EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
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86 The latest research indicates the two leading photorefractive approaches provide similar outcomes, but specific circumstances can give one advantages over the other R efractive laser patients c an derive similar visual results from LASIK and photorefractive keratec- tomy (PRK), according to the latest side-by-side comparison, which may assure surgeons that they can switch between the procedures as patient circumstances dictate. LASIK and PRK are comparably safe, effective, and predictable proce- dures for excimer laser correction of high astigmatism greater than 3 D in myopic eyes, concluded a recent study in the Journal of Refractive Sur- gery. 1 The study by Lars Wagenfeld, MD, University Medical Center Hamburg-Eppendorf, Hamburg, Germany, and colleagues retrospec- tively analyzed up to six months of postop surgical results in 114 con- secutive randomly selected myopic eyes (up to –8.63 D spherical equiva- lent for LASIK and –8.38 D for PRK) with astigmatism of greater than 3 D (up to –5.75 D cylinder for both LASIK and PRK). Patients who were evenly divided between PRK and LASIK had no statistically significant difference in efficacy, safety, or predictability. Vector analysis of the cylindrical correction found no statistically sig- nificant difference in the surgically induced astigmatism, although the correction index showed a slight and significant advantage of LASIK over PRK. The findings of Dr. Wagenfeld and colleagues echo the recent expe- rience of Vance Thompson, MD, assistant professor of ophthalmol- ogy, Sanford School of Medicine, University of South Dakota, Sioux Falls, S.D. An original investigator in the FDA-monitored studies for PRK and LASIK in the 1990s, Dr. Thompson said both leading types of photorefractive surgery continue to provide good results. "I highly respect PRK and LASIK and have seen a lot of good from each, but over the years have seen complications from each, too," Dr. Thompson said. "What I like about this study is that it supports the philosophy that in general if a pa- tient is a good candidate for LASIK they are also a good candidate for PRK, even high myopic and/or astig- matic corrections. The reverse is not true though. There are many pa- tients who are not candidates for LASIK who can safely undergo PRK." One key to avoiding complica- tions is selecting the best procedure for each patient. The first step is assessing the patient's corneal thickness and corneal topographic regularity. Patients with normal to- pography anteriorly and posteriorly, who do not rub their eyes and who would have more than 300 µm of residual corneal stroma after a 110- µm femtosecond flap and laser ablation are typically good LASIK (or PRK) candidates, he said. In cases of thinner corneas, anterior basement membrane dys- trophy, and when activity-related trauma can be an issue, Dr. Thompson prefers PRK. "With anterior basement mem- brane dystrophy, PRK can be a great way to both treat the refractive error and re-establish a more normal epithelial adherence and regularity," Dr. Thompson said. "If you want to treat both the refractive error and the anterior basement membrane dystrophy, you have to make sure that the preoperative measurements are accurate enough for PRK. Other- wise, it is better to do a corneal scraping or PTK, allow things to heal really well, then get accurate measurements and do the refractive correction." Concerns that arise with either procedure include dry eye. Dr. Thompson avoids any refractive procedure in patients with epithelial staining. " I would consider this a stressed corneal surface and a setup for wound-healing issues," he said. "If I can treat the tear film and clear up the epithelial staining, I would pre- fer doing PRK so there is less dener- vation of the surface. I follow these patients more closely." Dr. Thompson also emphasizes that d ry eye can negatively impact wavefront analysis. Keys to the surgical phase The surgical pearls that improve visual outcomes for both PRK and LASIK include maximizing tear film and being cognizant of the extent of patients' higher order aberrations. When patients do not have visually s ignificant higher order aberrations, Dr. Thompson is comfortable with wavefront-guided or wavefront-opti- mized excimer laser ablations. In cases of significant higher order aberrations, the first step is identifying a cause and ruling out possibilities such as dry eye-induced aberrations or lenticular or corneal issues. Dr. Thompson uses the Pentacam (Oculus, Arlington, Wash.) and the HD Analyzer (Visiometrics, Terrassa, Spain) to measure forward scatter and rule out subtle irregularities that would result in a poor visual outcome after laser vision correction. In the absence of such pathology, Dr. Thompson treats the aberrations with a wavefront- guided system. In cases of visually significant higher order aberrations induced by previous corneal laser procedures, the only effective treatment is wavefront-guided technology, he said. "When enhancing multifocal implants, if there is any reduction in contrast sensitivity, I look closely at the tear film and epithelium. If I am suspicious that the surface is playing a role, I am aggressive with tear film management and will use PRK as a way to both enhance the refractive error and smooth any potential un- seen anterior membrane dystrophy for a pristine air/tear interface to help the ultimate visual outcome," Dr. Thompson said. Surgical preferences Each procedure has its own limita- tions. Dr. Thompson sees fewer dry eye issues clinically with PRK. He also offers PRK for avoiding epithe- lium growth under a flap in LASIK procedures performed many years ago that now have a small refractive e rror they would like treated. But PRK enhancements can carry a haze risk. "Fortunately, one of the benefits that small spot, Gaussian- shaped scanning laser technology brought to laser vision correction was very smooth ablations com- pared to our broad beam cases in the beginning. This helped visual out- c omes, but also lessened the amount of haze reactions we needed to deal with in PRK. This greatly expanded the range of quality PRK." "Even though I have the epithe- lium growing under the flap risk from a LASIK from 10 years ago, I don't have to worry about haze risk with these folks," Dr. Thompson s aid. "But I do try to be very diligent about assessing anterior and poste- rior topography in all cases to minimize the risk of ectasia." When treating residual refrac- tive error following refractive lens implantations, PRK is often preferred for low corrections. "I include patients in this deci- sion, of course, and if they have a sharp refractive endpoint and all my LASIK check points are satisfied, I am comfortable delivering LASIK for them," he said. "But if I am con- cerned at all that the epithelial sta- tus is limiting image quality, I will use PRK as my enhancer." Dr. Thompson concluded, "As you can see, I am a big fan of both PRK and LASIK. Each situation deserves a well-balanced, decision- making approach to recommend PRK and/or LASIK. Whether the patient is best suited for a conven- tional treatment of sphere and cylinder or a custom wavefront- guided or optimized treatment, the approach to delivering the energy on the stromal surface or under a flap deserves careful considera- tion." EW Reference 1. Katz et al. LASIK versus photorefractive keratectomy for high myopic (> 3 diopter) astigmatism. J Refract Surg. 2013 Dec;29 (12):824-31. Editors' note: Dr. Thompson has financial interests with Abbott Medical Optics (Santa Ana, Calif.), Alcon (Fort Worth, Texas), and WaveTec Vision (Aliso Viejo, Calif.). Contact information Thompson: vance.thompson@vancethompsonvision.com EW REFRACTIVE SURGERY 8 6 March 2014 by Rich Daly EyeWorld Contributing Writer PRK vs. LASIK updated Anterior basement membrane dystrophy c ase where PRK or PTK can re-establish a more normal epithelial regularity and adherence Source: Vance Thompson, MD 76-87 Refractive_EW March 2014-DL_Layout 1 3/6/14 3:18 PM Page 86