Botulinum toxin type A (BTX-A) is a potent
neurotoxin that blocks
acetylcholine release from presynaptic nerve terminals by cleaving the SNARE complex. BTX-A has been reported to have
analgesic effects independent of its action on muscle tone. The most robust results have been observed in patients with
neuropathic pain.
Neuropathic pain due to peripheral lesions has been the most widely studied. BTX-A has shown its efficacy on
pain and
allodynia in various animal models of inflammatory
neuropathic pain. The only randomized, double-blind, placebo-controlled trial in patients with focal painful neuropathies due to nerve
trauma or
postherpetic neuralgia demonstrated significant effects on average
pain intensity from 2 weeks after the
injections to 14 weeks. Most patients reported
pain during the
injections, but there were no further local or systemic side effects. The efficacy of BTX-A in painful
peripheral neuropathies has been more recently studied. Results were positive in the only study in an animal model of
peripheral neuropathy. One study in patients with diabetic painful
peripheral neuropathy demonstrated a significant decrease in Visual Analog Scale. In conclusion, one session of multiple
intradermal injection of BTX-A produces long-lasting
analgesic effects in patients with focal painful neuropathies and diabetic
neuropathic pain, and is particularly well tolerated. The findings are consistent with a reduction of peripheral sensitisation, the place of a possible central effect remaining to define. Further studies are needed to assess some important issues, i.e. BTX-A efficacy in patients with
small fiber neuropathies and the relevance of early and repeated
injections. Future studies could also provide valuable insights into pathophysiology of
neuropathic pain.