Body Mass Index is Associated With Direction-Specific Increases in With-the-Rule Corneal and Refractive Astigmatism in Schoolchildren

Purpose Examine the association between body mass index (BMI) and astigmatism directionality using vector analysis of refractive and corneal components ( J 0 , J 45 ) in a nationally representative sample of schoolchildren. Refractive and corneal J 0 was the primary outcome, with secondary aims to assess whether BMI influences astigmatism severity or other refractive and axial parameters. Methods This cross-sectional study recruited 1626 schoolchildren aged 6–7 ( n = 728) and 12–13 years ( n = 898) from 37 randomly selected schools in Ireland. Cycloplegic autorefraction and ocular biometry were obtained. Height and weight were measured and BMI calculated and analysed as a continuous predictor, with WHO-defined BMI categories used for secondary severity modelling. Refractive and corneal astigmatism were decomposed into vector components ( J 0 , J 45 ). Multivariate linear regression assessed associations between BMI and J 0 , J 45 , spherical equivalent refraction (SER), axial length (AL) and axial length/corneal radius (AL/CR) ratio, adjusted for sex, ethnicity, socioeconomic status and urban/rural location. Ordinal logistic regression examined associations between BMI category and clinically defined astigmatism severity. Results Higher BMI was associated with a positive shift in refractive J 0 , indicating increased with-the-rule astigmatism. Associations were small but consistent in both age groups: 12–13 years (corneal J ₀ β = 0.03, p < 0.001; refractive J ₀ β = 0.02, p < 0.001) and 6–7 years (corneal J ₀ β = 0.03, p = 0.008; refractive J ₀ β = 0.03, p = 0.007). Associations with J 45 were weak and inconsistent. BMI was not associated with SER, AL or the AL/CR ratio. Compared with non-overweight peers, children with obesity had higher odds of moderate-to-severe astigmatism (6–7 years: OR = 5.26, 95% CI: 1.70–9.37; 12–13 years: OR = 2.50, 95% CI: 1.00–4.34). Conclusions Higher BMI was independently associated with increased J 0 , indicating greater with-the-rule refractive and corneal astigmatism, without evidence of an effect on axial length or overall refractive status. These cross-sectional findings suggest a potential biomechanical influence on corneal toricity, highlighting the need for longitudinal studies to clarify causality and mechanisms.