Abstract
Purpose: :
Determining the visual impact of residual astigmatism in pseudophakic eyes is critical for the management of post-operative refractive error after cataract surgery. This study determined the impact of induced and habitual astigmatism on monocular distance and near acuity of pseudophakes with monofocal IOL implants.
Methods: :
Expt I: +2.5D myopic to -2.0D hyperopic astigmatism in 0.5D steps was induced along one of four axes (0º, 45º, 90º and 135º) by placing a trial lens over one eye (fellow eye was occluded) of 15 emmetropic pseudophakes (Mean+1SD: 57.9+9.0yrs), after five weeks or more of uncomplicated cataract surgery. High contrast distance (4m) and near (40cm) logMAR acuity was recorded for each combination of lens power and axis. Expt II: Monocular distance and near acuity and pupil size was recorded in 41 pseudophakes (60.9+9.8yrs) with varying magnitudes of habitual residual astigmatism after uncomplicated cataract surgery.
Results: :
Mean (+1SD) uncorrected distance and near acuity was -0.06+0.08 and +0.6+0.15, respectively in Expt I and +0.21+0.19 and +0.55+0.20, respectively, in Expt II. Distance acuity reduced monotonically with increasing astigmatism (p<0.001) for all axes tested (p=0.52), more so in induced myopic than in induced hyperopic condition (p<0.05 for all). Near acuity did not change with induced hyperopic astigmatism (p>0.1) while it improved significantly with up to +1D of induced myopic astigmatism (p<0.001), before saturation for all axes tested (p=0.66). The loss in distance acuity with 1D and 2D of induced myopic astigmatism was poorly correlated with the corresponding gain in near acuity for all axes tested (r<0.15, p>0.05 for both). The overall trend in distance and near acuity with habitual astigmatism in Expt II was similar to that of Expt I. The near acuity for a given magnitude of astigmatism, however, varied with pupil size, with smaller pupils showing better near acuity than larger pupils.
Conclusions: :
Near acuity of pseudophakic eyes changes with both the magnitude of astigmatism and pupil size. Improved near acuity with myopic astigmatism is associated with a proportional loss in distance acuity for all axes tested. The monotonic loss in distance acuity and the saturation of near acuity suggests that visual experience of pseudophakes with myopic astigmatism >1D may be sub-optimal with a large loss of distance acuity and no additional benefit to near vision.
Keywords: astigmatism • visual acuity • intraocular lens