prescribing for astigmatism
To Correct or Not to Correct Astigmatism
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO
When prescribing contact lenses to correct astigmatism, we all have different cutoffs for how much uncorrected astigmatism we (and our patients) can tolerate. To confound matters, this cutoff amount might change based on a patient's vision needs and refractive error.
I have become more aggressive in correcting spectacle cylinder with toric soft contact lenses. If a patient has 0.75D of cylinder, I often prescribe toric contact lenses to optimize the patient's vision. If a patient has 1.00D of cylinder, I generally prescribe a toric lens. On occasion, I will prescribe a 0.75DC toric contact lens for a patient who has 0.50DC of refractive astigmatism.
I am also more likely to prescribe toric soft contact lenses if the sphere component is relatively small such as for a patient who has a −1.00 −0.75 x 90 manifest refraction. However, if a patient's vertexed refractive error is −9.00 −0.75 x 90, I typically don't prescribe a toric lens because the cylinder is relatively small in comparison to the sphere power and the prescribed spherical lens will have a thinner overall profile to optimize corneal physiology.
When the exact cylinder correction is not an available parameter for the chosen lens design, I tend to prescribe the closest lower cylinder power. For example, if the patient's astigmatism is 1.50DC and the available cylinder options are 1.25DC and 1.75DC, I tend to prescribe the 1.25DC. In most instances, I am comfortable prescribing a toric soft contact lens when the refractive cylinder is 0.50DC to 0.75DC more than the maximum available cylinder power. I will generally add a −0.25D to the sphere power if available parameters force me to under-correct the cylinder by 0.50DC to 0.75DC.
What About GP Bitorics?
I select bitoric base curves when I need to optimize the lens-to-cornea fitting relationship. I find that it's also desirable to correct the entire amount of refractive cylinder, thereby achieving a spherical over-refraction. Therefore, I generally prescribe cylinder power effect (CPE) bitorics, as opposed to spherical power effect (SPE) bitorics. The optical quality of both designs is the same, as is the cost. Consider an SPE design if the residual cylinder is clinically insignificant or if the over-refraction axis is not the same or 90 degrees away from the corneal toricity axis.
When prescribing a GP lens for keratoconus (KC), it is very common to measure 1.00D to 2.00D of astigmatism in the over-refraction. It is tempting to correct this with a front-surface toric or bitoric GP design. Keep in mind that irregular corneas have irregular, not regular, astigmatism, so both front-surface and bitoric lens designs tend to not be rotationally stable. Also, the prism incorporated in a front-surface toric GP generally causes the lens to position inferiorly on the cornea.
For presbyopic KC patients, I may correct the cylinder in a pair of multifocal spectacles to be worn over their contact lenses. If a patient desires part-time correction of the astigmatism, such as for enhanced night-time vision or driving, I may slightly under-correct the cylinder to allow for an easier transition when the over-spectacles are removed.
Seize the Moment
Today's toric soft and bitoric GP designs are better than their predecessors. If you are not routinely prescribing toric contact lenses for your low astigmats, I recommend that you reconsider. You might be pleasantly surprised to find that your patients appreciate having small amounts of residual astigmatism corrected. CLS
Dr. Edrington is a professor at the Southern California College of Optometry. He has also worked as an advisor to B+L. E-mail him at firstname.lastname@example.org.