prescribing for astigmatism

Toric Lens Problem-Solving

BY THOMAS G. QUINN, OD, MS
FEBRUARY 1999

Preprogrammed calculators can be helpful in resolving visual complaints experienced by toric soft contact lens wearers. But any good tool is helpful only when it is applied properly. The following are some ideas on how to utilize this potential problem-solver.following are some ideas on how to utilize this potential problem-solver.

Probe Before You Punch

Before you start punching keys on the calculator pad, ask astigmatic patients this important question: "Is the blur constant or fluctuating?" If blurred vision is fluctuating, lens rotation is probably unstable or the lens is flexing irregularly. In either case, a fundamental change in lens design is needed. In such cases, an overrefraction will be variable, if obtainable at all, making a crossed-cylinder calculation meaningless. A new lens power won't solve the problem, so a different base curve or another design altogether will be required to achieve clear, stable vision.

If blur is constant, look at the contact lens on the eye. Is it clean? A dirty lens can cause constant blur, and an overrefraction with such a lens can be an exercise in frustration. If the lens is clean, is it rotating as expected? If so, consider the possibility that the lenses have been inadvertently switched from one eye to another. If you don't catch this, you'll end up ordering a new lens based on a calculation combining the overrefraction with the power of the lens that's supposed to be on that eye, not the one that actually is on the eye. Consequently, the patient won't be able to see out of the new lens either. Avoid this situation by comparing the power suggested by the calculator output to the patient's spectacle prescription.

If blur is constant and the lens looks good on the eye, the pre-programmed calculator can be enlightening. Perform an overrefraction. Is the result stable? Does it provide good acuity? If not, use a pinhole or remove the lens and check spectacle acuity to make sure you don't overlook the presence of pathology. I once had a toric soft contact lens patient complain of visual blur which turned out to be due to a retinal detachment!

Crossed-Cylinder Calculation

If the overrefraction is stable and provides good acuity, you're in business. If the overrefraction axis is oblique to the lens axis, it becomes necessary to perform a crossed-cylinder calculation. The preprogrammed calculators allow you to perform this with ease.

If the contact lens is rotating on the eye, you must compensate for the rotation to arrive at an accurate resultant (Koers and Quinn, Contact Lens Spectrum, January, 1997). If you are using a preprogrammed calculator that does not account for lens rotation, you'll need to perform this manually. This is a simple procedure. Table 1 outlines what steps will lead you to the proper lens power needed for the patient.

Remember, always examine the resultant to ensure that it is logical based on the spectacle prescription and the known position of the contact lens axis while on the eye. If it makes sense, you're well on your way to achieving success!

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