Factors aVecting 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 environment in the variance of pupil size after mydriasis, and to examine the eVects of other factors such as age, iris colour, and refractive error. Methods—506 twin pairs, 226 monozygotic (MZ) and 280 dizygotic (DZ), aged 49–79 (mean age 62.2 years, SD 5.7) were examined. Dilated pupil size was measured using a standardised grid superimposed over digital retroillumination images taken 50–70 minutes after mydriasis using tropicamide 1% and phenylephrine 10%. Univariate maximum likelihood model fitting was used to estimate genetic and environmental variance components. Results—Dilated pupil size was more highly correlated in MZ compared with DZ twins (intraclass correlation coeYcients 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 environmental factors explained 18–19% of the variance in this population. Age only accounted for 2–3% of the variance and refractive error and iris colour did not significantly contribute to the variance. Conclusions—Pupil size after mydriasis is largely genetically determined, with a heritability of up to 80%. (Br J Ophthalmol 2000;84:1173–1176) In clinical practice there is a wide variation in pupil size after mydriasis. Large pupils make cataract surgery easier and quicker and allow for more thorough examination of peripheral retina, which may be important in the success of retinal detachment surgery. Some conditions are known to reduce the size of dilated pupils, such as diabetes mellitus and pseudoexfoliation. However, it is not known to what extent genes and environment are important in determining the size of pupils in the population. Twin studies have been described as the “perfect natural experiment” to decide between the relative importance of genetic and environmental factors. They are based on comparison of the concordance between identical or monozygotic (MZ) twin pairs and nonidentical or dizygotic (DZ) twins. MZ twins share the same genes and DZ twins share only half of their genes; any greater similarity between MZ twins can therefore be attributed to this additional genetic sharing. A classic twin study was performed to examine the heritability of pupil size after mydriasis. Dilated pupil size was measured in a large sample of twins and quantitative genetic modelling used to estimate the relative role of genes and environment. In addition, other factors such as the eVect of iris colour, age, and refractive error on pupil size were studied.