Cataract is the word used to describe an area of increasing cloudiness in the lens of the eye — the lens of the eye is located behind the iris -the coloured part of the eye. A cataract may begin as a white spot at the edge of the lens or as a haze at the centre of the lens and gradually worsens until the vision is badly blurred.

Clouded vision can make it more difficult to read, drive a car or see the expression on a friend's face. Cataracts commonly affect distance vision and cause problems with glare. They generally don't cause pain, double vision with both eyes open or abnormal tearing.

Most cataracts develop slowly and don't disturb your eyesight early on. But as the clouding progresses, the cataract eventually interferes with your vision.

In the early stages, stronger lighting and eyeglasses can help you deal with the vision problems. But at some point, if impaired vision jeopardizes your normal lifestyle, you might need surgery. Fortunately, cataract removal is one of the safest, most effective and most common surgical procedures.

Signs and symptoms

A cataract usually develops slowly and causes no pain. At first, the cloudiness may affect only a small part of the lens and you may be unaware of any vision loss. Over time, however, as the cataract grows larger, it clouds more of your lens. When significantly less light reaches the retina, your vision becomes impaired.

Symptoms of a cataract include:

• Clouded, blurred or dim vision

• Increasing difficulty with vision at night

• Sensitivity to light and glare

• Halos around lights

• The need for brighter light for reading and other activities

• Frequent changes in eyeglass or contact lens prescription

• Fading or yellowing of colors

• Double vision in a single eye

If you have a cataract, light from the sun, lamps or oncoming headlights may seem too bright. Glare and halos around lights can make driving uncomfortable and dangerous. You may experience eyestrain or find yourself blinking more often to clear your vision.

Cataracts don't typically cause any change in the appearance of your eye or in the production of tears. Pain, redness, itching, irritation, aching in your eye or a discharge from your eye aren't signs or symptoms of a cataract, but may be signs and symptoms of other eye disorders.

A cataract isn't dangerous to the physical health of your eye unless the cataract becomes completely white, a condition known as an overripe (hypermature) cataract. This can cause inflammation, pain and headache. A hypermature cataract is extremely rare and needs removal as soon as possible.

• Headache

Causes

A cataract can develop in one or both eyes, and it may or may not affect the entire lens. The lens is located just behind the iris and the pupil. It's shaped like a magnifying glass — thicker in the middle and thinner near the edges. Tiny ligaments, which are bands of tough tissue fiber, hold it in place.

When your eyes work properly, light passes through the cornea and the pupil to the lens. The lens focuses this light, 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 consists of three layers. The outer layer (capsule) is a thin, clear membrane. It surrounds a soft, clear material (cortex). The hard 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. A cataract can form in any part of the lens.

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 yellow or greenish and begins to cloud your vision. As the cataract progresses, the lens may even turn brown. Seeing in dim light and driving at night may be especially troublesome.

• Cortical. A cortical cataract begins as whitish, wedge-shaped 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 nucleus. Both your distance and near vision can be impaired. Focusing problems and distortion are common. You may also have problems with glare and loss of contrast.

• Subcapsular. A subcapsular cataract starts as a small, opaque area just under the capsule shell. It usually forms at the back of the lens, right in the path of light on its way to the retina. This type of cataract may occur in both eyes but tends to be more advanced in one eye than the other. A subcapsular cataract often interferes with your reading vision, reduces your vision in bright light and causes glare or halos around lights at night.

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 more dense and involves a greater part of the lens.

Scientists don't know why a lens changes with age. One possibility is damage caused by unstable molecules known as free radicals. Smoking and exposure to 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.

• Nicotine dependence

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 rubella (German measles) 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.

Risk factors

Everyone is at risk of developing cataracts simply because age is the single greatest risk factor. By age 65 about half of all Americans have developed some degree of lens clouding, although it may not impair vision.

Other factors that increase your risk of cataracts include:

• Diabetes

• Family history of cataracts

• Previous eye injury or inflammation

• Previous eye surgery

• Prolonged use of corticosteroids

• Excessive exposure to sunlight

• Smoking

• Nicotine dependence

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