Minimum amount of astigmatism that should be corrected

Abstract

Purpose: To evaluate how small amounts of astigmatism affect visual acuity and the minimum astigmatism values that should be corrected to achieve maximum visual performance.

Setting: Optics Laboratory, University of Murcia, Murcia, Spain.

Design: Case series.

Methods: A wavefront sensor was used to measure astigmatism and higher-order aberrations (HOAs) in normal young eyes with astigmatism ranging from 0.0 to 0.5 diopter (D). Astigmatism was corrected for natural pupil diameters using a purpose-designed cross-cylinder device. Visual acuity was measured for high-contrast and low-contrast stimuli at best subjective focus with the natural and corrected astigmatism. From the aberrations, optical image-quality metrics were calculated for 3 conditions: natural astigmatism, corrected astigmatism, and astigmatism only (with all HOAs removed).

Results: The study evaluated 54 eyes. There was no significant correlation between the amount of astigmatism and visual acuity. The correction of astigmatism improved visual acuity for only high-contrast letters from 0.3 D, but with a high variability between subjects. Low-contrast visual acuity changed randomly as astigmatism was corrected. The correction of astigmatism increased the mean image-quality values; however, there was no significant correlation with visual performance. The deterioration in image quality given by astigmatism higher than 0.3 D was limited by HOAs.

Conclusions: In most subjects, astigmatism less than 0.5 D did not degrade visual acuity. This suggests that under clinical conditions, the visual benefit of precise correction of astigmatism less than 0.5 D would be limited.