Source: Anesthesiology  |  Posted 10 years ago

Dextromethorphan appears to be effective in a dose-related fashion in select patients with painful diabetic neuropathy, but not in patients with post-herpetic neuralgia.

The varying effectiveness of the drug suggests a difference in pain mechanisms for the two conditions. For that reason, selective approaches to pain-relevant []N[]-methyl-D-aspartate receptors would appear to be warranted, say Canadian and American researchers.

They studied patients with painful diabetic neuropathy (DN) and postherpetic neuralgia (PHN) in two crossover trials. The first was an efficacy trial comparing dextromethorphan, memantine, and lorazepam, which served as an active placebo. The second was a dose-response trial of the preferred active drug in responders from the first study, testing the drug at 0, 25, 50 and 100 percent of each patient's maximally tolerated dose.

Pain intensity was measured on a 20-point scale.

Nineteen patients with DN and 17 patients with PHN completed the efficacy trial. Median doses for DN were 400 mg/day dextromethorphan, 55 mg/day memantine, and 1.8 mg/day lorazepam. Median does for PHN were 400 mg/day dextromethorphan, 35 mg/day memantine, and 1.2 mg/day lorazepam.

Among patients with DN in the efficacy trial, dextromethorphan reduced pain intensity by a mean of 33 percent from baseline, memantine reduced pain intensity by a mean of 17 percent, and lorazepam reduced pain intensity by a mean of 16 percent. The proportions of subjects achieving greater than moderate pain relief were 68 percent with dextromethorphan, 47 percent with memantine, and 37 percent with lorazepam.

Among patients with PHN, mean reductions in pain intensity were six percent with dextromethorphan, two percent with memantine, and 0 percent with lorazepam. No comparison with placebo reached statistical significance in the efficacy trial.

In ten subjects with DN who responded to dextromethorphan, there was a significant dose-response effect on pain intensity, with the highest dose significantly better than that of lorazepam.