Remote F-wave changes after local botulinum toxin application

Objective: Although the therapeutic effects of botulinum toxin A can be explained by its action at the neuromuscular junction, central or more proximal effects have also been discussed. Methods: Eleven patients with torticollis spasmodicus and 3 patients with writerʹs cramp were studied before and 1 and 5 weeks after the first treatment with botulinum toxin. We measured compound muscle action potentials (CMAPs), motor conduction velocities (MCVs), the shortest (SFL) and the mean F-wave latencies (MFL) and F-wave persistence (30 trials) of untreated muscles for each side (ulnar nerve-abductor digiti minimi muscle, peroneal nerve-tibialis anterior muscle). Results: CMAPs and MCVs showed no significant changes. For both nerves, however, SFL and MFL were prolonged slightly 1 week after treatment and returned to about baseline after 5 weeks (t test). The F-wave persistence was reduced 1 week after treatment for the right ulnar and both peroneal nerves (t test). Conclusions: These results are not likely due to an impairment of neuromuscular transmission. Instead, we propose a decreased excitability of α-motoneurons supplying non-treated muscles. A reduction of muscle spindle activity or changes of the recurrent inhibition are discussed as possible causes.