Aetiology
Heat illness occurs when thermal loads overwhelm the body's thermoregulatory responses and homoeostasis is altered. Extremes of temperature and humidity make heat dissipation less efficient and can lead to heat illness.[1][2] Heat illness comprises a spectrum of diseases from minor to severe, including heat oedema, heat cramps, heat syncope, heat exhaustion, and life-threatening heat stroke.[1][2] This topic focuses on heat exhaustion and heat stroke in adults.
Physical effort generates intrinsic heat, and can rapidly lead to heat stroke when combined with environmental factors. Older adults and chronically ill patients are at increased risk by virtue of a range of physiological limitations that can also favour progression to heat stroke. Intrinsic factors, including chronic volume depletion, inability to increase cardiovascular output, and normal deficiencies in heat shock protein responses associated with ageing and lack of acclimatisation, can all inhibit the body's ability to respond to heat challenges.[6]
Pathophysiology
The pathophysiology of heat stroke is complex and includes protein denaturation, endotoxin release, and thermoregulatory failure, which contribute to systemic inflammatory response syndrome (similar to septic shock) leading to multi-organ failure and death.[2]
Classification
Clinical classification[1][2]
Classic heat stroke
Due to passive exposure to severe environmental heat, particularly in at-risk patients (e.g., older people) during a heat wave.
Exertional heat stroke
Due to strenuous physical exercise, particularly in younger adults, athletes, and people who exert themselves in the heat (e.g., firefighters, soldiers, construction workers).
Heat exhaustion
Milder form of heat illness.
Core temperature is normal or slightly elevated (37°C to 40°C, compared with >40°C in heat stroke).
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