Effects of Lens-Induced Astigmatism at Near and Far Distances.

Purpose: To investigate and compare the degradation of visual acuity (VA) in myopic presbyopes due to lens-induced astigmatism at near and at far distance.

Patients and methods: Fourteen corrected myopic presbyopes were recruited. VA (logarithm of the minimum angle of resolution) was measured binocularly for different conditions of lens-induced astigmatism: cylindrical powers of -0.25, -0.50, -0.75, -1.00, -1.50, and -2.00 diopters (and positive spherical power of half the cylindrical power) with two axis orientations (with-the-rule WTR and against-the-rule ATR) were added to their optical correction. Measurements were carried out at far and near distance both in photopic and mesopic conditions, and for high and low contrast (HC/LC) stimuli. The paired Wilcoxon signed-rank statistics test was used to evaluate difference between conditions.

Results: The measured VA as a function of the lens-induced astigmatism was described by regression lines in all investigated experimental conditions. The angular coefficients (slopes) of these lines represent the VA degradation, ie, the variation in logMAR corresponding to the addition of 1.00 diopters of cylindrical power. In photopic HC conditions, the VA degradation is significantly more pronounced at far distance than at near distance (0.22±0.06 diopters^-1 vs 0.15±0.05 diopters^-1, p = 0.0061 in WTR conditions; 0.18±0.06 diopters^-1 vs 0.12±0.05 diopters^-1, p = 0.0017 in ATR conditions), although VAs at near and at far with zero cylinder were similar (-0.14±0.10 vs -0.14±0.08, p = 0.824).

Conclusion: The better tolerance to lens-induced astigmatism blur at near than at far distance in photopic conditions with HC stimuli is tentatively attributed to a possible experience-mediated neural compensation associated to the tendency of the eye toward an inherent astigmatism at near.