American Academy of Ophthalmology Web Site: www.aao.org
Clinical Update: Refractive
PRK: Feeling Better and Healing Faster
Since photorefractive keratectomy was approved by the FDA in 1995, patient outcomes have steadily improved. Better visual results, fewer complications and reduced pain after the surgery have all contributed to increased interest in surface ablation.1
The Achilles’ heel of PRK, however, remains the time required for epithelial healing. It can take a week to recover an intact epithelium and several weeks before best-corrected acuity is achieved. But now there may be ways to speed epithelial healing after PRK, as well as further improve pain control, according to surface ablation specialists.
Problems and Improvements for Both PRK and LASIK
When PRK first became available, its drawbacks included slow reepithelialization, corneal haze and significant postoperative pain as well as delays in the return of vision.
The popularity of LASIK, unveiled a few years after PRK, was fueled in part by the limitations of PRK. With LASIK, patients could achieve good visual results quickly with a shorter healing and visual recovery period and less postoperative pain. After years of experience with LASIK, however, some of its shortcomings have come to light—namely complaints about night vision and quality of vision. And as wavefront technologies became available, ophthalmologists found that they could achieve better quality of vision with PRK than with wavefront-guided LASIK using a microkeratome.
Improvements in laser technology over the last decade have benefited PRK, allowing the surgeon to leave a smoother stromal bed after surgery, with improved quality of vision and reduced formation of haze, according to William B. Trattler, MD, in private practice in Miami and volunteer assistant professor of ophthalmology at the Bascom Palmer Eye Institute. And while newer technologies also have allowed LASIK to achieve comparable visual results, PRK remains a viable option for many patients who may not be candidates for LASIK, Dr. Trattler said.
Attractive candidates. PRK is especially useful in patients with thin corneas, large pupils, corneal scars, epithelial basement membrane disease or a his- tory of radial keratotomy, channel IOLs or corneal transplants.
Since LASIK with a metal microkeratome can sever 40 to 50 percent more collagen fibrils than surface ablation,2 PRK can potentially be used for patients with early forme fruste keratoconus, Dr. Trattler said, and reduce the chance of postoperative ectasia in those patients. Yet Dr. Trattler raised a caution: Even though patients with forme fruste keratoconus can achieve good short- to medium-term results with PRK, additional long-term studies will help determine the safety and effectiveness.
Other experts question whether patients with problematic corneal shapes should receive laser vision correction at all, said Marguerite B. McDonald, MD, clinical professor of ophthalmology at New York University. In any event, the preservation of more undisturbed corneal stroma may be why there are fewer reported cases of postoperative ectasia with PRK than with LASIK, Dr. McDonald said.
PRK for rough work and play. Some ophthalmologists also recommend PRK for patients who participate in contact sports or have occupations that may subject them to injury. These include vigorous sports such as basketball and football, or work on a police force or fire department where eye injuries are more likely than at desk jobs.
“With PRK, there is no flap, and thus no potential for the flap complications that can occur after LASIK if patients such as professional basketball players or police officers get poked in the eye during a game or a scuffle,” Dr. Trattler said.
At Fort Bragg in North Carolina, Army surgeons have moved toward surface ablation procedures for all refractive surgery candidates because of the rare but real potential for traumatic flap dislocations after LASIK. At Fort Bragg, five traumatic flap dislocations have occurred after 2,500 LASIK procedures. A review of 28,000 procedures also showed that soldiers with PRK had a 20 percent increased chance of uncorrected visual acuity of 20/15 or better than soldiers with a similar refractive error undergoing LASIK, according to an unpublished paper by Dr. Trattler and Scott D. Barnes, MD, staff ophthalmologist at the Womack Army Medical Center at Fort Bragg.3
7 Keys to Success
Preop prep. Careful preoperative care of the PRK patient is crucial in helping to speed epithelial healing after surgery. Patients with blepharitis should be treated with lid hygiene, scrubs and antibiotics; those with dry eye should receive artificial tears, cyclosporine drops (Restasis), ointments or punctal plugs, according to Neal A. Sher, MD, adjunct clinical professor of ophthalmology at the University of Minnesota in Minneapolis.
Because dry eye is so common in older patients, Dr. Sher uses punctal plugs in most patients over age 45 undergoing PRK. “That will get them through the first week postoperatively and create a better tear film,” he said.
Postop contact lens. It’s also crucial that the contact lens placed on the eye after surgery fits correctly. “That’s the most important aspect for epithelial healing—and for reducing pain,” Dr. Sher said. He examines his PRK patients immediately and again within half an hour after surgery to make sure the contact lens fits properly. When it’s cold or dry outside, he also patches the eye, with instructions to keep the patch on until the patient gets home. “If they’re not blinking adequately because of the topical anesthetics used during surgery and they go out in very cold weather, the contact lens can become dislocated,” he said.
Dr. Sher noted that if the lens does not fit properly, or if excessive amounts of proteins or lipids build up, anoxia and corneal edema can delay healing. “A poorly fitting contact lens should be replaced. A contact lens with excessive protein or lipid buildup also should be replaced, sometimes daily. Patients should be warned about exposure to smoke and cooking fumes, as they accumulate in the contact lens and can be toxic and cause healing delays.”
The choice of contact lens also can play a role in epithelial healing, as well as help to reduce eye irritation and pain. A silicone bandage contact lens usually achieves better results for epithelial healing and also reduces irritation better than a nonsilicone hydrogel lens, according to Dr. Trattler.
Easy on the anesthetics. Another tip for speeding epithelial healing is to instruct the patient not to overuse anesthetic drops after surgery. “When used excessively, they can lead to delays in healing. They should be regarded as emergency drops because excessive use really does slow down healing,” Dr. Trattler said.
Dr. Sher previously recommended topical proparacaine, 0.05 percent, but found that there was minimal pain relief. If a patient is having significant pain, and the contact lens is fitting well, he gives the patient 0.5 ml of 0.5 percent proparacaine with strict instructions to use it only four times during the day and for the patient to come back the next day.
Caution with the nonsteroidals, too. Drs. Sher and Trattler note that NSAIDs, which can help with postoperative pain control in PRK, can also occasionally cause delays in epithelial healing. In one study by Dr. Trattler, epithelial problems were seen when a topical NSAID, nepafenac sodium, was placed on the corneal bed.4 Placing nepafenac directly on the stromal bed prior to the application of the contact lens in PRK surgery can also contribute to early corneal haze. However, if nepafenac is placed on the eye after the contact lens, and is limited to three days or less, the risk of delays in healing and early corneal haze are low, Dr. Trattler said.
“In contrast, Acular LS, Xibrom and Voltaren can be placed directly on the cornea and prior to the bandage contact lens and, if needed, used for four to five days without a significant epithelial delay,” he added.
But inflammation is not good. For Dr. McDonald, decreasing inflammation in the eye after PRK is vital to speed healing. Her preoperative regimen for PRK patients includes 80 mg of prednisone taken 30 minutes prior to the PRK procedure, with the dose quickly tapered over the next six days. “As long as the patient is not diabetic, steroids can decrease healing times as well as dramatically reduce pain,” she said.
Yet this regimen has its detractors. Dr. Sher believes the potential for side effects with steroids outweigh their benefits. He noted that in patients with depression or bipolar disorder, short-term steroids can result in exacerbations of these conditions.
Cool the cornea. Chilling the cornea before and/or after ablation is also helpful for inflammation as well as pain control, Dr. McDonald said. Many surgeons recommend chilling the cornea with the Durrie popsicle or with cold balanced saline solution to reduce pain.
The chilled BSS can serve a second purpose: It is useful for rinsing off mitomycin-C that may have been applied to the eye after ablation.
A mighty mycin clears the haze. MMC has become nearly standard in PRK to reduce haze after surgery. According to Dr. Trattler, some studies have shown MMC to be safe with no significant side effects on keratocytes.
Dr. McDonald noted that some surgeons use it in every case, while others use it only in eyes with higher dioptric corrections or previous incisional corneal surgery or scars.
MMC is especially useful in cases involving more substantial-than-average ablation, agreed Parag A. Majmudar, MD, associate professor of ophthalmology at Rush University in Chicago. Dr. Majmudar has published a number of studies on MMC, and he notes that while there are concerns about delays in epithelial healing with MMC, there are few complications when the medication is applied to the central cornea following standard protocol.
One such study showed that the administration of MMC for haze prophylaxis following PRK did not have a significant effect on quantitative endothelial cell density or qualitative morphometric parameters.5
Dr. Sher has used 0.02 percent MMC in all cases of PRK over the last three years and has not had any complications from its use or anything more than trace haze, even on surface treatments over prior RK, PRK or LASIK.
What’s on the horizon for reducing healing time after PRK? Some experimental approaches, such as eyedrops and contact lenses fabricated from amniotic membrane, have the potential to speed epithelial healing. “Amniotic membrane drops and contacts are a very exciting area, and they could have tremendous potential. However, the research is still preliminary. Their routine use is still a long way off, and so we have to regard this modality as investigational,” Dr. Trattler said.
Oral pain agents. Beyond topical approaches to post-PRK pain, such as bandage soft contacts, cold saline, ice, anesthetics and NSAIDs, an increasing number of opthalmologists are considering antiseizure medications—gabapentin (Neurontin) and pregabalin (Lyrica)—for pain management. A study just published in the American Journal of Ophthalmology compared oxycodone plus acetaminophen (Percocet) after PRK with gabapentin in 141 patients. The patients’ rating of their pain in both groups was similar, although gabapentin was associated with significantly more frequent use of anesthetic eyedrops.6
Gabapentin and pregabalin work by blocking nerve impulses, but the jury is still out on their effectiveness in treating pain after PRK, according to Dr. Majmudar. Although some ophthalmologists find them effective, others think their effect is modest. They have fewer side effects than narcotics, Dr. Sher said, but they may cause drowsiness and are more costly than older pain medications; he generally does not find them useful.
Dr. Majmudar added that with all of the accumulated refinements, extreme delays or pain in healing are quite rare. “Most people heal pretty quickly after PRK.”
2 Marshall, J. Biomechanics and why surface ablation is so important. Presented at the Fifth International Congress on Epi-LASIK and LASEK Advanced Surface Ablation, September 2006, London.
3 Barnes, S. D. What one army center has learned. Presented at the Royal Hawaiian Eye Conference, January 2008, Kona.
4 Trattler, W. B. PRK and LASEK to enhance previous LASIK. Presented at the Royal Hawaiian Eye Conference, January 2006, Kona.
5 Goldsberry, D. H. et al. J Refract Surg 2007;23(7):724–727.
6 Nissman S. A. et al. Am J Ophthalmol 2008;145(4):623–629.
Dr. Majmudar is a speaker for Alcon, Allergan, AMO, Inspire and IOP, Inc. Dr. McDonald consults for Allergan, AMO, Bausch & Lomb, Oasis, Santen and Vistakon. Dr. Sher is a consultant for Ista and Medtronic. Dr. Trattler has received funding for research, consulting and/or speaking from Allergan, AMO, Glaukos, Inspire, Ista, Lenstec, Sirion and Vistakon.