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Hyperthermia is an acute condition resulting from excessive exposure to heat, it is also known as heat stroke. The homeothermal regulatory mechanisms become overwhelmed and unable to effecively deal with the heat, and body temperature climbs uncontrollably. This is a serious medical emergency that requires immediate hospitalization.

Signs include increasing body temperature, dehydration and lack of sweating, seizures, collapse and decreased consciousness which proceeds rapidly to multi-organ failure and death as the brain 'cooks'.

Heat stroke comes on suddenly. Vigilance is required in order to prevent and treat this rapidly dangerous condition. The first and foremost symptom of a serious heat stroke is that the patient has stopped sweating. Because the evaporation of water is endothermic, body heat is taken away by the evaporation of sweat. When the body is no longer capable of sweating, core temperature will begin to rise immediately and swiftly. The victim will become confused, often hostile, and may seem drunk. The temperature must be lowered immediately, the victim must be hydrated by drinking water. Other substances may be used in place of water if absolutely necessary, however alcohol and caffeine should be avoided at all costs.

A preceding condition is known as heat prostration or heat exhaustion.