Amblyopia, or lazy eye, is an acquired defect in vision caused by a deficient visual experience early in life.  It is usually found in one eye only but may be affecting both eyes. Amblyopia itself produces no change in the appearance of the eye, but it nearly always develops in association with some other condition, such as a high refractive error or eye turn, which is observed through an eye examination and which is responsible for an inadequate visual experience.

The word amblyopia (derived from Greek roots meaning “blunt sight”) is occasionally still applied to other disorders in which partial loss of vision is congenital or acquired through processes unrelated to visual experience.  The degree of amblyopia is determined by observing how well the patient can read a standardized eye chart with the best visual correction.  

The visual system is sensitive to the effects of abnormal visual experience only during a limited time in infancy and childhood when it is immature and still changing. For humans, this period extends roughly from birth through the end of the first ten years of life. Vulnerability is greatest during the first few months of life and decreases gradually thereafter, with apparently considerable variation from person to person in the degree of sensitivity at a particular age.

Abnormal early visual experience can affect monocular vision through either or both of two amblyopia causing mechanisms: In the first, lack of exposure to the sharply focused images necessary for normal development disturbs and limits the development of normal vision.

In the second, marked disparity in the quality or directionality of inputs from the two eyes prevents binocular fusion and results in abnormal competitive binocular interaction, which leads to active interference with, or exclusion of, one eye’s input to higher visual centers that persists during single eye viewing. This is caused by a persistent eye turn, also called strabismus. This disturbs development of normal vision in the brain’s visual center.   

Either mechanism can contribute to one eye or two eye amblyopia: The two eyes may be affected simultaneously by the first mechanism or sequentially by the second.

Refractive amblyopia may be corrected over time by wearing glasses or contact lenses with the correct prescription full time.  The earlier a patient wears the correct prescription, the better the outcome of achieving normal vision be.  In strabismic amblyopia with large eye turns, surgery may be needed to fully correct the eye and restore normal vision.  Patching the better eye will often encourage the patient to use the amblyopic eye and stimulate the visual system to develop correctly, resulting in improved vision.