Prescribing for Presbyopia

Is 0.75D of Astigmatism the Right Flinch Level?

BY THOMAS G. QUINN, OD, MS, FAAO

Astigmatism of 0.75D is often cited as the level at which astigmatic correction should be incorporated into a contact lens. Is there any science to support this assertion? Richdale et al (2007) enrolled 30 subjects who had astigmatic refractive error from 0.75DC to 2.00DC. They then compared visual acuity performance of these individuals while they wore spherical versus toric soft contact lenses.

During the data analysis, the subjects were separated into two groups: those who had a low level of astigmatism (≤1.00D) and those who had a moderate level (1.25D to 2.00D). It’s no surprise that those in the moderate group benefited significantly from astigmatic correction (11-letter improvement while viewing a high-contrast target).

For the low-astigmatic-level group, in a well-lit (photopic) setting, while observing a high-contrast target, acuity improved about half a line (three letters). In a low-light (mesopic) setting, while viewing the same target, the improvement nearly doubled (5.5 letters). That’s about a line of acuity.

Supporting Evidence

These findings agree with findings from an earlier study by Bayer and Young (2005). In this investigation, 150 subjects who had 0.75D or 1.00D of astigmatism were fit with toric soft contact lenses; 62 were wearing spherical soft contact lens at the start of the study. When comparing visual performance of these individuals, acuity was found to improve by nearly a line (from 20/25 to 20/20) with wear of the toric lenses.

Is It Worth It?

You might ask whether one line of acuity is worth the added time and expense of fitting someone with a toric lens? Bayer and Young’s study provides some insight on this question.

Of the 150 subjects enrolled in their study, 97% (146/150) were dispensed toric contact lenses. Of these, 91% needed only one diagnostic fitting to achieve a successful fit. Most of these (95%) were dispensed toric contact lenses with an axis within 10º of their spectacle axis.

These findings demonstrate that fitting a toric soft lens to a patient who has 0.75D to 1.00D of astigmatism is not only visually beneficial, it can be done with a high rate of success and takes little to no additional time over what is required to fit a spherical soft lens.

The Answer

When we find a spectacle change that can improve vision by a line of acuity, most of us wouldn’t hesitate to recommend that our patients update their glasses. Why set a different standard for our contact lens wearers?

We are vision care specialists, and our contact lens wearers deserve the best that we can offer. Let’s provide it. Fit the 0.75D astigmat with a contact lens that offers full visual correction…and the clarity that comes with it. Science says that it can be done effectively and efficiently. Science, and our mission, says that it’s the right thing to do. CLS

For references, please visit www.clspectrum.com/references and click on document #231.

Dr. Quinn is in group practice in Athens, Ohio. He is an advisor to the GP Lens Institute and an area manager for Vision Source. He is an advisor or consultant to Alcon and B+L, has received research funding from Alcon, AMO, Allergan, and B+L, and has received lecture or authorship honoraria from Alcon, B+L, CooperVision, GPLI, SynergEyes, and STAPLE program. You can reach him at tgquinn5@gmail.com.