EyeWorld Asia-Pacific December 2013 Issue

48 EWAP DEVICES December 2013 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, SC, USA. 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. Faria-Correia 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, Ore., USA. 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, NY, USA) 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 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, Optical coherence topography compared to histology. Source: Mohamed Abou Shousha, MD, and J. Wang