A swallow moves a bolus from the mouth to the stomach through a coordinated sequence of voluntary and reflex steps, and dysphagia appears when one of those steps fails.
The swallowing mechanism
Food is chewed and mixed with saliva in the mouth, and the voluntary part of the swallow ends when the tongue contracts and pushes the bolus into the hypopharynx. That contact triggers a reflex that does four things at once: it drives a peristaltic contraction through the oropharynx that clears it of food, it moves the larynx forward and upward so food does not enter the airway, it opens the upper esophageal sphincter (UES), and it starts the primary peristaltic wave of the esophagus.
After the bolus enters the esophagus, contractions carry it into the stomach. Primary peristalsis is the automatic wave triggered by the swallowing center and running the whole length of the esophagus. If the bolus is not fully cleared, local distension starts secondary peristalsis, which begins at the point of distension and travels distally. Tertiary contractions are disordered, non-peristaltic movements rather than a propulsive wave.
The muscles of the oral cavity, pharynx, UES and cervical esophagus are striated, and each group is supplied by specific cranial nerves: the tongue by the hypoglossal (12th) nerve, the oral cavity by the trigeminal (5th) and facial (7th) nerves, and the pharynx by the glossopharyngeal (9th) and vagus (10th) nerves.
The upper esophageal sphincter
The UES is formed by three muscles: the inferior pharyngeal constrictor, the cricopharyngeus and the proximal muscles of the cervical esophagus. It is innervated by the vagus, and its opening depends on the trigeminal, facial and hypoglossal nerves (5th, 7th and 12th), which pull the hyoid and larynx forward as the sphincter’s tone relaxes. The cervical esophagus is striated and under vagal control, whereas the distal esophagus and the lower esophageal sphincter (LES) are smooth muscle, controlled mainly by the myenteric plexuses.
At rest the LES stays closed through a myogenic contraction of its smooth muscle, and the right crus of the diaphragm supplements that closure as an external sphincter, helping to keep the LES shut during coughing.
Pathophysiology of dysphagia
Normal transport of the bolus of food depends on four things: the size and consistency of the bolus, the caliber of the lumen, the efficiency of peristaltic movements, and the timely inhibition of the sphincters. Dysphagia results when any of these fails. It can be described by where it happens — oral, pharyngeal or esophageal — or by what has gone wrong. Dysphagia from a large bolus or a narrow lumen is called structural dysphagia, whereas dysphagia from abnormal peristalsis, or from impaired inhibition of the LES or the UES, is called propulsive, or motor, dysphagia.