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The Merck Manual of Medical Information--Home Edition
Section 20. Eye Disorders
Chapter 223


A cataract is a cloudiness (opacity) in the eye's lens that impairs vision.

click on thumbnail to see full-size imageCataracts produce a progressive, painless loss of vision. Their cause usually isn't known, although they sometimes result from exposure to x-rays (such as high-dose radiotherapy to the eye) or strong sunlight, inflammatory eye diseases, certain drugs (such as corticosteroids), or complications of other diseases such as diabetes. They're more common in older people; babies can be born with cataracts (congenital cataracts).


Because all light entering the eye must pass through the lens, any part of the lens that blocks, distorts, or diffuses light can cause poor vision. How much vision deteriorates depends on where the cataract is and how dense (mature) it is.

In bright light, the pupil constricts, narrowing the cone of light entering the eye, so that it can't easily pass around the cataract. Thus, bright lights are especially disturbing to many people with cataracts, who see halos around lights, glare, and scattering of light. Such problems are particularly troubling when a person moves from a dark to a brightly lit space or tries to read with a bright lamp. People with cataracts who also take glaucoma medication that constricts the pupils may have greater vision loss.

A cataract at the back of the lens (posterior subcapsular cataract) particularly interferes with vision in bright light. It affects vision more than other cataracts because the opacity is at the point where light rays cross.

Surprisingly, a cataract in the central part of the lens (nuclear cataract) may improve vision at first. The cataract causes light to be refocused, improving vision for objects close to the eye. Older people, who generally have trouble seeing things that are close, may discover that they can read again without glasses, a phenomenon often described as gaining second sight.

Although cataracts usually aren't painful, rarely they cause swelling in the lens and increased pressure in the eye (glaucoma), which can be painful.

Diagnosis and Treatment

A doctor can see a cataract while examining the eye with an ophthalmoscope (an instrument used to view the inside of the eye). Using an instrument called a slit lamp, a doctor can see the exact location of the cataract and the extent of its opacity.

Usually, people who have a cataract can determine when to have it surgically removed. When people feel unsafe, uncomfortable, or unable to perform daily tasks, they may be ready for surgery. There's no advantage to having surgery before then.

Before deciding on surgery, a person with a cataract can try other measures. Eyeglasses and contact lenses may improve vision. For certain kinds of cataracts in people who don't have glaucoma, medications that keep the pupil dilated may help. Wearing sunglasses in bright light and using lamps that provide reflected lighting rather than direct lighting decrease glare and aid vision.

Cataract surgery, which can be performed on a person of any age, usually doesn't require general anesthesia or an overnight hospital stay. During the operation, the lens is removed, and usually a new plastic or silicone lens is inserted; this artificial lens is called a lens implant. Without a lens implant, people usually need a contact lens. If they can't wear a contact lens, they can try eyeglasses, which are very thick and tend to distort vision.

Cataract surgery is common and usually safe. Rarely, after the operation, a person may develop an infection or a hemorrhage in the eye, which can lead to a serious loss of vision. Older people in particular should make arrangements in advance to get extra help at home for a few days after surgery. For a few weeks after surgery, eyedrops or ointment is used to prevent infection, reduce inflammation, and promote healing. To protect the eye from injury, the person wears glasses or a metal shield until healing is complete, usually a few weeks. The person visits the doctor the day after surgery and then typically every week or two for 6 weeks.

Sometimes people develop an opacity behind a lens implant weeks or even years after it is implanted. Usually, such an opacity can be treated with a laser.

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