New research seeks methods to ease drawbacks of surface ablation

Surgeons at the AAO meeting spoke on eliminating haze and stabilizing refraction through enhanced techniques.

NEW ORLEANS — Surgeons can mitigate complications of surface ablation with new and innovative approaches through the use of intraoperative mitomycin-C or through surgical techniques such as corneal “smoothing,” according to three surgeons speaking here.

Complications of surface ablation can include undercorrection, haze, increased spherical aberration, pain, discomfort and slow healing. The surgeons presented study results at Refractive Subspecialty Day preceding the American Academy of Ophthalmology meeting.

Ronald R. Krueger, MD, and Miguel A. Teus, MD, discussed separate studies showing how varied doses of topical MMC affected postoperative refraction and corneal stability.

Fabrizio I. Camesasca, MD, showed results of a study examining phototherapeutic keratectomy-style smoothing used in conjunction with PRK for high myopia, defined as greater than 7 D.

MMC, a powerful, potentially toxic antimetabolite agent, is commonly used in a standard concentration of 0.02% in patients undergoing surface ablation for high myopia (more than 5 D or 7 D, depending on degree of myopia and surgeon choice). Still, some surgeons recommend caution in using MMC because of its toxicity.

More haze with low MMC dose

Dr. Krueger and colleagues conducted a 4-year retrospective study comparing the clinical effectiveness of low-dose MMC 0.002% compared with a standard 0.02% dose in patients who underwent PRK for moderate or high myopia.

The study included 95 eyes that underwent PRK with a standard concentration of MMC and 126 eyes that had the procedure with a low concentration. All eyes had moderate or high myopia.

“If we look at the haze over time, we can see that there was statistically more haze with the low dose of mitomycin at every single time point, 1 month, 3 months, 6 months, 12 months, compared with the high dose,” Dr. Krueger said.

The difference was even greater in cases of high myopia with deep surface ablation, he said.

“Low-dose mitomycin 0.002% is statistically better than no mitomycin in preventing haze after surface ablation for myopia greater than 3 D,” Dr. Krueger said. “Our preliminary results at least suggest that the standard dose, 0.02%, is statistically better than low dose for myopia greater than 6 D and for treatments greater than a depth of 75 µm.”

However, low and standard doses were equally effective for moderate myopia of 3 D to 6 D with an ablation depth of less than 75 µm, he said.

“The duration of mitomycin is probably a little less important than the standard concentration,” he said. MMC exposure ranged from 30 seconds to 2 minutes in all patients.

Ronald R. Krueger

The findings point to effective treatment regimens at this time, Dr. Krueger said.

“Further analysis needs to be done,” he said. “But at this point, we are suggesting: Go ahead and use the standard dose of mitomycin for your higher treatments above 6 D, and for myopia between 3 and 6 D, consider either low or standard dose mitomycin.”

Dr. Krueger and colleagues published their results in the American Journal of Ophthalmology.

LASEK with MMC for thin corneas

Data reviewed by Dr. Teus showed that myopic LASEK with intraoperative MMC performed on thin corneas yielded long-term refractive stability with no signs of ectasia at 15-month follow-up.

A retrospective study included 64 consecutive eyes with thin corneas that underwent LASEK with intraoperative MMC for myopia or myopic astigmatism. Dr. Teus and colleagues compared refraction, visual acuity and corneal topography outcomes at 3 months and 15 months postop to detect any myopic shift that might signify secondary corneal ectasia.

All patients had central corneal thickness (CCT) of less than 500 µm and normal topography, Dr. Teus said.

“Probably the limit of pachymetry of 500 µm is demonstrated for surface ablation,” he said. “Mitomycin-C is of great help. It clearly induces less haze, much better prediction and refractive results after this procedure. There has been no reported single case of ectasia after surface ablation with intraoperative mitomycin-C.”

However, potential sticking points include long-term adverse effects on corneal stability stemming from the degradation of keratocytes, he said.

Dr. Teus performed all LASEK procedures. All eyes received MMC 0.02% for 30 seconds over the ablated stroma.

Patients’ preoperative CCT averaged 485.8 µm ± 10.8 µm. Preoperative spherical refraction averaged –4.6 D ± 2.30 D, Dr. Teus said.

At 3 months postop, patients’ CCT averaged 402.9 µm ± 31 µm. Residual sphere was +0.2 D ± 0.5 D. Mean cylinder was –0.2 D ± 0.4 D. Uncorrected visual acuity was 0.93 ± 0.1. Best corrected visual acuity was 0.98 ± 0.1.

At 15 months postop, mean residual sphere held stable at +0.2 D ± 0.8 D. Mean cylinder was –0.40 D ± 0.7 D. UCVA was 0.96 ± 0.2. BCVA was 1 ± 0.1, Dr. Teus said.

No eyes showed signs of ectasia between 3 months and 15 months postop, he said.

“No other paper has studied this issue of stability of the refraction after mitomycin-C application in surface ablation,” Dr. Teus said. “Myopic LASEK with the use of mitomycin-C performed in thin corneas with normal preoperative topography seems to achieve long-term stable refractive results with no sign of ectasia 3 and 15 months postoperatively.”

Smoothing reduces regression and haze

PRK performed with a wide optical and transition zone prevented an increase in spherical aberration and, combined with smoothing, resulted in a “very regular” corneal surface with almost no haze, Dr. Camesasca said.

“We think the extremely wide transition zone induces less spherical aberration and has better vision quality and less halos at night,” he said. “Smoothing is the key to creating a regular surface, decreasing haze and regression. And we have demonstrated the long-term effectiveness of this treatment, even with very thin corneas.”

Co-author Paolo Vinciguerra, MD, developed smoothing to treat “irregularities” in the overlapping ablation zones, Dr. Camesasca said.

The retrospective study included 114 eyes of 69 patients with a mean age of 37.7 years. Inclusion criteria included high myopia of 9.55 ± 1.79 D and risk factors for undercorrection, haze, regression, glare and ectasia, he said.

“These patients required accurate patient selection,” he said. “It’s a complex treatment and follow-up must be careful. Why did we decide to use surface ablation? Because it’s possible to correct higher myopia values. We can treat very steep or flat corneas, as well as eyes with wide pupils. … Less corneal stroma is involved with a reduced biochemical effect … and the risk of ectasia is lower.”

PRK was performed with a Nidek EC-5000 excimer laser with a multiple optical zone approach, ablation zone of 4.8 mm to 7 mm and transition zone of 8 mm to 10 mm. A cross-cylinder technique was use to correct astigmatism. Smoothing was performed with the laser set at a frequency of 10 Hz and with masking fluid applied over a 10-mm diameter zone, Dr. Camesasca said.

“The technique of smoothing requires masking fluid that has the same ablation rate of the normal cornea,” he said.

Mean preoperative CCT was 560.4 µm ± 30.1 µm. Mean preoperative BCVA was 0.88 ± 0.16 with 9.53 D ± 1.18 D spherical equivalent.

At 3 years postop, mean UCVA was 0.79 ± 0.26. Mean BCVA was 0.92 ± 0.19, with a spherical equivalent of 0.56 D ± 0.90 D, the abstract said.

Mean haze peaked at 1 month postop (0.58 ± 0.35), then decreased steadily; at 3 years, mean haze was 0.11 ± 0.32, according to the abstract.

“Stability is impressive after 3 years,” Dr. Camesasca said. “There is practically no change, and haze decreased even more after 1 year.”

Astigmatism was reduced and coma increased slightly. Two cases required re-treatment because of undercorrection. No eyes had signs of ectasia or retinal detachment, he said.

For more information:

• Fabrizio I. Camesasca, MD, can be reached at Istituto Clinico Humanitas, Via Manzoni 56, 20089 Rozzano, Milan, Italy; 39-02-8224-2311; e-mail: fabrizio.camesasca@tiscali.it.
• Ronald R. Krueger, MD, can be reached at the Cleveland Clinic Foundation, 9500 Euclid Ave., Room i32, Cleveland, OH 44195; 216-444-8158; e-mail: krueger@ccf.org.
• Miguel A. Teus, MD, is a professor of ophthalmology at University of Alcalá, Madrid, Spain. He can be reached at Santa Hortensia 58, Madrid 28043; 34-91-510-6639; e-mail: mteus@vissum.com.

Reference:

• Thornton I, Puri A, Xu M, Krueger RR. Low-dose mitomycin C as a prophylaxis for corneal haze in myopic surface ablation. Am J Ophthalmol. 2007;144(5):673-681.

• Matt Hasson is an OSN Staff Writer who covers all aspects of ophthalmology. He focuses on regulatory, legislative and practice management topics.
• Lauren Wolkoff, Executive Editor of OSN U.S. Edition, also contributed to this report.