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Causes
By Mayo Clinic staff
A cataract can develop in one or both of your eyes. However, in most cases — except for those caused by injury or trauma — cataracts tend to develop symmetrically in both eyes. A cataract may or may not affect the entire lens.
When your eyes work properly, light passes through the cornea (the protective dome of clear tissue over the front of your eye) and the pupil (the hole in the center of your eye) to the lens. The lens is located just behind the iris (the colored part of your eye) and the pupil. It's shaped thicker in the middle and thinner near the edges. Tiny ligaments, which are bands of tough tissue fiber, hold it in place. The lens focuses light that passes through the cornea and pupil, producing clear, sharp images on the retina — the light-sensitive membrane on the back inside wall of your eyeball that functions like the film of a camera. As a cataract develops, the lens becomes clouded, which scatters the light and prevents a sharply defined image from reaching your retina. As a result, your vision becomes blurred.
The lens: Its makeup and its changes
The lens consists of three layers. The outer layer (capsule) is a thin, clear membrane. It surrounds a soft, clear material (cortex). The harder center of the lens is the nucleus. If you think of the lens as a piece of fruit, the capsule is the skin, the cortex is the fleshy fruit and the nucleus is the pit.
As you age, the lenses in your eyes become less flexible, less transparent and thicker. The lens is made mostly of water and protein fibers. The protein fibers are arranged in a precise manner that makes the lens clear and allows light to pass through without interference. With aging, the composition of the lens undergoes changes and the structure of the protein fibers breaks down. Some of the fibers begin to clump together, clouding small areas of the lens. As the cataract continues to develop, the clouding becomes denser and involves a greater part of the lens.
A cataract can form in any part of the lens.
Types of cataracts
Cataracts occur in three types:
- Nuclear. A nuclear cataract occurs in the center of the lens. In its early stages, as the lens changes the way it focuses light, you may become more nearsighted or even experience a temporary improvement in your reading vision. Some people actually stop needing their glasses. Unfortunately, this so-called second sight disappears as the lens gradually turns more densely yellow and further clouds your vision. As the cataract progresses, the lens may even turn brown. Seeing in dim light and driving at night may be especially troublesome. Advanced discoloration can lead to difficulty distinguishing between shades of blue and purple.
- Cortical. A cortical cataract begins as whitish, wedge-shaped opacities or streaks on the outer edge of the lens cortex. As it slowly progresses, the streaks extend to the center and interfere with light passing through the center of the lens. Problems with glare are common for people with this type of cataract.
- Subcapsular. A subcapsular cataract starts as a small, opaque area just under the capsule of the lens. It usually forms near the back of the lens, right in the path of light on its way to the retina. A subcapsular cataract often interferes with your reading vision, reduces your vision in bright light and causes glare or halos around lights at night.
Scientists don't know exactly why a lens changes with age. One possibility is damage caused by unstable molecules known as free radicals. Smoking and exposure to ultraviolet (UV) light are two sources of free radicals. General wear and tear on the lens over the years also may cause the changes in protein fibers.
Other causes of cataracts
Age-related changes in the lens aren't the only cause of cataracts. Some people are born with cataracts or develop them during childhood. Such cataracts may be the result of the mother having contracted German measles (rubella) during pregnancy. They may also be due to metabolic disorders. Congenital cataracts, as they're called, don't always affect vision, but if they do they're usually removed soon after detection.
