As with many practices, PRK is no longer our go-to surgery in all our cases, and instead occupies a place reserved for certain patient presentations and levels of myopia. For most cases that come to our clinic, we use either the new femtosecond-based, small-incision lenticule extraction procedure, or LASIK. For select patients, though, PRK is simply a better fit. Such patients have low myopia, below approximately -3 D, and one or more of the following characteristics:
• an occupation that involves a high risk of receiving an injury to their eyes, especially blunt injuries, such as the military or sports;
• epithelial basement membrane dystrophy or even miniscule signs of EBMD that tell you the patient has a risk of developing an epithelial abrasion with LASIK;
• young age, because young patients, on average, do slightly better than older ones in terms of corneal haze development; and
• subclinical keratoconus or a family history of keratoconus.
In addition to PRK being able to treat the patients described above more safely than LASIK, PRK has certain advantages over LASIK that make it appealing. These include:
• no flap folds;
• no epithelial ingrowth;
• no risk of late dislocation of the flap;
• no ectasia; and
• a shorter period of dry-eye symptoms.
The dry-eye advantage was shown in a study from 2012. In it, researchers treated 68 eyes of 34 patients with either LASIK or PRK, randomized by ocular dominance. Patients completed a questionnaire preoperatively and at each postoperative visit evaluating symptoms of dry eye, dry-eye severity, visual fluctuations and foreign body sensation. Both groups had significant increases in the frequency of dry-eye symptoms after surgery, with LASIK having a higher frequency at both one and three months. By the one-year visit, though, there was no increase in dry-eye symptoms over baseline in either group.1
Here is the way we approach key junctures of the PRK procedure, from preop prep to follow-up.
• Prep and debridement. We prepare the patient with two drops of topical anesthetic and then place a drop of antibiotic. In our region, the antibiotic we use is chloramphenicol but U.S. surgeons are likely to use something different. We then use a blunt, 8-mm corneal marker to delineate the area from which I’ll be removing epithelium. The surgeon applies an alcohol solution in that 8-mm zone for a few seconds, then uses a blunt Beaver blade to scrape away the epithelium, making sure to remove any remaining epithelial cells. We prefer the alcohol/scrape method to mechanical debridement both because it’s worked well in our hands and because of the findings from a randomized study. In this bilateral clinical study, researchers used confocal microscopy to compare mechanical epithelial debridement with alcohol-assisted debridement. They found that mechanical debridement retarded epithelial healing time and decreased stromal keratocyte density.2
• Centration. We then position the patient under the laser and use a combination of the laser’s own automatic centration and a manual adjustment. First, the laser zeroes in on the center of the pupil. The surgeon then moves it one-third of the way toward the corneal apex. The light reflex there is from a part of the cornea that’s perpendicular to the laser when the patient is fixated on the fixation light.
• The bandage lens question. Many surgeons will use a bandage contact lens that the patient is to wear for a few days after the PRK. In our experience, after two or three days patients will start to feel some discomfort from the lens. There’s also a possibly increased risk for infiltrates with the use of a contact lens immediately after PRK.3,4 Because of these factors, we have simply felt that it’s safer to perform PRK cases the way we have been doing them, without a bandage lens. The patient will, however, have more pain postop without a contact lens in the eye, which we hope to counter with postop q.i.d. non-steroidal anti-inflammatory drops, such as diclofenac, for the first two or three days, as well as oral painkillers.
In addition to the NSAID, we’ll send the patient home with chloramphenicol and fluorometholone drops, both q.i.d. for two weeks. They then taper those drops to b.i.d. for two weeks.
• Mitomycin-C. For very low myopia, we don’t use this powerful antimetabolite. However, if we were to ablate more than 50 µm, we’d soak a 7- or 8-mm diameter sponge in 0.02% MMC and apply it to the stroma for 20 seconds. We then flush it with saline.
• Follow-up. Our situation is a little different in that our patients often come a good distance to see us, so we want to see them on the first day postop, just to look for anything unusual and to assure them that it’s normal to have some pain. They then return home and, at one week, are seen by their local ophthalmologist to make sure their epithelium has healed. If it hasn’t healed at that point, their ophthalmologist will call us and we’ll suggest a way to proceed, which may involve placing a bandage contact lens. If everything is OK at one week, we’ll see them at three months.
We counsel patients to expect some visual acuity fluctuations during those postop months. And, if we’ve done a higher-level ablation for some reason, they should be prepared for some slightly blurred vision at three months due to possible haze. This development is normal and the haze will disappear with time. Typically, patients will also be slightly overcorrected initially, and then their vision will move toward the target refraction. They should be informed that whatever refraction they have at one week most likely won’t be their final result.
PRK and SMILE
Even though PRK is an older procedure and SMILE is one of the newest, it turns out they may be able to complement each other. In a few of our SMILE patients, there will be some irregular astigmatism or microfolds in the corneal cap that might be visually significant. In such cases, we’ll do a trans-epithelial PRK.
In trans-epithelial PRK, we use the laser in phototherapeutic keratectomy mode to ablate the epithelium, stopping the ablation at the stroma. At that point, we switch the laser over to PRK mode to place the refractive ablation on the stroma. If the patient has microfolds, a trans-epithelial PRK gives you a good chance at smoothing them out.
Using the laser in PTK mode to ablate the epithelium takes a bit of artistry, rather than just pure science. To do it, you first program the laser to cut away 60 or 70 µm, and begin the ablation. You have to watch the cornea carefully. When you see a change in reflection on the corneal surface, you know you’re through the epithelium and can stop the PTK and start PRK.
PRK is typically described as a very safe way to perform a refractive laser treatment since it doesn’t involve the creation of a LASIK flap and has been used for many years. However, PRK also carries the risk of keratitis if a patient doesn’t follow through with his postop drops. So, even though PRK is effective and very safe, it’s not without risk, which is always important to keep in mind. REVIEW
Dr. Hjortdal is a clinical professor at the University of Aarhus and president of the European Eye Bank Association.
1. Murakami Y, Manche EE. Prospective, randomized comparison of self-reported postoperative dry eye and visual fluctuation in LASIK and photorefractive keratectomy. Ophthalmology 2012;119:11:2220-4.
2. Einollahi B1, Baradaran-Rafii A, Rezaei-Kanavi M, et al. Mechanical versus alcohol-assisted epithelial debridement during photorefractive keratectomy: A confocal microscopic clinical trial. J Refract Surg 2011;27:12:887-93.
3. Liu X1, Wang P, Kao AA, et al. Bacterial contaminants of bandage contact lenses used after laser subepithelial or photorefractive keratectomy. Eye Contact Lens 2012;38:4:227-30.
4. Dantas PE, Nishiwaki-Dantas MC, Ojeda VH. Microbiological study of disposable soft contact lenses after photorefractive keratectomy. CLAO J 2000;26:1:26-9.