Exertional heat stroke in runners

Exertional heat stroke, also known as severe exertional hyperthermia, is a medical emergency that can occur during intense physical activity such as running. It happens when the body can no longer regulate its temperature during exercise, with an excessive rise in body temperature, usually above 40°C, combined with a dysfunction of the thermoregulatory system.

In runners, this can occur in very hot weather, during prolonged exercise or with insufficient hydration. Although rare, exertional heat stroke must be recognised because it can be life-threatening if not treated quickly.

When running, the body produces a great deal of heat. Normally, this heat is dissipated through sweating, skin blood flow and the evaporation of sweat, three mechanisms that keep body temperature within physiological limits despite the intensity of the effort.

When these mechanisms become insufficient — particularly because of ambient heat, humidity or overly prolonged exercise — body temperature rises rapidly and can cause exertional heat stroke.

Exertional heat stroke remains rare in runners, but it can occur during prolonged exercise in hot weather, particularly in races such as marathons.

Several factors can promote hyperthermia while running; a combination of several of these factors greatly increases a runner’s risk of heat stroke.

Environmental conditions

High heat, high humidity, no wind, direct sun exposure.

Exercise-related factors

Long or very intense running, racing or pushing beyond your limits, lack of heat acclimatisation.

Individual factors

Dehydration, lack of sleep, infection or fatigue, taking certain medications.

Signs may appear gradually or suddenly during exercise. Common symptoms include a feeling of intense heat, unusual fatigue, dizziness, headache, nausea, or chills and goosebumps.

Warning signs of severity

Confusion, loss of balance, unusual behaviour, cessation of sweating, loss of consciousness. The presence of neurological signs should raise suspicion of exertional heat stroke requiring immediate treatment.

Acclimatise to the heat

The body takes about 10 to 14 days to adapt to a hot environment. It is advisable to gradually increase the duration and intensity of training when it is hot.

Adapt your hydration

Regular hydration is essential: drink before running, drink regularly during exercise, and replace fluid losses after the session.

Adjust the intensity of exercise

In very hot weather: reduce intensity, favour early-morning or evening runs, and wear breathable clothing.

Listen to your body’s signals

Unusual fatigue, dizziness or chills while running should prompt you to slow down or stop training.