EyeWorld India September 2018 Issue

48 EWAP REFRACTIVE September 2018 Considering percent tissue altered as a risk factor for post-LASIK ectasia by Liz Hillman EyeWorld Senior Staff Writer 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 pre- operatively presented with normal bilateral topography. Marcony Santhiago, MD, associate professor, University of São Paulo, Brazil, and adjunct professor 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 consid- ered 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. “In the ectasia group, percent tissue altered * 40 was the most prevalent factor. … Percent tissue altered had the highest odds ratio (223), followed by residual stro- mal bed ) 300 µm (74) and ectasia risk score * 4 (8),” Santhiago et al. wrote. “Stepwise logistic regres- sion revealed percent issue altered * 40 as the single most significant independent variable (p<0.0001).” The study authors concluded that, “Percent tissue altered at the time of LASIK was significantly associat- ed with the development in ectasia in eyes with normal preoperative topography and was a more robust indicator of risk than all other vari- ables in this patient population.” Dr. Santhiago and co-authors pub- lished a couple of other papers on the topic as well. Dr. Santhiago said PTA has been increasingly recognized as a risk factor for developing post- LASIK ectasia in cases where patients presented with normal topography and no other risk fac- tors preoperatively and underwent uncomplicated surgery. Dr. Saad agreed that the refrac- tive surgery community is becom- ing familiarized with the concept of PTA due to the amount of com- munication and published papers describing it and due to the high interest in ectasia prevention. Dr. Saad said that PTA might be con- sidered a risk factor for iatrogenic ectasia, but it’s not more of a risk factor compared to 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 ecta- sia, but no validation study on an external group of patients was able to reproduce the same result,” Dr. Saad said. “Sensitivity and specific- ity are commonly used to evaluate screening factors, and as PTA is being used as a screening method to select good candidates for LASIK surgery, we calculated the sensitiv- ity 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 de- scribing 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 try- ing to find or predict anything with PTA,” Dr. Santhiago said. “It should not be investigated with sensitivity and specificity because it’s not a screening 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 complica- tion, but a patient with normal topography 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 pres- ence of normal topography and tomography and a PTA of more than 40%, LASIK can still be safely performed. More care should be given, he continued, to evaluation