Heritability of intraocular pressure in older female twins

Purpose: To examine the heritability of intraocular pressure (IOP) among older women not diagnosed as having glaucoma. Design: Cross-sectional twin study. Participants: 94 monozygotic (MZ) and 96 dizygotic (DZ) female twin pairs aged 63-76 years and not diagnosed as having glaucoma. Methods: Intraocular pressure was measured using a noncontact tonometer. The contributions of genetic and environmental factors to individual differences in IOP were estimated by applying an independent pathway model to twin data. Main Outcome Measures: Contribution of genetic and environmental effects to the variation in IOP among MZ and DZ twins. Results: Mean IOP of the study population was 14.1 mmHg (+/- standard deviation 3.1) with no differences observed neither between the MZ and the DZ individuals, nor between the left and the right eyes. The pair-wise correlations for IOP of the right eye were .61 in MZ and .25 in DZ and for the left eye .63 and .42. The phenotypic correlation between the left and the right eye IOP was high (r = 0.81), suggesting that they were indices of a single trait. Quantitative genetic modeling revealed that for both eyes 64% (95% confidence interval [Cl], 53-71) of the variance in IOP was explained by additive genetic effects and 18% (95% Cl, 11-27) by nonshared environmental factors in common. In addition, 18% (95% Cl, 15-23) of the variance in IOP was explained by nonshared environmental factors specific to each eye. Conclusions: Additive genetic influences explained most of the individual differences in IOP among older women not diagnosed as having glaucoma. Because elevated IOP is an important risk factor for glaucoma, genetic factors underlying IOP may have a significant role in determining the risk for glaucoma, a complex progressive disease leading to death of ganglion cells.