The main mechanism of action of botulinum toxins that's been well established is the inhibition of the release of acetylcholine from the presynaptic terminals. This action caused in the affected organ, typically in most cases muscle, is the weakening of contraction of the muscle and that could be helpful in some neurological conditions. A second mechanism of action that has been proven to some degree particularly in animal and in vitro studies has been the inhibition of release of other neurotransmitters from the synaptic vesicles that go into the synaptic cleft, specifically calcitonin, gene-related peptide and glutamates. When you inhibit the release of some of these neurotransmitters that are clearly linked to the development and the maintenance of a chronic pain state or neurogenic chronic inflammation you can potentially decrease the effects of this chronic neurogenic inflammation. Pain conditions that are appropriate for botulinum toxin therapy would include chronic migraine, myofascial pain, other types of headaches of greater occipital or occipital neuralgia, Reynaud's syndrome or Reynaud's phenomenon, low back pain, performance syndrome, thoracic outlet syndrome. So, actually the specific indications are becoming more numerous as the evidence accumulates. Most of those are of course non-FDA-approved, so they are off-label indications. But there's actually pretty good data out there that is showing their efficacy. Right now, the FDA has an approval for Botox specifically for chronic migraine.
A big part of the use of these agents, though, is their cost. If you could define what groups of patients have better response to these toxins, it would make it a more cost effective treatment. At present, most practitioners tend to use it as a last resort when other modalities have failed. One thing that I want to emphasize is that there are multiple different agents. They are dosed differently. They have different indications - approved and not approved indications - and that the units are certainly not interchangeable between one and the other, so you can't say 'we tried this one, and then we're going to swap to the other one.' They are completely different agents even though they are the same base compound. And despite the word 'toxin' they are extremely safe in practice, with a favorable side effect profile, and can provide good results in some of the indications.