Factors affecting pupil size after dilatation: the Twin Eye Study

Background/aims-Well dilated pupils make eye surgery easier. A classic twin study was established to examine the relative importance of genes and envi ronment in the variance of pupil size after mydriasis, and to examine the e ffects of other factors such as age, iris colour, and refractive error. Methods-506 twin pairs, 226 monozygotic (MZ) and 280 dizygotic (DZ), aged 4 9-79 (mean age 62.2 years, SD 5.7) were examined. Dilated pupil size was me asured using a standardised grid superimposed over digital retroilluminatio n images taken 50-70 minutes after mydriasis using tropicamide 1% and pheny lephrine 10%. Univariate maximum likelihood model fitting was used to estim ate genetic and environmental variance components. Results-Dilated pupil size was more highly correlated in MZ compared with D Z twins (intraclass correlation coefficients 0.82 and 0.39 respectively). A model specifying additive genetic and unique environmental factors showed the best fit to the data, yielding a heritability of 78-80%. Individual env ironmental factors explained 18-19% of the variance in this population. Age only accounted for 2-3% of the variance and refractive error and iris colo ur did not significantly contribute to the variance. Conclusions-Pupil size after mydriasis is largely genetically determined, w ith a heritability of up to 80%.