Effects of botulinum toxin on motor system excitability in patients with writer's cramp

Objective: To investigate botulinum toxin ( BTX) effects on central and peripheral motor excitability in writer's cramp. Methods: Using transcranial magnetic stimulation over the motor cortex and brainstem and peripheral electrical stimulation, the authors investigated measures of motor cortical and peripheral motor excitability on the affected and unaffected side before and 2 to 4 weeks after BTX A injection into the affected muscles in six patients with writer's cramp. The following motor excitability measures were studied: resting and active motor threshold, cortical silent period, intracortical inhibition and facilitation using the double- pulse technique, recruitment curves, motor evoked potentials following magnetic brainstem stimulation, and maximum M- response amplitude following supramaximal peripheral nerve stimulation. Results: BTX injection improved function and reduced the M response. No other measures of motor system excitability showed significant changes following BTX injection, and there was no difference in these measures between sides. Conclusions: BTX A effects on motor system excitability seem to be based mainly on its peripheral mechanisms of action. There was no difference in the motor system excitability between the clinically affected and unaffected sides in this group of patients.