prescribing for astigmatism
Specialty Contact Lenses For Correcting Astigmatism
BY CHRISTOPHER SNYDER, OD, FAAO
Three specialty contact lenses are highlighted for use in vision correction: the SoftPerm lens (CIBA Vision/Wesley Jessen), the Macrolens (C&H Contact Lens, Inc.) and the Epicon lens (Specialty Ultravision).
The larger diameters in the specialty lenses provide for a fitting relationship on the peripheral cornea and sclera, and this minimizes or eliminates vision problems associated with the poor centration and instability of smaller diameter RGP lenses. Additionally, each lens offers enhanced comfort owing to the soft lens-like diameters.
Due to their rigid nature, at least in the central area, these lenses will correct astigmatism, regular or irregular, that comes from the corneal topography (toricity; distortion). While central cornea is of primary importance for aspects of vision correction, peripheral corneal irregularity seen in post-RK, keratoconus, post-PK and post trauma patients can greatly impact the centering of smaller RGP lens optics in front of the pupil. These larger lenses fit with minimal corneal touch, sharing bearing pressure between the cornea and sclera.
One caveat for astigmatism correction with the SoftPerm is the low-Dk RGP center is susceptible to flexure, and therefore incomplete correction of astigmatism may occur.
TABLE 1:Lens Materials and Parameters
|LENS||MATERIAL||STANDARD DIAMETER(S) (mm)||BASE CURVES||LENS PERIPHERAL DESIGN|
|SoftPerm||25 percent water
(RGP Dk = 14)
(8.0 mm central
| 7.1-8.1 (0.1
in +6 to 13) powers
6.5, 6.7, 6.9 in 3 to 16 powers
|fixed (dependent upon central base curve|
(Dk = 58)
|13.9-15.0|| 38.00 to 51.00
(0.50 D steps)
(Dk = 52)
|13.5||6.4 - 7.6 mm
(0.2 mm steps)
|adjustable (flat, median and steep)|
Both the Macrolens and the Epicon allow you to fit a base curve for optimal fitting relationship to the cornea while independently prescribing for optimal peripheral lens fitting relationship. This capability to independently design the lens periphery is a necessity for the large, semi-scleral rigid lens and not as necessary for the SoftPerm hydrogel skirt owing to its ability to drape and align more easily with the complex scleral curvatures.
The standard Macrolens design is called the Macrolens A and is also available as the Macrolens D and Macrolens H for different fitting relationships. While promoted as a mainstream lens, it is also suggested for post-refractive surgery, keratoconus, and post-penetrating keratoplasty.
The Epicon lens was designed primarily for application in keratoconus but has also been successfully used for management of post-trauma and post-surgical irregular astigmatism and moderate to high regular astigmatism. Another design, the Ultracon lens, has been designed primarily for application in cases of low to moderate amounts of astigmatism and should be available in the near future.
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.