Cataract is one of the most common causes of impaired vision as well as the third leading cause of blindness in the United States. Cataract surgery is one of the safest and most successful of all operations. The National Eye Institute has estimated that if the progression of cataract could be slowed enough to delay the need for surgery by even 10 years, the current annual number could be reduced by 45 percent.

Little is known about the relative importance of various potential risk factors in the development of cataract. Most current information on risk factors has come from anecdotal reports or from relatively small case-control studies. One major project, the Framingham Eye Study, has identified several factors that were significantly associated with subsequent cataract formation, including diabetes and dietary factors. Diabetes has long been thought to increase the risk of developing cataract.

Recently, aspirin has been proposed as a drug that can prevent cataract formation or slow its progression. Aspirin may affect tryptophan levels in patients with cataract, or it may inhibit aldose reductase, an enzyme associated with the development of diabetic cataract. Thus, data from this study sought to determine whether one 325-mg aspirin tablet, taken on alternate days, protects against cataract formation. The data also sought to reveal other additional cataract risk factors that emerge after simultaneous controlling for other variables.

The other primary objective of this trial was to assess the antioxidant effects of beta-carotene (50 mg on alternate days) on cataract development. In addition, factors that have been suggested to be cataractogenic were assessed in prospective cohort studies. These factors included age, blood pressure, blood cholesterol, height, diabetes, medication use, cigarette smoking, and history of previous eye trauma or surgery. In addition, the possible associations between history of vitamin E and selenium intake and cataract were explored.

This trial was part of the Physicians Health Study, an ongoing, randomized, placebo-controlled clinical trial of aspirin in the prevention of cardiovascular mortality and of beta-carotene in the prevention of cancer. Following randomization, each of the 22,071 physicians enrolled was assigned to one of four groups to take either aspirin or its placebo and beta-carotene or its placebo. Follow-up questionnaires were sent 6 and 12 months after randomization and every 12 months thereafter. The randomized aspirin component of the trial was terminated early (January 1988), after an average followup of approximately 5 years, because of a statistically extreme 44 percent reduced risk of a first myocardial infarction in the aspirin group.

Since this study is conducted by mail among physicians nationwide, examinations cannot be performed on all patients to determine when they have reached an end point. Reported diagnoses of cataract are confirmed by medical record review. The primary analysis will be of incidence of cataract in the aspirin and placebo groups. In addition, the Cox proportional hazards model will be used to determine whether there is a difference in time to cataract diagnosis between the two groups. It has been postulated that the potent antioxidant properties of beta-carotene might make it effective in preventing cataract development. The investigators will thus determine whether there is a difference in the numbers of cataracts between the beta-carotene/placebo groups and the aspirin/placebo groups.