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For further information, see CMDT Part 37-02: Disorders due to Heat

Key Features

Essentials of Diagnosis

  • Spectrum of preventable heat-related illnesses: heat cramps, heat exhaustion, heat syncope, and heat stroke

  • Heat stroke: the most serious disorder, defined as hyperthermia with cerebral dysfunction in a patient with heat exposure

  • Best outcome: early recognition, initiation of rapid cooling, and avoidance of shivering during cooling

  • Best choice of cooling method: whichever can be instituted the fastest with the least compromise to the patient; delays in cooling result in higher morbidity and mortality in heat stroke victims

General Considerations

Heat syncope

  • Sudden collapse may result in unconsciousness from cutaneous vasodilation and volume depletion with consequent systemic and cerebral hypotension

  • May occur during or immediately following exercise

Heat cramps

  • Muscle cramps result from dilutional hyponatremia as sweat losses are replaced with water alone

Heat exhaustion

  • Exhaustion results from prolonged strenuous activity in a hot environment with inadequate water or salt intake

  • Characterized by dehydration, sodium depletion, or isotonic fluid loss with accompanying cardiovascular changes

  • May progress to heat stroke if sweating ceases and mental status changes

Heat stroke

  • A life-threatening medical emergency

  • Classic heat stroke occurs in patients with impaired thermoregulatory mechanisms or in extreme environmental conditions

  • Exertional heat stroke occurs in healthy persons undergoing strenuous exertion in a hot or humid environment


  • Persons at greatest risk for heat stroke

    • Very young

    • Elderly (age > 65 years)

    • Chronically ill

    • Patients taking medications that interfere with heat-dissipating mechanisms

Clinical Findings

Symptoms and Signs

Heat syncope

  • Systolic blood pressure usually < 100 mm Hg; weak pulse

  • Skin typically cool, moist

Heat cramps

  • Slow, painful skeletal muscle contractions

  • Severe muscle spasms last 1–3 min

  • Muscles tender, hard, and lumpy; may be twitching

  • Skin moist, cool

  • Victim alert, with stable vital signs, but may be agitated and complain of focal pain

  • Body temperature may be normal or slightly increased

Heat exhaustion

  • Rectal temperature > 37.8°C (> 100°F), tachycardia, and moist skin

  • Symptoms associated with heat syncope and heat cramps may be present

  • Additional symptoms

    • Nausea, vomiting

    • Malaise

    • Myalgias

    • Hyperventilation

    • Thirst

    • Weakness

  • Central nervous system symptoms

    • Headache

    • Dizziness

    • Fatigue

    • Anxiety

    • Paresthesias

    • Impaired judgment

    • Psychosis (occasionally)

  • Hyperventilation secondary to heat exhaustion can lead to respiratory alkalosis

  • Lactic acidosis may also occur due to poor tissue perfusion

Heat stroke

  • Presenting symptoms include all those seen in heat exhaustion with additional symptoms of

    • Dizziness

    • Weakness

    • Emotional lability

    • Confusion

    • Delirium

    • Blurred vision

    • Convulsions


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