Focal dystonia: the role of botulinum toxin.

Botulinum toxin (BTX) has been found to be effective in a wide range of focal dystonias. Debate surrounds the selection of injection sites. In general, localization is satisfactory by clinical examination, but poor response, requiring localization of deep muscles, may necessitate use of electromyography for localization. Delineation of optimal doses of BTX is a work in progress; as studies have tended to show efficacy at lower doses than used in the past, the trend is to use lower doses. This is important, because development of antibodies to BTX, the main reason for secondary resistance to this treatment, is more frequent with larger doses and shorter inter-injection intervals. Although the mechanism of denervation of the neuromuscular injunction by BTX is relatively well understood, secondary changes at the level of the basal ganglia, thalamus, and cortex, and their role in response to BTX, need further exploration.