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EW REFRACTIVE 40 May 2018 by Liz Hillman EyeWorld Senior Staff Writer can still be safely performed. More care should be given, he continued, to evaluation of topography and tomography to avoid surgery in sub- clinical keratoconus cases. He also thinks more work could be done to improve the calculation of PTA and eliminate what he called "confound- ing factors present in the current PTA calculation." "PTA is currently calculated as a bidimensional metric and looks only at the altered tissue centrally," Dr. Saad said. "An evaluation of percent of volume altered will certainly represent a more reliable indicator of biomechanical weakening induced by the LASIK procedure." To bring in a third-party opin- ion on the topic, EyeWorld reached out to Dan Reinstein, MD, London Vision Clinic, London, U.K. In gen- eral, Dr. Reinstein said, "We must continue to improve our screening systems to avoid preventable ectasia; in an ideal world, we would bring this ratio to zero. "While post-LASIK ectasia can occur without pre-existing kerato- conus, the two main risk factors for ectasia are pre-existing keratoconus and removing excess tissue," both of which were first described by the late Jose Ignacio Barraquer Moner, MD, Dr. Reinstein said. Regarding Dr. Santhiago's con- clusions of PTA of 40% or more, Dr. Reinstein said referring to PTA in- stead of traditional residual stromal calculations makes sense. However, he takes issue with the criteria being based only on Dr. Santhiago's analy- sis of cases of ectasia. "There was no analysis of the whole population of eyes treated that generated those cases, and therefore we have no indication as to the percentage of eyes that have been treated with a PTA more than 40% that did not develop ectasia. The PTA cutoff of 40 has good sen- sitivity, but the likelihood is that it has low specificity." Dr. Reinstein also said ret- rospective interpretation of the corneal maps used in Dr. Santhiago's research, as acknowledged by the study authors, showed that most eyes had risk factors for keratoconus, which itself is a major risk factor for development of post-LASIK ectasia. others such as high myopia or thin corneas, based on his research. "In only one series of patients, PTA appeared to be a significant, independent risk factor for ectasia, but no validation study on an ex- ternal group of patients was able to reproduce the same result," Dr. Saad said. "Sensitivity and specificity are commonly used to evaluate screen- ing factors, and as PTA is being used as a screening method to select good candidates for LASIK surgery, we cal- culated the sensitivity and specificity of PTA in our group of patients. We found that sensitivity of PTA was less than 48% and specificity was 79%." This is the crux of Dr. Saad's and Dr. Santhiago's perspectives. Dr. Santhiago's counterpoint to this argument is that he is describing PTA not as a predictor or screening method but as a risk factor. "A screening method is when we are attempting to find a disease before we have … clinical findings of it," Dr. Santhiago said, going on to explain that the tools for identifying screening methods are sensitivity and specificity. "I'm not talking about trying to find or predict anything with PTA," Dr. Santhiago said. "It should not be investigated with sensitivity and specificity because it's not a screen- ing method." Risk factors, on the other hand, he said, mean that the chance of this happening is higher. What Dr. Santhiago wants people to know is that ectasia is a rare complication, but a patient with normal topogra- phy and a PTA of more than 40% undergoing LASIK might be at least 28 times more likely to develop ectasia, according to his research. In patients with a PTA of 40% or more, Dr. Santhiago said he performs PRK. "I don't want to scare people. This is supposed to be a tool to help. … I have some older surgeons saying 'I have done patients with high PTA and they are fine,'" Dr. Santhiago said. "The only thing I'm saying is this is a risk factor: The higher the PTA, the higher the chance of this problem. But it's rare." Dr. Saad said in the presence of normal topography and tomography and a PTA of more than 40%, LASIK prevalent factor. … Percent tissue altered had the highest odds ratio (223), followed by residual stromal bed ≤ 300 μm (74) and ectasia risk score ≥ 4 (8)," Santhiago et al. wrote. "Stepwise logistic regression revealed percent issue altered ≥ 40 as the single most significant independent variable (P < .0001)." The study authors concluded that, "Percent tissue altered at the time of LASIK was significantly associated with the development in ectasia in eyes with normal preoperative topography and was a more robust indicator of risk than all other variables in this patient population." Dr. Santhiago and coauthors published a couple of other papers on the topic as well. 2–3 Dr. Santhiago said PTA has been increasingly recognized as a risk fac- tor for developing post-LASIK ectasia in cases where patients presented with normal topography and no other risk factors preoperatively and underwent uncomplicated surgery. Dr. Saad agreed that the refrac- tive surgery community is becoming familiarized with the concept of PTA due to the amount of communica- tion and published papers describing it and due to the high interest in ectasia prevention. Dr. Saad said that PTA might be considered a risk fac- tor for iatrogenic ectasia, but it's not more of a risk factor compared to As a relatively new metric, PTA is a debated topic as to whether it should be viewed as a risk factor for post-LASIK ectasia P ercent tissue altered (PTA) is a possible risk factor for post-LASIK ectasia in eyes that preoperatively presented with normal bilateral topography. Marcony Santhiago, MD, associate professor, University of São Paulo, Brazil, and adjunct profes- sor of ophthalmology, University of Southern California, and Alian Saad, MD, Rothschild Foundation, Paris, France, got into a heated debate over whether PTA of 40% or more should be considered a valid risk factor at the XXXV Congress of the ESCRS. EyeWorld caught up with both of them later to dive deeper into some of the nuanced points and positions on the topic. PTA is flap thickness plus ablation depth divided by central corneal thickness. Dr. Santhiago first published his analysis that stated a PTA of 40% or more carries an increased risk of post-LASIK ectasia in 2014. 1 "In the ectasia group, percent tissue altered ≥ 40 was the most Considering percent tissue altered as a risk factor for post-LASIK ectasia Presentation spotlight LASIK procedure at the conclusion of closing the flap Source: Gregory Parkhurst, MD

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