prescribing for astigmatism

Prescribing Power for Toric Soft Lenses

BY THOMAS G. QUINN, OD, MS
OCTOBER 1998

The ultimate goal in fitting an astigmat with a toric soft lens is to enhance visual performance. However, a well-fitted toric lens prescribed with the incorrect power will defeat, or at least minimize, the best of intentions. Here are a few tips on how to bias the odds of success in your favor.

Bias Cylinder Power Down

A toric soft lens must move enough on the eye to flush debris from the cornea-lens interface, but not so much that it leads to lens axis instability. If a patient has a spectacle cylinder finding of -1.50DC but the lens you're fitting is available only in -1.25DC or -1.75 DC, which do you choose? Choosing the lower power will induce less residual astigmatism should the lens temporarily rotate off axis.

The Plane!

In highly ametropic individuals, we're accustomed to adjusting the spherical component of the prescription to account for vertex adjustments from the spectacle plane to the corneal plane. However, the astigmatic value can be effected as well (Table 1). This has become more significant now that toric soft lenses are available for high levels of refractive error.

Turning Point

In assessing the stability of lens axis location, I like to digitally displace the lens and observe its response. First view the right lens axis position through the slit lamp and measure rotation by aligning the lamp housing with the microscope's line of sight and rotating a narrow beam to align with the lens axis marking. Note the axis position on the slit lamp scale. With clean hands, reach up and lightly rotate the lens out of position in the opposite direction from which it seems to be rotating. Now repeat these steps OS.

Return to the right lens and observe the location of the axis mark. If it has recovered to the position noted prior to dislocating the lens, axis orientation will likely be stable. If the lens is in a different location, have the patient blink firmly. If the lens moves toward the original axis location, performance may still be acceptable. If the lens remains displaced, you may need to change the lens base curve, diameter or design. Repeat OS. If a diagnostic lens varies within five degrees in its axis position, order an axis that assumes the lowest degree of rotation. After further settling, the lens will usually seat in the less rotated position.

The Hyperopic Astigmat

Hyperopic patients often accept 0.25D to 0.50D more plus in a contact lens than in spectacles. Plus power toric lenses are also more likely than minus power torics to have incomplete flexure due to their increased center thickness, so you may need less cylinder in the contact lens.

Overrefraction

If refracting over a toric soft lens doesn't yield a crisp, stable endpoint, the lens won't provide clear vision, regardless of power. A change in fit is required. If the overrefraction is stable and provides good acuity but the overrefraction axis is oblique to the lens axis, perform a crossed-cylinder calculation, compensating for any rotation. Ensure the resultant is logical based on the spectacle prescription and the position of the lens axis on the eye.

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