Eyeworld

77 EW FEATURE February 2018 • Laser vision correction using the Pentacam. Though these technologies are not new, Dr. Rocha explained that she has more recently combined her Pentacam measure- ments with that of the Corvis ST (Oculus), a non-contact tonometer that has a dynamic ultra-high speed Scheimpflug camera to show real time deformation of the cornea. The corneal tomography combined with the corneal deformation make up the tomographic biomechanical index (TBI). 3 "By combining the biomechan- ical properties and the tomographic findings, you have this TBI index that is more sensitive in detecting 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% sen- sitivity with 96% specificity in eyes with normal topography and very asymmetric ectasia in the fellow eye. In a recent paper on which she was an author, TBI was applied to clinical cases, 4 and Dr. Rocha said she now uses TBI parameters to evaluate all of 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 deformation parameters], and if that cornea is strong enough for surgery, PRK, LASIK, or SMILE," Dr. Rocha ex- plained. "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, U.K., 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 other 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 ker- atoconus 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 ability to remodel, masking early keratoconus that might not be iden- tified 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 a 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 signifi- cant number of patients are cleared for cornea surgery by the confirma- tion of a 'normally' classified epithe- lial profile," Dr. Reinstein said. "As you may surmise from our scanning protocol, we think every patient should have epithelial thick- ness mapping prior to surgery," Dr. Reinstein said. "Given the signifi- cant change in diagnostic 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 confirmation of nor- mality when things are equivocal. "Finally, by having the ability to map epithelium we are also equip- ping ourselves with the ability to perform layered pachymetric map- ping 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 Asso- ciates, 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 patients, 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 import- ant 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 staining, continued on page 78 Basic LASIK preoperative evaluation Ronald Krueger, MD, discusses the salient points of the basic preoperative evaluation for refractive surgery. EWReplay.org

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