The cause of most cataracts is unknown, but at least two factors associated with aging contribute to cataract development. Clumping of proteins in the lens leads to scattering of light and a decrease in the transparency of the lens. Also, the breakdown of lens proteins leads to the accumulation of a yellow-brown pigment that clouds the lens.
Researchers have found certain chemical changes 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.
When cataracts form, levels of calcium and sodium in the lens increase, and levels of potassium, vitamin C, and protein decrease. In addition, lenses with cataracts appear to be deficient in the antioxidant glutathione. However, studies on the use of medications or vitamins to alter the levels of these substances in the lens have not produced promising results.
Currently, no effective drug therapy exists to prevent cataracts from forming. However, cigarette smoking, certain medications, eye injuries, sunlight, diabetes, and even obesity can increase the risk of cataracts. Several years ago, a study of elderly men suggested that exposure to lead from paint in old houses or from gasoline dramatically increased the risk of cataracts. But this risk has likely diminished for people who live in the United States, because lead exposure also has lessened over the years.
Cigarette smoking raises the risk of developing a cataract. Why cigarette smoking has an adverse effect on the lens is not clear. One possibility is that smoking may 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 a study of individuals taking oral prednisone for one to two years, cataracts developed in 11 percent of those taking less than 10 mg a day, 30 percent of those taking 10 to 15 mg a day, and 80 percent of those taking more than 15 mg a day. Shortterm use of oral corticosteroids is unlikely to lead to cataracts.
Several studies show that inhaled corticosteroids also can raise the risk of cataracts. In addition, the larger the dose of an inhaled corticosteroid, the greater the likelihood of cataract surgery. The results from these studies may concern people with asthma who take inhaled corticosteroids for that condition. However, the benefits of using inhaled corticosteroids for asthma outweigh the long-term risk of cataracts, which are treatable.
Cataracts can also develop as a result of applying topical corticosteroids to the eyelids or using corticosteroid-containing eyedrops (however, the more common side effect from corticosteroids is elevated intraocular pressure, which can lead to glaucoma). To reduce the risk of these adverse effects, topical corticosteroids 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 certain chemicals can cause clouding of the lens, either immediately or later on. Rapid formation of cataracts commonly occurs after a penetrating eye injury.
Sunlight and Ionizing Radiation
Population studies have shown that prolonged exposure to the ultraviolet (UV) radiation in sunlight more than doubles the risk of cortical cataracts. In one study, the risk of developing cortical cataracts was two times greater in people with the highest levels of sunlight exposure than in those with the lowest exposure levels. However, the study found that nuclear cataracts were not linked to sunlight exposure.
Ionizing radiation (from x-rays, for example) also can cause cataracts.
Individuals with diabetes are at increased risk for cataracts, particularly the posterior subcapsular type, and they tend to be affected at an earlier age than people who do not have diabetes. Some evidence indicates that the accumulation of sorbitol (a sugar formed from glucose) in the lens promotes cataract formation in people with diabetes.
Being overweight also may put you at risk for developing cataracts. You can determine whether you are overweight or obese by calculating your body mass index (BMI), a measure of weight in relation to height. Multiply your weight in pounds by 704 and divide that number by the square of your height in inches. Overweight is defined as a BMI between 25 and 29.9; obesity is defined as a BMI of 30 or greater.
A study of men between the ages of 40 and 84 compared levels of BMI with risk of developing cataracts. Having a BMI between 22 and 28 increased cataract risk by about 50%, and a BMI above 28 more than doubled the risk. A high BMI was associated most often with posterior subcapsular cataracts. Other researchers have found similar results in women.
Although the reason for the link between obesity and cataracts is unclear, researchers believe that maintaining a proper weight may reduce cataract formation by decreasing blood glucose levels or by improving the antioxidant properties of the blood.