The impact of on-axis astigmatism on peripheral aberrations and through-focus blur anisotropy

Abstract

Purpose : Imposed on-axis astigmatism has been found to affect emmetropization in animals and peripheral astigmatism was proposed to affect emmetropization of the human eye. The relationship between on-axis astigmatism at different meridians and peripheral astigmatism has not yet been systematically studied. The goal of this study is to investigate the impact of on-axis astigmatism on peripheral vertical astigmatism (V-astig_, Z₂²) and peripheral through-focus retinal image quality.

Methods : The left eye aberrations of nine adults were measured across ±30° along the horizontal meridian using a custom-built wide-field scanning wavefront sensor. Subjects varied in spherical equivalent power (-9.50 to +3.75D) and were divided into three groups based on their on-axis astigmatism power: 1) ≤0.5D astigmatism (control) in any meridian (n=4); 2) ≥0.75D with-the-rule (WTR) astigmatism (n=4); 3) ≥0.75D against-the-rule (ATR) astigmatism (n=1). Zernike coefficients were calculated for a 5.5mm diameter circular pupil, and then were used to compute the point spread functions (PSFs) through focus. Blur orientation along the radial and azimuthal directions were derived from vector analysis of the PSFs. Through-focus blur anisotropy was quantified with the slope of the blur orientation changes over ±2.5D.

Results : V-astig increased monotonically from 0° (-0.41±0.71µm) to 30° (0.86±0.91µm) in all subjects. Horizontal coma (Z₃¹) changed more sharply across the field in subjects with larger on-axis spherical aberration (Z₄⁰). Among the three groups, average V-astig at 30° was higher in the ATR group (2.03µm) compared to the controls (1.13±0.43µm). However, in the WTR group, magnitude of peripheral V-astig was reduced (0.12±0.95µm) compared to the control group. These different trends were tightly correlated to through-focus blur anisotropy being stronger in the center for the WTR group (0.23±0.13) and in the periphery for the ATR group (0.22). At the periphery, the control group showed more anisotropic blur (0.22±0.04) than the WTR group (0.13±0.13), but weaker than that of the ATR group (0.29).

Conclusions : On-axis ATR astigmatism increases peripheral through focus blur anisotropy while WTR decreases it. Significant on-axis ATR or increased peripheral blur anisotropy could play a role in emmetropization and myopia development although a longitudinal study in young children is necessary to test the hypothesis.

This is a 2020 ARVO Annual Meeting abstract.