Methods: A group of 53 twins were included in the study: 24 monozygotic (MZ) (mean age: 52.4±5.5 y/o) and 29 dizygotic (DZ) (mean age: 54.7±6.8 y/o). A complete ophthalmological exam was performed for all subjects, excluding all those with ocular pathologies that were known to increase intraocular scatter. Subjects with suspected cataracts were excluded, using as exclusion criteria having a Lens Opacities Classification System-III (LOCS-III) value for nuclear, cortical or subcapsular cataract above grade II. Both eyes in each subject were measured using three different and complementary techniques to evaluate intraocular scattering. A Hartmann-Shack wavefront sensor (AOnEye; Voptica SL, Murcia, Spain) provided a low-angle scatter parameter (less than 0.5 degrees), similar to the Objective Scattering Index (OSI) (Artal et al. PLoS One, 2011). A compact optical instrument based in the principle of optical integration (Sigma, Sinusmedii SL, Spain) allowed the estimation of a straylight parameter (SS) for a retinal angle of 5.5 degrees. Additional psychophysical measurements of straylight (SC) were also performed (c-Quant, Oculus GmbH, Germany). Due to high correlation between both eyes data from one single eye per subject was randomly selected. Intraclass Correlation Coefficients (ICC) were used as a descriptive statistics of twin resemblance and genetic models were fitted to estimate heritability.
Results: On average, ICCs between siblings were similar for the three measured scatter parameters. ICC values for OSI were 0.738 in MZ and 0.530 in DZ twins; for SS were 0.623 in MZ twins and 0.869 in DZ twins, and for SCwere 0.741 in MZ and 0.664 in DZ twins. The estimates of heritability were low (0.3) for all the measured scattering parameters.
Conclusions: Correlations of the estimation of intraocular scatter and straylight in the groups of identical and non-identical twins were similar. Heritability estimates were of limited magnitude. These results suggest that environmental and individual factors are probably dominant to determine the level of straylight in healthy eyes of middle-aged adults.