Why do we hiccup?

Why do we hiccup? Considering the fact that almost everyone experiences hiccups at one time or another, remarkably little is known about them. The name itself is onomatopoeic, which is appropriate considering that the only common understanding of the hiccup is of the characteristic sound. Hiccups can be predictably elicited in some individuals by overindulgence of food, alcohol, or both, sometimes providing evidence of such behaviour and making them a common object of humour. There are, however, instances in which hiccups become intractable (singultus) causing insomnia, wasting, exhaustion, and even death, prompting scientific scrutiny of this otherwise harmless curiosity. 1 In this issue Fass et al (see page 590) present original investigative work on the aVerent limb of the hiccup reflex. Fass et al used a barostat to characterise the parameters of oesophageal distention that could elicit hiccups in normal volunteers. They report that rapid phasic disten-sion of the proximal, but not distal, oesophagus could reproducibly induce hiccups in four of 10 subjects. Hiccups occurred during rapid inflation of the barostat bag and immediately resolved with deflation, strongly implicating oesophageal mechanoreceptors as the critical aVerents. The authors speculatively generalise this observation to the population as a whole, but in this one needs to be circumspect, recognising the long list of stimuli that have been reported to cause prolonged bouts of hiccups. A partial inventory of hiccup aetiologies encompasses trauma (skull fracture, closed head trauma, surgery), mass lesions (uraemia, drugs). 2–4 One individual is described in whom a hair tickling the tympanic membrane was ultimately revealed to be the cause of singultus. In view of this great diversity of causative stimuli, defining the aVerent limb of the hiccup reflex is no simple task and there is reason to suspect substantial intersubject variability. In the broadest sense, relevant aVerents can course with the vagus or phrenic nerves, the pharyngeal plexus from C2 to C4, and the sympathetic chain from T6 to T12. 3 5 In all likelihood, there is no universal stimulus for hiccups in adults, but rather, a long list of potential stimuli in susceptible individuals. The central elements and eVerent limbs of the hiccup reflex have been better characterised than the aVerent limb. The hiccup is an involuntary medullary reflex influenced by, but independent of, the respiratory centre of the medulla. In a meticulous investigation, Davis demonstrated that hiccup frequency could be modulated or completely suppressed by inhalation of CO …