Choosing the Correct Lens for Each Astigmatic Patient
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO

Matching the best contact lens option to each patient is a skill that improves patient satisfaction and practice success. I'll discuss pearls related to astigmatic patients and contact lens selection.

High Sphere, Low Cylinder (for example, –7.50 –1.00 x 180) Soft and silicone hydrogel spheres, soft aspherics, soft and silicone hydrogel torics and GP lenses all work well for most patients who have high sphere and low cylinder prescriptions.

Even though I've become more aggressive in prescribing torics for my soft lens patients who have low amounts of astigmatism, I'm more conservative when the spherical component is highly plus or minus because of the extra lens thickness and resulting decreased oxygen transmission. As available parameters expand, this refractive group would be good candidates to upgrade into silicone hydrogels.

High Sphere, High Cylinder (for example, +6.50 –4.00 x 90) These patients are ideal candidates for GP lenses, which generally provide sharper vision and better corneal physiology compared to the soft toric lenses available today. If a patient's corneal toricity exceeds 2.00D, I generally prescribe a bitoric GP lens design. Even though a spherical or aspheric base curve design may provide excellent vision, bitoric designs enhance lens centration, decrease peripheral cor-neal desiccation and minimize lens awareness by decreasing lens movement.

Low Sphere, Low Cylinder (for example, –150 –0.25 x 90) These patients are great candidates for spherical soft and spherical silicone hydrogel lenses. Spherical GP lenses may also provide good vision, but many of today's patients balk at the initial adaptation.

Low Sphere, High Cylinder (for example, +1.00 –4.50 x 180) To optimize vision, I prefer bitoric GP lenses for the majority of these patients. For a bitoric to succeed, you need a sufficiently toric cornea (at least 1.50D of toricity) to stabilize lens rotation. Bitoric (as opposed to spherical GP) lenses minimize physically induced localized corneal distortion for highly toric corneas.

Practitioners often prescribe toric soft contact lenses for these patients with mixed results. Vision may be decreased because of lens misalignment or rotation. For a patient who has 4.50D of refractive astigmatism, 10 degrees of cylinder axis misalignment will result in 1.50D of over-refraction (OR) cylinder at an axis oblique to the prescribed axis (10 degrees of misalignment results in one-third of the lens' correcting cylinder in the OR; 15 degrees of misalignment results in one-half of the cylinder in the OR; and 30 degrees of misalignment results in the full amount).

Medium, Medium (for example, –2.50 –1.50 x 90) Many of your patients won't fall into any of the groups I've mentioned. If a patient has –1.00D to –2.00D of cylinder in the manifest refraction, yet the cornea is relatively spherical, rush to your soft toric fitting set. Spherical or low toricity corneas don't provide enough of an anchor to stabilize lens rotation with a toric back-surface GP. Textbooks suggest prism-ballasted, front-surface toric GP lenses for these patients. (Ergo, soft toric.)

More Considerations

Don't overlook patients who could benefit from part-time lens wear. Part-time wearers generally do better with soft lenses because of faster adaptation and readaptation. Part-time wearers may also be less critical about the quality of their vision through soft torics.

Of course, you should also weigh other considerations such as the individual patient's vocation, hobbies and past contact lens experience in deciding which option to prescribe. CLS

Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at tedrington@scco.edu.