Photorefractive keratectomy (PRK) is a safe, effective option for the correction of residual refractive errors following previous refractive surgeries. This article reviews the literature and surgical techniques that support this conclusion, and offers recommendations for avoiding complications, improving visual outcomes, and maximizing patient satisfaction.
PRK after LASIK
The retreatment rates for residual overcorrection, undercorrection and induced astigmatism following LASIK ranges widely from 5% to 37.9%.1-2 The most commonly used technique for LASIK retreatment is flap relift or flap recutting. However, complications such as buttonhole creation, post-LASIK dry eye syndrome, thin residual stromal bed, and the inability to identify the flap edge can limit surgical options for retreatment.1-3 In addition, relifting an old flap increases the risk of epithelial ingrowth, flap tear, striae, and diffuse lamellar keratitis.1 To avoid these complications, photorefractive keratectomy (PRK) could be used.
In the past, use of PRK after LASIK has been complicated by severe stromal haze formation.4 The use of mitomycin C (MMC) 0.02% (0.2 mg/ml) has gained popularity in the prevention of corneal haze development.
The eye is prepared as for a standard PRK procedure. Diluted alcohol is placed in an appropriately sized well on the cornea to loosen the epithelium. It is important to completely remove the epithelium so that only Bowman's membrane remains. Any residual epithelium will create an uneven ablation and irregular astigmatism. It is also possible to remove the epithelium entirely with the excimer laser via a transepithelial technique, which allows the surgeon to use the epithelium as a smoothing agent, or with a laser-scrape technique.
The ablation should promptly follow epithelial removal to prevent drying of the ocular surface. It is important to make certain that the hydration status of the corneal stroma is uniform during the procedure. Immediately following the ablation, the MMC 0.02 % (0.2 mg/ml) treatment is performed. The corneal surface and the entire conjunctiva are then vigorously irrigated with 10 ml of cold normal saline to remove any residual MMC. The eye is then treated and followed as in a standard PRK procedure.
Review of Published Results
These retrospective studies (Table 1) and case reports demonstrate the safety and efficacy of performing surface ablation after LASIK, especially with the use of MMC. Every patient should be counseled preoperatively about possible risks, benefits, and potential complications of this treatment. Corneoscleral perforation, melts, and delayed epithelial healing after the intraoperative application of MMC may occur. As the long-term effects of MMC on the cornea are not fully known, care should be taken to avoid exposure to the corneal stem-cell population that resides on the peripheral corneal surface. As with any refractive procedure, realistic patient expectations are necessary.
Table 1. Published Results for PRK Following LASIK.
PRK after PKP
Postoperative refractive error can be a significant adverse outcome after penetrating keratoplasty (PKP). The amount of astigmatism present can be unpredictable, and depends on a myriad of factors.
Anisometropia and high amounts of astigmatism can be fdifficult to correct with glasses alone. Irregular astigmatism can be corrected with rigid gas-permeable lenses. Some patients, however, are unable to tolerate contact lenses. In the past, relaxing incisions, arcuate keratotomy, and wedge resections have all been used with varying results.
The advent of the excimer laser has provided a new avenue for refractive correction. Both LASIK and PRK have been used in keratoplasty patients. LASIK offers the advantage of rapid visual rehabilitation, decreased scarring, decreased irregular astigmatism and minimized postsurgical regression.10 However, it does carry increased risks of wound dehiscence, induced graft rejection, dry eyes, and irregular wound healing leading to high-order aberrations.
For these reasons, the authors have found PRK to be a safer option. PRK has been used in post-keratoplasty patients since the 1990s and has shown good results in various studies. There have been some problems with increased corneal haze and postoperative regression over time as well as reported cases of graft rejection, although newer medications have helped decrease the incidence. A history of herpes keratitis should alert the surgeon to the danger of possible viral reactivation. The authors have successfully treated such patients by pretreating with appropriate antiviral medications and continuing the antiviral treatment as long as the patient requires topical steroids.
The authors recommend that the surgeon wait at least 1 year after the keratoplasty before attempting PRK. All sutures should first be removed and the patient's refraction, keratometry and topographical analyses should be stable for at least 3 months before considering refractive surgery. The patient must understand that the ultimate goal will not be to eliminate the need for glasses but to make glasses a more viable alternative and to decrease high-order aberrations. PRK can be carried out in a standard fashion, but if possible, the optical zone should be kept smaller than the size of the corneal graft.
After debridement of the epithelium, the authors advocate a single 12-second application of MMC 0.02% as an adjunct to prevent subepithelial fibrosis (unpublished data). The authors do not recommend a concentration higher than 0.02%, as it can result in nonhealing epithelial defects. Care must be taken to avoid compromising the integrity of the graft-host junction, which can result in wound dehiscence and refractive instability. After excimer laser ablation is complete, the authors advocate a postoperative drop of antibiotic, steroid, and topical NSAID for comfort. A bandage contact lens is placed and allowed to remain for at least the first week as the epithelial defect heals. Topical steroids are slowly tapered over the course of 4 months to prevent scarring and graft rejection.
In the authors' experience, PRK after PKP is effective, especially when used with wavefront analysis; patients can have substantial improvements to their vision and irregular astigmatism. The surgeon and patient must remember, however, that the refractive improvement generally requires a longer recovery time than with standard PRK.
PRK after RK
The introduction of radial keratotomy (RK) for the treatment of myopia was a tremendous advancement in the realm of refractive surgery. Unfortunately, post-RK induced hyperopia has been reported in as many as 43% of cases in the Prospective Evaluation of Radial Keratotomy (PERK) study with an incidence of another 1% to 2% per year.11 In choosing between PRK and LASIK for treating these patients, it must be recalled that past methods of enhancing previous RK surgery entailed deepening or extending the RK incisions. Thus, during excimer laser surgery, it is vital to minimize trauma to the incisions because extension or deepening of the incisions will cause unintended refractive changes.
A review of the literature shows that LASIK has been reported as a possible treatment modality for the induced hyperopia and residual myopia.12 The reported results are good; however, complications with flap creation including extension of the previously made RK incisions have been reported. PRK avoids these complications, and in the authors' experience, PRK is a safe, effective option for the correction of residual refractive error following RK.
In the authors' opinion, a laser-scrape technique or a manual scrape aided with diluted absolute alcohol induces much less stress on the RK incisions than application of a keratome or even the femtosecond laser apparatus.
In summary, treating the residual refractive error in post-RK patients remains a common scenario in a refractive practice. PRK is a viable option for the treatment of these patients.
- Martines E, John ME. The Martines enhancement technique for correcting residual myopia following laser assisted in situ keratomileusis. Ophthalmic Surg Lasers. 1996;27(5 Suppl):S512-S516.
- Gimbel HV, Basti S, Kaye GB, Ferensowicz M. Experience during the learning curve of laser in situ keratomileusis. J Cataract Refract Surg. 1996;22(5):542-550.
- Netto MV, Wilson SE. Flap lift for LASIK retreatment in eyes with myopia. Ophthalmology. 2004;111(7):1362-1367.
- F. Carones, L. Vigo, A.V. Carones and R. Brancato. Evaluation of photorefractive keratectomy retreatments after regressed myopic laser in situ keratomileusis. Ophthalmology. 2001;108(10):1732-1737.
- Chalita MR, Xu M, Krueger RR. Alcon CustomCornea wavefront-guided retreatments after laser in situ keratomileusis. J Refract Surg. 2004;20(5):S624-S630.
- Shaikh NM, Wee CE, Kaufman SC. The safety and efficacy of photorefractive keratectomy after laser in situ keratomileusis. J Refract Surg. 2005;21(4):353-358.
- Cagil N, Aydin B, Ozturk S, Hasiripi H. Effectiveness of laser-assisted subepithelial keratectomy to treat residual refractive errors after laser in situ keratomileusis. J Cataract Refract Surg. 2007;33(4):642-647.
- Beerthuizen JJG, Siebelt E. Surface ablation after laser in situ keratomileusis: retreatment on the flap. J Cataract Refract Surg. 2007;33(8):1376-1380.
- Srinivasan S, Drake A, Herzig S. Photorefractive keratectomy with 0.02% mitomycin C for treatment of residual refractive errors after LASIK. J Refract Surg. 2008;24(1):S64-S67.
- Alió JL, Javaloy J, Osman AA, Galvis V, Tello A, Haroun HE. Laser in situ keratomileusis to correct post-keratoplasty astigmatism; 1-step versus 2-step procedure. J Cataract Refract Surg. 2004;30(11):2303-2310.
- Waring GO 3rd, Lynn MJ, McDonnell PJ. Results of the prospective evaluation of radial keratotomy (PERK) study 10 years after surgery. Arch Ophthalmol. 1994;112(10):1298-1308.
- Perente I, Utine CA, Cakir H, Yilmaz OF. Complicated flap creation with femtosecond laser after radial keratotomy. Cornea. 2007;26(9):1138-1140.
Dr. Kaufman has received lecture fees from Allergan, Inc., and Alcon Laboratories, Inc. Drs. Kramarevsky, Fung, Raja, Shaikh, and Meduri state that they have no financial relationship with the manufacturer or provider of any product or service discussed in this article or with the manufacturer or provider of any competing product or service.