Abnormal postexcitatory and interhemispheric motor cortex inhibition in writer's cramp

Focal transcranial magnetic stimulation (TMS) of the motor cortex was used to study excitatory and inhibitory stimulation effects in 25 patients with writer's cramp and 25 healthy volunteers. We investigated excitatory and in hibitory corticospinally mediated motor effects in muscles contralateral to the stimulation side as well as interhemispheric inhibition of tonic motor activity in muscles ipsilateral to stimulation. Motor evoked potentials (M EPs) were recorded from both first dorsal interosseus muscles. Motor thresh olds at rest and amplitudes and latencies of MEPs obtained during maximal c ontraction were always bilaterally normal. The duration of postexcitatory i nhibition was significantly shortened (168+/-55 vs. 198+/-39 ms in normal s ubjects, P=0.001) and the duration of interhemispheric inhibition prolonged (30.3+/-6.6 vs. 25+/-3.9 ms in normal subjects, P < 0.001). Both observati ons would be compatible with a decreased inhibition of corticospinal and tr anscallosal outputs of the motor cortex. The results were not influenced by fatigue effects. Abnormal motor cortex inhibition seems to be a generalize d phenomenon in writer's cramp since it was detected in both hemispheres an d during a simple isometric motor task which did not evoke dystonic symptom s.