Parameters associated with the different astigmatism axis orientations.

PURPOSE To evaluate parameters associated with astigmatism axis orientation. METHODS A retrospective population-based study was conducted on 67,899 (53% males) Israeli Defense Force conscripts aged 16 to 22 years with >or=0.25 D of astigmatism, using prerecruitment examination and demographic data that included a validated general intelligence score. Refractive errors were classified by the cylinder axis with the least deviation from emmetropia (LDE), a scheme intended to avoid confounding spherical classification by cylinder power. Perinatal photoperiod was determined from birth date and astronomical tables. RESULTS With-the-rule (WTR) axis was associated with higher LDE (P < 0.001), higher cylinder power (P < 0.001), Eastern or Western compared with Israeli origin (P < 0.001), higher body mass index (P < 0.001), lower intelligence scores (P < 0.001), and longer perinatal photoperiod (P < 0.001), using univariate analysis. Multivariate logistic regression confirmed the independent association of these parameters and identified being of the female sex as another WTR axis association. Subjects' cylinder axis also was associated with their siblings' cylinder axis (odds ratio [OR] = 1.58; 95% confidence interval [CI] = 1.31-1.90), using siblings' refractive data for the 3852 available subjects in the regression model. However, the associations of these parameters with WTR, against-the-rule, or oblique astigmatism were neither parallel as a whole nor parallel between any two sets of axis orientations. Further, the directions of the associations of these parameters with astigmatism axis and power did not consistently conform to their previously reported associations for spherical ametropias. CONCLUSIONS The results suggest both that distinctive mechanisms may account for the different astigmatism axis orientations and that mechanisms influencing astigmatism development are likely to vary from those governing the spherical component of refraction.