the contact lens exam

Wavefront Error and Corneal Topographers

BY JOHN MARK JACKSON, OD, MS, FAAO

The study of ocular aberrations is playing an ever-increasing role in eyecare. Aberrometers allow a thorough examination of defects in refraction well beyond the sphere, cylinder, and axis of the manifest refraction — but they're also expensive. It will probably be several more years before most practices have one on-site.

Corneal topographers are not aberrometers, but some may be able to approximate one. Owners of the Medmont E300 topographer (Medmont/Precision Technology Services) have an additional map view in the newest version of the software called a Wavefront Error: Zernike Fit display. The Medmont documentation describes it as a ray tracing method to determine the aberrations from the anterior cornea.

I haven't used this display much because it can't determine the aberrations of the whole eye and it doesn't provide values for spherical aberration (SA), coma, etc. However, in cases where the majority of aberrations are from front surface irregularities (such ortho-k or keratoconus), it may be useful.

Refractive Astigmatism

Ortho-k works fairly well for my eyes. Overall vision is decent, but I go from having a refractive error of –3.00DS to +0.50 –1.00 × 030 in each eye. I can't say that I'm very fond of this astigmatism; the induced blur at all distances is not very comfortable for someone used to a spherical refractive error. The cause of this increase in cylinder has always eluded me. The tangential difference map only shows an increase of –0.25DC and a slightly oval flattened area (Figure 1), not enough to explain the refractive astigmatism.

Figure 1. Tangential difference map.

Studies show that ortho-k increases higher-order aberrations, particularly SA and coma. I wondered if the Medmont could pick up any aberration changes that would shed some light on my case. Figure 2 shows the change in Wavefront Error from pre- to post-treatment. What looks like an astigmatic bowtie pattern axis 030 is evident within the pupil. It appears that the combined changes in higher-order aberrations result in a change that is correctable with extra cylinder in the manifest refraction in my case.

Figure 2. Wavefront error difference map.

I have reviewed about 20 cases in which the change in refractive astigmatism doesn't match the change on the curvature maps. Each of them shows this bowtie at the correct axis on the difference map for Wavefront Error. I'm sure it's not technically correct to interpret the map in this manner, but it has certainly been useful to me clinically to visualize why astigmatism has changed in an unpredictable way. CLS

Dr. Jackson is an associate professor at Southern College of Optometry where he works in the Advanced Contact Lens Service, teaches courses in contact lenses and performs clinical research.