ALTERATIONS OF MOTOR CORTICAL INHIBITION IN PATIENTS WITH DYSTONIA

Cortical inhibitory mechanisms were investigated with the technique of paired transcranial magnetic stimulation in 10 patients with dystonia of the right arm. six patients had focal, task-specific dystonia (wri ter's cramp) and three had segmental and one had generalized dystonia. Paired stimuli were delivered in a conditioning-test design during sl ight voluntary activation of the tar-get muscle, with subthreshold con ditioning stimuli at short intervals (3-20 ms) and suprathreshold cond itioning stimuli al long intervals (100-250 ms). The amount of inhibit ion at short interstimulus intervals did not differ significantly betw een patients and normal subjects. With long interstimulus intervals, p atients showed more inhibition of the test response, which was signifi cant at the 150-ms interval. The cortical silent period following a si ngle suprathreshold magnetic stimulus was slightly shorter in patients . No significant difference was detected between the affected side and the unaffected side in patients with unilateral task-specific dystoni a, neither in the duration of the silent period nor in the response to paired magnetic stimuli. These results indicate that the different ty pes of motor cortical inhibition are produced by different inhibitory circuits. We propose that the alterations observed in patients with dy stonia are the result of impaired feedback from the basal ganglia to m otor cortical areas, with the ultimate effect of a flattening of the e xcitability curve of the cortical motoneuron pool during voluntary mus cle activation.