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'Minimal-touch' epithelial removal technique improves PRK outcomes
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TORONTO - A Canadian surgeon who has performed more than 8,000 photorefractive keratectomy (PRK) procedures since 1991 gives the operation high marks.
"The incidence of significant complications for low myopia is close to 0%. There is no way that this statistic can be matched by laser in situ keratomileusis (LASIK) at the present time," said Raymond M. Stein, MD, chief of ophthalmology at Scarborough General Hospital here. He also noted that PRK is much easier to perform than LASIK.
Dr. Stein prefers mechanical epithelial removal for low ranges of myopia (less than 2.5 D), enhancements with clear corneas and for all hyperopic PRKs in which about 9 mm of epithelium is removed. More specifically, he favors using a rotary brush. "It's extremely quick, there's less drying of the corneal surface than occurs with mechanical removal with a spatula or blade, and it's very easy to master after only a few cases," he said. "I also believe the brush accounts for the faster re-epithelialization."
For cases more than 2.5 D, Dr. Stein uses a transepithelial approach that does not require observation of the blue fluorescence. He calls this a "minimal-touch" technique, introduced within the past year. "One problem with a laser scrape is that if you go too deep past the epithelium, it creates a steep border between the unablated and the ablated areas," he explained. "This can lead to peripheral arcuate haze and scarring, which can induce a hyperopic shift."
Little to no debris
In contrast, "the minimal-touch technique (available internationally and pending U.S. approval) consists of performing a minimal myopic PRK in the tear film and superficial epithelium followed by a phototherapeutic keratectomy (PTK). This pushes the tear film out of the way in a very smooth fashion and accounts for the differences in the epithelial thickness," he said. For example, with a 6.5-mm optical zone, the transepithelial/PRK (-0.75 D) approach removes an additional 12 µm centrally. "There's minimal to no debris left over after the PTK of 38 µm, and usually one pass with the spatula is all that's required."
Dr. Stein has follow-up data at 6 months for his transepithelial technique. "These results are early, but it's my clinical impression that there's a lower incidence of corneal haze," he said.
Dr. Stein questions whether ablation through Bowman's layer is responsible for corneal haze. "Many practitioners would answer `yes,'" he said, "but then why would a -3 D PRK with ablation through Bowman's layer show an incidence of significant haze close to zero?"
Noting that there are a number of factors that seem to be associated with drying of the corneal surface - prolonged epithelial removal, longer stromal ablations - Dr. Stein said his preventative technique is to irrigate the surface of higher corrections (more than 7 D). "I have shown a decrease in corneal haze with this," he said.
Chilled BSS
After PRK, patients are more uncomfortable if they have debris trapped beneath their bandage contact lens, Dr. Stein explained. "So now I irrigate with chilled BSS [balanced salt solution, Alcon] at the conclusion of the case, rather than during the case, with the lid speculum in place. It has been my impression that the comfort level improves."
Management of overcorrected PRKs includes the use of epithelial debridement, the holmium laser and hyperopic PRK or LASIK. "I predict that either PRK or LASIK will prove to be the procedure of choice," said Dr. Stein, noting that he has witnessed cases of clear corneas that ended up with significant corneal haze after debridement.
Six cases of hyperopic PRK treated with the excimer laser indicate that all patients had a marked improvement in refractive error. The overcorrected PRKs prior to hyperopic PRK had a spherical equivalent that ranged from +1.25 D to +3.75 D. At 10 months postoperatively, however, refractive errors ranged from plano to -0.75 D in all six cases.
Dr. Stein also shared results of about 3,000 eyes at 1 year. Among patients with -1 D to -6 D, 98% were 20/40 or better, and 90% were 20/25. For patients -6 D to -10 D, 88% were 20/40 or better; and for patients -10 D to -14 D, 80% were 20/40 or better. Further, for patients requiring an enhancement, 96% were overall 20/40 or better.
Ideal candidates
Patients with 1 D to 14 D of myopia are ideal candidates for PRK, said Dr. Stein, as are patients with corneal stability following discontinuation of contact lens wear. "You want to make sure that the refractive error is stable," he said. Moreover, patients should exhibit no evidence of keratoconus or irregular astigmatism.
He also mentioned that patients with a history of keloid formation who desire PRK can often be accommodated by first conducting a test spot. "I use a 2-mm spot size out in the periphery and go down 10 to 20 µm," he said, assessing the healing response over 3 months. "If they heal well, without any corneal haze, I've gone back and performed PRK centrally with good results."
As for age, Dr. Stein prefers patients who are at least 22 years old, as there is an increased probability of refractive stability. "The only patients I operate on in the 18 to 21 age group are those who desire refractive surgery for occupational reasons," such as law enforcement and fire fighters. Because these job applicants only need to pass an initial visual test, a change in refraction after acceptance is not a concern.
Postop care
Dr. Stein divides his postoperative protocol into two patient groups: those treated for less than 6 D and those treated for greater than 6 D. Low myopes are administered FML Forte (fluorometholone, Allergan) drops tapered over 1 month. The antibiotic Ciloxan (ciprofloxacin HCl, Alcon) is also given four times daily for 5 days, as well as Voltaren (diclofenac sodium, Ciba Vision) four times daily for 24 to 48 hours. For patients greater than 6 D, the only difference is the extended use of the steroid FML Forte for a total of 2 months.
In addition, all patients are now given a unit dose of nonpreserved tetracaine (0.5%) eye drops. One drop is applied the first night at bedtime, a second drop the following morning. "More than 90% of patients are comfortable with this pain management," said Dr. Stein, who collects the residual dose from the patient during the day 1 office visit.
Further, all patients are treated with a bandage soft contact lens (Acuvue from Vistakon, Jacksonville, Fla.) with a base curve of 8.4 mm, to be worn until the epithelium is healed, which is generally 2 to 4 days. "We usually see patients every day until their epithelium is intact," Dr. Stein said.
Complications
Pain is the most likely postoperative complication of PRK; however, the regimen of irrigation of the cornea with cold BSS, a bandage soft contact lens, Voltaren eye drops and tetracaine eye drops has resulted in comfort for more than 90% of patients. "For patients who are uncomfortable, the pain resolves generally within 24 hours," Dr. Stein said.
Another potential complication is infection. "But I have not seen a single case of infection in the more than 5,000 cases of PRK I have performed since using a broad-spectrum antibiotic like Ciloxan, both preoperatively and postoperatively," he said. Sterile infiltrates can also create a problem; however, "they were far more frequent when a nonsteroidal drop was not used with a topical steroid. Today, it is an extremely rare complication."
There have been only two cases of central islands requiring treatment by Dr. Stein. Corneal haze is also an uncommon complication in a low degree of myopia. "It is extremely rare under -6 D," he said. "But in the higher myopes, it can occur in 5% to 10% of patients." Fortunately, "the haze is usually not visually significant and will resolve with time.
"The optometrist needs to understand that technology is not stagnant. PRK, with its long track record, has continued to improve," said Dr. Stein. "Patients can now expect excellent quality of vision for the next 30 to 40 years."
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For Your Information:
- Raymond M. Stein, MD is chief of ophthalmology at Scarborough General Hospital in Toronto. He can be reached at 40 Prince Arthur Ave., Toronto, Ontario, Canada M5R 1A9; (416) 921-2131; fax: (416) 966-8917. Dr. Stein did not disclose whether or not he had any financial interest in the products mentioned or if he is a paid consultant for any company mentioned.