Cataract Channel

Overview

A cloudiness or opacification of the lens is called a cataract. Derived from the Latin word meaning "waterfall," the term cataract arose from the ancient misconception that cataract symptoms were caused by evil liquids that mysteriously flowed into the eye. Cataracts can occur at any age-in fact, babies can be born with them-but they are most common later in life.

About 50 percent of people ages 65 to 74 and 70 percent of those 75 and older have cataracts. In 2004, an estimated 20.5 million Americans over 40 (approximately 17 percent) had a cataract; that number is expected to reach 30.1 million by 2020. However, not all cataracts affect vision significantly or require treatment.

About 1.5 million cataract surgeries are performed each year in the United States. Surgery is the only way to cure cataracts.

Need-to-know anatomy

Ordinarily, light rays reflected from an object enter the eye through the cornea and lens, which together focus the light onto the retina (the innermost layer of the eye, which consists of light-sensitive nerve tissue) to produce a sharp image. When a cataract develops, however, light rays are no longer precisely focused. Instead, the rays are scattered before reaching the retina.

The lens is made of protein fibers arranged in a specialized way so that the lens is transparent. The lens is composed of four distinct structures: At the center is the nucleus, which is surrounded by the cortex, then the lens epithelium, and finally the lens capsule. The three common types of cataracts are defined by where they occur in the lens: nuclear, cortical, and posterior subcapsular (in the rear of the lens capsule). Nuclear cataracts are the most common, and their incidence increases with age. Cortical cataracts also become more common with age and are related to lifetime exposure to ultraviolet light. Posterior subcapsular cataracts are most likely to occur in younger people and are often the result of prolonged use of corticosteroids (such as prednisone), inflammation, trauma, or diabetes. People often have more than one type of cataract in the same eye.

Causes

The cause of most cataracts is unknown, but at least two factors associated with aging contribute to cataract development. One, clumping (aggregation) of lens proteins leads to scattering of light and a decrease in the transparency of the lens. Two, the breakdown of lens proteins leads to the accumulation of a yellow-brown pigment that clouds the lens.

Certain chemical changes have been noted in the eyes of people with cataracts. These changes include a reduced uptake of oxygen by the lens and a rise in the water content of the lens, which is later followed by dehydration. Amounts of calcium and sodium in the lens increase, while levels of potassium, vitamin C, and protein decrease during cataract formation. In addition, glutathione (an antioxidant) appears to be deficient in lenses with cataracts. Studies on the use of medications or vitamins to alter the levels of these substances in the lens have not produced promising results, however.

The extent of vision damage and how quickly vision becomes impaired depend not only on the size and density of the cataract but also its location in the lens. For example, a cataract on the outside edge of the cortex has little effect on vision because it does not interfere with the passage of light through the center of the lens, while a dense nuclear cataract causes severe blurring of vision.

Risk factors

Cigarette smoking, medications such as corticosteroids, eye injuries, sunlight, diabetes, and even obesity can increase the risk of cataracts. Also, about 50 percent of people ages 65 to 74 and 70 percent of those 75 and older have cataracts.

Cigarette smoking is associated with an increased risk of cataracts. It is not clear why cigarette smoking has an adverse effect on the lens. One possibility is that smoking might reduce blood levels of nutrients required for lens maintenance.

Long-term use of corticosteroids , especially at high doses, is the most common drug-related cause of cataracts. In one study of individuals taking oral prednisone for prolonged periods, cataracts developed in 11 percent of those taking less than 10 milligrams a day, in 30 percent of those taking 10 to 15 mg a day, and in 80 percent of those taking more than 15 mg a day. Short-term use of oral corticosteroids is unlikely to lead to cataracts.

Inhaled corticosteroids also can raise the risk of cataracts. In one study, people who had used inhaled corticosteroids had a 50 percent greater prevalence of nuclear cataracts and a 90 percent greater prevalence of posterior subcapsular cataracts (in the rear of the lens capsule) than those who had not used inhaled corticosteroids. These results may cause concern for people with asthma, who often rely on inhaled corticosteroids to treat their condition. However, the benefits of inhaled corticosteroids for asthma outweigh the long-term risk of cataracts, which are treatable.

Cataracts can also develop from applying topical corticosteroids to the eyelids or using corticosteroid-containing eyedrops (though the more common side effect is elevated intraocular pressure, which may lead to glaucoma). To reduce the chance of these adverse effects, topical corticosteroids applied to the eye and corticosteroid-containing eyedrops should be used only under the supervision of an ophthalmologist.

Blunt trauma to the eye or damage to the eye from alkaline chemicals can cause opacification of the lens, either immediately or later on. Rapid cataract formation commonly occurs after a penetrating eye injury.

Population studies have shown that prolonged exposure to the ultraviolet radiation in sunlight more than doubles the risk of cortical cataracts. In one study, the more sunlight exposure, the higher the risk of cortical cataracts. However, the study found that nuclear cataracts were not linked to sunlight exposure. Ionizing radiation (from X-rays, for example) also can cause cataracts.

People with diabetes are at increased risk for cataracts, particularly the posterior subcapsular type, and these cataracts tend to occur at an earlier age in people with diabetes than in the general population. Some evidence indicates that the accumulation of sorbitol (a sugar formed from glucose) in the lens promotes cataract formation in people with diabetes.

Excess weight also may increase the odds of developing cataracts. Although the reason for the link between obesity and cataracts is unclear, it is thought that a low-calorie intake may reduce cataract formation by decreasing blood glucose levels or improving the antioxidant properties of the blood.