A defect in the initiation of swallowing is usually attributable to weakness of the tongue and may be a feature of myasthenia gravis, motor neuron disease or rarely, inflammatory disease of the muscle; it may be caused by palsies of the 12th cranial nerve (metastases at the base of the skull or meningoradiculitis, carotid dissection), or to a number of other causes. In all these cases there is usually an associated dysarthria with difficulty pronouncing lingual sounds. The second type of dysphagia, associated with nasal regurgitation of liquids, indicates a failure of velopalatine closure and is characteristic of myasthenia gravis, tenth nerve palsy of any cause, or incoordination of swallowing because of bulbar or pseudobulbar palsy. A nasal pattern of speech with air escaping from the nose is a usual accompaniment.
Viewed from a physiologic perspective, the causes of aspiration fall into four main categories: (1) weakness of the pharyngeal musculature because of lesions of the vagus on one or both sides; (2) myopathy (polymyositis, myotonic and oculopharyngeal dystrophies) or neuromuscular disease (amyotrophic lateral sclerosis and myasthenia gravis); (3) a medullary lesion that affects the NTS or the cranial motor nuclei (lateral medullary infarction is the prototype)—but syringomyelia-syringobulbia and, rarely, multiple sclerosis, polio, and brainstem tumors may have the same effects; or (4) less-well-defined mechanisms of slowed or discoordinated swallowing arising from corticospinal disease (pseudobulbar palsy, hemispheral stroke) or from diseases of the basal ganglia (mainly Parkinson disease) that alter the timing of breathing and swallowing and permit the airway to remain open as food passes through the posterior pharynx. In the latter cases, a decreased frequency of swallowing also causes saliva to pool in the mouth (leading to drooling) and adds to the risk of aspiration.
Because of its frequency, the neurologist will encounter stroke in a cerebral hemisphere as a cause of discoordinated swallowing. The problem is most evident during the first few days after a hemispheral stroke on either side of the brain (Meadows). These effects last days or weeks and render the patient subject to pneumonia and fever. In the clinical and fluoroscopic study by Mann and colleagues, half of patients still had manifest abnormalities of swallowing 6 months after their strokes. For this reason, it has become customary for patients to have swallowing evaluations in the days after acute stroke. Some insight into the nature of swallowing dysfunction after stroke is provided by Hamdy and colleagues, who correlated the presence of dysphagia with a lesser degree of motor representation of pharyngeal muscles in the unaffected hemisphere, as assessed by magnetic stimulation of the cortex.
Pain on swallowing occurs under a different set of circumstances, the one of most neurologic interest being glossopharyngeal neuralgia as discussed in Chaps. 7 and 44.
Videofluoroscopy has become a useful tool in determining the presence of aspiration during swallowing and in differentiating the several types of dysphagia. The movement of the bolus by the tongue, the timing of reflex swallowing, and the closure of the pharyngeal and palatal openings are judged directly by observation of a bolus of food mixed with barium or of liquid barium alone. However, authorities in the field, such as Wiles, whose reviews are recommended (see also Hughes and Wiles), warn that unqualified dependence on videofluoroscopy is unwise. They remark that observation of the patient swallowing water and repeated observation of the patient while eating can be equally informative. Having the patient swallow water is a particularly effective test of laryngeal closure; the presence of coughing, wet hoarseness or breathlessness, and the need to swallow small volumes slowly are indicative of a high risk of aspiration.