Neuromuscular respiratory failure in critically ill patients Neurologists increasingly are being called upon to determine if there is an underlying neuromuscular cause for respiratory failure in a critically ill patient. Malnutrition, hypophosphatemia (induced by hyperalimentation), and hypokalemia always need to be kept in mind as causes of muscular weakness. Aside from the acute neuromuscular diseases listed above, Bolton and colleagues have delineated a critical illness polyneuropathy, which accounts for as many as 40 percent of cases of an inability to wean a patient from the ventilation. Most of these patients have had an episode of sepsis or have multiple organ failure (see Chap. 46). The EMG demonstrates widespread denervation with relative sparing of sensory potentials. Less often, a critical illness myopathy occurs in relation to the administration of high-dose corticosteroids (see Chap. 45). This myopathy occurs mainly in patients who are receiving neuromuscular postsynaptic blocking drugs such as pancuronium simultaneously with high-dose steroids but corticosteroids alone have been implicated.
The Neurologic Basis of Swallowing
The act of swallowing, like breathing, continues periodically through waking and sleep, largely without conscious will or awareness. Swallowing occurs at a natural frequency of about once per minute while an individual is idle; it is suppressed during concentration and emotional excitement.
The fundamental role of swallowing is to move food from the mouth to the esophagus and thereby to begin the process of digestion, but it also serves to empty the oral cavity of saliva and prevent its entry into the respiratory tract. Because the oropharynx is a shared conduit for breathing and swallowing, obligatory reflexes exist to ensure that breathing is held in abeyance during swallowing. Because of this relationship and the frequency with which dysphagia and aspiration complicate neurologic disease, the neural mechanisms that underlie swallowing are of considerable importance to the neurologist and are described here. The reader is also referred to other parts of this book for a discussion of derangements of swallowing consequent upon diseases of the lower cranial nerves (see Chap. 44), of muscle (see Chap. 45), and of the neuromuscular junction (see Chap. 46).
A highly coordinated sequence of muscle contractions is required to move a bolus of food smoothly and safely through the oropharynx. This programmed activity may be elicited voluntarily or by reflex movements that are triggered by sensory impulses from the posterior pharynx. Swallowing normally begins as the tongue, innervated by cranial nerve XII, sweeps food to the posterior oral cavity, and brings the bolus into contact with the posterior wall of the oropharynx. As the food passes the pillars of the fauces, tactile sensation, carried through nerves IX and X, reflexly triggers (1) the contraction of levator and tensor veli palatini muscles, which close the nasopharynx and prevent nasal regurgitation, followed by (2) the upward and forward movement of the arytenoid cartilages toward the epiglottis (observed as an upward displacement of the hyoid and thyroid cartilages), which closes the airway. With these movements, the epiglottis guides the food into the valleculae and into channels formed by the epiglottic folds and the pharyngeal walls. The airway is closed by sequential contractions of the arytenoid–epiglottic folds, and below them, the false cords, and then the true vocal cords, which seal the trachea.