EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/618732
39 EW FEATURE January 2016 • Management of the irregular cornea ones—and we'll look at our 1-week and 1-month outcomes and go from there." Acquiring the Topolyzer is the first step during the exam prior to instilling any drops and before any manipulation of the cornea, Dr. Wexler noted. "Unlike wavefront- guided technology, which looks at the whole system, this is just looking at the front surface of the eye so the tear film has to be in good shape." Once the various axes are focused, the image is automatically acquired, he explained. For each patient, 6 to 8 scans are acquired per eye. It may take a little longer than with wavefront-guided tech- nology where just a few good images are needed, Dr. Wexler continued, adding that taking at least 4 images is a safeguard to make sure you have a good overall composite picture. "If you don't get a good image, [the machine] will flag it," Dr. Wexler said. "You toss that one out and keep going." Dr. Stein likewise stressed the need for both the physician and technicians to get comfortable with taking measurements with the topography-guided unit. He advised taking repeat measurements to make sure that they are consistent. "That's the first step because if you're getting inconsistent measurements, the treatment is going to be off," Dr. Stein said. He emphasized the need to treat dry eye cases prior to attempt- ing a topography measurement. "When a patient has a dry eye and a punctate keratopathy or a rapid tear break-up time, it's hard to get consis- tent measurements," Dr. Stein said, adding it's advisable for patients to be out of their gas permeable con- tact lenses for about 1 month and their soft lenses for about 1 week prior to taking a topography scan. In the comfort zone Once practitioners get comfortable, Dr. Stein thinks it's just a question of selecting the right cases, steering away from the more complicated ones at first. "The small optical zone, the decentered ablations, and the central islands are probably the last group to tackle," he said. Dr. Stein often uses topography- guided ablation to treat keratoconus or pellucid marginal degeneration. In performing this technique in such cases, he said not to operate on anyone with too thin of a cornea. "In crosslinking, usually 60% of the patients gain an improvement in best vision, but when we add topography-guided PRK to the cross- linking, we can greatly increase the percentage of patients who achieve an improvement in best corrected acuity," Dr. Stein said. "We like to have corneas gener- ally over 450 microns before doing a topography-guided PRK." This is important because the first step is to remove the epithelium with a laser, something Dr. Stein has found re- moves around 50 microns of tissue. Topography-guided PRK itself is gen- erally limited to about 50 microns of tissue. "That means the cornea has gone from 450 to 350, and that's considered a little too thin for cross- linking," Dr. Stein said. However, with the riboflavin drops that they have to swell the cornea, they have close to 100% success when the cor- nea is about 320 microns or greater prior to crosslinking. Dr. Stein recommended that when treating keratoconus patients, it's best to aim for less than a 10-di- opter difference across the cornea. "If you take a look at the topogra- phy at the 6-mm optical zone, we prefer the difference between the steepest point and the flattest point be generally less than 10 D," he said. "If that's the case, we can flatten the steep area by a maximum of 5 D and steepen the flat area by about 5 D." He advised that PTK be used to remove the epithelium in crosslinking cases instead of doing this manually, which would change the topography. "With a PTK we use the laser to remove the epithelium, and we go down 50 microns," Dr. Stein said. Before activating the laser, physicians should make sure the cornea is not too wet and not too dry. "What we don't want is the laser beam to come down and hit a large puddle on the surface of the cornea," Dr. Stein said, adding you want to make sure that the tear film is there but not thick. If it is too thick, wipe a Merocel sponge across the cornea to thin it out before activating the laser. In addition, for topography- guided PRK, deciding on the size of the optical zone can be a bit of a balancing act, Dr. Stein noted. "We prefer the 6 or 6.5 mm optical zone with the laser since we get a more permanent effect and less chance of regression, but this does remove more tissue," he said, adding that you want to limit the topography- guided PRK ablation to 50 microns. When performing topography- guided PRK, he stressed the need to use mitomycin-C on every case. "We use a circular disc that has been soaked in mitomycin-C, and we typically apply this for 30 to 60 sec- onds depending upon the amount of treatment," Dr. Stein said. "This prevents haze, prevents new colla- gen synthesis, and keeps the cornea clear." He recommended cooling the surface after the procedure to decrease discomfort. "We put some Merocel sponges with saline in a freezer. When we take those out, they're like little popsicles, and we apply the ice to the cornea at the end of the topography-guided PRK," Dr. Stein said. Dr. Stein hopes in the future the FDA will approve use of crosslink- ing in the U.S. When this happens, he thinks that a high percentage of crosslinking patients will undergo topography-guided PRK to improve quality of vision. Dr. Wexler, mean- while, is looking forward to fully incorporating topography-guided ablation into his practice. "I'm ex- cited about adding it to our practice and being able to offer wavefront- optimized, wavefront-guided, and topo-guided treatments and trying to develop an understanding of which modality is best for which patient," Dr. Wexler concluded. EW Editors' note: Dr. Stein has no finan- cial interests related to this article. Dr. Wexler has financial interests with TLC Laser Eye Centers. Contact information Stein: raymondmstein@gmail.com Wexler: wexlermd@aol.com