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. 2000 Oct;84(10):1173–1176. doi: 10.1136/bjo.84.10.1173

Factors affecting pupil size after dilatation: the Twin Eye Study

C Hammond 1, H Snieder 1, T Spector 1, C Gilbert 1
PMCID: PMC1723260  PMID: 11004106

Abstract

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 effects 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 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 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%.



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Figure 1  .

Figure 1  

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Path model for the measured variables of dilated pupil sizes for the right eye of twin 1 (pupil 1) and twin 2 (pupil 2) and age which are represented in squares. Latent factors are represented in circles: A, C, and E are the additive genetic, common environmental, and unique environmental influences. D, the dominant genetic influence, was also tested but is omitted to simplify the diagram. The correlation between the latent genetic factors is 1 for MZ pairs and 0.5 for DZ pairs. For the common environmental factors it is 1 for MZ and DZ pairs. Regression coefficients of the observed variables on the different latent factors are shown in lower case: h is the additive genetic effect, V the age effect, c the common environment effect, e the unique environmental path coefficient, and sd the standard deviation of age.

Figure 2  .

Figure 2  

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Scatter plots of the pupil sizes of the right eye plotted for twin 1 against twin 2 in MZ twin pairs (left) and DZ twin pairs (right). r = correlation coefficient.

Selected References

These references are in PubMed. This may not be the complete list of references from this article.

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