During the past 2 years, my practice has increased its percentage of PRK procedures from less than 5% to now more than 25% of the total refractive procedures performed. I attribute this resurgence to the many recent innovations in PRK as well as consumers' growing awareness of LASIK-related complications. However, despite the emergence of ?new? PRK, the complications associated with the procedure since its inception in the 1980s continue to occur. For this reason, it is important that surgeons do not forget the old lessons of performing PRK, even as they embrace its improvements.
Today's PRK offers five major benefits compared with the pre-LASIK PRK of the 1980s and early 1990s. First, 20% alcohol significantly aids surgeons in loosening the epithelial cell layer for removal. Second, the new small-spot scanning lasers cause less thermal injury to the cornea and leave a smoother ablated surface than the earlier broad-beam excimer lasers. Third, the current use of mitomycin C (MMC) reduces the risk of subepithelial fibrosis and its associated haze and regression. Fourth, improved soft contact lens technology provides a better fit and superior oxygen delivery for patients. Finally, the effectiveness of antibiotics, pain control, and anti-inflammatory eye drops have all improved.
POTENTIAL ADVANTAGES OF PRK OVER LASIK
Certain strengths of PRK may make it a more attractive procedure than LASIK. For example, patients with thin corneas may be less at risk for postoperative keratoectasia because PRK does not involve weakening the cornea with a LASIK flap. PRK also avoids certain complications unique to LASIK, including DLK, epithelial ingrowth, flap striae, flap buttonholes, and free caps. Risk-averse patients may prefer the “no cut” nature of PRK to the blade- or laser-cut flap created during LASIK. Additionally, PRK offers advantages when performing wavefront customized ablations, as suggested by the early work of Marguerite McDonald, MD, and Daniel Durrie, MD (see “A Rose by Any Other Name: PRK Nomenclature” on page 61).
My personal approach to PRK is a basic three-step technique that includes an additional fourth step for ablations of 50 to 100 µm. First, I loosen the epithelium using 20% alcohol and remove it with a Tooke knife (ASICO, Westmont, IL). I perform the ablation with the LADARVision4000 excimer laser (Alcon Laboratories, Inc., Fort Worth, TX) and, if necessary, apply MMC for 1 to 2 minutes for ablations between 50 and 100 µm. My parameters for PRK are as follows: (1) myopic patients receive less than or equal to 8.00 D of treatment; (2) hyperopic patients require less than or equal to 5.00 D of treatment; (3) the ablation depth must not exceed 100 µm; and (4) at least 400 µm of stromal bed must remain.
The complications of PRK can be categorized as intraoperative, early postoperative (from days to weeks), and late postoperative (weeks to months). Each of the intraoperative steps features potential but avoidable complications. The alcohol should be confined to the trephine well and properly irrigated to avoid toxicity to the limbal stem cells. The epithelial cells should be removed gently but thoroughly. The laser ablation must be carefully centered and tracked to avoid decentration (this step is critical for customized wavefront ablations). The MMC must be compounded at 0.2 mg/mL or 0.02% strength. Finally, as with the 20% alcohol, it is important that the MMC be confined to the area of the 6-mm corneal sponge and well irrigated at the conclusion of its use.
It bears repeating that the major potential complications of “new” PRK are easily avoided through careful surgical technique. Of greater concern are two early postoperative complications, infectious keratitis and delayed epithelialization, and the late postoperative complication of subepithelial fibrosis, which causes haze and regression. Careful technique can reduce but not eliminate these problems. The following four cases illustrate the persistence of old complications despite recent innovations in PRK.
My colleagues and I performed a myopic PRK for low myopia on a 47-year-old white female with keratoconjunctivitis sicca and moderately elevated pinguecula. The procedure on the right eye was uneventful, but the left eye lost its soft contact lens twice, possibly because of the pinguecula. Epithelialization covered 95% of the treatment area by day 11. A 1-mm epithelial defect remained for the next 3 days. On day 14, I treated dendritic keratitis with Q2h Viroptic (King Pharmaceuticals, Inc., Bristol, TN) (Figure 1). The epithelial keratitis progressed to stromal keratitis, and I added topical steroids to the treatment regimen. By day 32, the patient's UCVA was 20/15 OD and 20/20 OS. She had no subjective awareness of a difference in acuity between her right and left eyes. A significant delay in epithelialization should prompt surgeons to consider infectious keratitis, and they should always include herpetic keratitis in the differential diagnosis.
A 54-year-old white female with unilateral hyperopia of +2.25 -1.00 X 100 OD required hyperopic PRK. Although the procedure was uncomplicated, the patient showed 3/4+ grade stromal folds on the first postoperative day. As her epithelium gradually advanced, the underlying edema resolved. Despite the use of both a bandage soft contact lens and patching, a 1- X 2-mm epithelial defect remained on the second day. Two small infiltrates appeared within the epithelial defect, and cultures were positive for Streptococcus viridans. My colleagues and I successfully treated the patient with vancomycin and QUIXIN (Santen Inc., Napa, CA). The infiltrate resolved, and the patient's UCVA returned to 20/30 within 1 week.
Although most PRK patients achieve complete epithelial coverage within 5 days, rare cases may have a persistent epithelial defect. With each passing day, the potential for secondary bacterial keratitis increases.
A 29-year-old white female underwent PRK OU (-5.00 D) with the prophylactic use of MMC. At 1 month postoperatively, the patient was 20/25 +1 uncorrected and had a clear cornea. By 4 months postoperatively, her vision was 20/30 with -1.00 D of myopia and 2+ corneal haze (Figure 2). My colleagues and I prescribed prednisolone acetate 1% t.i.d. and gradually tapered it over 6 months to complete cessation. At 10 months, the subepithelial fibrosis had diminished, and the patient's UCVA had returned to 20/15 with a plano refractive error. Although the use of prophylactic MMC is not uniformly effective at preventing subepithelial fibrosis with associated haze and regression, the treatment can help reduce the aggressiveness of the fibrosis.
A 45-year-old white male underwent PRK for unilateral hyperopia of +3.75 D OS. At 3 months, his cornea was clear, his UCVA was 20/20, and he had no refractive error. By 12 months, his UCVA was 20/60, there was dense fibrosis through the inferior 180º of the treatment area, and his refractive error had regressed to +4.00 -1.50 X 90 (Figures 3 and 4). My colleagues and I removed the epithelium and subepithelial fibrosis through manual scraping and polishing with a diamond burr. We then applied MMC for 2 minutes. On the first postoperative day, the patient's distance and near acuities had returned to 20/25.
Following hyperopic PRK, a patient can develop severe, late subepithelial fibrosis and regression. Because the ablation depth was 61 µm, it may have been advisable to treat the corneal surface prophylactically with MMC.
PRK has re-emerged as a valuable technique for laser vision correction. Improvements in epithelial removal, smoother ablations, superior soft contact lenses, and better medications lead to faster epithelialization, less patient discomfort, and better visual results. Nonetheless, the complications traditionally associated with PRK remain, and surgeons need to take precautions to prevent them. An awareness of the potential for infectious keratitis or subepithelial fibrosis can help them to promptly identify and effectively treat these problems.
Dasa V. Gangadhar, MD, practices at the Grene Vision Group in Wichita, Kansas. He holds no financial interest in any product mentioned herein. Dr. Gangadhar may be reached at (316) 691-4444; email@example.com.