Suprahyoid muscles motor evoked potentials in response to transcranial magnetic stimulation

The objective of this study was to reveal whether corticobulbar projection to the suprahyoid muscles (SHM) is contralateral or bilateral.

Thirty-nine healthy subjects between 27 and 77 years of age participated. All subjects underwent transcranial magnetic stimulation (TMS) in both cerebral hemispheres using surface EMG recording in bilateral SHM. One subject underwent TMS in cerebral hemisphere at the same time using needle and surface EMG recording in the contralateral and ipsilateral SHM. Eight subjects underwent TMS in both cerebral hemispheres using surface EMG recording in bilateral SHM, within 6 months of the first day.

We obtained larger response in contralateral SHM than in ipsilateral SHM in the surface EMG recording. However, in the needle EMG recording, only contralateral SHM responses were evoked. TMS of either hemisphere evoked contralateral SHM motor-evoked potentials (MEPs) in all subjects [SHM latency: (left) 8.5 ± 0.9 ms, (right) 8.6 ± 1.1 ms]. There was no significant difference in latency between the first and second tests. In a case of right medullary infarction with left cortical stimulation, MEPs of right SMH were absent.

Corticobulbar projections to the SHM appear to be dominated by contralateral projections in healthy adults.