EyeWorld Asia-Pacific December 2011 Issue

3 December 2011 Letter from the Editor Dear Friends O ur final issue of EyeWorld Asia-Pacific for 2011 focuses on different facets of refractive surgery. Perhaps one of the major shifts in refractive surgery highlighted in this issue is the increasing relevance of surface ablation for the refractive surgeon. LASIK remains the most popular refractive procedure and utilizes either a microkeratome or a femtosecond laser to create a flap consisting of epithelium and Bowman’s so that the laser ablation is performed on the underlying stromal surface. Flap complications, although rare, can still occur, and the issue of long-term ectasia in predisposed individuals remains a concern. There are distinct differences in the biological response to LASIK compared with surface ablation, with reduced keratocyte loss in the underlying stroma and limited keratocyte regeneration in the flap. Because the epithelial surface remains intact, there is minimal discomfort, a reduced incidence of haze, and rapid visual recovery, all of which have been documented in randomized prospective studies. In contrast, PRK simply requires mechanical epithelial debridement, either with a spatula or a rotary brush exposing bare Bowman’s as a smooth surface for laser ablation. A bandage contact lens is typically used to reduce discomfort and assist epithelial healing. The major concerns with PRK are discomfort, delay in visual recovery, and the possibility of haze, particularly with higher levels of myopia. In vivo confocal microscopy demonstrates significant keratocyte death as well as the deposition of new extracellular matrix and the activation and transformation of keratocytes. Clinically, the visual outcome of PRK is equal to or better than LASIK in patients with less than 6 dioptres of myopia and rivals that of LASIK with higher levels if performed with the addition of topical mitomycin. One of the attractions of PRK is, of course, the elimination of the risk of flap complications. LASEK utilizes a short exposure to alcohol to first loosen the hemidesmosomes and the attachment of the epithelium to the basement membrane so that the epithelium can be separated at the level of the lamina lucida, pushed aside for the laser ablation, and then repositioned. The concept is to provide a mechanical barrier in the hope that this will address the limitations of PRK. The hypothesis is that the epithelial barrier will reduce the release of cytokines and growth factors, which are thought to play a role in the transformation of keratocytes into myofibroblasts. However, immunohistochemistry has demonstrated that epithelial cells are damaged by the exposure to alcohol with discontinuity of the basement membrane as well as the upregulation of cyclo-oxygenase, which may actually increase the inflammatory response compared with PRK. Certainly, when one examines the literature, the majority of case reports suggest that pain is no better or possibly even worse, healing generally slower and the incidence of haze similar. Epi-LASIK was introduced by Pallikaris and uses a modified microkeratome to create an epithelial flap without alcohol, cleaving the basement membrane at the lamina densa at a deeper level than LASEK, in the hope that maintaining a viable epithelial flap with an intact basement membrane would lead to faster healing with reduced pain and haze. Laboratory studies confirm that the epithelium remains vital, although there is some discussion whether the cleavage plane is truly confined to the deeper reticular layer of the basement membrane. Despite the biological differences, clinically, discomfort and the healing response appear no different from LASEK and is possibly even slower with epi-LASIK compared with PRK. The visual outcome and incidence of haze are also similar. Although there are clearly differences in the surgical techniques between PRK, LASEK, and epi-LASIK, with significant differences noted with histology and immunohistochemistry, the outcomes are at best equivalent and in some reports worse than PRK. It is common practice today for many surgeons to discard the epithelial flap, further minimizing the differences between the three methods of surface ablation which are often grouped together as Advanced Surface Laser Ablation (ASLA). There are several reasons for the renewed interest in ASLA. Although surface ablation is associated with more pain and discomfort than LASIK, in my experience, this issue is more manageable than we experienced in the early days of PRK. I suspect this is in large part due to improved silicone hydrogel bandage contact lenses, as well as a refinement of postoperative pain management protocols. Similarly, the risk of haze following surface ablation has diminished significantly with the judicious use of topical mitomycin. Topical mitomycin at 0.02% applied to the cornea after surface ablation for short periods of approximately 12 seconds have now been used for several years without significant complications. Sophisticated ablation patterns with scanning lasers result in a smoother surface than the original broad-beam treatments and may be another factor in the lower incidence of postoperative haze. An attractive feature of ASLA is the ability of patients to maintain accurate fixation during the procedure and the prospect of “non-touch” transepithelial techniques. The risk of flap complications with femtosecond lasers is reduced compared with mechanical microkeratomes, and yet flap creation always has an inherent albeit low risk of complications The most important reason, however, for surgeons to consider surface ablation rather than LASIK in individual cases is an the increasing awareness that ectasia following LASIK remains a concern despite an increased recommended residual bed of 300 microns and new topography devices based on slit-beam scanning principles as well as placido disc imagery. New algorithms have been introduced to help differentiate between normal and abnormal corneas and measurements of corneal hysteresis are now available. Despite all these diagnostic refinements, the boundary between normal corneas and those at risk from the development of ectasia following LASIK at times remains ill defined. Excimer laser ablation can therefore be performed either intrastromally or on the corneal surface and it is worth considering the differences between the various procedures available with respect to their biological behavior and clinical outcomes. LASIK remains the preferred option for most patients undergoing laser surgery for refractive errors. Nevertheless surface ablation remains a useful alternative with an upward trend due to the above considerations. As this will be our last issue for the calendar year, I would like to wish all readers of EyeWorld Asia-Pacific a healthy, happy and restful festive season. A special thanks to our regional co-editors, board members and editorial staff at APACRS for their dedication to producing such a high quality educational news journal. Warmest regards Graham Barrett, MD President, APACRS Chief Medical Editor, EyeWorld Asia-Pacific