Article

PRESCRIBING FOR ASTIGMATISM

ASTIGMATISM: TO CORRECT OR NOT TO CORRECT?

Toric contact lenses can often enhance a patient’s quality of vision. A challenge to practitioners is determining when to correct astigmatism using contact lenses. Let’s discuss the indications and limitations for correcting contact lens residual astigmatism.

Soft Contact Lenses

Because spherical soft contact lenses do not significantly mask corneal astigmatism, practitioners must choose when a toric soft contact lens is indicated. Patients who have less than 0.75D of refractive astigmatism will usually accept a spherical equivalent of their vertexed spectacle refraction in their spherical soft contact lens prescription. In deciding whether or not to proceed with a toric soft lens, take note of whether the patient tends to oscillate between 0.50DC and 0.75DC or between 0.75DC and 1.00DC on Jackson cross-cylinder (JCC) testing. The latter group would tend to benefit more from toric correction.

Also consider the amount of cylinder relative to the sphere component of the refraction. For example, a patient who has a refraction of –1.00 –0.75 x 180 will likely appreciate full cylinder correction more compared to a patient whose refraction is –8.00 –0.75 x 180.

There are a few guidelines to consider when fine-tuning the cylinder axis. The “LARS” rule (left add, right subtract) is frequently used to modify the cylinder axis when there is rotation. As a reminder, the adjustment is applied to the manifest refraction axis.

Another method to determine the final cylinder axis is the Snyder rule (Snyder, 1989). If the cylinder in the sphero-cylindrical over-refraction is equal to the full amount of the toric lens cylinder, then the lens is probably rotated 30º from the optimal axis. Using the Snyder rule, a 10º (one-third of 30º) rotation of a toric contact lens with 0.75DC would induce only 0.25DC of astigmatism, whereas a 10º rotation of a 2.25DC lens would induce 0.75DC of astigmatism.

GP Contact Lenses

GP contact lenses, whether corneal or scleral, provide the benefit of correcting astigmatism with the tear lens. However, if an unacceptable amount of residual astigmatism (> 0.75D) is present, a toric GP contact lens design is generally indicated.

Corneal GPs In cases of corneal toricity > 2.00D, a bitoric corneal GP lens may be indicated to enhance the fit and to provide rotational stability. Front-surface toric GPs and cylinder power effect (CPE) bitorics offer correction for residual astigmatism.

When prescribing a spherical base curve with a front-surface toric GP lens for a patient who has a mild amount of corneal toricity, prism is incorporated to enhance rotational stability. Rotational stability can be evaluated by placing a dot at the prism base. The addition of prism may alter the fit of the lens by causing it to decenter inferiorly.

Scleral GPs Because the majority of patients have asymmetric scleral curvatures, scleral GP lenses have the added option of toric haptics to increase rotational stability. Scleral lens rotation can be evaluated by markings that are typically located along the flat meridian. Some manufacturers use prism ballast or front-surface thick and thin zones to stabilize lens rotation of the front-surface cylinder.

A Final Option

In cases in which rotational stability is difficult to achieve with either soft or GP contact lens options, we recommend overlay spectacles. If your patient is presbyopic, you can correct both the astigmatism and the presbyopia in the overlay spectacles. CLS

To obtain references for this article, please visit www.clspectrum.com/references and click on document #261.