Lasting modulation of human cortical swallowing motor pathways following thermal tongue stimulation.

Purpose: Thermal tactile oropharyngeal stimulation has been clinically used to facilitate swallowing initiation in dysphagic patients. We previously demonstrated that thermal stimulation applied to the oral cavity provokes an immediate excitability in pharyngeal motor cortex. The aim of the current study was to investigate whether thermal stimulation can produce longer lasting effects on the cortico-pharyngeal neural pathway. Material and Methods: Healthy volunteers (n=8/12) underwent baseline pharyngeal motor evoked potential (PMEP) measurements evoked by transcranial magnetic stimulation. In the first experiment, subjects received thermal stimulation alternating 30 s of 15 degrees Celsius (°C) and 36°C applied to the tongue surface for either 10 minutes, 5 minutes, or sham. In the second experiment, one of three intermittent thermal stimulus patterns was delivered; cold (alternating 30 s of 15 °C and 36°C), warm (continuous 36 °C) or hot (alternating 30 s of 45 °C and 36°C) for 10 minutes. In both experiments, PMEPs were re-measured every 15 minutes up to 60 minutes following thermal stimulation. Results: Repeated measures ANOVA for each stimulus time in the first experiment showed a significant increased change in PMEP amplitude at 30 minutes following only 10-minutes stimulation compared to sham (p < 0.05). In the second experiment, we found that cold stimulation was more effective than the other stimulation (p < 0.05) at increasing PMEP amplitudes. Conclusions: Ten minutes cold stimulation on the tongue can induce a delayed (30 min) increase in pharyngeal cortical excitability, providing a clinically useful therapeutic window for its application in dysphagic patients.