Toric Soft Lenses for Extended Wear?

BY THOMAS G. QUINN, O.D., M.S.
NOV. 1997

No matter how much a patient may beg me, I don't do it. Even if they're approved for it, I've found that toric soft lenses just don't cut it on an extended wear basis. Here's why.

FALLING SHORT OF THE  DK DEMAND

The Summer 1997 issue of Contact Lens Quarterly lists six toric soft contact lenses that are approved for flexible or extended wear. All are made of 55 percent water content materials with oxygen permeability (Dk) values ranging from 16 to 18. The thinnest lens has a center thickness of 0.06mm. So, the best case scenario would provide an oxygen transmissibility (Dk/L) of 18.8/0.06mm = 31.3.

Work by Holden and Mertz (1984) suggests that preventing corneal edema in an open eye requires a lens with a Dk/L of 24, or about a 10 percent equivalent oxygen percentage (EOP). Flexible or extended wear toric soft lenses meet this need, but the stakes go up when the eye is closed.

Ideally, a Dk/L of 87, or an EOP of about 18 percent, should prevent overnight swelling greater than the four percent normally experienced by the non-contact lens wearing eye. A Dk/L of 34 provides an EOP of about 12 percent. This is a realistic compromise because swelling will occur but will dissipate rapidly once the eye opens. Despite the compromise, the shortfall is actually more significant with flexible or extended wear toric soft lenses because all but one of the approved designs are prism ballasted, adding lens thickness and further reducing oxygen transmissibility.

BEWARE CORNEAL STRIAE AND LIMBAL VESSEL CHANGES

What are the red flags that will alert you that the cornea is suffering from hypoxia? Corneal striae (Fig. 1) is perhaps the earliest sign that the cornea's oxygen needs are not being met. Some of the longer term changes include corneal microcysts, topographical changes and spectacle blur. Limbal vessel changes (Fig. 2) can occur as the eye attempts to deliver more oxygen to the cornea via encroachment of the surrounding blood vessels.

FIG. 1: CORNEAL STRIAE.

FIG. 2: LIMBAL VESSEL CHANGES.

A LESSON FROM REAL LIFE EXPERIENCE

Corneal oxygen needs vary in the population, so we might conclude that some corneas can continue to function well under the stress of a toric soft lens worn on an extended wear basis. In my experience, however, extended wear toric soft lenses have shown nearly a 100 percent failure rate for meeting the cornea's physiological needs under closed eye conditions.

One significantly myopic patient of mine, wearing an extended wear-approved spherical lens on the right eye and an extended wear-approved toric soft lens on the left eye, wore her lenses on a one-month extended wear cycle. Of course, she did this without my knowledge.

She presented at follow-up with approximately 1.0 mm of corneal neovascularization circumferentially in the right eye and up to 3.0mm of neovascularization inferiorly in the left eye. I discussed the adverse findings with her, and told her that although spectacle correction would be safest, I was optimistic about refitting her with rigid gas permeable contact lenses. RGPs supply significantly higher levels of oxygen and do not cover the involved limbal tissue. However, she opted for full-time spectacle wear.

You can prescribe toric soft lenses for appropriate patients, but be cautious about prescribing them for overnight wear under closed eyes. If your astigmatic patients want extended wear, prescribe RGPs! CLS

References are available from the editors at Contact Lens Spectrum. To receive them via fax, call (800) 239-4684 and request document #30.

Dr. Quinn is in group practice in Athens, Ohio, and has served as a faculty member at The Ohio State University College of Optometry.