EyeWorld India March 2018 Issue

that can cause changes in quality of vision.” Ruling out keratoconus Dr. Rocha performs Placido-based corneal topography (Atlas, Zeiss, Jena, Germany) and Scheimpflug corneal tomography, using Penta- cam (Oculus, Wetzlar, Germany). Though these technologies are not new, Dr. Rocha explained that she has more recently combined her Pentacam measurements with that of the Corvis ST (Oculus), a non- contact tonometer that has a dy- namic ultra-high-speed Scheimpflug camera to show real-time deforma- tion of the cornea. The corneal tomography combined with the corneal deformation make up the tomographic biomechanical index (TBI). 3 “By combining the biomechani- cal properties plus the tomographic findings, you have this TBI index that is more sensitive in didacting patients who are at higher risk for developing ectasia,” Dr. Rocha said, noting that an optimized TBI cut-off value of 0.29 provided 90.4% sensitivity with 96% specificity in eyes with normal topography and very asymmetric ec- tasia in the fellow eye. She recently had a paper accepted for publication in the International Journal of Kera- toconus and Ectatic Corneal Diseases in which TBI was applied to clinical cases. 4 Dr. Rocha said she now uses TBI parameters to evaluate all her refractive surgery candidates. “Corneal topography and to- mography provide different indices that look at the thickness profile and elevation maps … but the Corvis will give us information on corneal biomechanics [cornea de- formation parameters], and if that cornea is strong enough for surgery, PRK, LASIK, or SMILE,” Dr. Rocha explained. “Sometimes you can have a thin cornea but it’s a normal strong cornea … and some patients have a thick cornea but that cornea is weak.” Daniel Reinstein, MD, MA (Cantab), FRCSC , London Vision Clinic, London, UK, acknowledged the Corvis ST and Pentacam as a biomechanical diagnostic option, but said that while it might increase sensitivity of identifying keratoco- nus, its specificity is still too low to be an attractive tool in a refractive surgery clinic, in his opinion. “Too many false positive diag- noses of keratoconus would likely be picked up,” he said. “On the oth- er hand, I see the most important advances happening in epithelial mapping for keratoconus detection, particularly given that we now have a device that combines this with tomography. “Currently, the most advanced OCT device for screening for kera- toconus in my view is the MS-39 [CSO, Firenze, Italy],” Dr. Reinstein said. “The most accurate epithelial mapping device available is still the ArcScan Insight 100 System [ArcScan, Golden, Colorado] with a measurement precision of less than 1 µm . It is also the only system with an integrated keratoconus screening automatic epithelial profile classi- fier, which has a 94.6% sensitivity and 99.2% specificity for detecting keratoconus. 5 “The MS-39 combines mapping of the epithelium, a Placido front surface, and OCT tomographic back surface information, which are all captured simultaneously and spatially registered,” he continued. “While epithelial maps by OCT are not as accurate as those by very high-frequency ultrasound, the MS- 39 provides an excellent integration of all modalities.” 6,7 Because of the epithelium’s abil- ity to remodel, masking early kera- toconus that might not be identi- fied by other devices, Dr. Reinstein said epithelial thickness mapping may be used to confirm suspected keratoconus or show thickening over a suspicious area to help rule out keratoconus and enable corneal refractive surgery to be performed. In addition to epithelial thick- ness mapping with the MS-39 or RTVue (Optovue, Fremont, Califor- nia), Dr. Reinstein said his clinic uses a 20-point keratoconus screen- ing protocol on every refractive sur- gery consult. This includes Placido topography, tomography, corneal OCT, and corneal hysteresis. “Any patient in whom there is any question based on the above testing protocol undergoes ArcScan Insight 100 scanning, which is then used to make a final decision. Some patients are ‘saved’ from corneal surgery by the ArcScan Insight, but a significant number of patients are cleared for cornea surgery by the confirmation of a ‘normally’ classi- fied epithelial profile,” Dr. Reinstein said. “As you may surmise from our scanning protocol we think every patient should have epithe- lial thickness mapping prior to surgery,” Dr. Reinstein said. “Given the significant change in diagnos- tic category afforded by epithelial thickness mapping, I think that no refractive surgery clinic can afford not to use it. The ArcScan Insight 100 increases our annual surgical volume by 7% by providing confir- mation of normality when things are equivocal. “Finally, by having the abil- ity to map epithelium we are also equipping ourselves with the ability to perform layered pachymetric mapping of the cornea for flap, residual bed and other interface biometry (such as scars, etc.) when evaluating postoperative corneas.” Addressing the ocular surface All patients seeing Jennifer Loh, MD , Loh Ophthalmology Associ- ates, Miami, will receive ocular surface/dry eye testing. “We place it at a very high im- portance; it’s one of the first things I evaluate when examining pa- tients, especially if they’re coming in for a surgery consult,” she said. “I think the tear film and ocular surface are critical, and every refractive surgeon should be paying attention to it as a poor tear film and ocular surface will lead to refractive misses,” said Preeya K. Gupta, MD , associate professor of ophthalmology, Duke University School of Medicine, Durham, North Carolina. “To that end, it is impor- tant to screen pre-surgical patients, especially those who have a refrac- tive goal, to make sure they don't have dry eye disease or meibomian gland disease (MGD). To assess the ocular surface quality of a patient, Dr. Loh said she will perform fluorescein stain- ing, tear breakup time testing, topography, and meibography. The latter, she said, is one of the newer dry eye diagnostic tools that she thinks “can give us a lot of clues to the health of the ocular surface, and it gives the patient an image of their disease, if they do show such signs.” continued on page 44 March 2018 EWAP REFRACTIVE 43