prescribing for astigmatism

Piggybacking With Soft Torics

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

GP lenses are generally the optimal therapy for the vision needs of patients who have irregular corneal surfaces. Although a spherical GP lens will take care of most visual needs, it is not uncommon to find uncorrected astigmatism in the over-refraction that prevents patients from achieving their best vision. As practitioners, we often struggle to find a solution that will provide optimal vision while maintaining corneal health. Providing a spectacle correction over contact lenses is often an unpopular solution because of the need to wear two types of vision correction simultaneously. Prescribing a front-surface toric GP lens theoretically would provide the desired result, but the prism incorporated in the lens to stabilize rotation can result in inferior lens decentration. Because of these difficulties, practitioners are often reluctant to try to solve the problem of uncorrected residual astigmatism and instead prescribe spherical lenses.

A piggyback system consists of a soft lens worn underneath of a GP lens to improve comfort with GP lens wear, or to center a lens by providing a more regular surface (such as using a high-plus lens with a central "button"). Piggyback systems may also provide relief for GP-intolerant patients or to new wearers who need assistance building up wearing time.

The soft component of the piggyback system may also mask uncorrected astigmatism. A 2009 study by Professor Daniel Brazeau showed that a soft piggyback lens will change an over-refraction by about 23 percent of the soft lens power. For example, when combined with a GP lens, a +4.00D spherical soft lens will change the over-refraction by approximately +1.00D.

Case in Point

A patient who had keratoconus and a long history of spherical piggyback lens wear presented with complaints of blurred vision at distance and near with his left eye. His visual acuity was 20/30− through his habitual piggyback correction. The over-refraction revealed 1.00D of uncorrected astigmatism. The patient declined the option of wearing a spectacle lens prescription over his contact lenses. To maximize the amount of oxygen transmitted to the cornea, we used a silicone hydrogel lens with a cylinder power of −2.25D as the piggyback lens. When combined with his GP lens, the resulting cylinder in the over-refraction was reduced to −0.50D. His visual acuity improved to 20/20− and he was thrilled with the improvement.

Although a piggyback lens with a cylinder power of −4.00D would have been the most appropriate mathematical option in this example, we decided that a toric lens with higher oxygen transmissibility would minimize adverse effects on corneal physiology. The highest amount of cylinder available to us in a toric silicone hydrogel lens was −2.25D. It's possible to prescribe a toric soft lens with a higher cylinder power, but because high-cylinder options are currently only available in lower-Dk materials, we opted to prescribe a silicone hydrogel. Note that in many cases changing a piggyback lens from a spherical to a toric design may require alterations to the GP design to optimize the fitting relationship.

A Useful Tool

GP lens wearers who have irregular corneal surfaces and uncorrected refractive cylinder deserve the clearest vision available. Toric soft piggyback lenses are another tool that we can use to achieve this goal while still maintaining patients' corneal health. The next time a patient presents with uncorrected refractive cylinder, rather than settle for an equivalent diopter sphere, consider prescribing a toric soft lens as part of his piggyback system. Your patient will thank you for the improved vision and enhanced lens comfort. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #175.

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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.