prescribing for astigmatism
Contact Lens Design Choices for Borderline Astigmats
BY THOMAS G. QUINN, OD, MS, FAAO
How do you fit a soft contact lens candidate who is a borderline (0.50D to 1.00D) astigmat? Do you attempt to correct the cylindrical error with a toric lens or should you see if the patient can tolerate the slight blur? Or is there another option?
A toric soft contact lens (or a GP lens) will best serve most patients who have 1.00D or more of astigmatic error. Many studies have shown that correcting 0.75D of astigmatism improves visual performance. Even patients who have as little as 0.50D of residual astigmatism can experience visual symptoms if left uncorrected.
The Sphere Factor
Spherical refractive error can greatly affect visual performance in the presence of uncorrected astigmatism. The lower the spherical error, the more likely it is that a patient will be dissatisfied with uncorrected astigmatism.
Patients whose spherical refractive error hovers within 1.00D to 2.00D will most likely need astigmatic intervention. Patients who have 5.00D to 6.00D of spherical error can often tolerate as much as 0.75D of residual astigmatic refractive error. Table 1 lists my suggested lens selection guidelines for astigmatic patients.
The Aspheric Option
A front surface aspheric design contact lens such as the Frequency Aspheric (CooperVision), the Specialty AB (CIBA Vision) and the Definition AC (Optical Connection, Inc.) usually best serves low spherical error patients who have astigmatic errors of 0.50D or 0.75D. Traditional thinking suggests that these lenses do not actually correct astigmatism, rather they improve the optical performance of the contact lens by reducing spherical aberrations. However, a recent study (Patel et al 2002) suggests that aspheric lenses may mask nearly half of a patient's astigmatism, particularly for with-the-rule and oblique astigmatism patients.
Seeing is Believing
Patients who wear front surface aspheric contact lenses may not read the acuity chart better than they would when wearing a spherical contact lens, but they often report that aspheric contact lenses enhance the quality of their vision. I've always wondered how real this difference is...until one of my patients called to complain that her vision with the contact lenses mailed to her directly from the company was not as good as her vision with the diagnostic lenses that I had dispensed to her. I asked her to bring her new contact lens supply to the office. Sure enough, the company had sent her spherical contact lenses.
Now I'm a believer, and I ask my ordering technician to emphasize the "A" when ordering aspheric contact lenses.
Dr. Quinn is in group practice in Athens, Ohio, and has served as a faculty member at The Ohio State University College of Optometry.