Possible causes of discordance in refraction in monozygotic twins: Nearwork, time outdoors and stochastic variation

PURPOSE. To evaluate the impact of differences in nearwork and time spent outdoors on difference in refraction in monozygotic (MZ) twins. METHODS. Data on MZ twins aged 7 to 18 years from the Guangzhou Twin Eye Study were used in this analysis. A standard questionnaire was administered by personal interview to estimate time spent on nearwork and time spent outdoors. Spherical equivalent (SE) was measured by autorefraction under cycloplegia. The interaction between age and nearwork or time spent outdoors was also estimated. RESULTS. A total of 490 MZ twin pairs (233 male and 257 female) were eligible in this analysis, the mean age was 13.14 ± 2.49. In the mixed-effects model, nearwork difference was a risk factor of discordance in myopic SE (β =-0.11 diopter (D)/h, P = 0.009), the overall association between time outdoors difference and SE discordance was not significant (β=- 0.89 (D)/h, P = 0.120) although an interaction between time spent outdoors difference and age was detected (β=- 0.07 (D)/h, P = 0.002). Furthermore, difference in nearwork and time outdoors explained about 1.8% and 2.5% of the variation in SE discordance, respectively. CONCLUSIONS. Given the very marked genetic similarity of MZ twins, and the small effects of known risk factors on SE discordance, we suggest that the SE discordance across MZ twins largely results from stochastic variations at the genomic or epigenetic levels, or from uncollected environmental factors.