amblyopia

DEFINITION: Decreased visual acuity (typically ≥2 lines difference on Snellen/optotype chart) in one or both eyes that persists despite optimal refractive correction and is not attributable to a structural abnormality of the eye or visual pathway. It is a developmental disorder of cortical visual processing.

PATHOPHYSIOLOGY: During the critical period of visual development (peak in the first few years of life, plasticity through ~7-8 years), the visual cortex requires clear, concordant input from both eyes. Abnormal input leads to competitive inhibition: the brain suppresses input from the weaker/misaligned eye. Ocular dominance columns in layer IV of the primary visual cortex (V1) for the deprived eye shrink, while those of the normal eye expand (Hubel and Wiesel experiments with monocular deprivation in kittens/monkeys; Nobel Prize 1981). Lateral geniculate nucleus (LGN) layers receiving input from the amblyopic eye also show atrophy. The effect is irreversible if the deprivation continues beyond the critical period.

CAUSES (3 main types): 1. Strabismic (most common): misaligned eyes (esotropia > exotropia). Brain suppresses the deviated eye to avoid diplopia. Eyes are misaligned, so there is no stereopsis. 2. Refractive: anisometropia (unequal refractive error between eyes, typically hyperopia difference ≥1-1.5 D) or bilateral high refractive error (isoametropia). The eye with the greater error gets a blurred image. May be missed because the eyes look straight. 3. Deprivational (least common, worst prognosis): media opacity or obstruction, e.g., congenital cataract, ptosis covering the visual axis, corneal opacity, vitreous hemorrhage. Requires urgent intervention (cataract removal within the first weeks to months of life).

CLINICAL FEATURES: - Usually asymptomatic; the child doesn't complain because the other eye compensates. - Poor depth perception/stereopsis; may have strabismus, head tilt, squinting, or difficulty with fine visual tasks. - Crowding phenomenon: better recognition of isolated optotypes than lines of letters. - Anterior segment and fundus exam is normal (diagnosis of exclusion, but exam is needed to rule out organic causes such as retinoblastoma, cataract, optic nerve hypoplasia). - Relative afferent pupillary defect is typically absent or very mild.

DIAGNOSIS/SCREENING: - Red reflex testing (Brückner test) at every newborn and well-child visit; absent, dull, or asymmetric red reflex or leukocoria needs urgent evaluation (cataract, retinoblastoma). - Corneal light reflex (Hirschberg test) and cover-uncover test for strabismus. - Photoscreening/autorefractors and visual acuity testing (age-appropriate optotypes) at age 3-5 years (USPSTF recommends vision screening at least once in children 3-5 years to detect amblyopia or its risk factors). - Cycloplegic refraction to detect refractive errors.

MANAGEMENT (earlier is better): 1. Correct the underlying cause: eyeglasses for refractive error (often sufficient alone initially), surgery for cataract or ptosis, and strabismus surgery for alignment (alignment alone doesn't treat amblyopia). 2. Force use of the weaker eye: patching (occlusion) of the better eye for several hours/day, or pharmacologic penalization with atropine drops in the better eye (blurs near vision). Both have similar efficacy (PEDIG trials). 3. Newer options: binocular/dichoptic digital therapy. - Treatment can still be beneficial beyond age 7-8 but is less effective. - Complications of therapy: occlusion amblyopia in the patched eye (reversible if monitored), and recurrence after stopping treatment, so follow-up is needed.

BOARD PEARLS: - Most common cause of monocular vision loss in children; most common cause of unilateral vision loss in adults under 40-ish. - Amblyopia is a cortical problem, not a retinal one. - Strabismus is the most common cause; deprivation is the worst prognosis. - Absence of the red reflex or leukocoria: think congenital cataract or retinoblastoma. Treat urgently to prevent deprivational amblyopia. - Adults with strabismus acquired after the critical period get diplopia, not amblyopia, because the brain is no longer plastic enough to suppress. - Hubel and Wiesel: monocular deprivation during the critical period causes permanent shrinkage of ocular dominance columns of the deprived eye. - Don't confuse with amaurosis (blindness from organic disease) or with other causes of decreased acuity, such as optic neuritis, which has an RAPD.