Amblyopia

Reduced best corrected visual acuity in one or both eyes that cannot be attributed directly to structural eye disease, caused by abnormal visual experience during the sensitive period of visual development (birth to about 7 or 8 years, with plasticity persisting in some patients into the teens).

• Affects about 2 to 4 percent of the population; it is the commonest cause of unilateral visual loss in children and adults under 40.
• Types:
  • Strabismic: constant unilateral deviation causing suppression of the deviating eye.
  • Anisometropic: unequal refraction (hypermetropia difference above 1.00 dioptre, myopia above 3.00, astigmatism above 1.50).
  • Isoametropic (bilateral): high bilateral refractive error (hypermetropia above +5.00, myopia above -6.00, astigmatism above 2.50).
  • Stimulus deprivation: cataract, ptosis covering the pupil, corneal opacity, vitreous haemorrhage, eyelid haemangioma; the most severe type.
  • Meridional: uncorrected astigmatism in early life.
• Definition used clinically: interocular difference of two or more logMAR lines (or 0.2 logMAR) with a cause, or best corrected acuity worse than 6/12 (20/40) in a child over 3 years.

• Usually asymptomatic; detected by screening, strabismus, or poor performance on vision testing.
• Crowding phenomenon: acuity is better on single optotypes than on a line of letters.
• Reduced stereoacuity; reduced contrast sensitivity; neutral density filter effect minimal (differentiates from organic disease, where a 3.0 neutral density filter reduces acuity markedly).
• Normal fundus and anterior segment (except causes of deprivation); no relative afferent pupillary defect in most (mild defect possible in dense amblyopia).
• Fixation preference test: child objects to covering the better eye; inducing a strong eye preference in infants.

• Age appropriate acuity testing: preferential looking (Teller cards) in infants, HOTV or Lea symbols in preschoolers, Snellen or logMAR in older children; test each eye separately and with a crowded chart.
• Cycloplegic refraction (cyclopentolate 1 percent) with correction of any refractive error and a dilated fundus examination to exclude retinal or optic nerve disease.
• Cover test for strabismus; red reflex and slit lamp for media opacity; assessment of binocular function and stereopsis.
• Neuroimaging or electrodiagnostic tests (visual evoked potentials, electroretinography) when acuity is poorer than the refraction and media explain.
• Screening: photoscreening, autorefraction, and vision testing at 3 to 5 years.

• Remove the obstacle: cataract extraction, ptosis surgery, treatment of haemangioma with propranolol (2 to 3 mg/kg/day in divided doses), correction of media opacity.
• Optical correction: full cycloplegic correction in spectacles for 12 to 16 weeks (refractive adaptation), as acuity often improves on its own; repeat acuity measurement before adding other therapy.
• Occlusion therapy (patching the better eye):
  • Moderate amblyopia (6/12 to 6/24, 20/40 to 20/80): 2 hours daily.
  • Severe amblyopia (worse than 6/24): 6 hours daily (up to full time for deprivation amblyopia in infants, with close monitoring to avoid reverse amblyopia).
  • Review every 4 to 8 weeks; continue until acuity plateaus for two visits.
• Pharmacological penalisation: atropine 1 percent one drop daily or two days a week in the better eye (equally effective to patching for moderate amblyopia, better adherence); watch for photophobia, flushing, and reverse amblyopia.
• Optical penalisation (blurring spectacle lenses), Bangerter filters.
• Binocular treatments: dichoptic digital games and movies that present different contrast to each eye (used for children aged 4 to 12 years).
• Treat strabismus with surgery or glasses after the visual acuity has been improved.
• Maintenance: taper over 3 to 6 months to prevent recurrence; recurrence occurs in about 25 percent; follow until 8 to 10 years.
• Treatment is still worthwhile in older children (7 to 17 years) and sometimes adults with 2 to 6 hours of patching and perceptual learning, though gains are smaller.