Botulinum toxin treatment in the facial muscles of humans: evidence of an action in untreated near muscles by peripheral local diffusion.

In a population of subjects with blepharospasm and facial hemispasm treated for the first time with botulinum toxin type A (BT) in the orbicularis oculi muscle, we performed an electrophysiologic study (compound muscle action potential and motor evoked potential) to assess whether BT effect could be detected in near untreated muscles (orbicularis oris and masseter). There was a significant BT action in nearly untreated muscles with different peripheral innervation that can be explained by local diffusion of the drug.