The heritability of age-related cortical cataract: The twin eye study

PURPOSE. A classical twin study was performed to establish the relative imp ortance of genes and environment in cortical cataract. METHODS. Five hundred six pairs of unselected female twin volunteers (226 m onozygotic and 280 dizygotic) with a mean age of 62 years (range, 49 -79 ye ars) were examined. Cortical cataract was assessed using the slit-lamp- bas ed Oxford Clinical Cataract Classification and Grading System (clinical gra ding) and the Wilmer Automated Grading System, which analyzed digital retro illumination images of subjects' lenses (digital grading). The worse eye ca tegorized score for each individual was used in maximum likelihood path mod eling of the correlations within twin pairs. These correlations were used t o determine the underlying liability to cortical cataract. RESULTS. Prevalence of significant cortical cataract (greater than or equal to5% of lens area) was similar in monozygotic and dizygotic twins, occurri ng in 19.4% and 20.6% with the clinical grading system and 24% and 23% usin g the digital grading system, respectively. Modeling suggested liability to cortical cataract is explained by additive and dominant genes, individual environment, and age. Estimates of the broad sense heritability of cortical cataract were 58% (95% confidence interval [CI], 51%-64%) for the clinical grading system and 53% (35% CI, 45%-60%) for the digital system. Dominant genes were estimated to contribute to 38% (95% CI, 1%-64%) of the genetic e ffect with the clinical grading and 53% 05% CI, 28%-60%) with the digital g rading. Individual environment explained 26% and 37% and age 16% and 11% of cortical cataract variance in clinical and digital gradings, respectively. CONCLUSIONS. Genetic effects are important in the development of cortical c ataract and involve the action of dominant genes.