Methods to quantify the relation between disease progression in paired eyes.

The authors compared, in the context of diabetic retinopathy, alternative methods of quantifying the extent to which disease progression in one eye increases the risk of subsequent progression in the other eye. Data were gathered on 478 US patients with insulin-dependent diabetes mellitus who participated in the 1983-1988 Sorbinil Retinopathy Trial and were followed up for a median of 41 months. During that time, diabetic retinopathy progressed in 93 right eyes and 77 left eyes. Crude incidence rates of progression for right eyes were 7.7 times higher after the left eye had progressed and, for left eyes, were 4.4 times higher after the right eye had progressed. In eye-specific proportional hazards models that adjusted for increasing rates of progression over time and for baseline risk factors, the comparable relative risks associated with progression in the other eye were 2.6 (95% confidence interval (CI): 1.5, 4.7) for right eyes and 1.4 (95% CI: 0.72, 2.9) for left eyes. Two alternative proportional hazards models that included data on both eyes and accounted for their correlation produced estimated relative risks of 1.9 (95% CI: 1.2, 2.9) and 2.7 (95% CI: 1.8, 3.5), respectively. The more complex models for joint survival integrate information on both eyes and provide more stable estimates than do separate analyses of right or left eyes.