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26 EW CORNEA August 2013 Device focus Updates on anterior segment imaging by Michelle Dalton EyeWorld Contributing Writer Technology is advancing the field rapidly—and that's helping researchers and clinicians diagnose corneal disorders more thoroughly and more easily T he ability to quickly diagnose—and then treat just as quickly—diseases such as keratoconus is due in part to the sophisticated imaging devices currently available. Forme fruste keratoconus (FFKC) is "not the same as a topographic pattern of a keratoconus suspect, which is a topographic diagnosis," said Fernando Faria-Correia, MD, refractive surgery research fellow, Storm Eye Institute, and Magill Vision Center, Charleston, S.C. Rather, FFKC is an incomplete or abortive form of keratoconus presenting with a "relatively normal anterior curvature map," he said. "In FFKC, it's critical to consider enhanced tests beyond front surface curvature evaluation, such as corneal tomography, which provides a 3D corneal analysis," Dr. FariaCorreia said. As numerous studies show that "keratoconic disease may be first detectable at the posterior surface, Scheimpflug imaging appears today as the most sensitive technology for detecting subclinical keratoconus," said David Smadja, MD, anterior segment and refractive surgery unit, Bordeaux Hospital University, National Reference Center for Keratoconus, Bordeaux, France. Topography remains the "current clinical standard" for diagnosing keratoconus, which is a "greatly feared contraindication for LASIK," said Yan Li, PhD, research assistant professor of ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland. However, topography may not detect all patients at risk for keratectasia, and more sensitive and reliable methods are still needed. And while both the Pentacam (Oculus, Wetzlar, Germany) and the Orbscan (Bausch + Lomb, Rochester, N.Y.) can provide corneal thickness maps, "they cannot map the finer structure of the cornea, like the corneal epithelium," she said. In contrast, OCT provides micron-level resolution and allows Optical coherence topography compared to histology Source: Mohamed Abou Shousha, MD, and J. Wang excellent delineation of corneal surfaces. "Analyzing the corneal epithelial and stromal thicknesses and shapes separately can facilitate the detection of the disease in its early stage." Her recent studies demonstrated that OCT can map corneal epithelial thickness with good repeatability in both normal and keratoconic eyes.1 Keratoconus was characterized by apical epithelial thinning. The resulting deviation from the normal epithelial pattern could be detected with very high accuracy using the pattern standard deviation variable (100% specificity and 100% sensitivity). Scheimpflug imaging calculates the curvature based on elevation data from a slit image, and the pixel resolution requirement in a slit image must be extremely high for identifying a 0.25 D difference in curvature, Dr. Smadja said. With the advent of crosslinking to treat keratoconus, "we need a very sensitive and specific way to diagnose keratoconus early in the disease," said Mohamed Abou Shousha, MD, Saint Louis University Eye Institute. Being able to image Bowman's layer will produce an earlier diagnosis of the disorder, he said. High frequency ultrasound has a resolution of just 21 microns, while older optical coherence tomography (OCT) is about 18 microns, meaning in keratoconic patients this layer would be missed since it is significantly thinner, Dr. Abou Shousha said. His group is evaluating an ultra high-resolution OCT as a potential method to image Bowman's layer. Others have used ultra high-res (UHR) OCT, but "could not capture the thinning Bowman's layer because they were imaging just the central part; with UHR OCT, the more resolution you have, the less width and depth you can image, but our new imaging technique allowed us to image that layer up to the periphery of the cornea," Dr. Abou Shousha said. "Integrating Placido topography with Scheimpflug imaging improves the accuracy of central anterior corneal curvature calculation and takes the best of both technologies for improving the sensitivity of subclinical keratoconus detection," Dr. Smadja explained, adding that identifying subclinical keratoconus "remains one of the most challenging situations faced by the ophthalmologist when considering a refractive surgery procedure." Dr. Abou Shousha's Bowman's layer ectasia index (not yet published) was "100% sensitive and specific. Our previous work used the absolute thinning on the Bowman's layer that rendered an index that is 90% sensitive and specific. Preliminary results [of the new index using UHR OCT] discuss the relative rather than an absolute thinning on the layer, relative to the patient's own average thickness to minimize the effect of the differences in Bowman's thickness among different subjects, and that resulted in a significantly better index," he said. Further, this new index is "descriptive of the severity of keratoconus, making it a quantitative factor as well," he said. OCT and UBM Newer OCT technology—spectral OCT, spectral-domain OCT, and frequency-domain OCT, all of which are synonymous with Fourier-domain OCT—can acquire scans 10 to 100 times faster than time-domain OCT, Dr. Li said. "Newer OCT systems capable of producing axial resolutions of 1-5 μm are now available. With such high resolution, OCT provides reliable identification of fine angle structures such as the Schwalbe's line, trabecular meshwork, and Schlemm's canal. In contrast, the axial resolutions of the earlier OCT systems were limited to 15-20 μm and did not allow reliable identification of those smaller angle structures. This marked increase in resolution is due to the combination of broader bandwidth and shorter wavelength," she said. Dr. Faria-Correia prefers spectral domain OCT to visualize angle structures, but noted "the new swept-source OCTs are even better," although he does not yet have clinical experience with them. Ultrasound biomicroscopy (UBM) is an effective method to study the posterior chamber and ciliary body, Dr. Faria-Correia said, and can be useful for the preop evaluation when considering posterior chamber phakic IOL implantation. continued on page 28