72nd Annual Meeting of the Medical Society of Toho University Professor Special Lecture Pharyngeal Sensation and Dysphagia in Aspiration Pneumonia

Although the etiology of aspiration pneumonia is multifactorial, there is a strong association between dysphagia and the development of aspiration pneumonia. However, it is still not clear why swallowing function decline with aging. Although recent research focused on dysphagia due to sarcopenia, which was defined as sarcopenic dysphagia, we should pay attention not only to the motor aspect, but also to the sensory aspect of dysphagia. Impaired pharyngeal sensitivity results in a delay of triggering swallowing reflex, which is a most serious problem in dysphagia. It is postulated that sensory receptors located in the pharyngeal mucosa or submucosa are similar to those in glabrous skin. In addition to the mechanoreceptors such as Merkel cell, Ruffini ending, and Meissner and Pacinian corpuscles, another sensory nerve subtype is thought to subserve pruriceptive pain and thermosensation in pharyngeal mucosa. In these free nerve endings, the transient receptor potential (TRP) channels were identified as cellular sensors of temperature and pain. We found that mechanorecptors and thermosensing TRP channels in the pharynx work synergistically to enhance the afferent signal to the swallowing center located at the brainstem. Aging deteriorates pharyngeal sensitivity by affecting sensory receptors, the peripheral nervous system, and microcirculation, resulting in altered detection, reduced sensory conduction, and abnormal efferent response. In addition, pharyngeal sensation is affected by central nervous system deficits. Heretofore, understanding of the molecular mechanism concerning the sense of touch in the pharynx was seriously limited. Mechanisms of touch sensation in the pharynx are, in some parts, similar to those of skin, but there must be differential mechanisms. In order to stratify the treatment of dysphagia, understanding the detailed mechanism of pharyngeal sensation is crucial. Future studies revealing the molecular mechanisms of pharyngeal sensation are warranted to prevent aspiration pneumonia.