prescribing for astigmatism

RGPs vs. Soft Torics

BY THOMAS G. QUINN, OD, MS
DECEMBER 1999

The astigmatic patient in your chair has expressed interest in wearing contact lenses. What do you fit him with? Practitioners faced with this decision seem to belong to one of two groups: those who prefer to fit RGPs and those who prefer to fit soft torics. To which of these groups do you belong?

Hopefully, you'd choose the design that best meets the patient's visual, comfort and physiological needs. Deciding which design best meets these goals depends on the patient's desired wearing schedule, activities, visual demands, comfort demands and refractive error. The following provides some ideas to get you started on the right path:

A Fitting Primer

Have your patients complete a questionnaire at the time of their appointment. Be sure to inquire about occupational and recreational activities to get an idea of what visual situations the patient finds himself in on a daily basis. Ask if he has previously worn contact lenses and if he is presently interested in pursuing contact lens correction. If so, is he interested in daily wear, extended wear or part-time wear? This information will have a major influence on your decision-making process.

It's also important to consider the patient's refractive status and each lens design's potential for achieving good vision, comfort and physiology.

Specific Considerations

As a general rule, consider spherical RGPs for patients with less than 2.00D of with-the-rule (WTR) astigmatism because the steeper vertical meridian aids RGP positioning and enhances lens movement during the blink cycle. Great vision, simple design and low cost are all benefits of this design.

Conversely, against-the-rule (ATR) astigmatism often compromises RGP lens performance in these areas, usually making soft torics a better choice. In the case of a highly astigmatic patient (>3.00D), any mislocation of a toric soft contact lens can have dramatic visual consequences. Fortunately, many of these patients are candidates for RGPs regardless of whether they have WTR or ATR astigmatism. Why? Because now a toric RGP design can be employed.

The ability to dictate the lens-to-cornea relationship in the horizontal and vertical direction independent of one another is empowering! Whether the cornea is WTR or ATR, an RGP lens can be designed to relate to the cornea much like the ideal relationship of a spherical RGP on a slightly WTR cornea.

Managing Severe Conditions

How should one manage the highly hyperopic (> +4.00D) or myopic (> -7.00D) astigmat? A soft toric is the obvious choice for the part-time wearer due to its enhanced comfort. But an RGP is the obvious choice for those patients interested in extended wear because of its significantly greater oxygen profile. In the case of the high hyperope interested in daily wear, a soft toric may demonstrate irregular flexure, resulting in fluctuating vision. Although a soft toric will provide the best initial comfort, an RGP lens can be lenticulated to improve comfort, lens positioning and reduce lens mass. Furthermore, highly ammetropic patients are generally very motivated, which increases their chances of adaptation. The enhanced oxygen supplied by an RGP reduces the risk of limbal vessel changes (e.g. in the high myope), corneal distortion (e.g. in the high hyperope) and ultimately, corneal edema and ulceration. Which lens type do you think serves the patient best?

Dr. Quinn is in group practice in Athens, Ohio, and has served as a faculty member at The Ohio State University College of Optometry.