Visual acuity in simple myopic astigmatism: influence of cylinder axis.

PURPOSE The relationship between astigmatic refractive errors and their associated visual acuity has been studied in recent years in the context of refractive power space. The influence of the axis of astigmatism remains a matter of controversy. Our goal in this study is to provide additional experimental evidence to clarify this subject. The influence of the simulated axis orientation was compared with other factors that affect visual acuity such as the particular design of the test and the differences between eyes. METHODS Simple myopic astigmatism from 0 to -3.00 D, in steps of -0.25 D, and with five different axes between 0 degrees and 90 degrees , were simulated on four healthy eyes of young observers. In each case, visual acuity was recorded for three different tests. Refractions were expressed in the form of vectors and visual acuity was represented as a function of strength. RESULTS No significant differences in visual acuity were found for astigmatism of the same power but different axes. In fact, our results show these differences are even less important than those recorded for the same astigmatism induced in different eyes. The highest discrepancies in visual acuity were found when different charts were used to test the same astigmatic error. CONCLUSIONS The strength of the vector representing the astigmatic refractive state describes very accurately the performance of visual acuity across simple myopic astigmatic errors. In these cases, visual acuity can be associated with a single refractive parameter. This fact could be useful, especially in statistical studies involving visual performance.