The axis of astigmatism in right and left eye pairs

This study was designed to investigate the relationship between the astigmatic axes of right and left eye pairs, with particular attention given to determining the degree to which either direct or mirror symmetry (enantiomorphism) of the astigmatic axes exists. A sample of 192 "nonvisually" selected adults participated, with refractive error and corneal curvature data being measured using autokerato-refractive equipment. Total, corneal, and residual astigmatism were investigated, with residual astigmatism being taken as the vector difference between total and corneal astigmatism. There was no evidence for a predominance of either mirror or direct symmetry of the astigmatic axes within this sample. The patterns of astigmatic axis distribution of right and left eyes were remarkably similar but, within this context, there was no definite evidence for a definable association between the axis of the left and right pairs of individuals. These findings remained unchanged when the effect of the modulus of astigmatism was incorporated, either through weighting the frequency distributions or through analyzing a subgroup of the overall population (those individuals with greater than 0.50 D of astigmatism). We believe these findings contradict commonly held clinical impressions regarding the symmetry of astigmatic axes, and should be considered when performing statistical analysis of astigmatic data.