Exertional Heat Illness

During exercise, working muscle produces large amounts of heat, which the body loses mainly by sweating and the evaporation of that sweat, along with radiation and convection. When heat gain outstrips heat loss, for example in hot, humid conditions where evaporation is impaired, core temperature rises. Recognised risk factors include high environmental heat and humidity, high-intensity or prolonged exercise, dehydration, a lack of acclimatisation, heavy clothing or protective equipment, being unwell such as with a recent fever, obesity, and some medications and supplements.

The core severity continuum runs from heat exhaustion to exertional heat stroke. Exercise-associated muscle cramps, historically called heat cramps, are painful cramps during or after exercise; they are common and usually benign, are not necessarily caused by heat or electrolyte loss, and do not represent an inevitable step towards heat stroke. Heat exhaustion is the inability to keep exercising effectively because of heat, with fatigue and cardiovascular strain but without significant central nervous system dysfunction, and the core temperature is usually below 40 degrees Celsius. Exertional heat stroke is the most severe, defined by central nervous system dysfunction with a high core temperature. This is typically above 40 degrees Celsius, but that figure is a guide rather than a rigid cut-off, and cooling must not be withheld if a delayed or post-cooling temperature is below it. Untreated, it can rapidly damage multiple organs and be fatal.

Exercise-associated muscle cramps present as painful, sometimes severe muscle cramps. Heat exhaustion presents with fatigue, dizziness, headache, nausea, heavy sweating, a fast pulse and the inability to keep going, but the athlete stays mentally coherent and the core temperature is usually below 40 degrees Celsius. Exertional heat stroke presents with a high core temperature and central nervous system dysfunction, which can include confusion, disorientation, irritability or aggression, unsteadiness, collapse, seizures and loss of consciousness. Importantly, an athlete with exertional heat stroke may still be sweating, so the presence of sweat does not exclude it.

The critical differential is exercise-associated hyponatraemia, caused by drinking too much during prolonged exercise, which can also cause confusion and collapse but reflects a low blood sodium rather than overheating, and giving more fluid makes it worse. Temperature and fluid history alone do not separate the causes, since a normal temperature has several possible explanations for collapse, so point-of-care sodium and glucose testing should be used where available, without delaying cooling.

The key field investigation is an accurate core temperature, and rectal temperature is the preferred measurement during and immediately after intense exercise, since oral, tympanic (ear), axillary (armpit) and skin readings are inaccurate and can falsely reassure. If a rectal measurement is not available, cooling must still begin immediately when severe heat illness is suspected, rather than waiting. Exertional heat stroke is diagnosed on central nervous system dysfunction with a high core temperature, and point-of-care sodium and glucose testing, where available, helps identify hyponatraemia and hypoglycaemia without delaying cooling.

In hospital, blood tests assess sodium, kidney and liver function, muscle breakdown through creatine kinase (CK), and clotting, since exertional heat stroke can cause muscle breakdown, organ injury and disseminated intravascular coagulation (DIC). None of this must delay cooling, which begins on clinical recognition.

Exertional heat stroke is a medical emergency, and the priority is immediate, aggressive cooling alongside calling emergency services and attending to airway, breathing and circulation. Cold water immersion is the most effective method and is started as soon as possible, removing clothing and equipment, with continuous rectal temperature monitoring during immersion and cooling stopped at 38.5 to 39 degrees Celsius to avoid overcooling. Where immersion is not available, the effective alternative is to move the athlete into shade, strip clothing, spray or douse the whole body with water and fan vigorously, since ice packs alone give inadequate cooling.

The guiding principle is cool first, transport second, because outcome depends on the time spent above a critical core temperature, so on-site cooling to a safe temperature before transport saves lives, as long as this does not delay definitive care where adequate cooling cannot be achieved. All severe cases still require transfer to hospital after cooling for assessment and serial organ-function testing. Heat exhaustion is managed by stopping exercise, moving to shade or a cool area, removing excess clothing, lying the athlete down with the legs raised, giving oral fluids if they can drink, and cooling, and most recover; they are then observed for at least 30 minutes, safety-netted with medical follow-up, and not allowed to return to strenuous activity the same day, while heat stroke and hyponatraemia are excluded. Exercise-associated muscle cramps are treated with rest, gentle stretching and fluids.

The crucial caution is that if exercise-associated hyponatraemia is suspected, from collapse with a normal or low temperature or a history of heavy drinking, large volumes of dilute fluid must not be given, as this worsens it, and severe cases need hospital treatment that may include concentrated (hypertonic) saline.

Prevention centres on acclimatisation, introduced gradually over one to two weeks, sensible hydration that avoids both dehydration and overdrinking, with drinking to thirst a reasonable guide, suitable clothing, and scheduling and modifying activity according to the heat and humidity, often guided by the wet bulb globe temperature (WBGT), a measure that combines temperature, humidity and radiant heat. At events, a heat plan matters, with shade, cooling, a rectal thermometer and ideally a cold water immersion facility, along with identifying participants at higher risk.

Return to sport after exertional heat stroke requires resolution of symptoms, normal kidney, liver, muscle and clotting blood results, and medical clearance, followed by graded exercise, initially in a cool environment before progressive heat exposure and reacclimatisation. Heat-tolerance testing is reserved for selected cases. Recovery can take several weeks, and the aim is a safe, monitored return rather than a fixed timeline.

• •The 2024 United Kingdom Faculty of Pre-Hospital Care consensus on exertional heat illness sets out recognition, on-site cooling and the cool-first-transport-second approach.
• •The International Olympic Committee consensus on heat informs athlete protection and event planning.
• •Position statements from sports medicine bodies also address the recognition and immediate cooling of exertional heat stroke.
• •Guidance on the wet bulb globe temperature and on exercise-associated hyponatraemia supports prevention and the management of the main mimic.

• •Exertional heat illness ranges from exercise-associated muscle cramps and heat exhaustion to exertional heat stroke, a life-threatening emergency.
• •Exertional heat stroke is central nervous system dysfunction with a high core temperature, and cooling is not withheld just because a delayed or post-cooling temperature is below 40 degrees Celsius.
• •Rectal temperature is the preferred field measurement, and an athlete with heat stroke may still be sweating.
• •Cool first, transport second, with cold water immersion the most effective treatment, and all severe cases still go to hospital afterwards.
• •For collapse, consider exercise-associated hyponatraemia, hypoglycaemia and cardiac causes, and do not give large volumes of fluid if hyponatraemia is possible.
• •Prevention relies on acclimatisation, sensible hydration, suitable clothing and scheduling by heat and humidity.