AMBLYOPIA

•   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.

CLASSIFICATION:

1. Strabismic amblyopia 

2. Stimulus deprivation 

3. Anisometropic amblyopia 

4. Bilateral ametropic amblyopia 

5. Meridional amblyopia

Strabismic amblyopia: 

            Amblyopia seen in those patients with unilateral constant squint who strongly favour one eye for fixation. 

Typical Features: 

1. Grating acuity is better than Snellen's acuity 
2. Always unilateral 
3. More often in esotropes than exotropes 
4. Very rare in hypertropia (anomalous head posture)
5. Do not occur in alternate strabismus.

Stimulus deprivation:

            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 anisohypermetropia 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 orientation. Even a small amount of U/L astigmatism may cause amblyopia

Bilateral ametropic amblyopia: 

                       Amblyopia results in 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

Clinical Characteristics: 

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

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

3. 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.

4. Fixation Pattern - 

• Central fixation – foveolar fixation 
• Eccentric viewing – Extrafoveal point because of central suppression scotoma. Fovea still not lost its principal visual direction. Patients look past the object they have been asked to fix. 
• Eccentric fixation – Fovea lost its principal visual direction

5. Absolute central scotoma 

6. Localisation of the object of regard - normal in patients with amblyopia & eccentric fixation but abnormal in eccentric viewing.

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

8. Light Perception & Form vision - Dissociated.

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

10. Light and Dark Adaptations - Usually normal. 

EVALUATION AND DIAGNOSIS:

1. Evaluation of V.A & Refraction 
2. Neutral density filter test 
3. Test for crowding phenomenon 
4. A/S and fundus examination 
5. Evaluation of fixation 
6. Other sensory anomalies

Management of Amblyopia:

• Prevention and Early Detection 
• Treatment of Amblyopia

Prevention:

             Best Way – Vision Screening programs right from birth: I-ARM

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.
• If the vision in the amblyopic eye worse than 6/18, occlusion must be total.
• if the vision in the amblyopic eye better than 6/18, occlusion must be partial.

1. In patients with visual improvement assessed at monthly follow-up visits, occlusion should be continued till equal vision and equal fixation preference are achieved. Younger the patient, the better is the visual improvement. In patients with no improvement on 3 monthly follow up, further occlusion is unlikely to be fruitful. Management Occlusion Treatment – Once the vision has been equalized occlusion therapy for 2-3 hours has to be continued till at least 9yrs.
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3. Penalization: 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.
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5. Methods – 1) Atropine penalization 2) Optical Penalization
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7. 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.

1. 2) Optical Penalization – Prescribing more pluses to sound eye to force amblyopic eye to fix for distance targets.
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3. 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.
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5. Pleoptics: Only of historical interest. 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).
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7. Pharmacological Manipulation: Levodopa, a precursor for catecholamine dopamine has been studied as an adjunct patching, but remains 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 has poorer prognosis than myopes 
• Occlusion therapy is better than other methods.