Transcranial magnetic stimulation-evoked inhibition of voluntary muscle activity (silent period) is impaired in patients with ischemic hemispheric lesion

Transcranial magnetic stimulation of the motor cortex is well established as a valuable method for noninvasive examination of the central motor system. In addition to exciting corticospinal cells and evoking a direct motor response, the magnetic stimulus delivered during voluntary activity produces a prolonged postexcitatory inhibition (silent period) of activity. We investigated silent period changes in patients with ischemic stroke of different clinical degrees. Standardized transcranial magnetic stimulation during sustained muscle contraction was performed at the vertex. Electromyographic activity was recorded via surface electrodes placed over the abductor digiti minimi muscle on both sides. We examined 50 patients with stroke (divided into three subgroups according to the degree of impairment) and 50 healthy control subjects. In the control group we found no statistically significant interside difference in the duration of the silent period, whereas a marked interindividual variation was found. In patients with prior minor stroke who showed no residual motor disturbance, we found a significant prolongation of the postexcitatory inhibition recorded from the affected side compared with the healthy side. This interside discrepancy was even more pronounced in patients with minor hemiparesis and patients with moderate hemiparesis. Our findings suggest that the measurement of the silent period elicited by transcranial magnetic stimulation is a useful and sensitive neurophysiological parameter in the management of stroke. Particularly in the subgroup of patients with no residual clinical signs of central motor impairment, it is capable of detecting subclinical motor function disturbances.