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By Cheryl Guttman

Philadelphia - With increasing recognition of LASIK's limitations along with the advent of research into wavefront-guided ablations, refractive surgeons are now looking at PRK with renewed interest.

In a session aptly entitled PRK Reborn held during the annual ASCRS Symposium on Cataract, Refractive and IOL surgery, a panel of ophthalmic surgeons reviewed the latest information on the clinical utility of PRK.

David Edmison MD, Medical Director, Focus Eye Centre, Ottawa, Canada described a number of scenarios where PRK might have advantages over LASIK. The top two indications on his list were eyes with a too-thin cornea and those with basement membrane disease.

"Passing a microkeratome over an eye with a potential epithelial slough can leave you with a very difficult situation. PRK avoids postoperative problems because it allows the attachment of the epithelium to the stroma and can even be considered a treatment for basement membrane disease, especially if the epithelium is continually sloughing off," he noted.

PRK provides correction of the refractive error and also treatment for the pathology. PRK removes the fibrotic tissue in eyes with anterior scarring and the area is subsequently filled in with epithelium.
In eyes with early to moderate keratoconus, PRK can be a feasible alternative to LASIK and perhaps a definitive treatment, according to some studies.

"PRK removes the Bowman's layer - the site of the pathology - and results in a bond between the epithelium and stroma, thereby stabilising the cornea," Dr Edmison said.
Jonathan Carr MD, Laser Eye Consultants, Washington, DC presented data from two retrospective studies pointing to the efficacy and safety of PRK in eyes with a cornea too thin for LASIK, and in those with epithelial basement membrane degeneration (EBMD).

As a caveat, however, he observed that the ability to derive definitive conclusions was limited for both investigations due to small sample sizes and short follow-up.
In both cohorts, the ablations were performed using the S2 laser (VisX). Surgeons used alcohol, laser scrape (thin corneas only), or the Amoils brush for epithelial removal.

The analysis of PRK in the setting of a thin cornea included 67 eyes of 34 patients with corneas measuring no more than 500 microns at the thinnest value on Orbscan II topography. The average myopia treated was -4.5 D (range -1 D to -8 D).

The mean thinnest corneal thickness was 468 microns (range 432 microns to 500 microns).
Some 88% of eyes had UCVA of 20/20 at last follow-up with no enhancements (mean 3.3 months, range one to 12 months), with 98.5% achieving 20/40 or better UCVA. Analyses examining the potential for various factors to influence UCVA outcomes showed that the level of myopia and age were highly correlated with postoperative UCVA.

However, calculated residual corneal thickness was not a significant predictor. For the study cohort, it ranged from 375 microns to 443 microns and averaged 413 microns, Dr Carr reported.
"This data supports the conclusion that when corneal thickness excludes a patient from LASIK, PRK leaving a residual corneal thickness of at least 375 microns can effectively correct low to moderate myopia.

"However, safety outcomes for this small sample are lacking, and with its short follow-up, there is not sufficient information to answer the important question of whether the results are stable," he emphasised.

Discussing eyes with EBMD, Dr Carr pointed out that patients with that finding who undergo LASIK might achieve acceptable Snellen visual acuity outcomes, but complain consistently their vision quality is not good.

The second study cohort consisted of a heterogeneous group of 19 eyes of 10 patients, all with classic signs of EBMD identified with fluorescein staining.
Mean preoperative spherical equivalent averaged -3.25 D and ranged up to -5.9 D. The mean corneal thickness was 508 microns (range 476 microns to 551 microns) at the thinnest Orbscan reading.

Efficacy and predictability were good post-PRK. After an average of four months (range one to 10), 58% of eyes had UCVA of 20/20 or better, with 95% reaching 20/30 UCVA. Predictability was good, with 84% of eyes within 0.5 D of intended refraction.

Safety was also favourable, with none of the eyes losing more than two lines of BSCVA and losses of two lines typically representing a change from 20/15 to 20/20. Only one patient reported erosion symptoms preoperatively and the level of severity was unchanged after PRK, Dr Carr reported.

Safe and effective so far
"In addition to having a small, heterogeneous population, this study is limited by use of a variety of epithelial removal techniques that may or may not influence the outcome.
"While we can state that so far PRK appears safe and effective for correcting myopia in eyes with EBMD, longer term follow-up is important, particularly for assessing the risk of new erosion symptoms, which as we know from experience with therapeutic PTK can rear their ugly heads many months later," Dr Carr said.

Other pathological conditions where PRK is preferred over LASIK include eyes with corneal scars or dystrophy involving the anterior portion of cornea as well as those with keratoconus or pellucid marginal degeneration.

As in eyes with EBMD, PRK in those settings might kill two birds with one stone - providing correction of the refractive error but also treatment for the pathology, Dr Edmison said.

"In eyes with anterior scarring, PRK removes the fibrotic tissue and the area is subsequently filled in with epithelium. In eyes with early to moderate keratoconus, PRK can be a feasible alternative to LASIK and perhaps a definitive treatment according to some studies.
"PRK removes the Bowman's layer - the site of the pathology - and results in a bond between the epithelium and stroma, thereby stabilising the cornea," Dr Edmison explained.
Recognising the potential for factitiously low applanation tonometry post-LASIK, PRK may also offer a safer refractive surgery alternative in patients with uveitis requiring steroids, and for those with glaucoma.

"Known as the FLAT syndrome, this is a situation where fluid builds up in the space between the flap and the underlying cornea and acts as a disconnection to accurate applanation topometry. As a result, IOP can rise to well over 30 mm Hg, but appear factitiously low," Dr Edmison explained.

He mentioned there are also lifestyle-related contraindications to LASIK. Due to the potential for flap dislocation, persons whose work or recreational activities expose them to an elevated risk of ocular trauma are better off foregoing LASIK and undergoing PRK instead, Dr Edmison said.

PRK rides the wavefront
He also noted that as researchers focus attention on wavefront-guided refractive surgery, the concept has emerged that surface ablation may yield a better result than LASIK.

"These customised ablations involve very intricate treatments, but their effect might be lost once the relatively thick flap is laid back down as its surface will not follow that of the underlying bed. Surface ablation with PRK avoids this situation and might be a better option," Dr Edmison said.
Along these lines, Zoltan Nagy MD, Budapest, Hungary, presented data indicating that wavefront-guided PRK for the treatment of hyperopia is safe, effective, predictable and results in outcomes that are superior to traditional PRK.

His series included 40 eyes of 20 patients. The treatments were performed with a Shack-Hartmann wavefront analyzer (Aesclepion) and the MEL-70 scanning flying spot laser (Meditec). He used a 6 mm ablation zone, 9 mm transition zone, with the ablation focused on the centre of the cornea.

Mean SE averaged +2.68 D preoperatively and was reduced to -0.1 D at six months after surgery. Mean UCVA improved from 0.25 to 0.9. He also observed a treatment benefit in the BSCVA analysis - mean BSCVA increased from 1.05 to 1.16. No eyes lost any Snellen lines of BSCVA while five eyes gained one line and three eyes gained two or more lines. Those benefits occurred despite a near doubling in the root mean square value from 0.134 to 0.254, Dr Nagy reported.

"Compared to a cohort of eyes treated using traditional PRK, this group achieved superior UCVA and BSCVA outcomes. The differences, however, were modest and it appears the benefit of using wavefront-guided treatment might be less in hyperopes compared to the promise it holds for improving outcomes in eyes with myopia or myopic astigmatism," he said.

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