Abnormal motor evoked responses to transcranial magnetic stimulation in focal dystonia.

We evaluated motor responses evoked after magnetic cortical stimulation in dystonia, emphasizing the relationship between resting and facilitation state. We studied 15 normal controls (mean age, 37.9 years; range, 23 to 63) and 13 dystonic patients (mean age, 43.4 years; range, 20 to 56). Surface electrodes were placed over the right first dorsal interosseous muscle to measure motor evoked potentials and inhibitory silent periods obtained with magnetic stimulation. The amplitude ratio of motor evoked potentials measured during facilitation and at rest with low-intensity magnetic stimulation was significantly higher in dystonic patients (15.09) when compared with normal subjects (5.43; p = 0.04). The ratio of duration of silent periods evoked with 120% motor threshold (MT) and MT + 25% magnetic stimulus intensity was significantly higher in dystonic patients (78.4%) when compared with normal subjects (69.7%; p = 0.04). We conclude that with low-intensity magnetic stimulation the relationship between amplitudes of motor potentials evoked at rest and during facilitation, as well as the responses of pathways that mediate silent periods, are disturbed in focal dystonia.