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Brain
temperature in healthy and diseased conditions: A review
on the special implications of MRS for monitoring brain
temperature
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- Yulug B, Velioglu HA,
- Sayman D, Cankaya S, Hanoglu L.
- Biomed Pharmacother
- 2023 Jan 27;160:114287
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- Tous
les articles d'Andrew Gallup
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Brain temperature
- Brain temperature determines not only an individual's
cognitive functionality but also the prognosis and
mortality rates of many brain diseases. More
specifically, brain temperature not only changes in
response to different physiological events like yawning
and stretching, but also plays a significant
pathophysiological role in a number of neurological and
neuropsychiatric illnesses. Here, the authors have
outlined the function of brain hyperthermia in both
diseased and healthy states, focusing particularly on the
amyloid beta aggregation in Alzheimer's disease.
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- De la
température du cerveau
- La température du cerveau détermine non
seulement la fonctionnalité cognitive d'un
individu mais aussi le pronostic et le taux de
mortalité de nombreuses maladies du cerveau. Plus
précisément, la température du
cerveau ne change pas seulement en réponse
à différents événements
physiologiques comme le bâillement et
l'étirement, mais joue également un
rôle physiopathologique important dans un certain
nombre de maladies neurologiques et neuropsychiatriques.
Ici, les auteurs décrivent la fonction de
l'hyperthermie cérébrale dans les
états pathologiques et sains, en se concentrant
particulièrement sur l'agrégation
bêta-amyloïde dans la maladie d'Alzheimer.
Regional brain temperature measurement is novel method
that yields pathophysiological insights and associated
therapeutic opportunities for a neuroprotective approach
to degenerative diseases [1]. Monitoring regional
cerebral temperature has not yet been found to guide
goal-directed cerebral protection. However, this is due
to the lack of reference data regarding the application
of targeted temperature management associated with the
difficulty of collecting data through invasive direct
measurements [1,2]. Previous studies have
employed noninvasive techniques such as magnetic
resonance imaging and spectroscopy, infrared
spectroscopy, microwave radiometry and ultrasound
thermometry to determine the temperature of the brain
[3]. Neural temperature measurement sensors are
another feasible, noninvasive option. For instance,
thermocouples, resistance temperature detectors (RTDs),
and semiconductor-based optical sensors are all
noninvasive implantable temperature monitoring devices,
although they have not yet been used on humans due to
concerns about potential side-effects
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- In addition to offering the new therapeutic insights
described above, recent studies suggest that deviations
in brain temperature may also be of diagnostic value in
neurological disorders in the clinical setting
[4&endash;9], although a clear dissociation of
these deviations from healthy physiological variations
over time is essential [9]. The major error here
seems to involve an assumption of brain temperature based
on body core, which leads to the neglect of the
pathophysiological importance of brain-specific regions.
Several studies have suggested that brain cell function
is highly dependent on temperature, as suggested in
conditions in which the brain temperature of
brain-injured patients was found to be significantly
increased using intracranial probes allowing direct, but
invasive, measurement from a single cerebral locus
[10]. In contrast to these invasive approaches,
magnetic resonance spectroscopy (MRS) may offer an
alternative monitoring system in which spatially resolved
brain temperature data can be obtained noninvasively
[1,2]. A recent study by Thrippleton et al.
evaluated the feasibility of using MRS to measure brain
temperature and mapping, and described it as a reliable
method, especially at 3 T.
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- In this setting, the mechanism by which regional
temperature is measured involves the temperature-reliant
chemical shift of water in contrast to the reference
metabolite n-acetyl aspartate (NAA), which is not
temperature-dependent [11]. Temperature
measurement and monitoring based on the water proton
chemical shift is divided into two different imaging
techniques - spectroscopic imaging and the phase mapping
method, which is more commonly used. Based on this
method, brain temperature for each cerebral tissue voxel
can be calculated using a formulation between the above
parameters, as described in a recent study. In brief,
such a rational approach yields a mathematical value for
the difference in chemical shift between water and NAA,
thus providing an estimated value for brain temperature
in healthy subjects [1,11].
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- Brain temperature in healthy individuals
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- The average brain temperature in healthy individuals
is more than two degrees higher than that of the body
core, depending on factors such as the time of day, the
brain region involved, sex, menstrual cycle, and age
[2]. A similar difference can be observed at
night, when cerebral blood flow peaks [13]. This
is principally mediated by intact cerebral perfusion, a
compensatory mechanism especially effective in young,
healthy brains. It is therefore physiologically plausible
that lower temperature values may be observed for
specific brain regions (i.e. the hypothalamus) which are
closely associated with major vascular structures, such
as the Willis Circle [2,12,14]. This suggests the
importance of intact neurovascular integrity for an
effective heat-removal mechanism by creating spatial
gradients in brain temperature. It is also worth
mentioning that yawning and stretching are compensatory
thermoregulatory mechanisms against increased brain
temperature mediated by various neurotransimitters such
as acetylcholine, serotonin, dopamine and GABA
[35,36]. This is suggested by the same authors
showing that heavy nasal breathing terminated the yawning
reflex by reducing the brain temperature
[37].
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- Based on these physiological data, it is not
unreasonable to assume that recent research has
associated increased brain temperature with a less
efficient overnight cooling mechanism in the brains of
older individuals [12&endash;14]. This might open
a new window into the possibility of whether such a
mechanism may contribute to diseased conditions in the
brain, regardless of the kind of impairment, such as an
apoplectic character, as in neurovascular diseases, or a
relatively slower progression in degenerative
neurological disorders.
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- Another interesting subject is that elevated brain
temperatures have been measured following vaccine
administration [15]. Plank et al. hypothesized
that an increase in brain temperature should occur due to
neuroinflammation following typhoid vaccine
administration [15]. That study suggested that
peripheral invasive procedures may affect the central
nervous system, another important subject requiring
further analysis.
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- Entropy is another factor shown to be closely
associated with brain temperature. Entropy is classically
defined as a thermodynamic quantity expressing a system's
inability to convert thermic energy to mechanical work,
in other words, a degree of disorder or randomness in the
system. Heat causes greater randomness if added to a
system and thus higher entropy, indicating a relationship
between increased brain temperature and higher brain
entropy. One good example of this is an interesting
recent study by Saxe et al., who investigated the
relationship between intelligence and brain entropy using
resting-state fMRI of healthy adults. Those authors
observed a positive association between cognitive scores
and brain entropy, especially in the prefrontal cortex,
inferior temporal lobes and cerebellum [16]. This
suggests that increased entropy derived from complex
behavioral performance and intellectual capacity might
theoretically increase the temperature of the brain.
However, similar to other biological systems, this
association might be true to some extent, and it is still
unclear whether a process going beyond this fine line may
induce or be related to disease conditions, indicating
that further mechanistic studies are needed to shed light
on this chicken and egg paradox.
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