Contact Lens Practice Pearls
Advanced Corneal Reshaping for Astigmatism
By John Mark Jackson, OD, MS, FAAO
It is more complicated to use corneal reshaping to correct astigmatism than to correct myopia, but it can be done. Chen et al (2012) treated astigmatic children who had up to 3.50DC with an ortho-k design for astigmatism and had excellent results.
Astigmatism-specific designs can have unequal base curves, peripheral curves, or both. Ortho-k lenses use the shape of the tear film to reshape the cornea, but they work only if the tear film shape is correct. For high corneal cylinder, a standard design can produce the correct tear film in one meridian, but not in the other due to the difference in corneal elevation. Lenses that have unequal peripheral curves are able to create the proper tear film shape in all meridians.
My patient was a 49-year-old man who previously wore monovision GP lenses and wanted to try corneal reshaping. His spectacle Rx was OD −5.25 −1.50 × 180, OS −4.00 −1.75 × 030, +1.75D add. Figure 1 shows his OS topography (upper left map). Because of his relatively high astigmatism, we used the CRT Dual Axis design (Paragon Vision Sciences). Our post-treatment goal was −2.00D OD and plano OS for monovision.
Figure 1. OS Topography pre- and post-treatment.
The CRT Dual Axis design uses unequal Return Zone Depths (RZDs) and/or Landing Zone Angles (LZAs) for corneal elevation differences in the major meridians. Our patient’s elevation map showed a difference of 35 microns, which meant that the RZD difference should be 50 microns. The CRT software recommended OD 8.3mm base curve, 550 and 600 RZDs, 34 LZA and OS 8.4mm base curve, 525 and 575 RZDs, 34 LZA (Figure 2). The simulated fluorescein pattern looked correct for both eyes, so we ordered these lenses.
After two weeks of wear, his refraction changed to OD −2.00 −0.50 × 120, OS +0.25DS in the afternoon. Unaided acuity at all distances was OU 20/20. He was pleased with the results and had stable vision throughout the day.
Figure 2. CRT Dual-Axis design for OS.
Figure 1 also shows his post-wear topography (lower left) and the total change (right). Interestingly, the simulated K reading shows 2.40DC pre-treatment and 2.00DC post-treatment. While this makes it seem as though little astigmatism was treated, the astigmatism has been “pushed” out of the central visual axis, and the subjective refraction shows that it has all been corrected. If patients still manifest astigmatism after treatment, it may have not been pushed out far enough for effective correction. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #207.
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. You can reach him at firstname.lastname@example.org.