Changes in motor evoked potentials in the suprahyoid muscles by repetitive transcranial magnetic stimulation

Motor cortical excitability is generally increased by highly frequent repetitive transcranial magnetic stimulation (rTMS) of the motor cortex. However, there are few studies of the effect of high-frequency rTMS on serial changes of motor evoked potentials (MEPs) recorded from swallowing-related muscles. The purpose of this study was to investigate whether facilitation of the suprahyoid muscles is induced by rTMS of the motor cortex. The subjects were 15 healthy adult volunteers, all of whom were right-handed. A total of 150 pulses of rTMS at 5 Hz were applied to the left motor cortex for 30 seconds at the intensity of the resting motor threshold, followed by rest for 30 seconds. This set was repeated 5 times, providing 750 pulses in total. MEPs in the anterior belly of the digastric muscles on both sides were recorded before rTMS, immediately after rTMS, and then every 5 minutes for 60 minutes after rTMS. The mean MEP amplitude for the left muscles was significantly higher than that for the right muscles in all subjects. The MEP amplitude significantly increased immediately after rTMS on both sides. The excitation was 20 to 40% higher than the baseline MEPs and persisted until 35 minutes after rTMS. These results suggest that rTMS intervention induces long-term potentiation in the motor pathway that is conducted to the suprahyoid muscles. This kind of rTMS-evoked plasticity may have an application in treatment of patients with pseudobulbar palsy. (Accepted on October 13, 2010)