Cataracts are a clouding of the eye’s lens that prevents a clear image from forming on the retina, blurring vision. Most are related to aging. They are very common in older people. By age 80, more than half of all Americans either have a cataract or have had cataract surgery.

A cataract can occur in either or both eyes. It cannot spread from one eye to the other. Although most cataracts are related to aging, there are other types:

• Secondary cataracts can form after surgery for other eye problems, such as glaucoma. They also can develop in people who have other health problems, such as diabetes. Cataracts are sometimes linked to steroid use.

• Traumatic cataracts can develop after an eye injury, sometimes years later.

• Congenital cataracts sometimes appear in newborn babies or are developed in childhood, often in both eyes. These may be so small that they do not affect vision. If they do, the lenses may need to be removed.

• Radiation cataracts can develop after exposure to some types of radiation.

The eye’s lens is made of mostly water and protein, which is arranged in a precise way that keeps the lens clear and lets light pass through it. But as people age, some of the protein may clump together and start to cloud a small area of the lens. This is a cataract. Over time, it may grow larger and cloud more of the lens, making it harder to see.

Researchers suspect that there are several causes, such as smoking and diabetes. Or, it may be that the protein in the lens just changes from the wear and tear it takes over the years.

The most common symptoms are:

• Cloudy or blurry vision.
• Colors seem faded.
• Glare. Headlights, lamps, or sunlight may appear too bright. A halo may appear around lights.
• Poor night vision.
• Double vision or multiple images in one eye. (This symptom may clear as the cataract gets larger.)
• Frequent prescription changes in your eyeglasses or contact lenses.
• These symptoms also can be a sign of other eye problems. If you have any of these symptoms, check with an eye care professional.

Cataract is detected through a comprehensive eye exam that includes:

• Visual acuity test. This eye chart test measures how well you see at various distances.

• Dilated eye exam. Drops are placed in your eyes to widen, or dilate, the pupils. Your eye care professional uses a special magnifying lens to examine your retina and optic nerve for signs of damage and other eye problems. After the exam, your close-up vision may remain blurred for several hours.

• Tonometry. An instrument measures the pressure inside the eye. Numbing drops may be applied to your eye for this test.

Early symptoms may be improved with new eyeglasses, brighter lighting, anti-glare sunglasses, or magnifying lenses. If these measures do not help, surgery is the only effective treatment. Surgery involves removing the cloudy lens and replacing it with an artificial lens.

A cataract needs to be removed only when vision loss interferes with your everyday activities, such as driving, reading, or watching TV. Sometimes a cataract should be removed even if it does not cause problems with vision. For example, a cataract should be removed if it prevents examination or treatment of another eye problem, such as age-related macular degeneration or diabetic retinopathy.