prescribing for astigmatism
Are Torics an Option for Keratoconus?
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO
Keratoconus patients generally present with a significant amount of corneal toricity. Also, a moderate amount of residual astigmatism often occurs in the over-refraction of a spherical GP prescribed for keratoconus. Prescribing a bitoric or toric base curve lens design would ideally improve the base curve-to-cornea fitting relationship and correct the residual cylinder. However, if the toric base curve and peripheral curve systems too closely contour the shape of the cornea, peripheral seal-off could be the undesired outcome.
I haven't been a proponent of prescribing bitoric lenses for keratoconus patients. Keratoconus generally results in irregular, not regular astigmatism. One would suspect that this irregular corneal toricity wouldn't stabilize lens rotation with a bitoric or toric base curve lens on the eye and would not adequately correct residual astigmatism.
Two of the most common fluorescein patterns when fitting keratoconic patients are 1) the three-point touch pattern, featuring a small, round centralized area of lens bearing; and 2) a pattern somewhat characteristic of a "with-the-rule" (WTR) cornea that tends to be slightly oblique in axis orientation (approximately 20 degrees). Theoretically, the three-point touch cornea wouldn't stabilize lens rotation as well as the WTR cornea would.
I occasionally prescribe a bitoric when the fluorescein pattern reveals a WTR-type pattern at or 90 degrees away from the over-refraction cylinder axis. You may request that your laboratory mark the flat meridian of a back surface toric lens to help you verify the orientation and stability of the lens's rotational position. If these marks aren't reasonably aligned to the desired axis or if the marks are unstable after a blink, then correcting the residual astigmatism with a bitoric isn't indicated. You could still prescribe a spherical power effect (SPE) bitoric to improve the fitting relationship without rotation degrading your patient's vision.
A spectacle over-correction is a great option to correct residual astigmatism. If your patient is presbyopic, then consider prescribing the cylinder component of the over-refraction in a pair of multifocal spectacles.
I recently heard Paul Rose, BOptom, designer of the Rose K and Rose K2 Aberration Control lenses, describe prescribing bitoric and back surface toric lenses for keratoconus. I enjoy Rose K lenses because I can easily customize peripheral clearance by ordering flatter or steeper peripheral curve systems in a practitioner-friendly stepwise fashion.
Caveat Emptor (Let the Practitioner Beware)
Adequate peripheral clearance is important in prescribing GP lenses. If you prescribe toric base or peripheral curve lenses, then assess edge clearance using fluorescein to verify good tear exchange.
Also, perform slit-lamp biomicroscopy to look for corneal staining. If arc staining is present in the midperiphery, suspect a tight lens fitting relationship. Staining around the base of the cone may suggest debris and tear entrapment. To resolve arc staining near the lens edge, flatten the base curve or the peripheral curve system. If the arc staining is near the base curve-peripheral curve junction, increase the blend. To decrease staining around the base of the cone, decrease the optic zone diameter or flatten the base curve and/or peripheral curves.
Some Patients Will Benefit
I still prescribe spherical base curves for most of my keratoconus patients; however, some patients benefit visually or physiologically from wearing a bitoric lens design.
Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at email@example.com.