A cataract is a clouding of the lens of the eye. The lens is normally clear. It focuses light on the retina, light-sensitive tissue in the back of the eye. If the lens is obscured by cataract, vision will be blurred.

The word cataract also means waterfall. People with cataracts may describe the change in vision as being similar to looking through a waterfall.

The lens is made mostly of water and protein. Over time these proteins may form clumps that start to cloud a small part of the lens. This is a cataract. Cataracts often worsen and interfere with vision. Cataracts can also cause yellowing or browning of the lens, therefore changing the appearance of some colors. Cataract formation usually occurs in both eyes and is painless. However, in some cases a cataract forms in only one eye.

Most cataracts are related to aging. By age 80, more than half of Americans will have a cataract or have had cataract surgery, according to the National Eye Institute (NEI). Women and non-Hispanic whites are more likely to get cataracts, according to the U.S. Centers for Disease Control and Prevention (CDC). However, they are more likely to cause blindness in blacks.

The NEI reports that people with diabetes are twice as likely to develop cataracts and to develop them at an earlier age. Scientists are not certain why diabetes increases the risk of cataracts, but hyperglycemia appears to be a factor. Other potential complications of diabetes include diabetic retinopathy and glaucoma.

Cataracts can cause secondary glaucoma. They can also interfere with eye tests and procedures such as fluorescein angiography.

Cataracts are a leading cause of low vision among Americans. More than 20 million Americans have cataracts, more than macular degeneration, diabetic retinopathy and glaucoma combined.