Article

PRESCRIBING FOR ASTIGMATISM

RESIDUAL ASTIGMATISM WITH SCLERAL LENSES

Scleral GP contact lenses are often prescribed to neutralize irregular astigmatism caused by keratoconus, pellucid marginal degeneration, or post-refractive surgery ectasias. Increasingly, they are being prescribed for ocular surface disease as well as for high refractive errors. Scleral lenses vault the entire cornea, creating a tear lens that optically reduces corneal irregularities that contribute to astigmatism and aberration. However, they do not neutralize residual astigmatism that is not induced by the cornea. By prescribing a front-surface toric, you can correct residual astigmatism and improve your patients’ quality of vision.

Step 1: Determine whether a front-surface toric is indicated. If the over-refraction (OR) of your spherical diagnostic scleral lens reveals cylinder, this may be an indication that the lens is flexing on the patient’s eye. The center thickness of scleral lenses is usually sufficiently robust to prevent flexing of the lens; however, under certain circumstances (i.e., tight eyelids), flexure can occur. Ensure that lens flexure is not the cause of the astigmatism in the OR by checking over-keratometry values with a manual keratometer or topographer while the patient is wearing the scleral lens. If the result is not spherical, this indicates that flexure is present; consider increasing the lens center thickness. If no flexure is detected, there truly is residual astigmatism to be corrected.

Step 2: Determine the natural position of the lens. The sclera is typically toric and asymmetric (Bandlitz et al, 2017). You can evaluate for scleral toricity by observing the scleral landing pattern with a spherical-peripheral-curve diagnostic scleral lens (Figure 1).

Figure 1. A spherical-back-surface diagnostic lens demonstrates edge lift (seen here as a shadow beneath the lens), indicating that the lens peripheral curve(s) should be steepened in this meridian.

Alternatively, anterior segment optical coherence tomography (OCT) and topographers capable of analysis beyond the corneal surface can help provide a more exact measurement of scleral shape and toricity. If there is no, or minimal, scleral toricity, a front-surface toric can be rotationally stabilized using a prism-ballasted design. We are hesitant to add additional lens thickness with prism ballast due to oxygen and lens centration concerns. Therefore, we often prescribe toric peripheral curves so that the lens will “lock into place” at a consistent axis. Alternatively, it is possible to customize the landing zone to a patient’s “exact” scleral shape by using a molded impression-based design.

Ordering the Final Lens

If you are using a scleral fitting set that has toric peripheral curves, determine the axis orientation of the trial lens before ordering the front-surface cylinder power and axis. Ideally, wait for the lens to settle for at least 30 minutes.

If you do not have diagnostic peripheral-toric lenses available, we recommend first ordering a toric-peripheral-design lens with the patient’s spherical power equivalent for adequate vision, then determining on-eye lens rotation, and ordering a lens with the appropriate front-surface cylinder to correct the patient’s residual cylinder. With lens rotation data available, your lab consultants can also assist in correctly orienting the cylinder axis to provide patients with optimal vision.

We find it helpful to place a dot at six o’clock on the front surface of a toric lens to help patients achieve clear vision upon initial application of the lens. Once a scleral lens settles, it should not significantly rotate or move upon blinking; therefore, it is possible to provide your patients with consistent and crisp vision when you prescribe front-surface-toric scleral designs. CLS

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