EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
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74 | EYEWORLD | MARCH 2021 R EFRACTIVE Contact Carones: fcarones@carones.com Cummings: abc@wellingtoneyeclinic.com Reinstein: dzr@londonvisionclinic.com "Sometimes we see some sort of steepen- ing on corneal curvature maps, and you always have to question whether this is keratoconus or something else. The epithelial map helps discriminate because if we find some sort of corneal steepening and if steepening is related to epithelial thickness, we know this is not kera- toconus," he said. Dry eye disease can also cause variability in epithelial maps, Dr. Cummings noted. "Subclinical anterior basement disorder where there are no signs of map, dot, or finger- prints can also cause variability in the epithelial profile that can be misinterpreted as keratoco- nus," Dr. Reinstein said. A post-refractive surgery tool After corneal surgery, Dr. Carones said epithelial maps can provide information prior to enhance- ments. If, for example, there was a myopic shift post-refractive surgery, epithelial maps can help distinguish between true progression vs. thicker epithelium acting as a positive contact lens. "Sometimes it is just because the epithelium became thicker in the center compared to the mid-periphery, and removing the epithelium and doing the PRK enhancement may lead to a re-epithelialization where the epithelium is not even in the center so the amount of correction used by the laser would lead to an overcorrec- tion," Dr. Carones said. Dr. Carones said he includes epithelial mapping assessments in all of his post-refractive surgery visits. During the early postop period, epithelial maps help him assess the healing process. If the epithelium is uneven or irregular, he said he knows healing is still taking place. Once it regularizes, the healing process is over. After several years, epithelial mapping can help detect ectasia but is also used to drive decisions about whether an enhancement is needed or if the epithelium has just become too thick. Dr. Reinstein noted that the bulk of the compensatory epithelial changes after LASIK occur within the first postoperative day, most of the rest of the changes happen over the next few weeks, and very little changes after 3 months. 11 "Epithelial maps are especially helpful when making a final decision for enhancement in patients who had a hyperopic correction," Dr. Reinstein said. The central epithelial thickness can be a useful indicator as a measurement of potential risk for apical syndrome, which occurs if the epithelium is too thin (less than 25 µm). 12 "For example, in one case from our study, the maximum simulated keratometry of 50.80 D would most likely prevent the surgeon from treating further hyperopia; however, the central epithelial thickness of 41.7 µm would suggest that the cornea could be steepened further without resulting in epithelial breakdown," Dr. Reinstein said. "On the other hand, another case from this study demonstrated that the epithelial thickness can be thin (33.7 µm) although the cornea was still relatively flat postoperatively (46.40 D). The curvature limit would allow further hyperopic ablation, whereas the thin, central epithelium would indicate that further steepening might increase the risk of apical syndrome. Therefore, using epithelial thickness measurements, hyperopic retreatments might be performed without risk of apical syndrome while also allowing some patients to have retreatment who would otherwise have been rejected from further surgery due to high ker- atometry postoperatively. 13 "The epithelium always masks stromal surface abnormalities beneath, becoming thinner over bumps and thicker over troughs," Dr. Reinstein said. Relying on topography or wavefront-guided treatments alone can lead to suboptimal treatment and could even make things worse in attempting to correct corneal complications. "In such cases with localized irregularities, a transepithelial PTK treatment is the better option as this uses the irregular epithelial thickness profile as a natural masking agent to target the ablation onto the relative peaks on the stromal surface, thus producing a smoother stromal surface," he said. 14–16 Epithelial mapping is also helpful for ensur- ing safe flap thickness for LASIK on an eye with previous PRK or LASIK after SMILE. Dr. Cummings said that he thinks more ed- ucation is needed about the value of epithelial mapping to drive more widespread adoption. "People who I think would be most exposed are refractive surgeons initially and then corne- al surgeons who know the value of this technol- ogy … and getting to cataract surgery eventual- ly for quality of vision, too," he said. References (cont.) 14. Reinstein DZ, Archer T. Com- bined Artemis very high-frequen- cy digital ultrasound-assisted transepithelial phototherapeutic keratectomy and wavefront- guided treatment following multiple corneal refractive procedures. J Cataract Refract Surg. 2006;32:1870–1876. 15. Reinstein DZ, et al. Refractive and topographic errors in topog- raphy-guided ablation produced by epithelial compensation predicted by 3D Artemis VHF digital ultrasound stromal and epithelial thickness mapping. J Refract Surg. 2012;28:657–663. 16. Reinstein DZ, et al. Tran- sepithelial phototherapeutic keratectomy protocol for treating irregular astigmatism based on population epithelial thickness measurements by Artemis very high-frequency digital ultrasound. J Refract Surg. 2014;30:380–387. Relevant disclosures Carones: CSO Italia Cummings: Alcon Reinstein: ArcScan, Carl Zeiss Meditec continued from page 73