prescribing for astigmatism
Astigmatism Correction Means Great Visual Acuity
BY JOHN MARK JACKSON, OD, MS, FAAO
The title of this column is a no-brainier — patients see better when their vision is fully corrected. But I'm still surprised by the number of patients I see whose contact lenses don't correct their astigmatism. Even more troubling is the number who believe or have been told by their practitioner that contact lenses can't correct their astigmatism!
Are there really that many in our profession who think soft torics can't correct acuity well, that they're too time-consuming to fit and/or that they're too expensive? Or maybe these practitioners think "masking" the cylinder with a soft sphere will work? Or that using an aspheric design will do the trick?
Better Vision With Torics
It's quite clear from both clinical experience and research findings that patients see better when their contact lenses correct their astigmatism. Richdale et al (2007) evaluated the acuity of myopic astigmats wearing both soft spherical and toric lenses from four manufacturers. This study was a well-masked randomized trial and measured acuity with high- and low-contrast charts in high- and low-light conditions. The findings aren't surprising: astigmatic patients see better when wearing toric lenses. The effect was most dramatic for patients whose astigmatism was between –1.25DC and –2.00DC. For these patients, soft torics improved their11/Dec/2007 18:19 acuity by about two Snellen lines in both bright and dim light compared to spherical lenses. For low-cylinder patients the bright light results were more modest, yielding only a few letters more, but in dim light their acuity improved by just over one line. This improvement may not seem substantial, but in low-light conditions such as night driving, this can be a major benefit to patients' quality of vision.
Aspheric vs. Toric Lenses
What about aspheric lenses for low astigmats? Morgan et al (2005) evaluated high- and low-contrast acuity with spectacles, aspheric soft lenses and toric soft lenses. They used variable pupil sizes to simulate different lighting conditions. They found similar high- and low-contrast acuity with all corrections for small (2mm) pupils, but better acuity (about one line) with spectacles and toric lenses for 4mm and 6mm pupils.
My personal experience correlates with these findings; in well-lit, high-contrast conditions (such as the exam lane), low-cylinder astigmats often have pretty good acuity with aspherics, but patients often complain about night driving if they aren't wearing a toric. When you don't correct the cylinder component, the spherical aberration correction doesn't meet its full potential. We now have some designs that are both toric and aspheric, allowing for correction of both astigmatism and spherical aberration. This should provide enhanced acuity for these patients.
Give Astigmats Better Vision
Of course, good acuity with a soft toric requires the lens to be stable, and most of today's designs are quite rotation-resistant. Combine that with low-cost disposable torics, and you have a recipe for success for astigmats. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #145.
Dr. Jackson is an assistant professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses and performs clinical research.