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R E V I E W Open Access
The pathophysiological basis and
consequences of fever
Edward James Walter
*
, Sameer Hanna-Jumma, Mike Carraretto and Lui Forni
Abstract
There are numerous causes of a raised core
temperature. A fever occurring in sepsis may be
associated with a survival benefit. However, this is not
the case for non-infective triggers. Where heat
generation exceeds heat loss and the core
temperature rises above that set by the
hypothalamus, a combination of cellular, local,
organ-specific, and systemic effects occurs and puts
the individual at risk of both short-term and long-term
dysfunction which, if severe or sustained, may lead to
death. This narrative review is part of a series that will
outline the pathophysiology of pyrogenic and
non-pyrogenic fever, concentrating primarily on the
pathophysiology of non-septic causes.
Keywords: Hyperthermia, Fever, Organ failure,
Physiopathology, Heatstroke
Background
“Humanity has but three great enemies: fever, famine,
and war, and of these by far the greatest, by far the
most terrible, is fever.”(William Osler)
The normal human temperature is considered to be
37 °C, but may vary by up to 1 °C in healthy individuals
[1]. Elevated core temperature is a common finding in
intensive care, affecting up to 70 % of patients [2].
Despite the general usage of the terms ‘pyrexia’,‘fever’,
and ‘hyperthermia’, they are not yet universally defined.
The American College of Critical Care Medicine, the
International Statistical Classification of Diseases, and the
Infectious Diseases Society of America define fever as a
core temperature of 38.3 °C or higher, i.e. just above the
upper limit of a normal human temperature, irrespective
* Correspondence: ewalter@nhs.net
Department of Intensive Care Medicine, Royal Surrey County Hospital,
Egerton Road, Guildford, Surrey GU2 7XX, UK
of the cause [1]. Fever has its etymological basis in Latin,
meaning simply ‘heat’, and pyrexia comes from the Greek
‘pyr’, meaning fire or fever. Some sources use the terms
interchangeably, whereas others preserve ‘fever’to mean a
raised temperature caused by the action of thermoregula-
tory pyrogens on the hypothalamus; for instance, in sepsis
and inflammatory conditions [3].
Hyperthermia also has no agreed definition; it has been
defined as a core temperature above 38.2 °C, irrespective
of the cause [3]. Others use it for the classification of those
conditions that increase the body’s temperature above that
set by the hypothalamus, and therefore specifically exclude
those where fever is caused by pyrogens [4], being due to
heat exposure or unregulated heat production in excess of
heat loss. Common causes include classical and exertional
heatstroke, and drug-related illnesses (for example, malig-
nant hyperthermia and neuroleptic syndrome).
There is, however, increasing evidence that many
conditions considered non-pyrogenic may stimulate an in-
flammatory response, and the division into pyrogenic and
non-pyrogenic may therefore be less clear-cut than previ-
ously understood.
Generation of fever
Sepsis accounts for up to 74 % of fever in hospitalised
patients [5] and, of the remainder, malignancy, tissue is-
chaemia, and drug reactions account for the majority
[6]. Neurogenic fever, and fevers associated with endo-
crinopathy, are rarer.
Sepsis
Pyrogenicfeverisacommonresponsetosepsisincritically
ill patients, and the generation of fever occurs through sev-
eral mechanisms. The interaction of exogenous pyrogens
(e.g. micro-organisms) or endogenous pyrogens (e.g. inter-
leukin (IL)-1, IL-6, tumour necrosis factor (TNF)-α)with
the organum vasculosum of the lamina terminalis (OVLT)
leads to the production of fever. Exogenous pyrogens may
stimulate cytokine production, or may act directly on the
OVLT. The OVLT is one of seven predominantly cellular
© 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
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Walter et al. Critical Care (2016) 20:200
DOI 10.1186/s13054-016-1375-5
structures in the anterior hypothalamus within the lamina
terminalis, located in the optic recess at the anteroventral
end of the third ventricle. Being a circumventricular
organ it is highly vascular and lacks a blood–brain bar-
rier (BBB), permitting it to be stimulated directly by
pyrogenic substances. Its stimulation leads to increased
synthesis of prostanoids including prostaglandin
(PG)E
2
, which acts in the pre-optic nucleus of the
hypothalamus slowing the firing rate of the warm sensi-
tive neurons and resulting in an increase in body
temperature. The bioactive lipid derivative, ceramide,
which has a proapoptotic as well as a cell signalling
role, may act as a second messenger independent of
PGE
2
, and may be of particular importance in the early
stages of fever generation [7]. Lipopolysaccharides (LPS)
from gram-negative bacteria may stimulate peripheral
production of PGE
2
from hepatic Kupffer cells [8, 9].
LPS-stimulated fever may also be neurally mediated
[10]. Neural pathways may account for the rapid onset
of fever, with cytokine production responsible for the
maintenance, rather than the initiation, of fever [11].
Fever generation is also thought to occur by signal-
ling via the Toll-like receptor cascade, which may be
independent of the cytokine cascade [12] (Fig. 1).
The febrile response is well preserved across the animal
kingdom, with some experimental evidence suggesting it
may be a beneficial response to infection. Retrospective
data analysis shows that a raised temperature in patients
with infection in the first 24 h following admission to
the intensive care unit (ICU) is associated with a better
outcome compared with normothermia or hyperther-
mia above 40 °C [13], and that a temperature between
37.5 °C and 39.4 °C trends towards improved outcome
compared with normothermia [14]. In elderly patients
with community-acquired pneumonia, the observed
mortalityratewassignificantlyhigherinpatientswho
lacked fever (29 %) when compared with patients who
developed a febrile response (4 %) [15]. A temperature
greater than 38.2 °C has also been found to have a pro-
tective role against invasive fungal infections in the
ICU [16]. The raised temperature may provide protec-
tion by several mechanisms. Firstly, human infective
pathogens often demonstrate optimal replication at
temperatures below 37 °C; thus an elevated host
temperature inhibits reproduction [17]. Secondly, in-
creasing the temperature in vitro from 35 °C to 41.5 °C
increases the antimicrobial activity of many classes of
antibiotics [18]. Thirdly, a rise in temperature may also
Fig. 1 Proposed mechanisms for the generation of fever in sepsis. Stimulation of sentinel cells by exogenous pyrogens produces endogenous
pyrogens which stimulate fever production in the pre-optic area (POA) of the hypothalamus by the second messengers prostaglandin E
2
(PGE
2
),
and ceramide. PGE
2
is also produced from Kupffer cells in the liver in response to stimulation from lipopolysaccharide (LPS), which additionally
stimulates the POA via the vagus nerve. OVLT organum vasculosum of the lamina terminalis
Walter et al. Critical Care (2016) 20:200 Page 2 of 10
be associated with an increase in innate immunity asso-
ciated with microbial destruction [19]. Interestingly, at
temperatures above around 40 °C there is a further
mortality increase [13, 14], suggesting that at this stage
the deleterious effects of hyperthermia on organ and
cellular function outweigh any benefit conferred from
hyperpyrexia in acute sepsis. These potential benefits of
fever in sepsis may not be well recognised; in one survey
of fever monitoring in sepsis from UK ICUs, 76 % of ICU
physicians would be concerned about a temperature of
38–39 °C, and 66 % would initiate active cooling at that
point [20].
In contrast with a fever in response to sepsis, a
non-pyrogenic fever is not of any perceived teleological
benefit. A temperature of 37.5 °C or greater at any point
during an ICU admission trends towards a worse out-
come, and becomes significant at temperatures greater
than 38.5 °C [14].
Fever associated with inflammation
In critically ill patients, inflammation is commonly ob-
served to aid repair after traumatic or infective insults.
The four cardinal features of pain, heat, redness, and
swelling were originally described by Celsus around
2000 years ago and, at about the same time, Hippocrates
noted that the fever was of benefit. Fever is a ubiquitous
component of inflammation across the animal kingdom,
and enhances the host response. A large number of both
the cell-derived and plasma-derived inflammatory medi-
ators are pyrogenic; fever associated with inflammation
is probably mediated in a similar way to sepsis as de-
scribed above. Chronic inflammation is deleterious; the
recently described compensatory anti-inflammatory re-
sponse syndrome (CARS) restores homeostasis, and it is
likely that the magnitude and relative timings of the in-
flammatory and anti-inflammatory responses are both
important in determining the host outcome.
Fever in patients with malignancy is reported to be sep-
sis related in around two thirds of cases [21]. The tumour
is the direct cause of fever in less than 10 % of febrile
episodes; tumour necrosis and production of pyrogenic
cytokines is the likely pathogenesis [21].
Regulated autoimmunity is considered to be a natural
physiological reaction; however, pathological autoimmun-
ity occurs because of higher titres of more antigen-specific
antibodies, often of the IgG isoform, and a reduction in
self-tolerance. There are five pathogenic processes associ-
ated with autoimmune disease development, and in excess
of 80 diseases have been described; fever is considered to
be cytokine mediated in the majority of cases [22].
Autoinflammatory conditions differ from autoimmune
diseases. In the former, the innate immune system dir-
ectly causes inflammation without a significant T-cell re-
sponse, whereas in the latter the innate immune system
activates the adaptive immune system, which is in itself
responsible for the inflammatory process. The former
are also known as periodic fever syndromes, highlighting
the intermittent febrile nature of these conditions.
Examples include familial Mediterranean fever and
some arthopathies, including adult-onset Still’s disease.
Most autoinflammatory conditions are genetic, and a large
number are related to abnormalities in pro-inflammatory
cytokine handling, for example IL-1 or interferon (IFN)
signalling, or constitutive NF-kB activation, offering thera-
peutic targets.
Drug-induced fever
The causes of drug-induced fever are shown in Table 1
[23]. Pharmacological agents may cause fever by a num-
ber of pathophysiological mechanisms. These include
interference with the physiological mechanisms of heat
loss from the peripheries, interference with central
temperature regulation, direct damage to tissues, stimu-
lation of an immune response, or pyrogenic properties
of the drug.
A common mechanism in many of these drugs is con-
sidered to be stimulation of non-shivering thermogenesis
(NST), primarily in brown adipose tissue and skeletal
muscle. Under normal conditions, cellular oxidative
phosphorylation allows the synthesis of ATP from ADP
for cellular metabolism. NST uncouples the proton
movement from this pathway, allowing the energy to be
dissipated as heat, under the control of uncoupling pro-
teins, ultimately influenced by thyroid hormones and
catecholamines. A number of agents, including sympa-
thomimetics and those which act via the serotonin path-
way, are thought to cause fever by modifying the NST
pathway at a central, peripheral, or cellular level [24].
Fever after brain injury
Fever after acute brain damage, from trauma or a vascu-
lar event, is common, and is independently associated
with a worse outcome. The mechanism of fever gener-
ation is probably multi-factorial; 41 % of deaths after
traumatic brain injury (TBI) in one series displayed
hypothalamic lesions, suggesting thermal dysregulation
in some cases [25]. Alterations in cellular metabolism, a
shift to anaerobic metabolism, and ischaemic–reperfu-
sion injury are all associated with thermogenesis [26].
The cerebral production of a large number of inflamma-
tory and pyrogenic cytokines is increased acutely [27];
IL-6 in particular is associated with fever production
after a stroke, and with a worse outcome. After cerebral
haemorrhage, both the presence of blood and the pres-
ence of its degradation products are associated with heat
production [28]. Recent work suggests a protective role for
uncoupling of mitochondrial oxidative phosphorylation fol-
lowing neurotrauma under the regulation of uncoupling
Walter et al. Critical Care (2016) 20:200 Page 3 of 10
proteins [29]; the dissipation of the proton gradient pro-
duces heat.
Brain injury following a cardiac arrest is well recognised,
but the pathology is complex and probably involves mul-
tiple mechanisms, including cell death, excitotoxicity, cell
signalling changes, ischaemia–reperfusion, and alterations
in cellular metabolism [30]; this is very similar to those de-
scribed following brain injury from other causes, and, as
such, the mechanisms of thermogenesis are likely to be
similar. The teleological benefit of pyrexia following brain
injury is uncertain.
Endocrine fever
Thyroid hormones are essential for regulation of energy
metabolism. Hyperthyroidism is associated with hyper-
thermia; patients with thyroid storm have an average
body temperature of 38.0 °C; temperatures above 41 °C
have been reported [31]. The mechanism of thermogenesis
is not clear; the classical view is that metabolism of periph-
eral tissues increases through a peripherally mediated path-
way. Recent work suggests that thyroid hormones may
instead act centrally to increase the hypothalamic ‘set-
point’, and that centrally driven neurogenic activation of
uncoupling protein-1 acting on brown adipose tissue may
instead be responsible for the thermogenesis [32]. The con-
verse relationship is also present: levels of serum T3, even
in non-thyropathic individuals, decrease with increasing
body temperature and, above 40 °C, T3 levels would be
consistent with severe hypothyroidism. The levels of T4
and thyroid-stimulating hormone (TSH) are unchanged
with changes in body temperature [33].
Adrenal insufficiency is rarely associated with fever,
but the hyperthermia may be related to the underlying
pathology; autoimmunity accounts for the majority of
primary insufficiency. A malignant process, or an infec-
tious process, account for a proportion of the remainder;
all of the patients in the original description had adrenal
tuberculosis [34].
A fever has been reported in 28 % of patients hospita-
lised with a pheochromocytoma [35]; a large tumour, the
presence of necrosis, and higher metabolite excretion in-
crease the likelihood of pyrexia [35].
Mechanisms of damage from fever
There are a number of pathophysiological mechanisms
for the deleterious effects of a fever, classified as follows
(Fig. 2):
Direct cellular damage
Local effects, e.g. stimulation of cytokines and
inflammatory response
Systemic effects, e.g. gut bacterial translocation
Cellular damage
Hyperthermia is directly cytotoxic, affecting membrane
stability and transmembrane transport protein function.
Consequently, ionic transport is disrupted leading to in-
creased intracellular sodium and calcium with a reduced
intracellular potassium concentration. Protein and DNA
synthesis is disrupted at various stages in the pathway;
while RNA and protein synthesis may recover quickly
after cessation of hyperthermia, DNA synthesis remains
disrupted for longer [36]. The nuclear matrix shows dam-
age at lower temperatures than other parts of the cell, with
significant endothermic changes observed at 40 °C [37].
Direct cell death in humans occurs at temperatures of
Table 1 Causes of drug-induced hyperthermia
Class Examples of causes
Antimicrobial
agents
β-lactam antibiotics (piperacillin, cefotaxime)
Sulphonamides
Malignant
hyperthermia
Suxamethonium
Volatile anaesthetic agents
Neuroleptic
malignant
syndrome
Dopamine antagonists (chlorpromazine, haloperidol)
Atypical agents (serotonin and dopamine
antagonists) (olanzapine, risperidone, paliperidone,
aripiprazole, quetiapine)
Serotonin
syndrome
Antidepressants (monoamine oxidase inhibitors,
tricyclic antidepressants, selective serotonin
reuptake inhibitors, serotonin noradrenaline
reuptake inhibitors, bupropion)
Opioids (tramadol, pethidine, fentanyl,
pentazocine, buprenorphine oxycodone,
hydrocodone)
Central nervous system stimulants (MDMA,
amphetamines, sibutramine, methylphenidate,
methamphetamine, cocaine)
Psychedelics (5-methoxy-diisopropyltryptamine,
lysergide)
Herbs (St John’s Wort, Syrian rue, Panax ginseng,
nutmeg, yohimbine)
Others (tryptophan, L-dopa, valproate, buspirone,
lithium, linezolid, chlorpheniramine, risperidone,
olanzapine, antiemetics (ondansetron, granisetron,
metoclopramide), ritonavir, sumatriptan)
Propofol infusion
syndrome
Propofol
Anticholinergic
agents
Anticholinergics (atropine, glycopyrrolate),
Antihistamines (chlorpheniramine),
Antipsychotics (olanzapine, quetiapine),
Antispasmodics (oxybutynin),
Cyclic antidepressants (amitriptyline, doxepin)
Mydriatics (tropicamide)
Sympathimometic
agents
Prescription drugs (e.g. bronchodilators)
Non-prescription drugs (e.g. ephedrine in cold
remedies)
Illegal street drugs (e.g. cocaine, amphetamines,
methamphetamine (‘ecstasy’), mephedrone)
Dietary supplements (e.g. ephedra alkaloids)
Piperazine
compounds
Anti-emetic (cyclizine)
Anti-helminths
Legal ‘club drugs’(‘Legal X’,‘Legal E’,‘Frenzy’)
Synthetic
cathinones
Street drugs (mephedrone, ‘meow-meow’)
Bupropion (anti-depressant and anti-smoking agent)
Taken from [23] with permission
Walter et al. Critical Care (2016) 20:200 Page 4 of 10
around 41 °C, with the rate of cell death increasing mark-
edly with even modest further increases in temperature
[36, 38]. The thermal energy required for cell death is
similar to that required for protein denaturation, suggest-
ing that hyperthermic cell death may occur primarily
through its effect on protein structure, although cell death
occurs primarily through necrosis or from apoptosis de-
pending on the cell line and the temperature [36]. Cells in
mitosis are more thermosensitive than cells in other phases
of replication. Given that organ dysfunction occurs at tem-
peratures lower than that required for in-vitro cell death,
milder degrees of hyperthermia are also likely to affect cell
structure and function with a degree of reversibility.
Local effects
Effect of cytokines and the inflammatory response
The role of cytokines in heat stress is unclear, with an
inconsistent response to thermal stress. The levels of a
number of pro-inflammatory and anti-inflammatory cy-
tokines are elevated at the time of hyperthermia from
heatstroke. Acute phase reactants may also increase. Of
these, some (for example, INFγ, IL-1β) are raised in a
proportion of patients, whereas IL-6 may be elevated in
all patients [39]. Furthermore, there is some correlation
with outcome; the rise in IL-6 and the duration of the
increased expression is related to mortality, independent
of the maximum core temperature obtained [40]. Mice
pre-treated with IL-6 before exposure to heat take longer
to reach 42.4 °C, showing less organ damage, and at-
tenuation in the increase of other cytokines [41].
Antagonism of IL-1 also improves survival [42].
The cytokine profile of the two forms of heatstroke,
classical and exertional, show similarities, and mirrors
that produced by exercise [43]. The profile also shows
similarities to that produced by endotoxaemia, which is
considered to be of importance in the cytokine expres-
sion—abolition of endotoxaemia significantly reduces
cytokine production [43].
Development of other hyperthermic states may also be
associated with inflammatory mediators. Neuroleptic
malignant syndrome (NMS) may be at least partly driven
by an acute phase response; acute phase response medi-
ators are reported to rise, and peak at 72 h. Conversely,
levels of anti-inflammatory agents such as serum iron
and albumin initially decline then return to the normal
range, coinciding with clinical improvement [44]. It is
proposed that the acute phase response may be triggered
by the heat stress per se, or by muscle breakdown, or by
interaction between a virus and the drug, or the immune
system [45]. IL-6 and TNFαlevels have also been found
to be significantly increased in NMS [46], as has IL-6 in
malignant hyperthermia (MH) [47].
Protection by heat shock proteins
Heat shock proteins (HSP) are a family of cell-derived
proteins that offer protection against a range of insults,
Fig. 2 Diagrammatic representation of the mechanisms of damage from hyperthermia
Walter et al. Critical Care (2016) 20:200 Page 5 of 10
including heat. They are expressed in response to the insult,
and their effect may depend on their location. Intracellu-
larly located HSPs have a protective role, including correct-
ing misfolded proteins, preventing protein aggregation,
transport of proteins, and supporting antigen processing
and presentation, and limiting apoptosis. In contrast,
membrane-bound or extracellular HSPs may be immunos-
timulatory, and appear to induce cytokine release or pro-
vide recognition sites for natural killer cells. HSPs may also
have both pro-apoptotic and anti-apoptotic actions [48, 49].
Vascular changes
Animal studies suggest that changes to the vasculature
occur rapidly after the onset of hyperthermia and, while
some organs are more tolerant to heat stress than others,
the majority of organs show similar changes consisting of
capillary dilatation, vascular stasis, and extravasation into
the interstitium, observed after 30 min at 40.5 °C [50].
Systemic effects
Gastrointestinal bacterial and endotoxin translocation
Non-pyrogenic hyperthermia increases gut bacterial
translocation and the gastrointestinal (GI) tract and BBB
appear to be more permeable to toxins than during
normothermia [51, 52]. Bacterial and endotoxin trans-
location are also implicated in the development of
multi-organ dysfunction in non-pyrogenic hyperthermia.
For example, antibiotic administration to dogs with heat-
stroke appears to improve their survival, suggesting that
bacteraemia may have a role even in non-pyrogenic condi-
tions [53]. In a similar study, raising the core temperature
in monkeys from 37.5 °C to 39.5 °C and then up to 44.5 °C
increased plasma LPS concentration. In the animals pre-
treated with oral kanamycin, which is very poorly
absorbed, and heated to 44.5 °C, no increase in plasma
LPS concentrations were seen and there was improved
haemodynamic stability, suggesting that the plasma LPS
originated from the GI tract [54]. Epidemiological studies
after classical heatstroke have demonstrated that over
50 % of heatstroke patients show evidence of concomitant
bacterial infections [55]. Furthermore, procalcitonin,
which has a high sensitivity and specificity for detecting
bacteraemia, was elevated in 58 % of patients with classical
heatstroke, which was associated with mortality [56]. How-
ever, microbiological and clinical evidence of infection was
not significantly higher in this group, and therefore it is un-
clear whether this represents undiagnosed bacteraemia or
procalcitonin elevated in the absence of infection.
Genetics
Genotypic and phenotypic differences may account for
how tolerant a particular individual is to heat exposure.
Individuals who demonstrate heat-intolerance may show a
reduction in HSP levels and, in addition, their vasculature
may be less reactive to heat stress [57]. Well-described
genotypic differences are seen in particular conditions.
MH affects up to 1 in 5000 patients, and is more common
in males and in young people, although it can affect all
age groups including neonates [58]. It has also been ob-
served in other species, such as dogs, cats, horses and pigs.
Mutation in the ryanodine receptor (RYR) accounts for up
to 70 % of cases, with more recent genetic abnormalities
also having been identified [59]. RYRs in the sarcoplasmic
reticulum of skeletal muscle form calcium channels and
are the main mediators of calcium-induced calcium re-
lease in animal cells. In MH, the RYR functions abnor-
mally such that calcium is released in a greater than
normal amount and heat is generated during the process-
ing of this excess calcium. The first documented survivor
of MH was in Australia in 1961; a young man required
surgery for a fractured tibia. Ten of his family members
had previously developed uncontrolled hyperthermia and
died during general anaesthesia with ether [60].
Exertional heatstroke (EHS) is increasingly observed in
endurance athletes [61]. EHS has clinical and biochem-
ical similarities to MH, and there are case reports of pa-
tients with both conditions. While some patients with
EHS display mutations in the RYR1 gene, the genetics
probably differ from MH, although some authorities ad-
vise that heatstroke patients should go on to be tested
for MH as they may be susceptible to its development
[62]. Recently, there has been some interest in another
similar sarcoplasmic skeletal muscle protein, calseques-
trin (CASQ1), which appears to modulate the function
of RYR1. Ablation of CASQ1 in mice increases the risk
of MH-like episodes when exposed to both heat and
halothane, supporting the possibility that there is a gen-
etic basis to EHS similar to that of MH [63].
Other hyperthermic states may also have a genetic
basis. Genetic mutations or polymorphisms in the dopa-
mine D2 receptor, serotonin receptor, and cytochrome
P450 2D6 have been studied in cases of NMS [64]. Such
cases may run in families, suggesting a genetic mechanism
for predisposition to the syndrome. In a study of patients
who had developed NMS, the frequency of the A1 allele
of the DA2 receptor was significantly higher in the pa-
tients who developed NMS (56.8 %) than in the control
group of patients with schizophrenia who had not
(35.1 %). The proportion of patients who were A1 carriers
was significantly higher in the patients with NMS com-
pared with those without (93.3 % vs 57.2 %) [65]. However,
the relationship between NMS and serotonin receptor
mutations remains currently undetermined. Early work in
patients who are genetically deficient in the cytochrome
P450 2D6 enzyme suggests that they may be more suscep-
tible to the effects of serotonin-containing drugs [66].
EHS is more common in men than women; whether
this is the protective effect of oestrogen, or the reduced
Walter et al. Critical Care (2016) 20:200 Page 6 of 10
muscled muscle bulk in women compared with men, or
genetic differences is not clear.
Deleterious consequences of pyrexia
Most patients fully recover after a period of hyperther-
mia, but patients exposed to higher temperatures and
for longer periods of time are more at risk of complica-
tions, which may lead to multi-organ failure and death
in extreme cases. The similarities between the different
hyperthermic aetiologies suggest that the pathological
features are at least partly a result of hyperthermia,
irrespective of the cause.
The risk from hyperthermia may be significant; heat-
stroke is the most severe form of heat illness with a mor-
tality rate of up to 58 % [67] to 64 % [68]. Classical
heatstroke, often seen in meteorological heat waves, is re-
sponsible for thousands of excess deaths each year. Most
survivors appear to recover fully, but there is increasing
concern over long-term organ dysfunction, susceptibility
to further injury, and delayed mortality.
Immediate cooling remains the mainstay of treatment,
a delay in a reduction in the temperature being associated
with increased mortality [68]. In classical heatstroke, cool-
ing to below 38.9 °C within 60 min is associated with a
trend towards improved survival [69]. Hyperthermia is as-
sociated with the inflammatory cascade [43]; heatstroke in
particular is considered a pro-inflammatory and pro-
coagulant condition. Given this, steroids [70], mannitol
[70], and recombinant activated protein C [71, 72] have
all been studied as putative treatments, and have shown
benefit in trials; however, none are currently recom-
mended for clinical practice. Anti-pyretic drugs would
not be expected to have a significant effect in non-
pyrogenic hyperthermia and, although non-steroidal
anti-inflammatory drugs (NSAIDs) have not been ex-
tensively studied, aspirin may have beneficial effects on
survival in animal studies [73]. Neither aspirin nor
paracetamol have been shown to be of any proven
benefit in humans and are therefore not recommended
in temperature control in heatstroke.
Specific organ dysfunction
Hyperthermia has many systemic effects, which may
present as specific organ dysfunction.
Gastrointestinal tract
Systemic hyperthermia increases the permeability of the
GI tract, and increases the rate of gut bacterial transloca-
tion. Blood flow to the GI tract is reduced at temperatures
above 40 °C [74] and hyperthermia damages cell mem-
branes, denatures proteins, and may increase oxidative
stress. This leads to loss of the GI barrier integrity and in-
creases the potential for endotoxaemia, which initiates re-
lease of pro-inflammatory cytokines leading to a systemic
inflammatory cascade [51]. GI oedema and petechial
haemorrhage are also described [75].
A theoretical mechanism following hyperthermia to the
GI tract appears to be increased free radical production
from the splanchnic viscera, which may stimulate oxida-
tive stress and contribute to cellular dysfunction [74]. Free
radical production can be increased in the presence of
heavy metals and this may exacerbate cytotoxicity. Heavy
metals themselves may also translocate across a dysfunc-
tional BBB, and are implicated in the development of
hyperthermia-induced neurocognitive dysfunction [76].
Renal
The glomerular filtration rate reduces after an increase of
2 °C, and worsens further with increasing temperature.
Plasma concentrations of creatinine and urea conse-
quently increase [77]. Morphological studies demonstrate
glomerular capillary dilatation, haemorrhage into the in-
terstitium, and vascular stasis, in small and large vessels
[50]. Stimulation of the renin–angiotensin system in
hyperthermia reduces renal blood flow [78]. Direct ther-
mal injury, renal hypoperfusion, and rhabdomyolysis also
probably contribute to acute kidney injury (AKI).
The development of EHS (>40 °C) in endurance ath-
letes significantly increases the risk of AKI compared
with those without EHS. Military data suggest that one
in six hospitalised EHS victims will develop AKI [79] in
comparison with marathon runners generally; the Com-
rades marathon have reported an average of only one
runner each year admitted with renal failure [80].
Classical heatstroke is also associated with the develop-
ment of AKI; for example, of 22 patients admitted to an
ICU after heatstroke during a heatwave, serum creatinine
levels were significantly higher 24 h after admission, and
18 % required renal replacement therapy (RRT). The de-
gree of renal impairment was worse in non-survivors than
in those who survived [68]. Of 58 patients hospitalised
with classical heatstroke during the 1995 Chicago heat
wave, 53 % had at least moderate renal impairment [55].
AKI has been reported in one series of patients with
neuroleptic malignant syndrome to occur in 7 out of 24
(30 %) patients, of whom 2 (8 %) required RRT [81].
Renal failure sufficient to require RRT has also been de-
scribed after hyperthermia due to NMS [82], MH [83]
and recreational drug use [84].
Cardiovascular system
In the acute phase, patients tend to be hypotensive, with
a hyperdynamic circulation and a high cardiac output.
The hypotension is probably a combination of redistri-
bution of blood, and nitric oxide-induced vasodilatation.
The electrocardiogram in heatstroke and MH may show
a variety of abnormalities, including conduction defects,
QT and ST changes, T-wave abnormalities, and malignant
Walter et al. Critical Care (2016) 20:200 Page 7 of 10
arrhythmias [85]. In addition, cardiac dysfunction and as-
sociated pulmonary oedema have also been described [86].
In common with other organs, myocardial vessels are
dilated, and extravasation occurs into the myofibril
structure. Fragmentation of the myocardial fibres occurs
[50]. Serum troponin I levels are significantly raised and,
interestingly, more so in non-survivors [68]. Whether
this represents myocardial cytotoxicity, myocardial dis-
ruption, or another problem is not currently clear.
Brain
Neurological and cognitive dysfunction may occur acutely
after an episode of hyperthermia and may lead to chronic
damage, reported to occur in 50 % of survivors discharged
from an ICU after heatstroke [87]. The pathophysiological
mechanisms are presumed to be similar to those described
above, but, in addition, the integrity of the BBB is dis-
rupted allowing translocation of systemic toxins to enter
the cerebral circulation. If neurological symptoms fail to
improve after the acute episode, cerebellar dysfunction
predominates. This is thought to be a result of the sensi-
tivity of the Purkinje cells to thermal damage.
Liver failure
Liver dysfunction is common. At temperatures above
40 °C, elevations in plasma aspartate transaminase (AST)
and alanine transaminase (ALT) are observed [88] and the
hepatocellular damage has been sufficient to require trans-
plant in some cases; however, results from transplantation
are disappointing, with only a minority surviving long-
term [89]. Hence, conservative management has been ad-
vocated in patients who would otherwise meet the criteria
for transplantation [89].
Similar to histological changes in other organs, small
and large vessel dilatation is seen, with stasis and haemor-
rhage [50]. A reduction in liver blood flow is also impli-
cated [90]. Liver dysfunction may continue to deteriorate
even after cessation of the hyperthermia [68].
Haemostatic system
Coagulopathy is common, with a reported incidence of
45 % in classical heatstroke [55], and probably contrib-
utes to the multi-organ dysfunction in hyperthermia.
Thrombocytopenia, increased plasma fibrin degradation
products, prolonged clotting times, and spontaneous
bleeding are often seen. This probably reflects hepatic
dysfunction, as coagulopathy is rare without liver de-
rangement and is temporally related to alterations in
liver function [91]. Hyperthermia inhibits platelet aggre-
gation, which becomes increasingly marked at higher
temperatures, and may begin to happen at 38 °C [92].
Disseminated intravascular coagulation (DIC) may also
be driven by release of pro-coagulant cellular compo-
nents from damaged muscle.
Long-term follow-up
Even in survivors of the acute episode, hyperthermia re-
duces life expectancy and worsens functional outcome.
In one epidemiological study of patients with classical
heatstroke, the 28-day mortality was 58 %, increasing to
71 % at 2 years [67]. An episode of exertional heatstroke
is associated with an increased risk of mortality of 40 %
after recovery from the initial episode [93].
Heatstroke is reported to cause moderate to severe
functional impairment in 33 % of survivors at 1 year
[55], with 41 % of survivors requiring institutional care
at 1 year [66]. There may be little or no improvement
after discharge from hospital [55].
Conclusions
A mild elevation in core temperature is of benefit in sepsis.
Non-pyrogenic hyperthermia isassociatedwithshort-term,
medium-term, and long-term effects in a variety of organs.
The damage occurs via a number of local and systemic
mechanisms. Additionally, there appears to be emerging
evidence of an overlap in the mechanisms of heat gener-
ation in different conditions. The evidence is that in sepsis
thebeneficialeffectsofpyrexiamaybalancethesedeleteri-
ous factors. However, in non-sepsis, the accumulation of
the deleterious consequences of hyperthermia occurs early,
at even mild degrees of fever. Hyperthermia above 40 °C
appears to carry a high mortality by whatever cause. Early
recognition, immediate cooling, and organ support are the
mainstays of treatment, and to this end an improved under-
standing of the pathophysiology will continue to develop.
Abbreviations
AKI, acute kidney injury; BBB, blood–brain barrier; CASQ1, calsequestrin; EHS,
exertional heatstroke; GI, gastrointestinal; HSP, heat shock proteins; ICU,
intensive care unit; IFN, interferon; IL, interleukin; LPS, lipopolysaccharides;
MH, malignant hyperthermia; NMS, neuroleptic malignant syndrome; NSAID,
non-steroidal anti-inflammatory drug; NST, non-shivering thermogenesis;
OVLT, organum vasculosum of the lamina terminalis; PG, prostaglandin; RRT, renal
replacement therapy; RYR, ryanodine receptor; TNF, tumour necrosis factor
Authors’contributions
EJW, SH-J, MC and LF contributed to the literature review and the drafting of
the manuscript. All authors read and approved the final manuscript.
Authors’information
EJW has an interest in pre-hospital and ICU hyperthermia, and has provided
medical cover for major sporting and public events. He is lead author of the
FSEM UK heatstroke consensus guidelines, and has also published and
spoken internationally in this area.
Competing interests
The authors declare that they have no competing interests.
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