[Botulinum toxin for the treatment of pain syndromes].

Although botulinum toxin (BoNT) is being used for therapeutic purposes for more than 20 years, the list of potential new indications continues to increase and includes various pain syndromes. The pain relief experienced by patients with dystonia and spasticity from intramuscular BoNT injections suggested that other chronic skeletal-muscles pain conditions may also benefit. BoNT inhibits the release of acetylcholine at the neuromuscular junction thereby reducing striatal muscle contractions and the proposed analgesic property was initially attributed to muscular relaxation. A specific analgesic BoNT effect is difficult to conclude from studies where pain is conditioned by other associated symptoms like dystonia, muscle contraction or spasticity. One alternative is to critically appraise clinical trials where BoNT was studied as the active intervention and pain evaluated as an outcome. From this analysis there is convincing evidence for the effectiveness of BoNT in the treatment of pain associated with cervical dystonia. For all other pain syndromes there have been relatively few, small sized, placebo-controlled studies (myofascial pain syndrome, chronic neck and low back pain, piriformis syndrome and fibromyalgia) and the results of these studies have been contradictory or non conclusive. To establish the analgesic properties of BoNT there is a need for appropriately designed, exploratory randomized controlled studies in well accepted human models of nociceptive or neuropathic pain. This does not exclude the subsequent need to conduct pragmatic trials to evaluate the effectiveness of BoNT in conditions where the improvement of pain or any associated clinical sign or symptom may be of clinical relevance.