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Results of retrospective studies comparing haze after PRK in eyes treated with 0.002% mitomycin C (MMC) versus no MMC and in eyes treated with 0.002% MMC versus 0.02% MMC suggest an approach for using MMC in clinical practice.
Dr. Krueger discussed the findings from two studies designed to evaluate the level of corneal haze and the efficacy, predictability, and safety of using topical low-dose, 0.002% MMC and high-dose (standard), 0.02% MMC applied to the stromal bed after surface laser treatment. All patients underwent treatment at the Cole Eye Institute.
"Further analyses are needed, but preliminarily, we are suggesting using the standard 0.02% concentration of MMC for eyes undergoing correction for more than –6 D of myopia and either low-dose MMC, 0.002%, or no MMC when treating lesser amounts of myopia," said Dr. Krueger.
The first retrospective analysis compared 84 myopic eyes treated with 0.002% MMC for varying durations of exposure (30 seconds to 2 minutes) depending on the amount of correction against 91 eyes that underwent surgery without MMC and showed significantly less haze in the group where MMC was used. Preoperatively, there were some significant differences in baseline characteristics between the two groups, especially in spherical equivalent (SE) and pachymetry, but they were adjusted for using multivariate statistical techniques.
Haze was graded using the Fantes scale at postoperative visits 1, 2, 3, 6, 12, and 24 months after surgery. At every time point, haze scores were significantly lower in the MMC-treated eyes compared with the controls. When eyes were divided into three subgroups based on level of myopia (–3 to –6 D; –6 to –9 D; >–9 D), the difference favoring use of MMC for minimizing haze persisted across all three subgroups.
"One might conclude from these results that low-dose MMC should be used in all patients undergoing surface ablation," Dr. Krueger said. "However, findings from our second retrospective analysis showed this was not the whole story."
The second analysis was prompted by a study performed in the Cleveland Clinic laboratory of Steven Wilson, MD, which investigated the effects of different concentrations of MMC in an animal PRK model. The results showed that control eyes treated with balanced salt solution had more smooth muscle actin staining (greater myofibroblast density) than eyes treated with low-dose or high-dose MMC. Use of MMC was associated with some keratocyte depletion, however.
"These results might suggest less is better, but our second retrospective analysis indicated that was not necessarily true," said Dr. Krueger.
The second retrospective study compared 95 eyes undergoing PRK with standard-dose MMC during the years 2002 to 2004 against 126 eyes operated on during 2005 and 2006 with PRK and MMC 0.002%. The exposure time again varied, and multivariate statistical techniques were used to adjust for baseline differences between groups.
In follow-up visits through 12 months, there was significantly more haze in the eyes treated with low-dose MMC compared with the standard-dose group at all time points. Subgroup analyses, however, showed that significant differences at the 1, 3, 6, 9, and 12 month visits were present only in eyes treated for an SE >–6 D or when the ablation depth exceeded 75 μm. In eyes with less than –6 D of myopia or shallower ablations, significant differences in haze scores were present between the low-dose and standard-dose MMC groups only at 1 and 3 months after surgery.