prescribing for astigmatism
Aspheric Hydrogels "Correct" Minimal Astigmatism?
BY CHRISTOPHER SNYDER, OD, MS, FAAO
Experienced clinicians can recall at least a few patients who have had small amounts of astigmatism "masked" by spherical soft lenses. Clinical studies have shown that masking astigmatism does apparently occur with some patients wearing some lens brands, but this outcome is not predictable and, in fact, occurs rarely.
In my previous column, I encouraged prescribing toric instead of spherical soft lenses for patients with low cylinders to optimize the patients' "bottom line" vision. While this allows us to do the "best," some successful spherical patients with minimal to low astigmatism resist the cost of toric lenses. We may still be able to do "better" than standard spherical soft lenses for these patients by prescribing aspheric soft lenses.
Be aware that asphericity does not correct the astigmatism. The eyecare practitioner will likely not be able to measure much, if any, actual improvement in visual acuity. The aspheric nature of the lens simply enhances focus by minimizing aberrations, and the visual improvement with these lenses is from an improved contrast sensitivity, often reported by the patient as "better vision."
Choose from aspheric aberration-reducing soft lenses such as the Specialty AB (Specialty Ultravision) and Frequency 55 Aspheric (CooperVision) lenses, to name just two.
Those in contact lens practice for more than just a few years would recall the Panafocal aspheric rigid lens design as a cousin to these soft lenses. The Panafocal offered a front aspheric surface to accomplish the same aberration-reducing goal to benefit patients who experienced small amounts of residual astigmatism.
What is Spherical Aberration?
Spherical aberration occurs when the light rays coming through the periphery of the lens are bent more than those entering through the lens center. This converges the peripheral light to a point slightly forward of the central ray's focus (Figure 1). A gentle flattening of the front radius from center to periphery weakens the peripheral lens light-bending power, correcting the aberrant focus of the peripheral rays (Figure 2). Voila! A better locus of focus (and that's no hocus pocus!)
Figure1. Peripheral light converges forward of the central ray's focus.
Figure 2. Correction of the aberrant focus of the peripheral rays.
Aspheric hydrogels do not correct the astigmatism the cylinder is still evident on over- refraction but offer an opportunity to enhance the optics of the minimal astigmat by minimizing spherical aberration and improving contrast sensitivity. The payoff may be most noticeable to the patient as subjectively-enhanced vision. The bonus with aspheric optics is also present when prescribed for patients with spherical refractive errors as well.
While aspheric soft lenses are not designed to correct astigmatism, they may be prescribed for low (0.25D to 0.50D) to moderate astigmats (0.75 D) who are not ready to upgrade to toric lenses.
Spherical lenses can offer better than 20/20 vision for spherical refractive error patients, but using aspheric lenses for these non-astigmats may offer even better vision.
Dr. Snyder is a professor of optometry and serves as chief of contact lens patient care at the School of Optometry at the University of Alabama at Birmingham.