EyeWorld Asia-Pacific September 2018 Issue

EWAP REFRACTIVE 49 September 2018 of topography and tomography to avoid surgery in subclinical kera- toconus cases. He also thinks more work could be done to improve the calculation of PTA and elimi- nate what he called “confounding 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 reli- able 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 general, Dr. Reinstein said, “We must continue to improve our screening systems to avoid prevent- able ectasia; in an ideal world, we would bring this ratio to zero.” “While post-LASIK ectasia can occur without pre-existing kera- toconus, the two main risk factors for ectasia are pre-existing kerato- conus 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 instead of traditional residual stro- mal calculations makes sense. However, he takes issue with the criteria being based only on Dr. Santhiago’s analysis 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 retro- spective interpretation of the cor- neal maps used in Dr. Santhiago’s research, as acknowledged by the study authors, showed that most eyes had risk factors for keratoco- nus, which itself is a major risk fac- tor for development of post-LASIK ectasia. Dr. Reinstein and his team performed a retrospective analysis of their entire database history, which includes more than 15,000 patients treated since 2002. If they had applied 40% PTA as a cutoff, 26.5% of their total database would have been denied LASIK surgery, he said. Seven patients had ectasia in one or both eyes; of those, 70% had a PTA of 40% or greater. “This means PTA has a sen- sitivity of 70% and specificity of 73.5% in our database, both of which are unacceptably low and certainly much lower than the mo- dalities described above,” he said. Dr. Reinstein advocates for a multifactorial screening approach that includes all possible corneal anatomical and morphological features—front surface topog- raphy, back surface topography, corneal thickness progression and tomography, and epithelial thick- ness mapping—combined with biomechanical factors. With this approach, Dr. Reinstein said PTA of 40% appears to add little further information. However, he acknowl- edged, PTA could offer a conserva- tive safety marker for surgeons LASIK procedure at the conclusion of closing the ap Source: Gregory Parkhurst, MD who do not have access to a full suite of diagnostics. Dr. Santhiago added: “There are some things that need to be clarified. Ectasia is a rare condition, which means that in any situation it occurs rarely. What we have shown is its higher incidence under the conditions of high PTA. That was clear. The ro- bustness of an association between a factor and an adverse event is not measured through sensitivity or specificity but through odds ratio or relative risk. A risk factor will never become a screening method. Finally, if a high PTA is the only risk factor and topography is bi- laterally normal, the patient could have been submitted to surface ablation.” EWAP References 1. Santhiago MR, et al. Association between the percent tissue altered and post-laser in situ keratomileusis ectasia in eyes with normal preoperative topography. Am J Ophthalmol. 2014;158:87–95. 2. Santhiago MR, et al. Role of percent tissue altered on ectasia after LASIK in eyes with suspicious topography. J Refract Surg. 2015;31:258–65. 3. Santhiago MR. Percent tissue altered and corneal ectasia. Curr Opin Ophthalmol. 2016;27:311–5. Editors’ note: The physicians have no financial interests related to their com- ments. Contact information Reinstein: dzr@londonvisionclinic.com Saad: dralainsaad@gmail.com Santhiago: marconysanthiago@hotmail. com