prescribing for astigmatism
Correcting Residual Astigmatism
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO
I've become more aggressive in prescribing toric soft contact lenses for patients who have low amounts of residual astigmatism (RA) such as 0.75D. The outdated "4:1 rule" (prescribe a spherical soft lens if the refractive cylinder is less than or equal to one-fourth of the sphere component of the refraction) shouldn't apply with the toric soft lens options we have available today.
The overall lens thickness of most toric soft lens designs decreases oxygen transmissibility and increases neovascularization concerns. This year we'll have the option of prescribing silicone hydrogel toric lenses, but initially the available parameters won't likely encompass high refractive errors. So in the meantime, I tend to prescribe spherical silicone hydrogel lenses for patients who have large spherical refractive errors (in excess of +6.00D or 8.00D) and cylinder needs of <1.25D. The enhanced effects on corneal physiology offset the minor decrease in vision clarity.
If one eye has negligible refractive astigmatism and the other eye has mild-to-moderate (1.00D or less) cylinder, then I may consider prescribing spherical lenses for both eyes. (This situation is most likely to occur when prescribing tinted or silicone hydrogel lenses because of current parameter limitations.) I educate the patient about the visual benefits of a toric soft contact lens and the fact that one eye will have sharper vision, but that he should have acceptable binocular vision.
You should also consider leisure and vocational vision needs when deciding whether to fully correct astigmatism. Part-time contact lens wearers might not need correction for astigmatism. However, make sure all patients are satisfied and have functional vision.
GP Contact Lenses
As with soft contact lenses, most GP patients are satisfied with RA of <0.75D. Again, a patient's individual vision needs will dictate his acceptance of uncorrected RA.
If a patient has sufficient corneal toricity (>1.50D) to minimize lens rotation, then I'll prescribe a bitoric GP to correct 0.50D or more RA if it's at or near the same axis (or 90 degrees away) as the patient's corneal toricity axis.
It's common to over-refract 1.00D to 2.00D of RA in keratoconus patients and in other GP patients who have irregular corneal conditions. Generally, I don't design a toric GP lens to correct this RA. Even though keratometry or topography often indicates significant corneal toricity, it's irregular astigmatism and will probably cause unacceptable rotational stability with a bitoric GP lens.
If spectacle trial-framing the residual cylinder correction significantly improves the patient's vision or eliminates ghost or multiple images, then I'll prescribe a spectacle over-correction. If the patient is presbyopic, then I'll correct the RA cylinder with multifocal spectacles. If the fluorescein evaluation of the lens-to- cornea fitting relationship exhibits a toric pattern, then I may consider a bitoric GP lens. However, the corneal toricity and the over-refraction cylinder axes must be similar or 90 degrees apart and lens rotation must be stable to obtain optimal vision. If the lens rotates or the correcting cylinder doesn't align with the RA, then an unacceptable, uncorrected cross-cylinder will result.
If a clinically significant amount of RA is present for one or both eyes, then the patient is less likely to achieve satisfactory vision with a spherical monovision correction. Therefore, I'm more likely to prescribe torics for a borderline case of astigmatism.
The Best Correction Possible
Present all viable options to astigmatic patients who are considering lens wear and individualize each contact lens prescription after weighing the patients' vision and lifestyle needs. Keep in mind that the common 0.75D cut-off of RA isn't nirvana for all of your contact lens patients.
Dr. Edrington is a professor at the Southern California College of Optometry. E-mail him at email@example.com.