Neurogenic Dysphagia

Normal Swallowing

Swallowing is a surprisingly complicated and intricate phenomenon. It comprises a mixture of voluntary and reflex, or automatic, actions engineered and carried out by some of the more than 30 pairs of muscles within the oropharyngeal, laryngeal, and esophageal regions along with five cranial nerves and two cervical nerve roots that, in turn, receive directions from centers within the central nervous system ( ). Reflex swallowing is coordinated and carried out at a brainstem level, where centers act directly on information received from sensory structures within the oropharynx and esophagus. A differentiation can be made between voluntary swallowing, which occurs when a person desires to eat or drink during the awake and aware state, and spontaneous swallowing in response to accumulated saliva in the mouth ( ). Volitional swallowing is, not surprisingly, accompanied by additional activity that originates not only in motor and sensory cortices but also in other cerebral structures ( ).

The process of swallowing can conveniently be broken down into three or four distinct stages or phases: oral (which some subdivide into oral preparatory and oral propulsive), pharyngeal, and esophageal. These components have also been distilled into what have been designated the horizontal and vertical subsystems , reflecting the direction of bolus flow in each component (when the individual is upright while swallowing). The horizontal subsystem comprises the oral phase of swallowing and is largely volitional in character; the vertical subsystem comprises the pharyngeal and esophageal phases , which are primarily under reflex control.

In the oral preparatory phase, food is taken into the mouth and, if needed, chewed. Saliva is secreted to provide both lubrication and the initial “dose” of digestive enzymes; the food bolus is then formed and shaped by the tongue. In the oral propulsive phase, the tongue propels the bolus backward to the pharyngeal inlet where, in a piston-like action, it delivers the bolus into the pharynx. This initiates the pharyngeal phase, in which a cascade of intricate, extremely rapid, and exquisitely coordinated movements sealoff the nasal passages and protects the trachea while the cricopharyngeal muscle, which functions as the primary component of the upper esophageal sphincter (UES), relaxes and allows the bolus to enter the esophagus. As an example of the intricacy of movements during this phase of swallowing, the UES, prompted in part by traction produced by elevation of the larynx, actually relaxes just prior to arrival of the food bolus, creating suction that assists in guiding the bolus into the esophagus. The bolus then enters the esophagus, where peristaltic contractions usher it distally and, on relaxation of the lower esophageal sphincter, into the stomach. Swallowing is synchronized with respiration, such that expiration rather than inspiration immediately follows a swallow, thus reducing the risk of aspiration—another example of the finely tuned coordination involved in the swallowing mechanism ( ).

Neurophysiology of Swallowing

Central control of swallowing has traditionally been ascribed to brainstem structures, with cortical supervision and modulation emanating from the inferior precentral gyrus. However, positron emission tomography (PET), transcranial magnetic stimulation (TMS), and functional magnetic resonance imaging (fMRI) studies of volitional swallowing reveal a considerably more complex picture in which a broad network of brain regions is active in the control and execution of swallowing.

It is perhaps not surprising that in PET studies, the strongest activation of volitional swallowing occurs in the lateral motor cortex within the inferior precentral gyrus, wherein lie the cortical representations of tongue and face. There is disagreement among investigators, however, in that some have noted bilaterally symmetrical activation of the lateral motor cortex ( ) whereas others have noted a distinctly asymmetrical activation, at least in some of the subjects tested ( ).

Additional and perhaps somewhat surprising brain areas also are activated during volitional swallowing ( ). The supplementary motor area may play a role in preparing for volitional swallowing, and the anterior cingulate cortex may be involved with monitoring autonomic and vegetative functions. Another area of activation during volitional swallowing is the anterior insula, particularly on the right. It has been suggested that this activation may provide the substrate that allows gustatory and other intraoral sensations to modulate swallowing. Lesions in the insula may also increase the swallowing threshold and delay the pharyngeal phase of swallowing ( ). PET studies also consistently demonstrate distinctly asymmetrical left-sided activation of the cerebellum during swallowing. This activation may reflect cerebellar input concerning the coordination, timing, and sequencing of swallowing. Activation of putamen has also been noted during volitional swallowing, but it has not been possible to differentiate this activation from that seen with tongue movement alone.

Within the brainstem, swallowing appears to be regulated by central pattern generators that contain the programs directing the sequential movements of the various muscles involved ( ). The dorsomedial pattern generator resides in the medial reticular formation of the rostral medulla and the reticulum adjacent to the nucleus tractus solitarius and is involved with the initiation and organization of the swallowing sequence ( ). A second central pattern generator, the ventrolateral pattern generator , lies near the nucleus ambiguus and its surrounding reticular formation ( ). It serves primarily as a connecting pathway to motor nuclei such as the nucleus ambiguus and the dorsal motor nucleus of the vagus, which directly control motor output to the pharyngeal musculature and proximal esophagus. The enteric nervous system also plays a role in controlling esophageal function, apparently involving both motor and sensory components ( ).

It has become evident that a large network of structures participates in the act of swallowing, especially volitional swallowing. The presence of this network presumably accounts for the broad array of neurological disease processes that can produce dysphagia as a part of the clinical picture.

Mechanical Dysphagia

Structural abnormalities—both within and adjacent to the mouth, pharynx, and esophagus—can interfere with swallowing on a strictly mechanical basis despite fully intact and functioning nervous and musculoskeletal systems ( Box 15.1 ). Within the mouth, macroglossia, temporomandibular joint dislocation, certain congenital anomalies, and intraoral tumors can impede effective swallowing and produce mechanical dysphagia. Pharyngeal function can be compromised by processes such as retropharyngeal tumor or abscess, cervical anterior osteophyte formation, Zenker diverticulum, or thyroid gland enlargement. An even broader array of structural lesions can interfere with esophageal function, including malignant or benign esophageal tumors, metastatic carcinoma, esophageal stricture from numerous causes, vascular abnormalities such as aortic aneurysm or aberrant origin of the subclavian artery, or even primary gastric abnormalities such as hiatal hernia or complications from gastric banding procedures. Gastroesophageal reflux can also produce dysphagia. However, individuals with these problems are more likely to be seen by the gastroenterologist than the neurologist.

Neuromuscular Dysphagia

A variety of neuromuscular disease processes of diverse etiology can involve the oropharyngeal and esophageal musculature and produce dysphagia as part of their broader neuromuscular clinical picture ( Box 15.2 ). Certain muscular dystrophies, inflammatory myopathies, and mitochondrial myopathies can all display dysphagia, as can disease processes affecting the myoneural junction, such as myasthenia gravis (MG).