The effects of unilateral and bilateral cerebellar rTMS on human pharyngeal motor cortical activity and swallowing behavior

The cerebellum is recognised to bilaterally modulate sensorimotor function and has recently been shown to play a role in swallowing. Unilateral cerebellar repetitive trans-cranial magnetic stimulation (rTMS) excites corticobulbar motor pathways to the pharynx but the effects of bilateral versus unilateral cerebellar rTMS on these pathways are unknown. In this three-part cross-over study, healthy participants (n = 13) were randomly allocated to receive unilateral or bilateral 10 Hz cerebellar rTMS. Participants were intubated with pharyngeal electromyography and/or manometry catheters for motor evoked potentials (MEPs) and pressure recordings. In part 1 of the study, single pulse TMS was used to measure baseline motor cortical pharyngeal MEP (PMEP) and hemispheric cerebellar MEP (CMEP) amplitudes, before cerebellar rTMS was administered. Repeat measures of PMEP amplitude were performed at 15-min intervals for an hour post unilateral and bilateral rTMS. Thereafter, in two further studies, a cortical 'virtual lesion' (V/L) was applied prior to cerebellar rTMS with pre and post PMEPs (part 2) and measurements of swallowing accuracy (part 3) using a behavioural task. Compared to baseline, unilateral and bilateral cerebellar rTMS provoked increases in pharyngeal cortical excitation (P = 0.028, 0.0005, respectively). Bilateral rTMS was significantly more effective than unilateral in causing cortical excitation (P = 0.0005) and in reversing the suppressive neurological (P = 0.0005) and behavioural (P = 0.0005) effects of a cortical V/L. Our findings suggest bilateral cerebellar rTMS has greater facilitatory effects on corticobulbar motor pathways to the pharynx than unilateral stimulation with the potential to be a more effective clinical therapy if its effects are reproduced in populations with neurogenic dysphagia.