High-frequency focal repetitive cerebellar stimulation induces prolonged increases in human pharyngeal motor cortex excitability

Brain neurostimulation has been shown to modulate cortical swallowing neurophysiology in post-stroke dysphagia with therapeutic effects which are critically dependent on the stimulation parameters. Cerebellar neurostimulation is, however, a novel, unexplored approach to modulation of swallowing pathways as a prelude to therapy for dysphagia. Here, we randomised healthy human subjects (n=17) to receive one of five cerebellar repetitive TMS (rTMS) interventions (Sham, 1Hz, 5Hz, 10Hz and 20Hz) on separate visits to our laboratory. Additionally, a subset of subjects randomly received each of three different durations (50, 250, 500 pulses) of optimal frequency versus sham cerebellar rTMS. Prior to interventions subjects underwent MRI-guided single-pulse transcranial magnetic stimulation (TMS) to co-localise pharyngeal and thenar representation in the cortex and cerebellum (midline and hemispheric) before acquisition of baseline motor evoked potential (MEP) recordings from each site as a measure of excitability. Post-interventional MEPs were recorded for an hour and compared to sham using repeated measures ANOVA. Only 10Hz cerebellar rTMS increased cortico-pharyngeal MEP amplitudes (mean bilateral increase 52%, P=0.007) with effects lasting 30min post-intervention with an optimal train length of 250 pulses (P=0.019). These optimised parameters of cerebellar rTMS can produce sustained increases in corticobulbar excitability and may have clinical translation in future studies of neurogenic dysphagia.