Topical 0.025% capsaicin in chronic post-herpetic neuralgia: efficacy, predictors of response and long-term course.

In order to evaluate the efficacy, time-course of action and predictors of response to topical capsaicin, 39 patients with chronic post-herpetic neuralgia (PHN), median duration 24 months, were treated with 0.025% capsaicin cream for 8 weeks. During therapy the patients rated their pain on a visual analogue scale (VAS) and a verbal outcome scale. A follow-up investigation was performed 10-12 months after study onset on the patients who had improved. Nineteen patients (48.7%) substantially improved after the 8-week trial; 5 (12.8%) discontinued therapy due to side-effects such as intolerable capsaicin-induced burning sensations (4) or mastitis (1); 15 (38.5%) reported no benefit. The decrease in VAS ratings was significant after 2 weeks of continuous application. Of the responders 72.2% were still improved at the follow-up; only one-third of them had continued application irregularly. Treatment effect was not dependent on patient's age, duration or localization of PHN (trigeminal involvement was excluded), sensory disturbance or pain character. Treatment response was not correlated with the incidence, time-course or severity of capsaicin-induced burning. If confirmed in controlled trials, the long-term results of this open, non-randomized study might indicate that the analgesic effect of capsaicin in PHN is mediated by both interference with neuropeptide metabolism and morphological changes (perhaps degeneration) of nociceptive afferents.