Cataracts - Introduction

A cataract is an opacity, or clouding, of the lens of the eye.

The lens of an eye is normally clear. If the lens becomes cloudy or is opacified it is called a cataract.

The prevalence of cataracts increases dramatically with age. It typically occurs in the following way:

The lens is an elliptical structure that sits behind the pupil and is normally transparent. The function of the lens is to focus light rays into images on the retina (the light-sensitive tissue at the back of the eye).
In young people, the lens is elastic and changes shape easily, allowing the eyes to focus clearly on both near and distant objects.
As people reach their mid-forties, biochemical changes occur in the proteins within the lens, causing them to harden and lose elasticity. This causes a number of vision problems. For example, loss of elasticity causes presbyopia, or far-sightedness, requiring reading glasses in almost everyone as they age.
In some people, the proteins in the lens, notably those called alpha crystallins, may also clump together, forming cloudy (opaque) areas called cataracts. They usually develop slowly over several years and are related to aging. In some cases, depending on the cause of the cataracts, loss of vision progresses rapidly.
Depending on how dense they are and where they are located, cataracts can block the passage of light through the lens and interfere with the formation of images on the retina, causing vision to become cloudy.

Click the icon to see an image of eye anatomy.

Cataracts can form in any of three parts of the lens and are named by their location.

Nuclear cataracts. They form in the nucleus (the inner core) of the lens. This is the most common variety of cataract associated with the aging process.
Cortical cataracts. These form in the cortex (the outer section of the lens).
Posterior subcapsular cataracts. These form toward the back of a cellophane-like capsule that surrounds the lens. They are more frequent in people with diabetes, who are overweight, or those taking steroids.