Heatstroke is hyperthermia accompanied by a systemic inflammatory response causing multiple organ dysfunction that may result in death. Defining characteristics include temperature > 40° C and altered mental status; sweating may be absent or present. Diagnosis is clinical. Treatment includes rapid cooling, IV fluid resuscitation, and support as needed for organ dysfunction.
Heatstroke occurs when compensatory mechanisms for dissipating heat fail and core temperature increases substantially. Inflammatory cytokines are activated, and multi-organ dysfunction may develop. Organ dysfunction may occur in the central nervous system (CNS), skeletal muscle (rhabdomyolysis), liver, kidneys, lungs (acute respiratory distress syndrome), and heart. Hyperkalemia and hypoglycemia may occur. The coagulation cascade is activated, sometimes causing disseminated intravascular coagulation.
Heatstroke is sometimes divided into 2 variants (1), although the usefulness of this classification is controversial (see table Some Differences Between Classic and Exertional Heatstroke):
Classic
Exertional
Classic heatstroke takes 2 to 3 days of exposure to develop. It often occurs during summer heat waves, typically in older, sedentary adults with no air-conditioning and often with limited access to fluids. It can occur rapidly in infants or children left in a hot car, particularly with closed windows. Other vulnerable populations include pregnant people and those with obesity. All of these groups lack robust thermoregulatory mechanisms (1).
Exertional heatstroke occurs more abruptly and affects healthy active people (eg, athletes, military recruits, factory workers) who are typically performing strenuous activity or work in hot weather (2, 3, 4). It is a common cause of death in young athletes. Intense exertion in a hot environment causes a sudden massive heat load that overwhelms physiologic heat loss mechanisms. Rhabdomyolysis is common; acute kidney injury and coagulopathy are somewhat more likely and severe. Heat exhaustion can transition to heatstroke as heat illness progresses and is characterized by impairment of mental status and neurologic function.
Some Differences Between Classic and Exertional Heatstroke
Characteristic | Classic Heatstroke | Exertional Heatstroke |
|---|---|---|
Onset | 2–3 days | Hours |
Patients usually affected | Older, sedentary adults; young children; those with medical conditions | Healthy active people (eg, athletes, military recruits, factory workers) |
Risk factors | No air-conditioning during summer heat waves Limited access to fluids | Intense exertion, particularly without acclimatization |
Skin | Usually hot and dry but sometimes moist with sweat | Often moist with sweat |
Heatstroke may occur after using stimulant medications or substances (eg, cocaine, phencyclidine [PCP], amphetamines), monoamine oxidase inhibitors, or anticholinergic medications (eg, antihistamines, antimuscarinics) that cause a hypermetabolic state or impair the ability to sweat. Usually, an overdose is required, but exertion and environmental conditions can be additive.Heatstroke may occur after using stimulant medications or substances (eg, cocaine, phencyclidine [PCP], amphetamines), monoamine oxidase inhibitors, or anticholinergic medications (eg, antihistamines, antimuscarinics) that cause a hypermetabolic state or impair the ability to sweat. Usually, an overdose is required, but exertion and environmental conditions can be additive.
(See also Overview of Heat Illness.)
References
1. Sorensen C, Hess J. Treatment and Prevention of Heat-Related Illness. N Engl J Med. 2022;387(15):1404-1413. doi:10.1056/NEJMcp2210623
2. Casa DJ, DeMartini JK, Bergeron MF, et al. National Athletic Trainers' Association Position Statement: Exertional Heat Illnesses [published correction appears in J Athl Train. 2017 Apr;52(4):401. doi: 10.4085/1062-6050-52.4.07.]. J Athl Train. 2015;50(9):986-1000. doi:10.4085/1062-6050-50.9.07
3. Yankelson L, Sadeh B, Gershovitz L, et al. Life-threatening events during endurance sports: is heat stroke more prevalent than arrhythmic death?. J Am Coll Cardiol. 2014;64(5):463-469. doi:10.1016/j.jacc.2014.05.025
4. Gamage PJ, Fortington LV, Finch CF. Epidemiology of exertional heat illnesses in organised sports: A systematic review. J Sci Med Sport. 2020;23(8):701-709. doi:10.1016/j.jsams.2020.02.008
Symptoms and Signs of Heatstroke
Central nervous system (CNS) dysfunction, ranging from confusion or bizarre behavior to delirium, seizures, and coma, is the hallmark of heatstroke. Ataxia may be an early manifestation. Tachycardia, even when the patient is supine, and tachypnea are common. Sweating may be present or absent. Core temperature is > 40° C.
Diagnosis of Heatstroke
History and physical examination, including core (usually rectal) temperature measurement
Laboratory testing for organ dysfunction
Diagnosis is usually clear from a history of exertion and environmental heat (1). Heatstroke is differentiated from heat exhaustion by presence of the following:
CNS dysfunction
Temperature > 40° C
When the diagnosis of heatstroke is not obvious, or mental status changes are present but core temperature is < 40° C, other disorders that can cause CNS dysfunction and hyperthermia should be considered. These disorders include the following:
Acute infection (eg, sepsis, malaria, meningitis, toxic shock syndrome)
Medications and substances
Malignant catatonia
Status epilepticus (interictal)
Stroke
Thyroid storm
Laboratory testing includes complete blood count, prothrombin time, partial thromboplastin time, electrolytes, blood urea nitrogen, creatinine, calcium, creatine kinase (CK), and hepatic profile to evaluate organ function. A urethral catheter is placed to obtain urine, which is checked for occult blood by dipstick, and to monitor output. Tests to detect myoglobin are unnecessary. If a urine sample contains no red blood cells but has a positive reaction for blood and if serum CK is elevated, myoglobinuria is likely. A urine drug screen may be helpful. Continual monitoring of core temperature, usually by rectal, esophageal, or bladder probe, is desired.Laboratory testing includes complete blood count, prothrombin time, partial thromboplastin time, electrolytes, blood urea nitrogen, creatinine, calcium, creatine kinase (CK), and hepatic profile to evaluate organ function. A urethral catheter is placed to obtain urine, which is checked for occult blood by dipstick, and to monitor output. Tests to detect myoglobin are unnecessary. If a urine sample contains no red blood cells but has a positive reaction for blood and if serum CK is elevated, myoglobinuria is likely. A urine drug screen may be helpful. Continual monitoring of core temperature, usually by rectal, esophageal, or bladder probe, is desired.
Diagnosis references
1. Roberts WO, Armstrong LE, Sawka MN, et al. ACSM Expert Consensus Statement on Exertional Heat Illness: Recognition, Management, and Return to Activity. Curr Sports Med Rep. 2023;22(4):134-149. Published 2023 Apr 1. doi:10.1249/JSR.0000000000001058
Treatment of Heatstroke
Aggressive cooling
Transport (or evacuation) to a hospital setting
Intravenous fluid/electrolyte replacement
Aggressive supportive care
Classic and exertional heatstroke are treated similarly. The importance of rapid recognition and effective, aggressive cooling cannot be overemphasized. The initial treatment goal is to lower to core temperature to < 39° C within 30 minutes (1, 2, 3).
Cooling techniques
The main cooling techniques are
Cold water immersion
Evaporative cooling
Ice water or cold water immersion results in the lowest morbidity and mortality rates and is the treatment of choice when available. Large cooling tanks are often used at outdoor activities such as football practices and endurance races (4). In more remote areas, patients may be immersed in a cool pond or stream (1). Immersion can be used in an emergency department if suitable equipment is available and the patient is stable enough (eg, no need for endotracheal intubation, absence of seizures). The rate of heat loss during cooling may be decreased by vasoconstriction and shivering; shivering (as well the discomfort of ice water immersion) can be decreased by giving a benzodiazepine (eg, diazepam or lorazepam) (). Immersion can be used in an emergency department if suitable equipment is available and the patient is stable enough (eg, no need for endotracheal intubation, absence of seizures). The rate of heat loss during cooling may be decreased by vasoconstriction and shivering; shivering (as well the discomfort of ice water immersion) can be decreased by giving a benzodiazepine (eg, diazepam or lorazepam) (4).
Evaporative cooling is also very effective and works best if the patient has adequate peripheral circulation (requiring adequate cardiac output). Evaporative cooling can be accomplished quickly by spraying tepid water over the patient and using a large industrial fan (often used by the janitorial department). The use of warm or tepid water maximizes the skin-to-air vapor pressure gradient and minimizes vasoconstriction and shivering. With this technique, most patients who have heatstroke can be cooled in < 60 minutes. Wet icy towels, or ice or chemical cold packs can be applied to the neck, axillae, and groin or to hairless skin surfaces (ie, palms of hands, soles of feet, cheeks) that contain densely packed subcutaneous vessels to augment cooling, but are not adequate as the sole cooling method.
Adjunctive cooling measures without significant evidence of benefit include cooled IV fluid administration, body cavity lavage with cold saline, and the direct application of ice packs to skin (1). Antipyretics and dantrolene are not recommended for use in heat stroke (). Antipyretics and dantrolene are not recommended for use in heat stroke (1, 4).
Cooling measures should be stopped once temperature reaches approximately 39° C to avoid overcooling and causing iatrogenic hypothermia.
Other measures
Necessary resuscitation should proceed while cooling is done. Neuromuscular blockade with endotracheal intubation and mechanical ventilation may be needed to control shivering and prevent aspiration in obtunded patients. Supplemental oxygen is given because heatstroke increases metabolic demand. IV hydration with isotonic saline solution should be initiated to help decrease core temperature; cooled saline may be used in conjunction with other modalities but is not a primary therapy for heat stroke. Fluid deficits range from minimal (eg, 1 to 2 L) to severe dehydration. IV fluids should be given as boluses, assessing responses and the need for additional boluses by monitoring blood pressure, urine output, and central venous pressures.
Patients should be evacuated or transported to a hospital, admitted to an intensive care unit and observed for multiple organ dysfunction, disseminated intravascular coagulation, and rhabdomyolysis. Hemodialysis may be required. Antipyretics are not indicated as they do not treat the underlying problem, can contribute to liver or kidney damage, and may divert attention and resources from more effective treatment..
Treatment references
1. Eifling KP, Gaudio FG, Dumke C, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2024 Update. Wilderness Environ Med. 2024;35(1_suppl):112S-127S. doi:10.1177/10806032241227924
2. Roberts WO, Armstrong LE, Sawka MN, et al. ACSM Expert Consensus Statement on Exertional Heat Illness: Recognition, Management, and Return to Activity. Curr Sports Med Rep. 2023;22(4):134-149. Published 2023 Apr 1. doi:10.1249/JSR.0000000000001058
3. Barletta JF, Palmieri TL, Toomey SA, et al. Society of Critical Care Medicine Guidelines for the Treatment of Heat Stroke. Crit Care Med. 2025;53(2):e490-e500. doi:10.1097/CCM.0000000000006551
4. Sorensen C, Hess J. Treatment and Prevention of Heat-Related Illness. N Engl J Med. 2022;387(15):1404-1413. doi:10.1056/NEJMcp2210623
Prognosis for Heatstroke
Mortality and morbidity are significant in heatstroke patients but vary markedly with age, underlying disorders, maximum temperature and, most importantly, duration of hyperthermia and promptness of cooling.
Key Points
Heatstroke differs from heat exhaustion by the presence of CNS dysfunction and temperature > 40° C.
If the diagnosis of heatstroke is not obvious in febrile, obtunded patients or the core temperature < 40°, consider a wide variety of other disorders, such as infection, intoxication, thyroid storm, stroke, seizures (interictal), neuroleptic malignant syndrome, and serotonin syndrome.
Rapid recognition of heatstroke and effective, aggressive cooling are extremely important.
Preferentially use ice water or cold water immersion cooling to rapidly cool the patient.
Patients will require intensive care monitoring with aggressive supportive care.
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