prescribing for astigmatism
Who Goofed? I've Got to Know! Reply to a Reader's Toric Concerns
BY TIMOTHY B. EDRINGTON, OD, MS
I authored an article on prescribing toric soft contact lenses in the June 1999 supplement to Contact Lens Spectrum. Dr. Charles McBride of Beaverton, Ore., wrote to me and commented that he was amused by two statements in the article.
Statement A: Rotation that's stable and within 30 degrees of base down, (base down marking located between 5 and 7 o'clock positions), is acceptable. Dr. McBride countered with, "If we're just talking mathematics here, what's wrong with a lens that's stable and rotated to 45 degrees?" Dr. McBride is correct. If a toric soft contact lens consistently positions with the prism base 35, 45 or 85 degrees away from the 6 o'clock position on the cornea and is rotationally stable and you have compensated the cylinder axis for the new home of the prism base, the vision and corneal physiology should be acceptable.
Statement B: The sphero-cylinder overrefraction data should be entered into a calculator that determines through cross-cylinder calculations the resultant prescriptions. Dr. McBride points out that I did not clarify that some calculators don't inherently take into account the physical rotation of the lens. For instance, assume you start with a weighted lens: plano sphere (oriented 30 degrees clockwise), which yields an over-refraction of plano -4.00 x 180. What happens when you order this resultant from the calculator and slap it on the eye? It rotates 30 degrees just like the weighted plano sphere did and the numbers don't correspond.
Again, I agree with Dr. McBride. Some calculators don't take prism base rotation into account, which is satisfactory if the prism base locates close to the 6 o'clock position. But, it's not accurate if the lens rotates a clinically significant amount, which is more of an issue with larger amounts of cylinder power.
If the corrective cylinder in a toric soft contact lens isn't aligned with the eye's refractive astigmatism, the overrefraction cylinder axis will be oblique to the eye's astigmatism axis. The amount of the overrefraction cylinder will be related to the amount of axis misalignment. If the correcting cylinder of the lens is 30 degrees away from the refractive cylinder axis, then the overrefraction cylinder will be approximately the same as the correcting cylinder amount. If mislocated by 15 degrees, the overrefraction cylinder will be approximately one-half of the correcting cylinder (see Table 1).
If the prescribed lens has the correct spherical and cylinder powers, but is rotated off axis, the spherical equivalent of the overrefraction will be plano. If the prescribed powers are overminused, the spherical equivalent of the overrefraction will be plus. In this case, consider more plus power in the sphere component of the prescription along with adjusting the axis according to the amount of lens rotation or overrefraction cylinder.
Toric soft contact lens reproducibility has improved in the past few years, and the increasing availability of quality frequent replacement options has made this modality a more attractive form of vision correction.
I greatly appreciate Dr. McBride's comments.
TABLE 1: Effect of Axis Mislocation on Overrefraction Cylinder
Manifest refraction -0.50 -3.00 x 80
Toric soft contact lens power -0.50 -3.00 x 80
Overrefraction if 0 axis mislocation plano DS
Overrefraction if 15 axis mislocation +0.75 -1.50 x oblique
Overrefraction if 30 axis mislocation +1.50 -3.00 x oblique
Dr. Edrington is a professor and chief of contact lens services at the Southern California College of Optometry. (email@example.com)