Capsaicin treatment for osteoarthritis pain: a meta-analysis

Abstract

Background: Capsaicin appears to be effective for osteoarthritis pain but it is uncertain whether the effect has a dose response, is consistent across joints, or changes over time. Purpose: To perform a quantitative overview the use of topical capsaicin in the therapy of painful osteoarthritis (OA) in adults, and to assess whether effects vary by formulation strength, joint site, or treatment duration. Methods: Literature databases (PubMed, EMBASE, ISI Web of Knowledge) were searched for randomized controlled trials for use of topical capsaicin (or capsaicin-like therapies) in OA using the keywords: “capsaicin”, “osteoarthritis” “double-blind method”, “randomized controlled trial [publication type]” or “controlled clinical trial [publication type]” up to 2012. Studies not in English were excluded. Trials were compared for pain scores (assessed using visual analog scales (VAS)), patient global evaluation of treatment effectiveness and application site burning using standardised mean differences (SMD), using RevMan. Results: Five double-blind randomized controlled trials and one case-crossover trial were identified, testing topical capsaicin four times daily. Formulations ranged from 0.025% - 0.075%, and trial durations from 4-12 weeks. Trials assessed OA of the knee (n = 3), hand (n = 1), and a mix of joints (n = 2). Patients were typically required to have at least moderate pain and either radiologically or clinically defined OA, or both. Treatment efficacy of capsaicin (compared to placebo) for change in VAS pain score was moderate, at 0.44 (95% CI 0.25-0.62) over 4 weeks of treatment. There was no heterogeneity between studies, indicating no between-study differences, including effect of OA site or treatment concentration. There was disagreement on effectiveness over time. Two studies reported treatment beyond 4 weeks, with the first study reporting an effect size of -9mm after 12 weeks, with the maximum difference between groups occurring at 4 weeks. The second study reported that between-group differences increased over time, up to 20 weeks. Capsaicin was reported as being safe and well-tolerated, with no systemic toxicity. The most common side effect reported was mild application site burning, affecting 35-100% of capsaicin-treated patients; the risk ratio was 4.22 (95% CI 3.25-5.48), reported in 5 trials. Incidence of burning peaked in week 1, with incidence rates declining over time, plateauing from weeks 2-12. Conclusions: Topical capsaicin treatment four times daily is moderately effective in reducing pain intensity up to 20 weeks regardless of site of application and dose in patients with at least moderate pain and clinical or radiologically defined OA. Capsaicin treatment is also generally well tolerated, suggesting that capsaicin should be used early in the OA treatment algorithm, especially for superficial joints such as the hand and knee.