Insights & Perspectives
Hiccups: A new explanation for the
The common hiccup is a ubiquitous
reﬂex; everyone experiences hiccups
innumerable times through life, but
unlike the other common reﬂexes like
sneezing (clearing material from the
nasal passages) and coughing (clearing
material from the airways) there is no
known physiologic advantage for the
common hiccup [1–4].
Rather than continuing as a vestigial
reﬂex whose purpose has evolved away,
I propose that the hiccup may be a sur-
prisingly complex reﬂex to remove air
from the stomachs of young suckling
The hiccup (or hiccough) is an ono-
matopoeic name that comes from the
sound made by the abrupt closure of
the vocal cords approximately 35 milli-
seconds after the forceful contraction of
the respiratory muscles. In the medical
literature, hiccups are referred to as ‘sin-
gultus’, although this term was origin-
ally used to describe the sharp intake of
breath often associated with long
periods of crying.
When hiccups continue for more
than 48 hours or occur frequently they
may be a sign of a serious disease. More
often they go unnoticed or are con-
sidered a minor annoyance that serves
no valuable purpose.
Hiccups seem to occur in most mam-
mals. They have been studied in cats,
rats, and rabbits [1, 5], and are often
observed in horses, dogs, and humans.
The rhythmic movement of hiccups can
be felt by pregnant mothers and seen on
ultrasound occurring in the fetus in
utero, before swallowing or respiratory
reﬂexes appear. The reﬂex is most
prevalent in newborns and they spend
as much as 2.5% of their time hiccup-
ping , it then diminishes in infancy
with occasional brief recurrences
through life . There do not appear
to be documented observations of hic-
cups in reptiles, amphibians, or birds.
Much of what is known about the
anatomy of the hiccup reﬂex comes from
the study of pathological hiccups, which
can arise as a result of infection or malig-
nancy near the diaphragm, or from
lesions in the brain. Afferent signals come
from the distal esophagus, stomach, and
the abdominal side of the diaphragm and
travel as part of the phrenic nerve, the
vagus, and sympathetic (T6-T12) chain
branches. The afferent limb path is vari-
able between individuals, as is the degree
of stimulus required to initiate the reﬂex.
The central component of the reﬂex
lies in the medulla. Electrophysiological
studies as well as the pattern of muscle
contraction suggest that the center for
the hiccup reﬂex is entirely separate
from the pathways involved in rhythmic
breathing . A series of patients
with lateral medullary infarction
(Wallenberg’s syndrome) and hiccups
suggest that middle level and dorsolat-
eral lesions can induce hiccups.
Once initiated, hiccups usually
occur at a rate of 4–60 per minute.
The frequency remains fairly constant
in the individual, but can be modiﬁed
by various conditions. Hiccups are sup-
pressed by elevations in serum carbon
dioxide  and can be triggered by gas-
tric distention, rapid eating, or drinking
carbonated beverages .
Efferent nerves travel from the hic-
cup center to the diaphragm, the exter-
nal intercostals, the scalene muscles,
glottic structures, and the esophagus.
The most signiﬁcant muscle group
involved is the diaphragm, and several
studies have shown that hiccups are
often unilateral, involving only the left
hemi-diaphragm [7, 8].
The result is the activation of the
respiratory muscles more vigorously
than with normal respiration, followed
approximately 35 milliseconds later by
closure of the glottis . This forceful
inhalation effort against the closed glot-
tis leads to a sharp reduction in intra-
thoracic pressure. At the same time,
normal esophageal peristalsis is sup-
pressed and the lower-esophageal
sphincter relaxes. Innervation to the
muscles of exhalation is inhibited.
The drop in intra-thoracic pressure
is experienced by the lungs, the heart,
and great vessels, lymphatic vessels, the
thymus, and the esophagus where it
traverses the thorax. The stomach lies
Queen’s University Department of Emergency
Medicine, Kingston, Ontario, Canada
Bioessays 34: 451–453,ß2012 WILEY Periodicals, Inc. www.bioessays-journal.com 451
Ideas & Speculations
underneath the diaphragm and is out-
side the thoracic cavity.
Previous theories of why
A hypothesis seeking to explain the pur-
pose of the hiccup should meet the fol-
lowing criteria in order to be considered
1. The hypothetical stimulus of the hic-
cup should be anatomically consistent
with the afferent limb of the reﬂex.
2. The activation of the efferent limb of
the reﬂex should resolve or help to
resolve the condition that leads to the
stimulus. Ideally the condition
should explain all of the components
of the efferent limb.
3. The hypothesis should offer an
explanation for the hiccup’s preva-
lence in mammals and its profoundly
increased incidence during infancy.
4. The resolution of the condition that is
hypothesized to stimulate the hiccup
should offer a tangible evolutionary
Many purposes have been proposed
for the hiccup, but to date none has met
all four of the criteria above.
Suggestions that hiccup represent a
form of epilepsy  or a failure of supra-
spinal inhibition  may suggest
causes for pathological hiccups, but
they do not explain the presence of
the reﬂex in the normal individual.
Similarly, the suggestion that the hiccup
could be a dysfunction of the reciprocal
inhibition of an inspiratory effort related
to breathing and a simultaneous glottic
closure related to swallowing  fails to
explain the afferent limb of the reﬂex.
Furthermore, hiccups are often present
in the absence of swallowing.
In 1899, Ferroni suggested that hic-
cups were a form of preparation for the
fetus to strengthen the muscles involved
in respiration, a hypothesis more recently
revisited by Kahrilas and Shi . The
respiratory exercise hypothesis does not
explain the existence of the afferent limb
of the reﬂex. Furthermore, the brief con-
traction of the respiratory muscles is
unlikely to have any beneﬁcial effect on
respiratory muscles, which have a high
concentration of slow twitch ﬁbers for
Other suggestions related to fetal devel-
opment have included clearance of
meconium (the ﬁrst feces that a newborn
produces; in times of fetal distress meco-
nium can be passed while in the uterus
and then breathed in) and training for
suckling, but these do not seem plausible
in light of the actions of the reﬂex. The
strong contraction of the respiratory
muscles would move meconium deeper
into the airway, and the majority of the
muscles triggered by the hiccup reﬂex are
not involved in suckling.
Straus et al.  proposed a phylo-
genic hypothesis that the hiccup is an
evolutionary remnant that originated
with gill ventilation. They make an
excellent argument for the phylogenic
development of the hiccup reﬂex from
ventilatory motor patterns of lower
vertebrates and suggest that the hiccup
is an evolutionary remnant. This hy-
pothesis – that there is no purpose for
the hiccup – should only be accepted in
the absence of an acceptable expla-
nation for its evolutionary persistence.
Others have suggested that the hic-
cup is a reﬂex to move boluses of food
trapped in the esophagus . This
theory is supported by the afferent
innervation of the reﬂex, suggesting
that the stimulus is a condition sensed
in the area of the lower esophagus,
stomach, or beneath the diaphragm. It
would also explain the simultaneous
relaxation of the lower-esophageal
sphincter. The main problem with this
theory is that the action of the hiccup
would move a food bolus toward the
middle of the chest – a food bolus in
the lower esophagus would move away
from the stomach where it can be safely
digested and toward the airway, where
it could become a dangerous obstruc-
tion. The fact that patients who present
with food stuck in the esophagus rarely
have associated hiccups and the high
prevalence of hiccups in suckling new-
borns who do not consume solids, do
not support this hypothesis.
The most notable action of
the hiccup reflex is the
sharp drop in intra-thoracic
The closure of the glottis with strong
contraction of the respiratory muscles
results in a sharp drop in the intra-
thoracic pressure, which suggests that
the purpose of this reﬂex is to move
something from outside of the thoracic
cavity toward the inside. There are ﬁve
conduits between the intra- and extra-
thoracic areas that contain ﬂuid or air
that might be moved, namely arterial,
venous, and lymphatic vessels, the tra-
chea, and the esophagus.
pressure ﬂow that would be expected to
change very little as a result of the brief
action of a hiccup. Venous and lym-
phatic ﬂow may be somewhat increased
by the action of the hiccup, but it is
unlikely that it is signiﬁcantly changed
by such a brief stimulus. In addition to
that, stimuli in the area of the afferent
limb are unlikely to be resolved by the
impact on blood or lymph ﬂow.
A number of the previously offered
explanations for the hiccup suggest the
trachea as the conduit of interest, but
this seems unlikely. The rapid closure of
the glottis serves to prevent most of the
air movement through the trachea,
movement that could be much greater
if the timing were different. Foreign
material partially obstructing the airway
would be moved deeper into the airway
by a hiccup, thereby impeding clear-
ance. Moreover, the airway is already
well protected by gag, cough, and
sneeze reﬂexes, the afferent limbs of
which are more appropriately situated.
This leaves the esophagus. The
negative intra-thoracic pressure of the
hiccup would move material from
the mouth or the stomach toward the
mid-section of the esophagus. The
existence of the swallowing reﬂex for
moving material from the mouth, as
well as the anatomy of the afferent limb,
suggest the purpose of the hiccup
relates to the lower esophagus instead
of the upper esophagus or mouth.
The contents of the stomach are
the materials of digestion and gas. The
vomiting reﬂex effectively removes
unwanted materials of digestion, leav-
ing stomach gas as the potential trigger
for physiologic hiccups.
The hiccup as a burping
Is it possible that the hiccup functions
to remove swallowed gas from the
D. Howes Insights & Perspectives.....
452 Bioessays 34: 451–453,ß2012 WILEY Periodicals, Inc.
Ideas & Speculations
stomach – essentially an evolved
burping reﬂex? The presence of an air
bubble in the stomach or distal esoph-
agus could stimulate mechanoreceptors
that activate the afferent limb of the
reﬂex (Fig. 1A). The contraction of
the respiratory muscles and closure of
the glottis would drop the intra-thoracic
pressure (Fig. 1B), pulling the air from
the stomach in to the mid-esophagus
(Fig. 1C), where it could then leave
through the mouth with the next
exhalation. This explanation is sup-
ported by the suppression of esophageal
peristalsis and the relaxation of the
lower esophageal sphincter that are
part of the reﬂex.
The presence of a burping reﬂex pro-
vides a signiﬁcant survival advantage.
Young mammals depend on milk
consumption for their nutrition. The
continuous nature of suckling means
that it has to be coordinated with
respiration and the result can be
swallowed air. A reﬂex that helps
remove swallowed air would signiﬁ-
cantly increase the stomach’s capacity
for milk. This also explains why the
hiccup is so much more frequent in
If this hypothesis is true, there must
be something about the presence of air
in the stomach that can be differentiated
from distention with food. A future test
of the hypothesis could be to identify
how the reﬂex differentiates air; either
directly or from a pattern of stimuli that
occurs with the movement of an air
The hiccup is a very common reﬂex. I
propose that hiccups are triggered by
the presence of air in the stomach.
This stimulates the sharp intake
typical of the reﬂex, moving swallowed
air out of the stomach and effectively
‘burping’ suckling infants, allowing
them to consume a greater volume of
milk in the meal. For adults, the infre-
quent annoying afﬂiction reﬂects per-
sistence of an infantile reﬂex and a
reminder that we may have eaten too
Figure 1. The hiccup may have evolved to remove air from the stomachs of young suckling
mammals. A: The presence of air (yellow) in the stomach beneath the diaphragm triggers the
afferent limb of the reﬂex, sending signals to the medulla (shown in red). B: Activation of the
reﬂex efferent limb (purple) causes the muscles of respiration to expand the chest (green
arrows), while simultaneously closing the opening of the trachea (green X). The result is a
sharp drop in pressure in the chest (symbolized by ). C: The negative intra-thoracic pressure
moves the air bubble to the thoracic esophagus. With relaxation after the hiccup, the air can
pass up the esophagus and out the mouth, leaving more room for milk.
There is, as yet, no proof for this hy-
pothesis, but hopefully it will stimulate
some thought about this ubiquitous
unexplained reﬂex, and provide a
framework for exploring its anatomy
and physiology in greater detail.
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.....Insights & Perspectives D. Howes
Bioessays 34: 451–453,ß2012 WILEY Periodicals, Inc. 453
Ideas & Speculations