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Heat-related illness is a spectrum of disease ranging from minor heat disorders, such as prickly heat and heat cramps, to severe heat imbalance that results in life-threatening heat stroke.


Malignant Heat Illnesses

Heat stroke develops in the setting of unmitigated heat imbalance. The cardinal features are hyperthermia (core temperature >40°C [104°F]) and end-organ injury. Height of the temperature and duration of heat exposure dictate degree of injury. Neural tissue, hepatocytes, nephrons, and vascular endothelium are most sensitive to heat stress. Prominent neurologic abnormalities include confusion, agitation, bizarre behavior, ataxia, seizures, and coma. Other bedside findings include hyperventilation, vomiting, diarrhea, and oliguria. Anhidrosis or profuse sweating may be seen. When coagulopathy/DIC develops, patients may have purpura, hemoptysis, or GI bleeding.

Heat stroke is further classified as exertional and nonexertional. Physical activity during high heat/humidity produces exertional heat stroke. Evaporation is the principal mechanism of heat loss, but is ineffective when humidity exceeds 75%. Nonexertional heat stoke is seen in the debilitated, chronically ill, and persons who are otherwise unable to escape from hot environments (e.g., closed vehicles, heavily bundled in crib, or isolated in a hot residence).

Minor Heat Illnesses

Heat syncope results from volume depletion, peripheral vasodilation, and decreased vasomotor tone. It occurs most commonly in the elderly and poorly acclimatized individuals. Postural vital signs may or may not be demonstrable on presentation to the emergency department. Exclude other causes of syncope.

Heat cramps are characterized by painful muscle spasms, especially in the calves, thighs, and shoulders during athletic events. They usually occur when individuals replace evaporative losses with free water but not with salt. Core body temperature may be normal or elevated.

Heat edema and heat rash are usually self-limited symptoms during the first few days of heat exposure and are characterized by swelling of dependent extremities (e.g., hands and feet) or rash found commonly over clothed areas of the body.


The differential diagnosis includes infection (e.g., sepsis, meningitis, encephalitis, malaria, typhoid, tetanus), endocrine disorders (e.g., diabetic ketoacidosis, thyroid storm), neurologic disorders (e.g., cerebrovascular accident, status epilepticus), and toxicologic causes (e.g., amphetamines for ADHD, dietary supplements, anticholinergics, sympathomimetics, salicylates, serotonin syndrome, malignant hyperthermia, neuroleptic malignant syndrome, alcohol or benzodiazepine withdrawal).


Malignant Heat Illnesses

Acute care requires accurate identification of hyperthermia with end-organ injury and rapid mitigation of the heat imbalance. Initial diagnostic studies are directed at detecting end-organ damage and excluding other disease processes. Early laboratory abnormalities associated with heat stroke include hypoglycemia, hypophosphatemia, hypokalemia, elevated liver enzymes, hypercalcemia, elevated hematocrit along with elevated creatine phosphokinase and myoglobin from rhabdomyolysis. Laboratory abnormalities of DIC and renal failure may develop ...

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