Objective: Botulinum toxin type A (BTX-A) has been reported to have analgesic effects independent of its action on muscle tone, possibly by acting on neurogenic inflammation. Such a mechanism may be involved in peripheral neuropathic pain.
Methods: A possible direct analgesic effect of BTX-A pain processing was investigated in 29 patients with focal painful neuropathies and mechanical allodynia using a randomized, double-blind, placebo-controlled design. Patients received a one-time intradermal administration of BTX-A (20-190 units) into the painful area. Outcome measures, evaluated at baseline, then at 4, 12, and 24 weeks, included average spontaneous pain intensity, quantified testing of thermal and mechanical perception and pain, allodynia to brushing (area, intensity), neuropathic symptoms, clinical global impression, and quality of life.
Results: BTX-A treatment, relative to placebo, was associated with persistent effects on spontaneous pain intensity from 2 weeks after the injection to 14 weeks. These effects correlated with the preservation of thermal sensation at baseline (p < 0.05). BTX also improved allodynia to brush and decreased pain thresholds to cold, without affecting perception thresholds. There were sustained improvements in the proportion of responders (number needed to treat for 50% pain relief: 3.03 at 12 weeks), neuropathic symptoms, and general activity. Most patients reported pain during the injections, but there were no further local or systemic side effects.
Interpretation: These results indicate for the first time that BTX-A may induce direct analgesic effects in patients with chronic neuropathic pain independent of its effects on muscle tone and suggest novel indications for BTX-A in analgesia.