prescribing for astigmatism
Nonplussed by High Plus?
BY PETER D. BERGENSKE, OD, FAAO

The hyperopic astigmat often presents a challenge to contact lens fitting: The lens geometries can prove physically and physiologically problematic, choices in "stock" soft toric lenses are limited, and, because of the relatively low numbers that typically present for contact lenses, fitter experience is also often limited.

On the other hand, the visual and cosmetic advantages of contact lenses for this population are at least as great as they are for their myopic counterparts. Of particular note is the greater incidence of refractive amblyopia in patients who have hyperopic astigmatism -- a situation in which contact lenses can play a significant therapeutic role.

Figure 1. Topography for patient EF.

Meet the Case Subjects

Our Pediatric and Strabismus Referral Center referred two young patients to our Contact Lens Service. EF is a 9-year-old Hispanic male who has intermittent esotropia and mild amblyopia. His refractive findings were OD +5.50 ­4.75 x 005 20/30 and OS +7.25 ­5.00 x 170 20/25.

CC is an 11-year-old Caucasian male who had the following refractive findings: +7.00 ­1.75 x 135 20/20 and OS +7.75 ­2.25 x 46 20/40.

Topography for both boys indicates regular corneal astigmatism well correlated with the refractive cylinders (Figures 1 and 2). We have successfully fit both of these young patients with rigid lenses. EF required bitoric lenses for a stable fit and we successfully fit CC with spherical lenses. Both are wearing contact lenses full time with good comfort and acuity.

	Figure 2. Topography for patient CC.

Still a Place for Rigid Lenses

A basic tenet for managing refractive amblyopia is optimal correction of the refractive error. It would be difficult to argue that we could accomplish that reliably with soft toric lenses in such cases.

Although several manufacturers produce high-quality toric soft contact lenses in these powers, lens rotation is always significant with this amount of cylinder correction. This can prove particularly disconcerting in the management of refractive amblyopia, where constant full correction of each eye is mandatory.

Another significant consideration is the physiological consequence of a soft toric lens in this power range. Current materials available for soft toric lenses already have limited permeability, and when combined with the substantial thickness required for a plus toric lens, oxygen transmission to the corneal surface would almost certainly be inadequate for most eyes. Corneal edema, distortion, vascularization, polymegathism and exhaustion are likely eventualities.

Maintaining Goals

Development of higher permeability soft materials will hopefully obviate these physiological concerns, yet stability of the correction will still be a significant issue, especially in corrections with a large cylinder component.

Dr. Bergenske, a past chair of the American Academy of Optometry's Section on Cornea and Contact Lenses, has practiced for more than 20 years in Wisconsin and now is on the faculty at Pacific University College of Optometry. E-mail him at berg1101@pacificu.edu.