Swallowing is a complex process involving the precise contractions of numerous muscles of the head and neck, which act to process and shepherd ingested material from the oral cavity to its eventual destination, the stomach. Over the past five decades, information from animal and human studies has laid bare the complex network of neurones in the brainstem, cortex and cerebellum that are responsible for orchestrating each normal swallow. Amidst this complexity, problems can and often do occur that result in dysphagia, defined as impaired or disordered swallowing. Dysphagia is common, arising from multiple varied disease processes that can affect any of the neuromuscular structures involved in swallowing. Post‐stroke dysphagia (PSD) remains the most prevalent and most commonly studied form of dysphagia and, as such, provides an important disease model to assess dysphagia physiology and pathophysiology. In this review, we explore the complex neuroanatomical processes that occur during normal swallowing and PSD. This includes how strokes cause dysphagia, the mechanisms through which natural neuroplastic recovery occurs, current treatments for patients with persistent dysphagia and emerging neuromodulatory treatments. image

Key points: Despite evidence that the human cerebellum has an important role in swallowing neurophysiology, the effects of cerebellar stimulation on swallowing in the disrupted brain have not been explored. In this study, for the first time, the application of cerebellar neurostimulation is characterized in a human model of disrupted swallowing (using a cortical virtual lesion). It is demonstrated that cerebellar stimulation can reverse the suppressed activity in the cortical swallowing system and restore swallowing function in a challenging behavioural task, suggesting the findings may have important therapeutic implications. Repetitive transcranial magnetic stimulation (rTMS) can alter neuronal activity within the brain with therapeutic potential. Low frequency stimulation to the ‘dominant’ cortical swallowing projection induces a ‘virtual‐lesion’ transiently suppressing cortical excitability and disrupting swallowing behaviour. Here, we compared the ability of ipsi‐lesional, contra‐lesional and sham cerebellar rTMS to reverse the effects of a ‘virtual‐lesion’ in health. Two groups of healthy participants (n = 15/group) were intubated with pharyngeal catheters. Baseline pharyngeal motor evoked potentials (PMEPs) and swallowing performance (reaction task) were measured. Participants received 10 min of 1 Hz rTMS to the pharyngeal motor cortex which elicited the largest PMEPs to suppress cortical activity and disrupt swallowing behaviour. Over six visits, participants were randomized to receive 250 pulses of 10 Hz cerebellar rTMS to the ipsi‐lesional side, contra‐lesional side or sham while assessing PMEP amplitude or swallowing performance for an hour afterwards. Compared to sham, active cerebellar rTMS, whether administered ipsi‐lesionally (P = 0.011) or contra‐lesionally (P = 0.005), reversed the inhibitory effects of the cortical ‘virtual‐lesion’ on PMEPs and swallowing accuracy (ipsi‐lesional, P < 0.001, contra‐lesional, P < 0.001). Cerebellar rTMS was able to reverse the disruptive effects of a ‘virtual lesion’. These findings provide evidence for developing cerebellar rTMS into a treatment for post‐stroke dysphagia.

Swallowing is a complex activity requiring a sophisticated system of neurological control from neurones within the brainstem, cerebral cortices and cerebellum. The cerebellum is a critical part of the brain responsible for the modulation of movements. It receives input from motor cortical and sensory areas and fine tunes these inputs to produce coordinated motor outputs. With respect to swallowing, numerous functional imaging studies have demonstrated increased activity in the cerebellum during the task of swallowing and damage to the cerebellum following differing pathological processes is associated with dysphagia. Single pulses of transcranial magnetic stimulation (TMS) have been applied to the cerebellum and have been shown to evoke motor responses in the pharynx. Moreover, repetitive TMS (rTMS) over the cerebellum can modulate cerebral motor (pharyngeal) cortical activity. Neurostimulation has allowed a better understanding of the connections that exist between the cerebellum and cerebral swallowing motor areas in health and provides a potential treatment for neurogenic dysphagia in illness. In this review we will examine what is currently known about the role of the cerebellum in the control of swallowing, explore new findings from neurostimulatory and imaging studies and provide an overview of the future clinical applications of cerebellar stimulation for treating dysphagia.

We sought to compare the effects of 10 Hz cerebellar vermis (vs. unilateral hemispheric and sham) repetitive transcranial magnetic stimulation (rTMS) on cortical neuroelectrical activity and thereafter 10 Hz cerebellar vermis (vs. sham) rTMS on swallowing behaviour. Healthy participants (n = 25) were randomly allocated to receive vermis, unilateral hemisphere or sham 10 Hz cerebellar rTMS. Recordings were made using pharyngeal electromyography and manometry catheters, obtaining motor-evoked potentials (MEPs) and pressure recordings. The amplitudes of MEPs elicited using single-pulse TMS delivered to the pharyngeal areas of the motor cortex bilaterally were measured pre- and post-cerebellar stimulation. As in previous studies, abductor policis brevis (APB) MEPs were measured to assess post-rTMS modulation specificity. Swallowing was assessed using a swallowing accuracy task. Measurements were made at baseline and 15-min intervals for an hour post-intervention. Measurements involved TMS being used to elicit 10 MEPs bilaterally over the pharyngeal areas of the motor cortex, over the APB cortical representation adjacent to the pharyngeal area with the lowest resting motor threshold and 5 MEPs bilaterally over pharyngeal areas of the cerebellar hemispheres. Swallowing accuracy was assessed by giving participants 10 attempts to swallow and hit a digital target. Cerebellar vermis rTMS caused significant suppression of cortical pharyngeal MEP amplitudes compared with unilateral rTMS and sham (P = 0.0005, 0.002). APB and cerebellar MEP amplitudes were unaffected as were pharyngeal and APB MEP latencies. Following cerebellar vermis rTMS there was a significant reduction in swallowing accuracy compared with sham (P = 0.001). Our findings demonstrate cerebellar vermis rTMS exerts a suppressive effect on pharyngeal motor cortical activity and swallowing behaviour.