March 2014

 

REFRACTIVE SURGERY

 

PRK vs. LASIK updated


by Rich Daly EyeWorld Contributing Writer

 
   

Anterior basement membrane dystrophy case where PRK or PTK can re-establish a more normal epithelial regularity and adherence Source: Vance Thompson, MD

The latest research indicates the two leading photorefractive approaches provide similar outcomes, but specific circumstances can give one advantages over the other

Refractive laser patients can derive similar visual results from LASIK and photorefractive keratectomy (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 procedures for excimer laser correction of high astigmatism greater than 3 D in myopic eyes, concluded a recent study in the Journal of Refractive Surgery.1 The study by Lars Wagenfeld, MD, University Medical Center Hamburg-Eppendorf, Hamburg, Germany, and colleagues retrospectively analyzed up to six months of postop surgical results in 114 consecutive randomly selected myopic eyes (up to 8.63 D spherical equivalent 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 significant 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 experience of Vance Thompson, MD, assistant professor of ophthalmology, 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 patient is a good candidate for LASIK they are also a good candidate for PRK, even high myopic and/or astigmatic corrections. The reverse is not true though. There are many patients who are not candidates for LASIK who can safely undergo PRK."

One key to avoiding complications 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 topography anteriorly and posteriorly, who do not rub their eyes and who would have more than 300 m of residual corneal stroma after a 110-m femtosecond flap and laser ablation are typically good LASIK (or PRK) candidates, he said.

In cases of thinner corneas, anterior basement membrane dystrophy, and when activity-related trauma can be an issue, Dr. Thompson prefers PRK.

"With anterior basement membrane 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. Otherwise, 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 prefer doing PRK so there is less denervation of the surface. I follow these patients more closely." Dr. Thompson also emphasizes that dry 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 significant higher order aberrations, Dr. Thompson is comfortable with wavefront-guided or wavefront-optimized 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 unseen 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 limitations. Dr. Thompson sees fewer dry eye issues clinically with PRK. He also offers PRK for avoiding epithelium growth under a flap in LASIK procedures performed many years ago that now have a small refractive error 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 compared to our broad beam cases in the beginning. This helped visual outcomes, 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 epithelium 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 said. "But I do try to be very diligent about assessing anterior and posterior topography in all cases to minimize the risk of ectasia."

When treating residual refractive error following refractive lens implantations, PRK is often preferred for low corrections.

"I include patients in this decision, 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 concerned at all that the epithelial status 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 conventional 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 consideration."

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

PRK vs. LASIK updated PRK vs. LASIK updated
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