Evaluation of risk factors for cataract types in a competing risks framework.

In prospective studies of incident cataract, a person may have developed two or all three of cortical, nuclear sclerotic and posterior subscapsular cataract at the time when clinically significant cataract is apparent. One can compare the impact of risk factors on different types of cataract with methods of competing risk survival analysis that account for tied events. We studied risk factors for incident cataract among 20,599 participants in the Physicians' Health Study; 1,923 of whom developed cataract that reduced visual acuity over a median follow-up of 13.0 years. Among incident cases, 45% had two types of cataract and an additional 12% had all three. Proportional hazards models, adjusted for tied failures through use of generalized estimating equations, provided comparisons across different cataract types. Stratified analyses by type with age as the time scale were used to accommodate the strong but different relationships of age with occurrence of different cataract types. Analyses found different relationships of diabetes and body mass index, but similar relationships of smoking, family history of coronary disease and multivitamin use, across different cataract types. A simplified model provided common estimates for relative risks associated with those characteristics with similar effects across cataract types. Whereas polytomous logistic regression has proven useful for comparisons of risks across cataract types in retrospective and cross-sectional studies, this approach based on methods of survival analysis has greater flexibility in prospective studies with variable follow-up and frequently tied event times for different cataract types.