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Monocular, binocular astigmatism affect contrast sensitivity differently
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Monocular astigmatism does not affect binocular contrast sensitivity in the same way binocular astigmatism does, according to a study.
Eighteen volunteers 22 to 42 years old with spherical equivalent between 0.00 D and – 6.00 D were recruited. Refraction was corrected with spectacles and binocular contrast sensitivity was measured.
Against-the-rule (ATR) or with-the-rule (WTR) astigmatism of +1.00 D, +2.00 D and +3.00 D was intentionally induced, first in both eyes and then in one eye only, and binocular contrast sensitivity was assessed again at five spatial frequencies. The area under the log contrast sensitivity function (AULCSF) was determined.
AULCSF decreased proportionally to the increase in astigmatic power in binocular ATR and WTR astigmatism conditions, while it was not affected by monocular astigmatism. ATR astigmatism resulted in significantly worse AULCSF than WTR astigmatism in binocular high-power (+2.00 D and +3.00 D) astigmatic defocus, while no difference between ATR and WTR was found with monocular astigmatic defocus.
According to the authors, there was no previous study assessing the effect of uncorrected monocular astigmatism on binocular contrast sensitivity. However, this is important, particularly for patients undergoing cataract surgery. If a pseudophakic eye has residual astigmatism, the astigmatism in the fellow eye should be aggressively corrected during cataract surgery and IOL implantation, the authors recommended.
“In recent years, toric intraocular lens, which can eliminate or reduce postoperative astigmatism, has been widely applied in clinical practice. Astigmatism, however, is not always and completely removed in each eye. Some patients have residual binocular or monocular astigmatism after operation. Therefore, it is crucial to clarify the influences of monocular or binocular uncorrected astigmatism on binocular contrast sensitivity,” the authors wrote. – by Michela Cimberle
Disclosure: The authors reported no relevant financial disclosures
Perspective
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Erin Jenewein, OD, MS, FAAO
This new study is the first to look at the effects of both monocular and binocular astigmatic defocus on binocular contrast sensitivity. Researchers induced both with- and against-the-rule astigmatism of +1.0 D, +2.0 D and +3.0 D in 18 adult subjects with normal visual acuity and binocularity and then tested binocular contrast sensitivity. They found that binocular contrast sensitivity was decreased only when astigmatism was induced binocularly and that monocular astigmatic defocus did not significantly decrease binocular contrast sensitivity.
Additionally, researchers found that binocular against-the-rule astigmatism decreased contrast sensitivity significantly more than with-the-rule astigmatism. Although significant monocular astigmatic anisometropia is only seen in a small percentage of our patient population, astigmatic anisometropia can often be seen in patients post refractive or cataract surgery.
The results of this study may help guide treatment for patients with astigmatism, as the researchers stated, “if [a] unilateral pseudophakic eye has residual astigmatism, astigmatism of the other eye should be aggressively corrected during cataract surgery.”
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