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See article on page 590
Why do we hiccup?
Considering the fact that almost everyone experiences hic-
cups at one time or another, remarkably little is known
about them. The name itself is onomatopoeic, which is
appropriate considering that the only common under-
standing 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 investi-
gative 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 implicat-
ing 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
(aneurysms, tumours, goitres, diverticuli), infections (sub-
diaphragmatic abscess, cholecystitis, pleurisy, meningitis,
encephalitis), irritative stimuli (heartburn, spicy food, gastri-
tis, peptic ulcers, pancreatitis), luminal distension (achalasia,
gastric distension, oesophageal rings and strictures), central
nervous system pathology (multiple sclerosis, cerebrovascu-
lar accidents, psychogenic), and metabolic aberrations
(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, rel-
evant aVerents can course with the vagus or phrenic nerves,
the pharyngeal plexus from C2 to C4, and the sympathetic
chain from T6 to T12.
35
In all likelihood, there is no univer-
sal 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 demon-
strated that hiccup frequency could be modulated or com-
pletely suppressed by inhalation of CO
2
(the physiology
behind the remedies of rebreathing into a paper bag and
breath holding).
6
Furthermore, the whole array of inspira-
tory muscles are activated on the eVerent side of the reflex:
the phrenic nerves to the diaphragm, the external intercos-
tal nerves (T1–T11) to the intercostal muscles, and the
scalenus anticus nerve to the scalene muscles which elevate
the clavicles.
46
These inspiratory eVerents are activated far
more vigorously during hiccups than during cyclic respira-
tion. Thirty five milliseconds after inspiratory activation,
the recurrent laryngeal nerve eVects glottic closure, result-
ing in the characteristic “hic” and eliminating any ventila-
tory eVect of the reflex. The large volume changes which
would occur in hiccups were it not for the associated glot-
tal closure is evident in tracheostomised individuals in
whom hiccups can cause severe hyperventilation; Davis
reports such a case in which hiccups resulted in an arterial
pH of 7.58 and a minute ventilation exceeding 20 litres.
6
Thus, although the hiccup reflex results in repetitive,
essentially maximal stimulation of the inspiratory muscula-
ture, it normally serves no respiratory function.
As detailed earlier, the hiccup has minimal impact on
ventilation because of the prompt glottic closure following
the intense inspiratory drive. Thus, speculation regarding
the purpose of the hiccup naturally shifted to the gastroin-
testinal tract because of the numerous gastrointestinal
stimuli observed to elicit them.
6
However, although a spell
of hiccups may interfere with eating, it is not a protective
reflex (despite the speculation presented by Fass et al that
it may prevent the entry of large food boluses into the gas-
trointestinal tract). Whereas vomiting, gagging, and
coughing are eVective (at times life saving) protective
reflexes of the gastrointestinal and respiratory tracts, the
hiccup has no discernible survival value. More likely than
not, spells of hiccups in adults result from activation of a
vestigial reflex that once served some purpose in ontogeny
or phylogeny. Since it is equally diYcult to construe a sce-
nario in which hiccuping is of use to mature beasts, the
more likely candidate would seem to be ontogeny. Hiccup-
ing is observed in utero and the tendency to hiccup contin-
ues after delivery. Fetal hiccuping can be demonstrated
ultrasonographically in utero as large inward and outward
movements of the chest occurring 1–6 times per minute
with spells lasting an average of eight minutes.
7
Premature
infants spend an average of 2.5% of their time hiccuping.
4
Hiccuping spells occur in utero and in premature babies
without any identifiable stimulus for initiation or for cessa-
tion. This suggests that during the perinatal period, when
the respiratory tract needs to mature rapidly, hiccuping
does have a survival value. Perhaps, hiccuping is essentially
a programmed isometric exercise of the inspiratory
muscles which are superfluous in utero, but aVorded no
time for maturation after birth. Beyond the perinatal
period, however, hiccuping is a vestigial reflex, incidentally
elicited by a wide and variable range of autonomic stimuli.
PETER J KAHRILAS
GUOXIANG SHI
Northwestern University Medical School,
Division of Gastroenterology and Hepatology,
Department of Medicine,
Passavant Pavilion, Suite 746,
303 East Superior Street,
Chicago, IL 60611, USA
Correspondence to: Dr Kahrilas.
Gut 1997; 41: 712–713712
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