EyeWorld Asia-Pacific June 2018 issue

June 2018 EWAP REFRACTIVE 55 Views from Asia-Pacific Mohamad ROSMAN, MBBS, MRCS (Ophth), MMed (Ophth), FRCS(Edinburgh), FAMS Senior Consultant Ophthalmologist, Head, Refractive Surgery Service and Laser Vision Centre Sr Consultant, Cataract and Comprehensive Service Assistant Professor, Graduate Medical School, Duke-NUS Clinical Senior Lecturer, YLL School of Medicine, NUS Adjunct Senior Clinician Investigator, Singapore Eye Research Institute (SERI) Head, Optometry Service, Singapore National Eye Centre 11 Third Hospital Avenue, Singapore 168751 Tel. no. +65-63228893 rosman_sg@yahoo.com T he Singapore National Eye Centre has been performing SMILE surgery since 2012. To date, we have not had any incidence of corneal ectasia post-SMILE. However, whether we can attribute this to SMILE surgery itself is debatable. It is important to note that during this same period, we also do not have any incidence of corneal ectasia post LASIK surgery. I attribute this to better patient selection based on evidence-based medicine on risk stratification for corneal refractive surgery. SMILE surgery involves a smaller corneal incision and better preserves the corneal architecture anterior to the lenticule. Some papers have shown that the anterior corneal stroma may be stronger than the posterior corneal stroma. But whether this means that we can push the threshold of refractive corneal surgery will require further studies. Currently, we employ stringent guidelines on our patient selection for SMILE. We do not perform SMILE on patients with suspicious corneal topography and maintain a residual stromal thickness (RST) of at least 250 μm posterior to the extracted lenticule. In most cases, our surgeons prefer to maintain at least 300 μm of RST. Furthermore, prior to 2017, the upper limit for SMILE was –10 D of spherical equivalent for the VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany) at our center. This restriction on the amount of lenticule which can be removed may have also further reduced the risks of ectasia. “ ...SMILE should not be performed on patients with suspicious corneal topography as there have been reports of corneal ectasia post-SMILE. ” - Mohamad Rosman, MBBS, MRCS(Ophth), MMed(Ophth), FRCS(Edinburgh), FAMS To summarize, I agree with Dr. Moshirfar that SMILE should not be performed on patients with suspicious corneal topography as there have been reports of corneal ectasia post-SMILE. While SMILE may have the potential to allow us to go below the 250- μm limit of RST, this will require long-term studies on the safety and the refractive stability of these procedures. At the moment, it seems that employing patient selection guidelines similar to LASIK for potential SMILE patients has been successful at preventing corneal ectasia at my center. Editors’ note: Dr. Rosman declared no relevant financial interests. Sri GANESH, MD Chairman, Nethradama Super Speciality Eye Hospital #256/14, Kanakapura Main Road, 7th Block Jayanagar, Bangalore 560070, India Tel. no. +91-80-26088000 Fax no. +91-80-26633770 chairman@nethradhama.org T he main advantages of SMILE over LASIK are better patient comfort, shorter procedure, less dry eye, and probably better corneal biomechanics. There have been over a million SMILE procedures performed worldwide over the last 8 years and it is the fastest growing refractive procedure and has been approved for correction of myopia in the U.S. since the past year. Reinstein et al. published a paper using mathematical modeling to demonstrate that the cornea following SMILE may be stronger then after LASIK or PRK. However, there are no clear methods or tools to demonstrate better corneal biomechanics of SMILE accurately in vivo post these three refractive procedures. Since the anterior one third of the cornea is three times stronger than the posterior one third of the cornea and in SMILE as there is no flap and only a small access incision of 2-5 mm with the anterior stronger lamellae being intact, some surgeons assume that the risk of ectasia may be lower than LASIK. So far there have been seven cases of reported ectasia following SMILE, so the incidence of ectasia appears to be quite small. In analyzing these cases, most of them except one had abnormal topographies with asymmetry which were suggestive of form fruste keratoconus and were not ideal cases for any corneal refractive surgery. The case with apparent normal topography which developed ectasia in one eye reported by Sachdev et al. showed unreliable quality of scan and may have had underlying abnormal topography. SMILE is a tissue subtraction technique and may precipitate the progress of form fruste keratoconus similar to LASIK or PRK. Hence, it is paramount to screen for abnormal topography and tomography. The Randleman’s risk assessment score is primarily for LASIK but can be applied to SMILE. This along with the BAD indices of the Pentacam may help screen out potential cases of ectasia. The PTA was ideally described for LASIK, but may not hold the same significance for SMILE as the anterior cap is presumed to be much stronger than a LASIK flap. However, it is ideal to leave behind a bed of 300 μm and an overall corneal thickness of over 410 μm until further studies on biomechanics prove otherwise. In cases of borderline topography and moderate Randleman’s risk we would prefer to perform corneal collagen crosslinking along with the SMILE procedure (SMILE Xtra). We published the first report on SMILE Xtra in the Journal of Ophthalmology in 2015 and have performed over 200 cases in the last 5 years with stable results. Editors’ note: The SMILE procedure is performed using the VisuMax femtosecond laser (Carl Zeiss Meditec, Jena, Germany). Dr. Sri Ganesh is a consultant for Zeiss. continued on page 56