FEB 26, 2014
This editorial’s authors write that clinical evidence suggests that correction of astigmatism of 1 D or greater will positively impact refractive surgery outcomes.
However, they note that in presbyopic patients not being considered for a simultaneous vision multifocal correction, surgeons should be careful not to reduce low levels of against-the-rule astigmatism as this may aid the patient's postsurgical spectacle independence.
They explain that in presbyopes who have a monofocal distance correction, induced astigmatism (without spherical compensation) causes a greater loss in distance visual acuity with myopic than hyperopic astigmatism, regardless of the axis of the astigmatism. At near distances, up to 1 D of myopic astigmatism improves near visual acuity, whereas hyperopic astigmatism makes near vision worse.
They note that a paper in this same issue of British Journal of Ophthalmology investigated the effect of uncorrected astigmatism using lenses to induce astigmatism without compensating for the change in mean spherical equivalent. The patients were presbyopes, but their eyes were cyclopleged to simulate presbyopia. Reading speed and threshold reading acuity decreased with increasing uncorrected astigmatism, even as low as −0.75 D. The effect was greater (worse) with with-the-rule compared to against-the-rule astigmatism for both reading speed and threshold reading acuity.
They also mention another study that found that when induced postoperative astigmatism was up to 1 D, multifocal IOLs achieved good visual acuity at both distance and near, although induced astigmatism reduced distance vision compared with monofocal IOL-corrected control patients.
They say that the age-related change to against-the-rule astigmatism is fortuitously beneficial to increasing the range of clear focus in the presbyope.