Background/aims Iris colour might contribute to refractive development, but it is uncertain whether it is related to astigmatism. We aim to examine the association of iris colour with the presence of astigmatism in a school-based sample of Chinese students.

Methods 2346 grade 7 students from 10 middle schools aged 13 to 14 years in Southwestern China participated in the study. We obtained standardised slit-lamp photographs and developed a grading system assessing iris colour (higher grade denoting darker). Astigmatism was defined as a cylinder power of more than 0.50, 0.75 or 1.00 dioptre (D). Logistic regression models with generalised estimating equation were fitted to assess the relationship between iris colour and astigmatism, accounting for the correlation between both eyes. ORs and 95% CIs were presented.

Results The overall prevalence of astigmatism for three different definitions was 30.4% (95% CI 28.6% to 32.2%) (<−0.5 D), 12.7 % (95% CI 11.3% to 14.0%) (<−0.75 D) and 5.3% (95% CI 4.4% to 6.2%) (<−1.0 D), respectively. In multivariate analysis adjusting for the effect of gender and height, darker iris colour was associated with an increasing trend of astigmatism (p for trend <0.05). Compared with individuals with iris colour of grade 4 or 5 (the darkest), those with grade 1 or 2 (the lightest) were significantly less likely to be affected by astigmatism (<−0.75 D) in gender-adjusted model (OR 0.67) and multivariate-adjusted model (OR 0.72).

Conclusion Darker iris colour might be a risk factor for astigmatism in Chinese adolescents.