prescribing for astigmatism

Contact Lenses for Astigmatism: Better Than Ever

BY GLENDA B. SECOR, OD
APRIL 1998

Many investigators have reported that as much as 45 percent of the contact lens-seeking population has an astigmatism correction of -0.75D or more. With the predominance of hydrogels in thethe market, there's obviously a substantial need for soft contact lenses that correct astigmatism because we all know that even marginally successful spherical soft lens wearers are more likely to drop out when acuity is less than optimal.

Soft Torics: Then and Now

The initial slow development of toric hydrogel lenses in the 1970s was followed by rapid advances in the 1980s. In the early days of toric fitting, we were all discouraged by snowflake lenses (no two alike). This problem caused both practitioners and patients to doubt the value of toric lenses. Fortunately, success with toric lenses has grown as a result of improvements in manufacturing, which have yielded better lens reproducibility and stability.We are now more assured of receiving the same performance with each and every replacement lens. The benefits of replacing lenses on a regular basis -- improved comfort, improved vision and emergency replacement coverage when lenses are lost or damaged -- are known to spherical contact lens wearers and can now be offered to their toric contact lens wearing friends.

The best method for selecting the appropriate type of lens is a diagnostic evaluation. Placing a diagnostic lens with the appropriate design on the patient's eye helps predict performance and reduce chair time. On the other hand, this way of fitting can be expensive. Empirical fitting is another option and its success has been reported as high as 85 percent for the first fit.

Soft vs. RGP: Every Lens Has its Place

If the refractive astigmatism is equal or less than the corneal toricity, a rigid lens is the best option because a spherical hydrogel placed on a toric cornea will most often have some degree of uncorrected cylinder which may deteriorate visual acuity. The tear lens formed by the relationship of the base curve of the rigid lens and the cornea can sometimes compensate for the uncorrected cylinder which would remain with a soft lens.

However, if the residual astigmatism is greater than the corneal toricity, a hydrogel toric is usually the lens of choice. Correcting the residual cylinder induced by a rigid lens requires that a front toric cylinder be applied to a spherical base curve. Unfortunately, this combination requires a complicated prism ballasted lens which may be prone to unstable acuity if the lens rotates. Since only the refractive cylinder power needs to be considered when prescribing hydrogels, residual astigmatism doesn't become a substantial issue. In contrast to rigid lenses, the relationship of the soft lens surface to the cornea typically doesn't create as significant a tear lens.

Maintain a Realistic Approach

Like presbyopic patients, astigmatic patients learn to realize that they have a more complicated prescription and require special lenses. Be sure to explain the limitations of toric lenses -- fitting is more complex, more time consuming and more costly. Explain that difficult fits may require lens changes for best visual acuity, comfort and fit. Your perception as offering the best care and information must be reinforced in the words and actions of both you and your staff. Being positive and realistic will result in more loyal patients who are great referral sources and who are less swayed by managed care plans or by alternate delivery sources for lens replacements.

Dr. Secor, a contact lens specialist in Huntington Beach, CA, is a diplomate in the Cornea and Contact Lens Section and is a charter member of the AOA's Contact Lens Section.