Many studies have assessed the visual impact of astigmatism by inducing it using loose trial lenses at set axes. There are conflicting opinions and some confusion about the results, but the reduction in vision is typically least with lens-induced with-the-rule astigmatism. In this study, we determined whether a participant’s habitual astigmatism influences the impact of lens-induced astigmatism on distance visual acuity (VA).
Participants included 35 adults (age range 19–79, average 41.3 ± 15.4 years), who were either astigmats (≥0.75DC; N = 21) or non-astigmatic controls (N = 14). Distance VA was measured using an automated FrACT (Freiburg VA) system with the participant’s habitual correction, and then with the addition of a +1.00DC lens at either 90 or 180 degrees (control) or at and perpendicular to the axis of habitual astigmatism (astigmats).
The reduction in VA with induced astigmatism was significantly affected by whether the participant was a with-the-rule astigmat, against-the-rule astigmat, or non-astigmat (p = 0.006). Lens-induced astigmatism axis ≈ 180 (160–20°) reduced the VA less than axis ≈ 90 (70–120°) in the control group (reduction in mean VA of 0.32 SD ± 0.10 with 95% confidence intervals of [0.27,0.37] compared to 0.42 ± 0.09 [0.37,0.47] logMAR) and with-the-rule astigmats (reduction in VA of 0.31 SD ± 0.06 [0.27,0.35] compared to 0.45 SD ± 0.12 [0.38,0.52] logMAR). The against-the-rule astigmats had a very similar reduction in VA with the ≈+1.00DC axis 180 condition compared to the ≈+1.00DC axis 90 condition (reduction in VA of 0.26 SD ± 0.09 [0.21,0.31] compared to 0.25 SD ± 0.14 [0.17,0.33] logMAR).
The impact of lens-induced astigmatism was dependent on the refractive status of the participants. When investigating the impact of lens-induced astigmatism, it is important to consider the participants’ habitual axis of astigmatism.