The silent period induced by transcranial magnetic stimulation in muscles supplied by cranial nerves: normal data and changes in patients.

Abstract

The silent period induced by transcranial magnetic stimulation of the sensorimotor cortex (Magstim 200, figure of eight coil, loop diameter 7 cm) in active muscles supplied by cranial nerves (mentalis, sternocleidomastoid, and genioglossus) was studied in 14 control subjects and nine patients with localised lesions of the sensorimotor cortex. In the patients, measurements of the silent period were also made in the first dorsal interosseus and tibialis anterior muscles. In the controls, there was a silent period in contralateral as well as ipsilateral cranial muscle and the duration of the silent period increased with increasing stimulus intensities. The mean duration of the silent period was around 140 ms in contralateral mentalis muscle and around 90 ms in contralateral sternocleidomastoid muscle at 1.2 x threshold stimulation strengths. Whereas the duration of the silent period in ipsilateral mentalis muscle was shorter than on the contralateral side it was similar on both sides in sternocleidomastoid muscle. In patients with focal lesions of the face associated primary motor cortex and corresponding central facial paresis, the silent period in mentalis muscle was shortened whereas it was unchanged or prolonged in limb muscles (first dorsal interosseus, tibialis anterior) with stimulation over the affected hemisphere. By contrast, in a patient with a lesion within the parietal cortex, the silent period in mentalis muscle was prolonged with stimulation of the affected side.