Based on bedside observations and on videofluoroscopy studies, an experienced therapist can make recommendations regarding the safety of oral feeding, changes in the consistency and texture of the diet, postural adjustments, and the need to insert a tracheostomy or feeding tube.
Vomiting is a complex, sequential act that may be triggered by numerous external, gastrointestinal, and neural stimuli. The main central nervous system structure of interest in eliciting the vomiting reflex is the area postrema, which is located at the base of the fourth ventricle. The neurons within the area postrema are chemosensitive and are activated by circulating toxins, which have direct access to these neurons because of the absence of a blood–brain barrier. Axons from the area postrema project to the nucleus of the solitary tract (NTS), which is also a convergence point of input from the pharynx, larynx, and gastrointestinal tract. The NTS engages groups of neurons in the medulla, which coordinates the sequential elements of vomiting; there is no “vomiting center,” as reviewed by Hornby. In addition to stimulation of the area postrema, vestibular, pharyngeal (gag reflex), and psychic stimuli can induce vomiting.
The final expulsion of gastric contents is effected through a combination of lowering of intrathoracic pressure by inspiration against a closed glottis and an increase in abdominal pressure during abdominal muscle contraction. Retroperistalsis begins in the small intestine and there is relaxation of the lower esophageal and pyloric sphincters; the stomach itself does not contract.
The vagus carries afferent information from the enteric system as well as conducting efferent signals from the NTS to the gastrointestinal structures. The neurons in the area postrema contain D2 dopamine, 5-HT3 serotonin, opioids, substance P, and acetylcholine receptors, as well as the aquaporin channel. This affords an explanation for the emetic properties of dopaminergic agents and the antiemetic activity of dopamine and serotonin antagonists. However, other potent antiemetics such as ondansetron, a 5-HT3 receptor antagonist, have their effect on vagal afferents.
Lesions near the area postrema, including tumors, hemorrhage, infarctions, and demyelination are the usual neurologic causes of vomiting. We have seen, and it has been reported in the literature, that vomiting may have a relation to the periventricular lesions of neuromyelitis optica due the enrichment of aquaporin-4 channels in this area (Iorio and colleagues). The mechanism of vomiting from raised intracranial pressure has not been fully explored but could be the result of transmission of pressure to the dorsal medulla.
Cyclic vomiting syndrome This syndrome of obscure cause is associated with abdominal migraine in children (see Chap. 9), and is a prominent component of Riley-Day dysautonomia. It is also well known as a factitious self-induced disorder, for example, in bulimia.
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