Purpose: To examine the roles of genetic and environmental factors in the optic disc in a classic twin study.
Methods: Five hundred six pairs of twins were recruited to participate from the Twins U.K. Adult Twin Registry at St. Thomas' Hospital, London. Photographs of the optic disc were obtained, and the covariance of optic disc, cup, and rim areas within monozygotic (MZ) and dizygotic (DZ) pairs was compared, and genetic modeling techniques were used to determine the relative contributions of genes and environment to the variation in optic disc parameters in this population.
Results: Mean optic disc, cup, and rim areas were 2.62 (range, 1.06-4.87), 0.98 (range, 0.25-4.72), and 1.64 (range, 1.43-6.16) mm(2), respectively. The MZ correlations were higher than those of DZ pairs for disc and cup areas (correlation coefficient, 0.73/0.81 and 0.41/0.50 for MZ and DZ twins, respectively). The correlation for optic rim was also higher in MZ (0.62) than in DZ (0.43) pairs. Modeling suggested heritability for the optic disc area of 0.73 and for the optic cup area of 0.66. The heritability of the rim area was lower at 0.34, with a significant shared environmental component of 0.27 and individual factors (including measurement error) explaining 39% of the variance of the rim area.
Conclusions: This study has demonstrated that genetic effects were important in the determination of optic disc parameters in this twin population, with genetic factors explaining 73%, 66%, and 34% of the variation of optic disc, cup, and rim areas, respectively. Environmental factors also seemed to be important.