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Amblyopia is a decrease in best corrected visual acuity, usually in an otherwise anatomically normal eye. It is generally classified by cause, with three primary types:
- Refractive amblyopia: resulting from uncorrected refractive error (improper focusing of light by the eye which generally requires correction by eyeglasses or contact lenses), with the following subtypes:
- Anisometropic amblyopia: resulting from asymmetric refractive error and resultant unilateral blurring. This condition is the most common cause of refractive amblyopia.
- Ametropic amblyopia: resulting from significant refractive error in both eyes
- Meridional amblyopia: resulting from large uncorrected astigmatism in one or both eyes
- Strabismic amblyopia: resulting from strabismus (misalignment of the eyes) and subsequent lack of an image that can be “fused” or integrated into a single image in the brain. This condition is most likely with early onset, constant strabismus. Approximately 50% of patients with strabismus will also have amblyopia.
- Deprivation amblyopia: resulting from optical or anatomic pathology (e.g., cataract, ptosis, corneal opacity, prolonged patching), which prevents the formation of a clear image in one or both eyes. Deprivation, especially if it begins early in life, is associated with the most severe amblyopia.
Amblyopia is the most common cause of unilateral vision loss in children and young adults.
Large population-based studies indicate that 0.8–3.3% of the adult population has amblyopia.
- Visual acuity in amblyopic eyes varies from minimal impairment (20/25) to legal blindness (<20/200) or worse. Other significant impairments in amblyopic eyes may include reduced contrast sensitivity, reduced or absent binocularity and depth perception, and impaired or distorted spatial perception. Peripheral visual fields are preserved, and vision is never completely lost (no light perception) from amblyopia alone.
- Asymmetric input between the two eyes (e.g., unilateral cataract, anisometropia, etc.) is more likely to cause amblyopia than symmetrically poor images due to competitive influences between the two eyes. As a result, amblyopia is usually unilateral.
- Bilateral amblyopia may result from severe, symmetric bilateral image degradation such as bilateral cataract, bilateral high ametropia (high refractive error), etc.