EyeWorld Asia-Pacific December 2020 Issue

T he ZEISS Live Webi- nar on June 7, 2020 provided a breadth of knowledge on refrac- tive surgery and its newer advancements including a clinical and scientific perspec- tive on Lenticule Extraction with SMILE ® (Carl Zeiss Meditec AG, Jena, Germany). 3D Nerve Imaging following Small Incision Lenticule Extraction Professor Jod Mehta from the Singapore National Eye Centre first presented “3D Nerve Imaging follow- ing Small Incision Lenticule Extraction (SMILE on VisuMax Femtosecond Laser, Carl Zeiss Meditec AG, Jena, Germany).” With SMILE, a small keyhole incision is made through which the lenticule is extracted. Be- cause SMILE creates a much smaller surface wound, there are benefits to patients including a smaller wound on the surface of the cornea, less post- operative discomfort and tearing, and potential for less dry eye problems. A few years back, Professor Mehta had investigated the difference between Lenticule Extraction with SMILE and LASIK with respect to nerve innerva- tion. At two weeks after surgery, the nerves in both a Lenticule Extraction and LASIK patient were intact in the inferior area with the small superior incision. However, at 8 weeks, there was a difference in the innervation in that Lenticule Extraction with SMILE provided greater innervation from the inferior side as well as the superior side, whereas LASIK showed very little innervation on the inferior side. So, there was a faster nerve recov- ery seen in Lenticule Extraction with patients compared to LASIK patients. In another comparison, it was shown that the LASIK group showed greater sprouting of sub basal nerves, indi- cating nerve regeneration, while the SMILE group had a greater number of longer sub basal nerves which is in- dicative of normal cellular physiology and better nerve regeneration. Finally, patients who under went surgery with SMILE showed better tear break-up time and ocular surface index at one month, three months, and six months post-surgery than LASIK patients. Professor Mehta also conducted a lenticular nerve study in which a 2D image was created from all Z-stacks after spinning-disk confocal laser microscopy. One can see the nerves traversing in both the X, Y, and Z axis as well as the interconnecting fibres between the nerves and shortened neurite crunching. From analyzing the data, comparing lenticular nerve density to power of correction gives a quadratic prediction plot which shows a minimal difference in nerve loss between -5 to -10 D (10mm/ mm 3 ), but a greater loss between -2 and -4 D (30mm/mm 3 ). This means that if you are doing a larger correc- tion, the innervation amount will be the same whether at -5 or -10 (See Figure 1). Additionally, nerves may be viable after freezing: after 48 hours at 4 degrees Celsius, one can still see nerve stimulation (35.6% of nerve im- ages still had calcium signal). At one month, however, 7% of the nerves still showed some active function. This data is important with respect to when it is necessary to put nerves back for lenticular implantation, hy- peropia, or presbyopia. New Advancements in Refractive Surgery: Supplement to EyeWorld Asia-Pacific December 2020 APACRS Nerves: Ectasia Model Dr. Natasha Pahuja continued the webinar series with a presentation on Nerves: Ectasia Model. Ectasia, a cellular or a microscopic phenome- na, at times may show up in patients who present with normal topography preoperatively. To study the ectasia model, Dr. Pahuja looked closely at the corneal nerves since they have a role in maintaining the ocular surface microenvironment. Corneal nerves connect the ocular surface to the rest of the body and brain in order to maintain ocular surface homeo- stasis. How, then, can we image and quantify corneal nerves? Using the Heidelberg Retinal Tomography (HRT) scan, “we can measure the different layers starting with the epithelium to the endothelium.” Dr. Pahuja further noted that to measure the sub-bas- al nerve plexus, one can map the corneal nerves using normal confocal microscopy which provides various metrics including the fiber density and branched density to analyze different corneal diseases. In Dr. Pahuja’s research, she studied the naturally occurring ec- tasia - keratoconus. Corneal nerves are affected in keratoconus in terms of decreased and significantly altered nerves. Looking at morphology and quantifiable data, Dr. Pahuja found that the corneal nerve fiber density and fiber length are predictors of subclinical disease. When looking at refractive surgery techniques, it was shown that LASIK results in signifi- cant nerve loss as well as increased tortuosity that is noticed at the end of five months and nerve beading after 8 months. These abnormal nerve find- ings are what causes dry eye symp- toms. With SMILE (since SMILE is the software for Lenticule Extraction), on the other hand, there is regener- ation of these nerves and activated keratocytes, which improve normal healing, at the end of 6 months. In other studies, it has been shown that 90% of the nerve bundles are severed after LASIK. In Dr. Pahuja’s published study, she also found that patients with normal preoperative topography showed decreased nerve density but increased dendritic cells, which are inflammatory and present in dry The news magazine of the Asia-Pacific Association of Cataract & Refractive Surgeons Figure 1. Lenticule nerve study showing less of a difference in nerve loss at a larger correction than at a lower correction. Source: Bandeira F et al. Invest Ophthalmol Vis Sci 2019 Copyright 2020 APACRS. All rights reserved. The views expressed here do not necessarily reflect those of the editor, editorial board, or publisher, and in no way imply endorsement by EyeWorld, Asia-Pacific or APACRS.