The association between residual astigmatism and refractive errors in a population-based study.

PURPOSE: To examine the relationship of spherical equivalent (SE) with refractive, corneal, and residual astigmatism.

METHODS: Using stratified cluster sampling, 6,311 individuals were invited and 82.2% participated in this study. Examinations including uncorrected and corrected distance visual acuity tests, refraction, slit-lamp examination, and funduscopy were performed for all participants. Refraction and keratometry were used to calculate refractive and corneal astigmatism, respectively, and their difference was regarded as residual astigmatism.

RESULTS: Refractive astigmatism showed a U-shaped relationship with SE; with every diopter (D) increase in astigmatism, the SE increased by 0.230 D in hyperopes and 0.664 D in myopes (P < .001). With every 1.0 D increase in residual astigmatism, the SE showed 0.376 D myopic shift in myopes (P = .004) and 0.077 D hyperopic shift in hyperopes (P = .224). Mean SE was highest among cases of with-the-rule (WTR) astigmatism (P < .001). Cases of high myopia and high hyperopia mostly had WTR and most emmetropes had against-the-rule (ATR) refractive astigmatism. Residual astigmatism was mostly WTR in emmetropic cases; at higher levels of SE error in myopes and hyperopes, WTR residual astigmatism decreased, whereas ATR and oblique residual astigmatism increased.

CONCLUSIONS: Increases in SE are associated with higher levels of refractive and residual astigmatism among myopic and hyperopic cases. Unlike refractive astigmatism, WTR residual astigmatism was mostly seen in near emmetropic individuals.