prescribing for astigmatism

In With the Old, In With the New

BY BRETT LARSON, OD, & TIMOTHY B. EDRINGTON, OD, MS, FAAO

The increased oxygen permeability of silicone hydrogel materials, improved rotational stability, and the ever-increasing parameters have all helped establish toric soft lenses as an option for most patients who have low-to-moderate amounts of astigmatism. However, with new technology there's a tendency to adopt the "out with the old and in with the new" philosophy and risk denying some patients the best in vision correction.

Case in point, a patient desiring new lenses presented with high amounts of meridional anisometropia and refractive cylinder. She had been wearing low-Dk, non-silicone hydrogel torics for several years with "acceptable vision." Slit lamp evaluation revealed a significant amount of neovascularization, probably due to long-term wear of low-Dk toric soft lenses. Surprisingly, she said that she was never given the option of GP torics to correct her refractive error.

We designed and prescribed GP lenses, and she was thrilled to have the increased visual clarity that GP lenses can provide. We were also more comfortable with the enhanced availability of oxygen provided by the tear pump and the higher-Dk GP material.

While toric soft lenses are a wonderful option for most astigmatic patients, they may not adequately solve the visual or ocular health needs of every patient. Currently, cylinder powers of greater than –3.00D are not available in high-Dk silicone hydrogels. Also, the vision achieved with cylinder powers greater than –3.00D tends to lack the crispness that spectacles and GP lenses can provide for those needing high amounts of cylinder correction.

Often, practitioners resist prescribing GP lenses because they assume that patients aren't interested. Fitting a toric GP has never been easier and many resources are available, including the GPLI Web site at www.gpli.info. Also, consultants at your GP laboratory can help you design or problemsolve your lens prescriptions.

Medically Necessary

Many GP lenses currently prescribed are for patients whose corneal conditions fall into the category of "medically necessary." For those needing rigid lenses for managing corneal complications such as keratoconus, pellucid marginal degeneration, or postsurgical problems, toric GP lenses are a valuable option.

Large-diameter GP lenses are becoming popular for the optical correction of irregular corneal surface complications. It's common to determine the correct power for these types of lenses by converting the sphero-cylinder over-refraction to an "equivalent diopter sphere." However, if the patient has a significant amount of stable refractive cylinder in the over-refraction (greater than 2.00D), designing a front-surface toric or bitoric may be an option.

Scleral lenses are also available in bitoric and front-surface toric designs. Toric peripheral curve designs may be specified when prescribing a scleral lens to create even bearing on the bulbar conjunctiva. Toric peripheral curves can also offer improved comfort and ocular health for patients wearing corneal GP lenses when the fluorescein pattern reveals excessive edge lift in one meridian and minimal edge lift 90 degrees away.

Providing the Best Option

Although soft toric contact lenses are a wonderful option for most low-to-moderate astigmatic patients, they're not always the optimal option. As vision care providers, we need to be certain that we don't keep patients from achieving their best possible vision. When making recommendations, step out of the soft toric comfort zone and consider a toric GP contact lens to maximize high astigmatic patients' vision. This will not only show patients that you have their best interest in mind, but will show them that you are up-to-date with "the old" and "the new" in lens care. CLS

Dr. Larson is a 2009 graduate of the Southern California College of Optometry and is the current Cornea and Contact Lens Resident at SCCO. Dr. Edrington is a professor at the Southern California College of Optometry. He has also worked as an advisor to B&L. E-mail him at tedrington@scco.edu.