Describe amblyopia with its types, treatment and prevention.

Amblyopia

• It is a disorder of the cortex, not of the eye — the retina and optic nerve are normal, and the deficit lies in the abnormal development of cortical connections. This is why it is reversible while the cortex remains plastic and irreversible afterwards
• The critical period extends to 7 to 8 years, with the greatest vulnerability in the first 2 years. The earlier the insult, the denser the amblyopia and the shorter the window for treatment
• Three mechanisms, in ascending order of severity:

Screening and Applied Aspects

• Amblyopia affects some 2 to 4% of children and is the commonest cause of monocular visual impairment in young adults — and it is entirely preventable by timely detection
• The lifetime risk that matters: a person with one amblyopic eye is at substantially greater risk of bilateral visual impairment, since disease or injury to the sound eye leaves nothing in reserve. This is the strongest argument for treating amblyopia, and is worth putting to a family who see little point in patching a child who "manages fine"
• Screening — red reflex at birth and at every child health check; visual behaviour in infancy; and formal acuity screening at 3 to 5 years, which is the age at which anisometropic amblyopia is detectable and still treatable. School screening in India under the national programme, with provision of free spectacles, is directed precisely at this
• Test acuity with a crowded chart; single-optotype testing misses amblyopia because of the crowding phenomenon
• Give the glasses and wait before patching; refractive adaptation over 12 to 18 weeks resolves a substantial share of amblyopia by itself
• Prescribe two hours rather than all day in moderate amblyopia; the evidence is clear and compliance is far better
• Offer atropine where patching fails; it is equally effective and often accepted where a patch is refused
• Review the patched eye; occlusion amblyopia of the sound eye is a real and iatrogenic hazard in the very young
• Look for a relative afferent pupillary defect; if one is present the diagnosis is not amblyopia and the optic nerve must be investigated
• Taper rather than stop, and follow to visual maturity; about a quarter regress after abrupt cessation
• Explain the reason for treatment to the family in terms of the second eye; a child who manages well with one eye gives parents little motive to persist, and the argument from lifetime risk is the one that works
• Amblyopia is cortical, not ocular, which is why the eye looks normal and why the condition is reversible only while the cortex is plastic
• An alternating squint does not cause amblyopia, because each eye is used in turn and neither is suppressed constantly
• Anisometropic amblyopia is silent, with straight eyes and no symptoms.
• Deprivation amblyopia is the densest form, and a congenital cataract therefore demands surgery within weeks rather than months
• A neutral density filter worsens organic disease more than amblyopia, which is a useful bedside discriminator
• No afferent pupillary defect occurs in amblyopia; if one is present the optic nerve must be investigated instead
• Two hours of patching matches six in moderate amblyopia, which transformed the burden on families
• Weekend atropine matches daily patching, and is often accepted where a patch is refused outright
• Occlusion amblyopia of the sound eye is an iatrogenic hazard in the very young, and is why the patched eye is reviewed
• About a quarter regress after stopping, so treatment is tapered and the child followed to visual maturity
• Compliance determines the outcome more than the regimen, and the child sees worse with the good eye covered.