EyeWorld Asia-Pacific June 2021 Issue

REFRACTIVE 38 EWAP JUNE 2021 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 keratometry 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 ensuring safe flap thickness for LASIK on an eye with previous PRK or LASIK after SMILE. Dr. Cummings said that he thinks more education 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 corneal surgeons who know the value of this technology … and getting to cataract surgery eventually for quality of vision, too,” he said. EWAP References 1. Reinstein DZ, et al. Epithelial thickness in the normal cornea: three-dimensional display with Artemis very high-frequency digital ultrasound. J Refract Surg. 2008;24:571–581. 2. Li Y, et al. Corneal epithelial thickness mapping by Fourier-domain optical coherence tomography in normal and keratoconic eyes. Ophthalmology. 2012;119:2425–2433. 3. Reinstein DZ, et al. Epithelial and corneal thickness measurements by high-frequency ultrasound digital signal processing. Ophthalmology. 1994;101:140–146. 4. Reinstein DZ, et al. Corneal epithelial thickness profile in the diagnosis of keratoconus. J Refract Surg. 2009;25:604–610. 5. Reinstein DZ, et al. Comparison of corneal epithelial thickness measurement between Fourier-domain OCT and very high-frequency digital ultrasound. J Refract Surg. 2015;31:438–445. 6. Scroggs MW, Proia AD. Histopathological variation in keratoconus. Cornea. 1992;11:553–559. 7. Haque S, et al. Corneal and epithelial thickness in keratoconus: a comparison of ultrasonic pachymetry, Orbscan II, and optical coherence tomography. J Refract Surg . 2006;22:486–493. 8. Reinstein DZ, et al. Epithelial, stromal, and total corneal thickness in keratoconus: three-dimensional display with Artemis very-high frequency digital ultrasound. J Refract Surg . 2010;26:259–271. 9. Silverman RH, et al. Epithelial remodeling as basis for machine-based identification of keratoconus. Invest Ophthalmol Vis Sci. 2014;55:1580–1587. 10. Reinstein DZ, et al. Corneal epithelial thickness profile in the diagnosis of keratoconus. J Refract Surg. 2009;25:604–610. 11. Reinstein DZ, et al. Change in epithelial thickness profile 24 hours and longitudinally for 1 year after myopic LASIK: three-dimensional display with Artemis very high-frequency digital ultrasound. J Refract Surg. 2012;28:195–201. 12. Reinstein DZ, et al. Epithelial thickness after hyperopic LASIK: three-dimensional display with Artemis very high-frequency digital ultrasound. J Refract Surg. 2010;26:555–564. 13. Reinstein DZ, et al. LASIK for the correction of high hyperopic astigmatism with epithelial thickness monitoring. J Refract Surg. 2017;33:314–321. 14. Reinstein DZ, Archer T. Combined Artemis very high-frequency 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 topography- 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. Transepithelial 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. Editors’ note: Dr. Carones practices at Carones Vision, Milan, Italy, and has relevant interests with CSO Italia. Dr. Cummings practices at Wellington Eye Clinic, Dublin, Ireland, and has relevant interests with Alcon. Dr. Reinstein practices at London Vision Clinic, London, UK, and has relevant interests with ArcScan and Carl Zeiss Meditec. OCT • MS-39 anterior segment OCT and tomographer (CSO Italia) • Avanti OCT System (Optovue) • SPECTRALIS OCT (Heidelberg Engineering) • CIRRUS HD-OCT (Carl Zeiss Meditec) VHF ultrasound • Insight 100 VHF digital ultrasound scanner (ArcScan) Epithelial mapping technologies