Hiccup (singultus) is a poorly understood phenomenon. It does not seem to serve any useful physiologic purpose, existing only as a nuisance, and is typically not associated with any particular disease. It may occur as a component of the lateral medullary syndrome (see Chap. 33) as in 7 of 51 cases studied by Park and colleagues, with masses in the posterior fossa or medulla, and occasionally with generalized elevation of intracranial pressure, brainstem encephalitis, or with metabolic encephalopathies such as uremia. Rarely, singultation may be provoked by medication, one possible offender in our experience being dexamethasone. Because the triggers of hiccup often seem to arise in epigastric organs adjacent to the diaphragm, it is considered to be a gastrointestinal reflex, more than a respiratory one. A physiologic study by
Newsom Davis demonstrated that hiccup is the result of powerful contraction of the diaphragm and intercostal muscles, followed immediately by laryngeal closure. This results in little or no net movement of air. He concluded that the projections from the brainstem responsible for hiccup are independent of the pathways that mediate rhythmic breathing.
Within a single burst or run of hiccups, the frequency remains relatively constant, but at any one time it may range anywhere from 15 to 45 per minute. The contractions are most liable to occur during inspiration and they are inhibited by therapeutic elevation of arterial carbon dioxide (CO2) tension. We cannot vouch for the innumerable home-brewed methods that are said to suppress hiccups (breathholding, induced fright, anesthetization, or stimulation of the external ear canal or concha, etc.), but where the neurologist is asked to help in an intractable case (usually in a male), baclofen is sometimes effective. Drugs that act to empty the stomach (e.g., metoclopramide) may work as well.
Disorders of Ventilation Caused by Neuromuscular Disease
Failure of ventilation in the neuromuscular diseases causes one of two symptom complexes: an acute one occurs in patients with rapidly evolving generalized weakness, such as Guillain-Barré syndrome and myasthenia gravis, and the other in patients with subacute or chronic diseases, such as motor neuron disease, myopathies (acid maltase, nemaline), and muscular dystrophy. The review by Polkey and colleagues provides a more extensive list of diseases that cause these problems. Patients in whom respiratory failure evolves in a matter of hours become anxious, tachycardic, and diaphoretic. They may display paradoxical respiration, in which the abdominal wall retracts during inspiration, owing to the failure of the diaphragm to contract, while the intercostal and accessory muscles create a negative intrathoracic pressure. Or, there is respiratory alternans, a pattern of diaphragmatic descent only on alternate breaths (this is more characteristic of airway obstruction). These signs appear in the acutely ill patient when the vital capacity has been reduced to approximately 10 percent of normal, or 500 mL in the average adult.
Patients with chronic but stable weakness of the respiratory muscles, demonstrate signs of CO2 retention, such as daytime somnolence, headache upon awakening, nightmares, and, in extreme cases, papilledema. The accessory muscles of respiration are recruited in an attempt to maximize tidal volume, and there is a tendency for the patient to gulp or assume a rounded “fish mouth” appearance in an effort to inhale additional air. In general, patients with chronic respiratory difficulty tolerate lower tidal volumes without dyspnea than do patients with acute disease, and symptoms in the former may occur only at night, when respiratory drive is diminished and compensatory mechanisms for obtaining additional air are in abeyance.