Cricopharyngeal sphincter muscle responses to transcranial magnetic stimulation in normal subjects and in patients with dysphagia

Abstract

Objective: Cricopharyngeal (CP) muscle of the upper oesophageal sphincter (UES) has a significant role in the pharyngo-esophageal phase of deglutition. The linkage between the CP muscle of UES and the motor cortex has not been previously studied electrophysiologically in healthy humans and in patients with neurogenic dysphagia. Methods: Needle recordings of EMG responses were carried out from the CP sphincter muscle following transcranial magnetic stimulation (TMS) over the vertex around the Cz electrode position (cortical MEP), and on the parieto-occipital skull and the occiput ipsilaterally (peripheral MEP) in 14 healthy control subjects and in 26 patients with and without neurogenic dysphagia. Needle recordings obtained from the cricothyroid muscle of the larynx were also evaluated in six healthy subjects. Results: The cortical motor latency of CP sphincter muscle was 10.7±0.5 ms with an amplitude of 0.8±0.2 mV in healthy subjects. Both the latency and amplitude of CP-MEP were facilitated during swallowing. The peripheral MEP of the CP muscle was very stable in all normal subjects (5.1±0.3 ms; 1.3±0.3 mV) and swallowing did not influence these parameters. The cortically elicited CP-MEP was significantly longer than the cortical MEPs obtained from the cricothyroid muscle of the larynx. In 10 dysphagic patients with corticobulbar tract involvement (6 ALS and 4 pseudobulbar palsy) and with pathologic and hyperreflexic EMG of the CP-sphincter muscle, the cortical MEP of CP muscle of the upper esophageal sphincter could not be elicited, although the peripheral CP-MEPs were obtained. TMS never produced a swallowing movement in neither healthy subjects nor patients. Conclusion: The CP muscle of the upper esophageal sphincter can produce MEPs by cortical TMS and by stimulation at the root/nerve levels of vagus nerve. The MEP latency values and central motor delay suggest that there is an oligosynaptic corticobulbar pathway to the motoneurons of CP muscles. When the pathway is affected by a pathology (i.e. ALS or pseudobulbar palsy) the CP sphincter becomes hyperreflexic due to disinhibition and the cortical MEP of the CP muscle disappears due to degeneration of the corticobulbar pathway. These mechanisms appear to be responsible for the pathogenesis of dysphagia.