A cataract is the clouding of the normally clear, natural crystalline lens of the eye. The lens is composed of water and protein. The protein is arranged in a highly organized pattern that allows light to pass through it with minimal distortion. As a result, the lens appears virtually clear. The lens can become cloudy (see Causes of Cataracts), blocking or scattering some light and preventing it from reaching the retina in sharp focus. This causes blurred vision and glare.

Simulation of vision clouded by a cataract. (Photo courtesy NEI)

Most cataracts progress slowly over a period of years, but their rate of progression is unpredictable. They can affect one eye or both eyes. As cataracts become more dense, they produce visual symptoms; these typically include blur, glare, halos around lights, and double vision. Colors can become dull, a brown-yellow tint is common, and driving can become dangerous. Untreated, cataracts can cause blindness. In the U.S., however, cataracts are usually treated when they begin to interfere with activities of daily living such as reading and driving.