Effects of Lens-Induced Astigmatism at Near and Far Distances

Abstract

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.

Effects of Lens-Induced Astigmatism at Near and Far Distances [Response to Letter]

“…The number of subjects does not allow the identification of potential differences between groups that are smaller than these specific thresholds reported in Tables 4–7. 2 Some differences exceeded these thresholds and were statistically significant. These are the differences discussed in the paper.…”

“…We thank Chaurasiya et al 1 for their interest in the paper “Effects of Lens-Induced Astigmatism at Near and Far Distances”, 2 for the positive comments, and also for highlighting some limitations that could serve as the starting point for further analysis.…”

“… 1 The age (mean ± standard deviations) of the participants was (57.0 ± 5.1) years (Table 2). 2 A study on age-related variations was not possible with a sample that has this limited age variability. A paragraph in the introduction concerns the age dependence based on data taken from the scientific literature.…”

“…A paragraph in the introduction concerns the age dependence based on data taken from the scientific literature. 2 In the discussion section, the age of the recruited subjects is mentioned for the purpose of comparing the findings of the work with those available in the literature. 2 …”

“… 2 In the discussion section, the age of the recruited subjects is mentioned for the purpose of comparing the findings of the work with those available in the literature. 2 …”