Eyeworld

MARCH 2021 | EYEWORLD | 73 R References (cont.) 6. Scroggs MW, Proia AD. Histo- pathological variation in kerato- conus. Cornea. 1992;11:553–559. 7. Haque S, et al. Corneal and epi- thelial thickness in keratoconus: a comparison of ultrasonic pa- chymetry, 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 ma- chine-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. statistically significant correlation between the thinnest epithelium and the steepest keratome- try, indicating that as the cornea became steep- er, the epithelial thickness minimum became thinner." From there, Dr. Reinstein and colleagues developed an automated machine-based classi- fier of epithelial profiles for detecting keratoco- nus well before specific topography or tomo- graphic changes. 9 "The Insight 100 incorporates our machine- based algorithm that classifies the epithelial profile as keratoconic with 95% sensitivity and 99% specificity, levels much higher than can currently be achieved by topography or tomog- raphy," he said. "The epithelium can then be used to ex- clude keratoconus in cases of suspect topogra- phy, such as inferior steepening, or to confirm keratoconus in cases of equivocal topography or tomography," 10 Dr. Reinstein continued. "Our studies on screening with epithelial profiles in the presence of inferior steepening showed that 85% of the time the inferior steepening was due to the epithelium being thicker inferiorly, and not keratoconus, allowing us to rule in cases that we would have previously rejected for corneal surgery." Dr. Carones reiterated that he finds epitheli- al mapping helpful in detecting keratoconus-like patterns that are not in fact keratoconus. explained that "epithelial mapping can demon- strate continued changes in the cornea after crosslinking that may not be apparent by just looking at surface topography." Dr. Carones also uses epithelial mapping in screening candi- dates for laser refractive surgery. For example, a patient might be seen as having ectasia after refractive surgery when, if epithelial mapping had been performed preop, it might have been identified as early keratoconus. While both VHF ultrasound and OCT can map the corneal epithelium, there are differ- ences in accuracy and mapping coverage, Dr. Reinstein said. "Our studies have demonstrated that VHF ultrasound epithelial measurement produces an accuracy of approximately 1 µm while OCT achieves approximately 3 µm," 5 Dr. Reinstein ex- plained, adding, "the main reasons for the lower accuracy of OCT relate to the fact that OCT is measuring epithelium and tear film together, the tear film itself being a big variable, as well as the effects of unknown refractive index with- in the epithelium. In our practice every single patient undergoes OCT epithelial mapping screening. Approximately 15% of patients go on to require definitive epithelial mapping by VHF ultrasound, which also gives us the best posteri- or chamber measurements at the same time for improved ICL sizing if the cornea ends up being classified as keratoconic." Dr. Reinstein pointed out that histopatho- logic analysis of keratoconic corneas has confirmed the clinical observations of epithe- lial breakdown over an excessively steepened cone. 6–7 He was the first to describe the epitheli- al profile in keratoconus, demonstrating how it was considerably different from that of normal corneas. 8 "The epithelium was thinnest at the apex of the cone, and this thin epithelial zone was surrounded by an annulus of thickened epi- thelium," he said. "While all eyes exhibited the same epithelial donut pattern, characterized by a localized central zone of thinning surrounded by an annulus of thick epithelium, the thickness values of the thinnest point and the thickest point as well as the difference in thickness between the thinnest and thickest epitheli- um varied greatly between eyes. There was a Epithelial mapping technologies OCT MS-39 anterior segment OCT and tomographer (CSO Italia) Avanti OCT System (Optovue) SPECTRALIS OCT (Heidelberg Engineer- ing) CIRRUS HD-OCT (Carl Zeiss Meditec) VHF ultrasound Insight 100 VHF digital ultrasound scanner (ArcScan) continued on page 74

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