Amblyopia

Amblyopia - Definition, pathophysiology, types, management.

1. AMBLYOPIA

2. 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 n o identifiable pathology of the eye or visual pathway.

3. CLASSIFICATION Strabismic amblyopia Stimulus deprivation Anisometropic amblyopia 4. Bilateral ametropic amblyopia 5. Meridional amblyopia

4. 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.

5. 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.

6. 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 .

7. 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.

8. 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

9. Bilateral ametropic amblyopia A mblyopia 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

10. PATHOGENESIS & PATHOPHYSIOLOGY Still not elucidated fully Amblyogenic Factors Role of retina Active cortical inhibition

11. Amblyogenic Factors Form vision deprivation – all forms Light deprivation – strabismic Abnormal binocular interaction - all monoocular forms

12. Neurophysiologic Studies - Hubel and Wiesel Deprivation 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.

13. 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.

14. 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

15. 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.

16. 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

17. 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.

18. 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

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

20. 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

21. 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 .

22. 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.

23. 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

24. 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

25. 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)

26. Evaluation of Central Vs Eccentic Fixation 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.

27. 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.

28. 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.

29. Management of Amblyopia Prevention and Early Detection Treatment of Amblyopia

30. 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

31. 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.

32. 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.

33. 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.

34. 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.

35. 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

36. 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 .

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

38. 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.

39. 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).

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

41. 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

42. 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.