EyeWorld Asia-Pacific September 2017 Issue

September 2017 EWAP FEATURE 13 Views from Asia-Pacific Myoung Joon “MJ” KIM, MD Professor, Asan Medical Center 88 Olympic-ro 43-gil, Songpa-gu, Seoul, South Korea Tel. no. +82-2-3010-3975 Fax no. +82-2-470-6440 mjmjkim@gmail.com I t is hard to believe that a normal human eye is a highly aberrated optical system. But it is true from the standpoint of optical engineering. Cameras with lenses which resemble the human eye’s optics could not be sold due to poor image quality. Even normal corneas have optical aberrations to some extent. Since the 2000s, laser refractive surgery has adopted tools that can measure sophisticated optical aberrations in eyes. Using this data and advanced laser algorithms, the system allows the correction of preexisting aberrations or the minimization of induced optical aberrations by laser ablation itself. This kind of laser ablation is called wavefront- guided. However, keratoconus was a contraindication of laser refractive surgery until several years ago. Pioneering surgeons started to treat keratoconus with laser ablation. Wavefront measured in keratoconus was inaccurate due to dynamic range of sensors. So, surgeons used topography instead of wavefront. This is topography- guided treatment. More specifically it is topography-guided PRK, because flap making in keratoconic eye is not safe. Its algorithm is clever because it combines both flattening (myopia correction) and steepening (hyperopia correction) procedures. Looking at the results, visual outcome is better than expected. In a typical keratoconic eye, coma aberration is dominant, which cannot be corrected by spectacle. Also, tear film is unstable in the steep region. Reshaping of cornea by the use of topography data can decrease optical irregularity. Corneal shape gets more regular and image quality through the treated cornea is enhanced. There is a concern of progression of keratectatic change after the treatment. To flatten the steep area, the laser removes corneal tissue from that area, which results in further thinning. I would cautiously expect that corneal microstructure is more stable once keratectatic progression is stopped. So, personally, I would consider PRK in keratoconic eyes in older ages of 30 years or more. Combination with crosslinking is a good idea to decrease risk of postoperative progression of keratoconus. However, we have to be more cautious about endothelial damage by ultraviolet radiation. Epithelium is doing important role in smoothening ocular surface. Epithelium is thicker in the flatter area and thinner in the steeper area. Corneal topography dramatically changes when epithelium is removed. For this reason, transepithelial PRK might be a good option. As I stated already, the optics of the human eye is not perfect. Although optics is the first and a very important step of vision , the brain performs another important role to enhance vision by compensating for the imperfection of the optics. In patients with keratoconus, a process called neural adaptation lets the patients see better. In other words, the image perceived by the patient is better than the image formed at the retina. Following topography-guided PRK in keratoconus, vision will recover slowly. We have to wait for completion of epithelial remodeling and neural adaptation to a brand new optics. Editors’ note: Dr. Kim declared no financial interests related to his comments. Sri GANESH, MD Chairman, Nethradhama Hospitals Pvt. Ltd. 256/14, Kanakapura Main Road, 7th Block Jayanagar, Bangalore – 560070, India Tel. no. +91-80-26088000 Fax no. +91-80-26633770 chairman@nethradhama.org P atients with corneal irregularities due to keratoconus, post-LASIK ectasia, and trauma can have a significant loss of best-corrected visual acuity (BCVA). Topo-guided treatments along with corneal crosslinkage can improve BCVA and quality of life and are becoming popular all over the world; with the recent FDA approval of topo-guided LASIK and PRK, surgeons in the U.S. now have access to this technology. Proper assessment is important for good outcomes. Ideally, both a corneal wavefront and an ocular wavefront measurement should be performed. Some equipment such as the iTrace (Tracey Technologies, Houston, Texas) and OPD Scan (Nidek Co., Ltd., Gamag ri, Japan) also give details of the internal aberrations from the lens. If the corneal aberrations are similar to the complete ocular aberrations, this indicates that most of the aberrations are because of corneal irregularity and a topo-guided PRK would be helpful for such patients. If there is a difference between the corneal aberrations and total ocular aberrations, then a wavefront-guided treatment may be necessary. The goal of topo-guided PRK is to regularize the cornea and improve BCVA rather than to provide good UCVA. It is very important to assess the corneal thickness and refractive error before deciding on whether to do a topo-guided smoothening to regularize the cornea (treatment of only the corneal aberrations) or to also correct the refractive error. Trying to correct the complete refractive error along with corneal aberrations with topo-guided PRK may remove excess tissue and also give rise to refractive surprises and is not very predictable. Assessment of the configuration and distribution of the cone is also very important as results are better with axial cones and symmetric distribution rather than severely decentred cones. Ideally, larger optical zones of 6.5 to 7 mm should be used to get a better regularization of the cornea. Removing tissue further weakens the cornea in ectatic conditions and ideally topo link PRK should be combined with corneal crosslinkage. In many cases, a combination of two or more procedures may be performed to optimize quality of vision and refractive result, such as intracorneal rings and implants, phakic IOLs, topo-guided PRK, and crosslinkage. The primary aim in treating irregular corneas is to regularize the surface and improve BCVA and the secondary goal is a refractive result. Editors’ note: Dr. Sri Ganesh is a consultant for Carl Zeiss Meditec (Jena, Germany) and Johnson & Johnson Vision (Santa Ana, California). continued on page 14 “ Proper assessment is important for good outcomes. ” - Sri Ganesh, MD