EyeWorld India December 2013 Issue

49 EWAP DEVICES December 2013 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. But UBM will not get as “close” as OCT, Dr. Shousha said—the technology uses sound waves compared to OCT’s laser images. “UBM can improve its technology, but it won’t get to those higher resolutions,” he said. Down the road OCTs with faster and wider scanning that could image the whole anterior eye in a 3D manner are likely to be commercialized in the future, Dr. Li said. OCT-guided surgery is now also a possibility, as several commercial femtosecond laser platforms already have built- in OCT to guide femtosecond cataract surgery, she added. Dr. Smadja expects a diagnostic system that combines morphological, optical and biomechanical evaluation of the cornea for securing and improving patient outcomes may be a realistic goal. “In addition, the popularization and implementation of ray tracing technology in the current devices might also be helpful in the near future for refining the post-LASIK IOL calculation as well as for improving further the customized corneal ablations in refractive surgery,” he said. Dr. Faria-Correia predicted OCT will evolve with “faster and even higher-resolution scans” that will “eventually replace other recent technology devices such as slit lamp scanning, and lower resolution Scheimpflug cameras.” Corneal biomechanics is another area of research interest, Dr. Faria-Correia said, with the Ocular Response Analyzer (Reichert, Depew, NY, USA) and the Corvis ST (Oculus, Wetzlar, Germany) currently marketed. With questions still remaining about how to properly measure intraocular pressure and understand/ interpret its impact on corneal deformation, “we will have a great integration of corneal tomography and biomechanical assessments with artificial intelligence, as developed by the BrAIN (Brazilian Study Group of Artificial Intelligence and Corneal Analysis),” he said. All of this will “ensure a more robust and refined identification” of FFKC and provide additional insights about an eye’s susceptibility in developing ectasia in the post-laser vision correction realm. An automated detection program for subclinical keratoconus combining “all the most discriminant parameters for improving the sensitivity of the detection using a sophisticated artificial intelligence software has been recently developed by our group with a system that combines a Placido-based corneal topography and dual Scheimpflug technology,” Dr. Smadja said, “with the aim of developing a diagnostic tool that will help the ophthalmologist in the decision-making process without requiring preliminary expertise in interpreting corneal imaging.” 2 EWAP References 1. Li Y, Tan O, Brass R, Weiss JL, Huang D. Corneal epithelial thickness mapping by Fourier-domain optical coherence tomography in normal and keratoconic eyes. Ophthalmol. 2012;119:2425-2433. 2. Smadja D, Touboul D, Cohen A, Doveh E, Santhiago MR, Mello GR, Krueger RR, Colin J. Detection of Subclinical Keratoconus using an Automated Decision Tree Classification. Am J Ophthalmol . 2013 June 7. Editors’ note: Dr. Abou Shousha has a provisional patent on using OCT Bowman’s layer indices in the diagnosis of keratoconus. Dr. Faria- Correia has no financial interests related to this article. Dr. Li has financial interests with Optovue (Fremont, Calif., USA) and Carl Zeiss Meditec (Jena, Germany). Dr. Smadja has no financial interests related to this article. Contact information Abou Shousha: hmfadel@yahoo.com Faria-Correia: f.faria.correia@gmail.com Li: +1-503-494-6394, liyan@ohsu.edu Smadja: davidsmadj@hotmail.fr