cataract, in medicine, opacity of the lens of the eye, which impairs vision. In the young, cataracts are generally congenital or hereditary; later they are usually the result of degenerative changes brought on by aging or systemic disease (diabetes). Cataracts brought on by aging are most common; most individuals over 60 exhibit some degree of lens opacity. Injury, extreme heat, ultraviolet light, X rays, nuclear radiation, inflammatory disease, and toxic substances also cause cataracts. There is growing concern that further disintegration of the ozone layer will increase the incidence of cataracts. Advanced cataracts are usually treated by surgical removal of the lens and implantation of an artificial lens. After cataract surgery, which is the most common surgical procedure in the United States, most patients do not require thick glasses or contact lenses.

Opacity of the eye's crystalline lens. Cataracts causing central visual-field defects are most likely to affect vision. Cataracts may occur in newborns and infants. Diabetes mellitus, prolonged exposure to ultraviolet rays, or trauma can cause them in adults, but they most often occur with age, resulting from gradual loss of transparency of the lens. Treatment is a surgical procedure to replace the lens with an artificial one.

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