discovering dry eye

Prism Sphere Soft Lenses and Dryness Symptoms

BY CHRISTOPHER SNYDER, OD, MS, FAAO
OCTOBER 1999

Increased thickness in soft lenses is believed to have a positive effect on lens wear-related dryness symptoms. What about lenses that are only thicker at the base? Prism is added to spherical powered soft contact lenses primarily to correct a vertical imbalance between the eyes. It may also improve patients' wearing comfort.

Vertical Imbalance

Using prism to correct hyper-phoria or hypertropia is not as effective as incorporating it into spectacle lenses. Base-down prism is the only orientation available for contact lenses, so indication of the prism base is not necessary, especially since patients who wear vertical prism in their spectacles often don't need it in their contact lenses!

In addition to prescribing a prism-ballasted toric soft contact lens for patients with only one astigmatic eye, practitioners will also prescribe a prism-ballasted spherical soft lens for the other eye to avoid inducing any vertical imbalance. But as previously noted, the prism (vertical imbalance) does not seem to be as significant a concern with lenses as it is with spectacles.

Minimizing Dryness Symptoms

Dryness symptoms are believed to be minimized and comfort improved when thicker soft lenses
(> 0.10 mm center thickness) are worn. Patients wearing prism-ballasted toric soft lenses rarely report symptoms of dryness. Is it the astigmatism or perhaps the prism in their lenses?

Increasing the lens bulk usually protects the post-lens tear pool from the effects of fluid evaporation from the front lens surface. Increased thickness just at the base of the lens, as with a prism design, may be particularly helpful with conditions of marginal tear volume and incomplete eyelid closure during blinks. Using the prism-ballasted design can improve comfort and wearing time without decreasing oxygen transmission across the lens.

Spherical power soft lenses with prism are a reasonable option to consider when fitting patients with dryness symptoms. Many of these lenses are listed in Table 1.

Dr. Snyder is a professor of optometry and serves as chief of contact lens patient care at the School of Optometry at the University of Alabama at Birmingham.