Astigmatic axis is related to the level of spherical ametropia

Against-the-rule (ATR) astigmatism has been shown to be a risk factor for subsequent myopia development. In this study, we evaluated the relationship between astigmatic axis and the level of spherical ametropia in both myopes and hypermetropes.

Astigmatic axes were analyzed in two distinct cohorts. First, 53 high myopes from families that were recruited for linkage analysis were compared with an age-matched control group derived from family members. Second, cross-sectional data were analyzed for 90,884 subjects attending 19 optometric practices in the north of England. Initially, the relationship between astigmatic axis and cylinder power and between axis and sphere power were analyzed in 21- to 40-year olds and 21- to 30-year olds, respectively, to control for the effects of age. Multivariate logistic regression analysis was then performed using data for all compound astigmats in the cohort to examine the effect of sphere power, cylinder power, age, and sex on the odds of subjects having either ATR or with-the-rule (WTR) astigmatism.

In the genetic study cohort, there was an excess of WTR astigmats in the high myopes compared with controls, but this only reached significance for the right eye. In the much larger optometric practice sample, the association of WTR astigmatism with high myopia was highly significant. A parallel increase in WTR astigmatism was also found for high hypermetropes. In addition, the odds of having WTR astigmatism were increased if subjects were young or had a high cylinder power. ATR astigmatism occurred more often with increasing age and in subjects with lower spherical ametropia. Indeed, for 21- to 30-year-old subjects with low myopia (> or =-2.00 DS in the least minus meridian), ATR occurred more often than WTR astigmatism.

Astigmatic axis was found to be related to the level of ametropia, with both a higher spherical component or higher cylinder power increasing the odds of astigmatism being WTR. Low ametropes, particularly myopes, were more likely to have axes ATR.