Partial or complete loss of vision in one eye caused by conditions that affect normal visual development. These conditions can include an imbalance in the positioning of the eyes, such as strabismus, in which the eyes are crossed inward (esotropia) or turned outward (exotropia). Amblyopia can also result from a major difference in refractive error between the two eyes, such as nearsightedness, farsightedness, or astigmatism. Less common causes of amblyopia include ptosis (drooping) of one eyelid, disease of the cornea (preventing light from entering the eye), congenital cataract, and injury to the eye of a young child.

In amblyopia, the brain favors one eye over the other. The other eye is ignored. It is not adequately stimulated and the visual brain cells do not mature normally. Anisometropia occurs when the eyes have an unequal refractive power. Because the brain cannot reconcile this difference, it develops a preference for the image coming from one eye only.

Amblyopia is the most common cause of monocular blindness, impaired vision in one eye. Amblyopia affects 2 to 3 of every 100 children in the US. The most frequent cause is strabismus. Treatment of strabismus may involve surgical correction of the eye muscle imbalance and wearing an eye patch over the stronger eye for 2 hours a day.