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 (J0, J45) in a nationally representative sample of schoolchildren. Refractive and corneal J0 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 (J0, J45). Multivariate linear regression assessed associations between BMI and J0, J45, 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 J0, indicating increased with-the-rule astigmatism. Associations were small but consistent in both age groups: 12–13 years (corneal J₀ β = 0.03, p < 0.001; refractive J₀ β = 0.02, p < 0.001) and 6–7 years (corneal J₀ β = 0.03, p = 0.008; refractive J₀ β = 0.03, p = 0.007). Associations with J45 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 J0, 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.