Amelioration
of pathological yawning after tracheostomy in a
patient with locked-in syndrome:
a
thermo-regulatory approach
Hari Prasad
Medical Research Faculty,
Microbiology,
K.S. Hegde Medical Academy,
Deralakatte, Nithyanandanagar P.O., Mangalore,
Karnataka, India
I have read with great interest the report
by Chang et al.
describing amelioration of pathological yawning
after tracheostomy in a patient with locked-in
syndrome [1]. It was an interesting
debate but what in- trigued me most was Chang et
al. spective of excessive yawning and its role
in neurological practice. My purpose is to give
another view of the meaning of excessive yawning
observed in locked-in syndrome. Yawning is a
stereotyped behaviour with an evolutionary
significance, and has been described since
antiquity [2].
Excessive yawning appears to be symptomatic
of conditions that increase brain and/or core
temperature, such as cerebral ischae- mia,
traumatic brain injury and sleep deprivation.
There is now a growing body of evidence
suggesting that yawning may be a
thermoregulatory mechanism, pro- viding
compensatory brain cooling when other provisions
fail to operate favourably [3].
In a recently published paper, I report the
protective thermoregulatory role of yawning in
patients on anti-depressant therapy [4].
Selective brain cooling appears to protect the
thermally vulnerable tissue of the brain from
thermal damage [5]. If so, the mechanism
would be expected to be most important in
locked-in syndrome, which is usually associated
with, infarction of the ventral pons. In my
opinion, an intense need for selective brain
cooling is the major functional cause to
initiate and execute yawning as an involuntary
motor phenomenon, despite destruction of
voluntary movements of facial and trigeminal
motor functions [6].
Well-controlled animal models of nasal and
tracheostomy breathing demonstrate a
significantly detrimental effect of tra-
cheostomy breathing on brain and body
temperatures, and on clinical outcome of
hyperthermia. The study by Laburn et al. (1988)
indicate that selective brain cooling depends on
upper respiratory tract cooling (i. e. via nasal
breathing) in normo- and in hyperthermic states.
Though it is true that patient tomy, I cannot
agree that it is connected with termination of
the pathological yawning. Conversely, the
tracheostomy breathing, which bypasses the
carotid rete mechanism, could cause a rise in
thalamic and cortical brain temperatures
[7].
One would therefore anticipate that a
predis- position towards abolition of body-brain
temperature difference during tracheos- tomy
breathing would produce a proportional increase
in brain tempera- ture thereby augmenting other
selective brain cooling mechanisms and,
ostensibly, placing one at increased risk for
aggravation, instead of amelioration of
pathological yawning. In this connection, I
would again like to mention the experiments made
by Laburn et al., who observed that,
intermittent nasal breathing after hours of
upper respiratory bypass results in a dramatic
and significant brain cooling [7].
This precipitous temperature fall has been
attributed to the evaporative cooling of the
re-ventilated nasal mucosal surfaces which had
been wetted by copious secretion, for the period
of bypass breathing. On the basis of this
evidence, I suggest that the cerebral cooling
stimulated by intermittent nasal air passage
following tracheostomy might be strong enough to
inhibit mechanisms that would normally trigger
yawning in patients with locked-in syndrome.
Further studies are needed before conclusions
can be drawn. Conflicts of interest The author
has no conflicts of interest to declare.
1. Chang CC,
Chang ST, Chang HY, Tsai KC. Amelioration of
pathological yawning after tracheostomy in a
patient with locked-in syndrome. European
Journal of Neurology 2008; 15:
e66&endash;e67.
2. Prasad H.
Yawning... and why yawns are contagious: a
theory on evolution and atavism. Medical
Hypotheses 2008; 71: 307.
3. Gallup
AC, Gallup GG Jr. Yawning and
thermoregulation. Physiology and Behavior 2008;
95: 10&endash;16.
4. Prasad H.
Drug-induced yawning: A vital protective reflex.
Medical Hypotheses 2008; 71: 457.
5. Gluckman PD, Wyatt JS, Azzopardi D, et
al. Selective head cooling with mild systemic
hypothermia after neonatal encephalopathy:
multicentre randomised trial. Lancet 2005; 365:
663&endash;670.
6. Smith E, Delargy M. Locked-in syndrome.
British Medical Journal 2005; 330:
406&endash;409.
7. Laburn HP, Mitchell D, Mitchell G, et al.
Effects of tracheostomy breathing on brain and
body temperatures in hyperthermic sheep. Journal
of Physiology 1988; 406: 331&endash; 344.
