Need for refractive perfection is fulfilled by epithelial PRK

Treating only the epithelium can correct up to 1 D of regression without requiring a flap lift.

The desire to correct less than 1 D of refractive error without lifting a corneal flap has led surgeons to treat a single layer of epithelial cells.

Ablating only the epithelium spares the flap and stroma, does not cause pain and fixes epithelial hyperplasia.

According to Richard L. Lindstrom, MD, of Minneapolis, epithelial photorefractive keratectomy (PRK) is “for someone who has pretty good visual acuity but wants that last little bit. Some of these patients are perhaps 20/20 in one eye and 20/25 in the other. They come in saying their left eye is a little better than their right eye and to please do something. But you’re afraid to do an aggressive procedure, such as lift the flap or recut a new flap, and possibly make them worse.”

In these scenarios, ablating only one layer of epithelial cells corrects that “last little bit.”

Hyperplasia at fault

In theory, most laser in situ keratomileusis (LASIK) and PRK patients have some amount of compensatory epithelial hyperplasia, Dr. Lindstrom said. A normal cornea has five or six epithelial layers. Postoperatively, some patients end up with seven or even 10 layers. This affects refraction.

At the same time, removing about 12 µm of tissue will correct 1 D of myopia. An epithelial cell is between 10 and 12 µm, so removing one layer of epithelial cells corrects about 0.75 D to 1 D without generating any healing response. Also, because nerves do not extend into the most superficial epithelial cells, there is no pain.

“It’s very low risk for the minimal refractive error,” Dr. Lindstrom said. “But we still have some of those patients that are pursuing perfection. I can do a 3-second treatment on the eye and offer rapid recovery. It really improves the risk-benefit ratio.”

Since starting the technique a year ago, Dr. Lindstrom said he has corrected up to 1 D of residual myopia. He has stopped using therapeutic contact lenses and nonsteroidal anti-inflammatory drugs (NSAIDs) after epithelial PRK as well, although some patients need lubricating drops.

“I’ve gotten a little more aggressive with enhancements on some of these lower level residual refractive errors,” he said.

“So far, the complication rate has been absolutely zero.”

Relationship defined

The discovery of intraepithelial PRK started several years ago in an effort to learn why LASIK cases regressed in the first 6 weeks, according to Jose L. Guell, MD, of Barcelona, Spain. Clinicians saw a linear relationship between the increase in central epithelial thickness and postoperative refraction.

“We were, of course, unable to know all the factors related to this regression that you can observe in some eyes and not in others,” Dr. Guell said. “The only important factor was that it was more common when you were correcting more, and less common when you were correcting less.”

Dr. Guell does intraepithelial PRK in cases with less than 1 D of spherical error and less than 0.5 D of astigmatism where lifting the flap is contraindicated in some way and when he is certain that he is seeing regression and not simply undercorrection.

Patients experiencing small islands of epithelial ingrowth risk further deterioration by lifting the flap, Dr. Guell said. Also, patients with blepharitis risk more interface problems.

“The speed of visual rehabilitation is higher without any doubt doing a standard enhancement than doing intraepithelial PRK,” Dr. Guell said. “The main advantage doing intra-epithelial PRK is that you are not taking the risk of re-lifting the flap.”

Refining the nomogram

Epithelial PRK is simple to perform and easily modified to a surgeon’s existing practice. The surgeons interviewed by Ocular Surgery News use a variety of laser platforms for the procedure. Dr. Lindstrom sets his laser for the regular PRK treatment plan, so he sets the laser to ablate 0.5 D for 0.5 D correction and 0.75 D for 0.75 D correction.

At the laser, he uses a drop of topical anesthesia and creates a smooth surface of tear film by passing a squeezed-out Merocel sponge across the surface of the cornea several times. Treating 0.5 D takes 3 seconds, and 1 D takes 6 seconds with the laser. After the ablation, he applies a drop of steroids, antibiotics and topical lubricants.

Kurt A. Buzard, MD, of Las Vegas has corrected about 25 patients with residual myopia or hyperopia using epithelial PRK. He overtreats the myopes by 0.25 D and undertreats the hyperopes by the same amount.

He is refining the nomogram to account for regressive effects over time. He has found that about half of the treatment’s effect disappears at 6 months.

“There definitely seems to be a group of patients who respond to the treatment,” Dr. Buzard said. “The problem is identifying what it is about the treatment that makes them respond and then trying to make that happen in everyone. The results are really variable. These are small treatments to start with.”

Unlike Dr. Lindstrom, Dr. Buzard said he applies contact lenses and steroids for 1 week postoperatively to reduce the pain. He also prescribes NSAIDs to control any pain.

Dr. Buzard compared epithelial PRK with changing a prescription of eyeglasses by 0.25 D to account for small changes over time.

“Over a 10-year period, little ups and downs may occur,” he said. “They may not be significant enough to lift the flap and do an entire surgery. It may be very nice to go in there and make little adjustments and not do a whole lot.”

For Your Information:

Richard L. Lindstrom, MD, can be reached at Minnesota Eye Associates, Park Avenue Medical Office Bldg., 710 E. 24th St., Ste. 106, Minneapolis, MN 55404 U.S.A.; +(1) 612-813-3600; fax: +(1) 612-813-3660.

Kurt A. Buzard, MD, can be reached at 6020 Spring Mountain Road, Las Vegas, NV 89146 U.S.A.; +(1) 702-362-3900; fax: +(1) 702-362-7405.

Jose L. Guell, MD, can be reached at the Instituto de Microcirugia Ocular, Munner 10, Barcelona 08022 Spain; +(34) 93-418-7199; fax: +(34) 93-417-1301.