prescribing for astigmatism

Toric Lens Prescribing Pearls

BY GLENDA B. SECOR, OD
AUGUST 1998

I recently had an enlightening discussion with two fitting consultants from a major contact lens company on the types of calls they receive from doctors regarding toric contact lenses. They told me that their greatest concern is the lack of basic knowledge and fear of failure callers often exhibit. While I can't overcome your fears, I can give you these basic pearls of knowledge. Having a systematic approach to toric lens fitting always seems to reduce my level of anxiety.

A Systematic Approach

• Perform a comprehensive eye examination to obtain best corrected visual acuity.
• Always consider toric soft lenses for any patient having at least 0.75D of cylinder, and remember to compensate for vertex distance in both meridians.
• Success with torics depends primarily upon rotational stability. The most likely cause of lens instability is the thickness differential which results from the cylinder power and axis. Prism ballasting is a primary tool for improving rotational stability, but adding more prism doesn't necessarily result in a more stable lens. The optimum amount seems to be about one prism diopter which can offset the thickness differential produced by toric lenses.
• The "Becherer twist" can help you gauge blur tolerance when rotation occurs. Just rotate the axis dial of the phoropter in both directions and note how quickly the patient reports a blur. More blur tolerance indicates a less sensitive patient and a better candidate for toric lenses.
• Use the smallest cylinder power that provides acceptable acuity to allow more rotational instability without symptoms. Rotation increases later in the day as protein and mucin deposition increases, making planned replacement a wise modality choice.
• If a lens rotates or misaligns but the patient is visually stable and satisfied, don't chase the rotation. The most likely cause is a marginally steep lens. There are fewer available base curves and diameter options in torics, so you must determine an acceptable tolerance level when fitting lenses that may not meet your academic standards for performance.
• Remember LARS and RALS and the difference between them. LARS: Estimate the amount of axis mislocation by visualizing a clock dial on the lens, with each deviation being equivalent to 30 degrees per hour. If the base down position rotates Left of the center line, Add to the spectacle prescription axis; if rotation is Right, Subtract. RALS: If the visual acuity is improved by physically rotating the base down position to your Right, you must Add to the spectacle prescription axis; if it improves with rotation to your Left, Subtract. Both LARS and RALS are academic guesstimates at best, but they can certainly confirm or support axis compensation decisions.
• Spherocylindrical overrefraction can guide you on how to proceed when the dispensed lens just doesn't deliver that knock-your-socks-off acuity you had promised. Rather than using the mathematical formula we once learned in optics class or placing ophthalmic trial and overrefraction lenses into a trial frame and reading the resultant in the lensometer, I prefer to use my preprogrammed calculator to determine the reorder. Even if the calculator fails to compensate for lens rotation, it will work well clinically if the rotation isn't excessive. The variation possible with target versus true specification-labeled lenses is another factor that affects the resultant.
• The fear of failure is almost eliminated by liberal return policies abundant within the industry. So when it comes to prescribing torics, listen to the folks at Nike and "Just Do It!"

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