Once the vision has ben equalised occclusion therapy for 2-3 hours has to be continued till atleast 9yrs.

Penalisation

• To force the amblyopic eye to greater use by penalizing the sound eye with the help of glasses nd a cycloplegic drug.

• Prerequisite – Eyes should be straight• Indications - As good as patching in moderate

amblopia• Methods – 1) Atropine penalization 2) Optical Penalization

1) Atropine penalization Near Penalization – Normal eye is atropined and

fully corrected for distance vision, while amblyopic eye is overcorrected with +2 or +3 D.

Distance Penalization – Normal eye is atropinized and overcorrected by 4 – 5 D, while amblyopic eye is fully corrected.

Total penalization – Normal eye is atropinized and undercorrectedby 4-5D, while amblyopic eye is fully correcteed.

2) Optical Penalization – Prescribing more pluses to sound eye to force amblyopic eye to fix for distance targets.

Active Stimulation Therapy

• Using CAM vision stimulator has been used in the past.

• Method – After occluding the sound eye, amblyopic eye is stimulated for 7 min by slowly rotating high contrast square wave raing of different spatial frequencies. Done once in a week.

Pleoptics• Only of historical interest• In this peripheral retina including eccentrically fixing

area around the fovea is dazzled with an intense light while protecting the foveal area.

• This is followed by direct stimulation of fovea by pleoptophore or after image(Cupper’s method).

Pharmacological Manipulation

• Levodopa, a precursor for catecholamine dopamine has been studied as an adjunct ti patchinf, but remains controversial.

Role of perceptual learning

• It employs practicing a visual discrimination task eg; Positional acuity, Contrast acuity, Stereo acuity etc.

• Recommended period for perceptual learning is 2hrs/day, 5 days/ week, for a period of 9 months.

• Still controversial

Prognosis of Amblyopia Treatment

• Younger the child better the prognosis• Deprivation amblyopia carries the poorest

prognosis• Strabismic amblyopia has best prgnosis• Presence of eccenric fixation worsens the prognosis• U/L hypermetropes ahs poorer prognosis than

myopes• Occlusion therapy is better than other methods.