prescribing for astigmatism

When to Correct Astigmatism With Soft and GP Lenses

BY TIMOTHY B. EDRINGTON, OD, MS, FAAO

There are many contact lens prescribing situations in which we choose not to correct astigmatism. For example, when soft and GP lens patients have only 0.50D or less of cylinder in their over-refraction (OR). Following are my guidelines for when to correct astigmatism.

Soft Contact Lenses

For the majority of patients who have 1.00D or more of refractive astigmatism, practitioners will prescribe toric soft contact lenses. However, I have become more aggressive over the past few years in prescribing toric soft lenses for patients who have as little as 0.50D of refractive astigmatism.

I routinely prescribe toric soft lenses for patients who have 0.75D and 1.00D of astigmatism, especially if they have critical vision demands and a low-to-moderate amount of spherical refractive error. I am, however, hesitant to prescribe low amounts (for example, 0.75D) of cylinder for patients who have high sphere prescriptions because it would result in a thicker lens profile.

GP Contact Lenses

Bitoric GP designs may help optimize the fit on a highly toric cornea and/or correct residual cylinder. If a cornea is 2.00D or more toric, then a bitoric GP lens will better align with the corneal contour, increase the uniformity of the edge lift, and improve lens centration. By optimizing the alignment of the cornea's topography, you tend to minimize spectacle blur. By increasing the uniformity of the edge lift, you tend to enhance lens comfort.

If the patient's OR through a spherical GP lens has 1.00D or more of cylinder, I generally prescribe a bitoric or front-surface toric GP lens design. If the patient's cornea is toric by 1.50D or more, I prescribe a bitoric. When prescribing a bitoric GP, generally the more toric the cornea, the more stable the lens rotation, leading to more consistent vision. A front-surface toric design needs prism to maintain rotational stability, but more prism may decrease lens comfort and/or cause the lens to position inferiorly.

Irregular Corneas Many keratoconus patients present with 1.00D or more of residual cylinder, often at an oblique axis. It is tempting to prescribe a bitoric GP to correct the OR cylinder. Even though the K reading indicates a large amount of corneal toricity, keep in mind that keratoconus leads to irregular astigmatism. Unless the midperipheral portion of the cornea is substantially toric, a bitoric design might not be rotationally stable.

An Underutilized GP Toric Design Toric peripheral curve GP lens designs may aid lens centration on corneas that have mild-to-moderate corneal toricity. They may also be prescribed to "even out" peripheral edge clearance for patients who have moderate amounts of corneal toricity or for keratoconus patients.

Toric peripheral curve GPs have been available for decades. They were once fabricated by crimping the lens and using diamond and brass tools to cut and polish the toric peripheral curve. With today's advanced technology, the automated numeric lathes can create toric peripheral curves without crimping the lens. GP lenses can also be fabricated with quadrant-specific toric peripheral curves to "tuck in" or reduce the peripheral edge lift in only one quadrant. This option may be utilized for patients who have irregular corneas or lenses that position superiorly because of a lid-attached fitting relationship.

Not as Hard as You Think

It can be surprisingly easy and satisfying to correct astigmatism, enhance the fitting relationship, and provide optimal vision without sacrificing comfort or corneal health. CLS

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Dr. Edrington is a professor at the Southern California College of Optometry. He has also worked as an advisor to B+L. You can reach him at tedrington@scco.edu.