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

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.

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.

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).

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.