Heat-Related Illness

A spectrum from mild cramps to life-threatening exertional heat stroke, when heat production exceeds heat loss during exercise.

The spectrum

Condition	Core temp	Key features
Heat cramps	Normal or mildly raised	Painful involuntary spasm, heavy sweating
Heat syncope	Normal	Transient faint, quick recovery (vasodilation, pooling)
Heat exhaustion	Usually under 40°C	Weakness, dizziness, headache, nausea, tachycardia. No CNS dysfunction
Exertional heat stroke	Over 40°C	Hyperthermia with CNS dysfunction, with or without organ failure. Emergency
Exertional hyponatraemia	Variable	Nausea, headache, confusion, seizure. Emergency

The key diagnostic point

Heat exhaustion versus heat stroke is decided by CNS dysfunction, not temperature alone. Any athlete with altered mental status during or after exercise in the heat has heat stroke until proven otherwise.

Measure rectal temperature. It is the only reliable field measure. Oral, axillary, tympanic and temporal readings are unreliable.

Risk factors

Environmental: high temperature, high humidity (dominant, blocks evaporation), low airflow, radiant heat, high WBGT
Individual: poor fitness, no acclimatisation, previous heat illness, obesity, sickle cell trait, dehydration, febrile illness, poor sleep
Drugs: diuretics, anticholinergics, antihistamines, beta-blockers, stimulants, alcohol
Activity: high intensity, protective clothing, no rest breaks, no cooling or medical cover

Prevention

Individual: heat acclimatisation over 1 to 2 weeks, which carries the strongest evidence, drink to thirst, light clothing
Team: buddy system, on-site ice baths, rectal thermometry, emergency action plan
Organisers: avoid peak heat, monitor WBGT, provide shade and fluids, mandatory breaks

Critical tip: cool first, transport second

Cooling begins immediately on scene and takes priority over transport.

Cold water immersion cools at about 0.35°C/min, far faster than other methods.
30-minute window: core temperature below 40°C within 30 minutes of collapse gives survival approaching 100%.
Transport time is lost cooling time.

Sequence:

Recognise altered mental status in a collapsed athlete.
Call for help in parallel with cooling.
Strip and immerse in ice water (shoulders and scalp under, airway clear). If unavailable, ice packs at neck, axillae and groin plus fanning.
Monitor rectal temperature continuously.
Stop at about 38.6 to 39°C to avoid hypothermia.
Transfer only after adequate cooling.

Do not confuse with hyponatraemia. Both cause collapse but need opposite fluid strategies. Check rectal temperature and serum sodium.

References

Roberts et al., ACSM consensus, Curr Sports Med Rep 2023, doi:10.1249/jsr.0000000000001058
Belval et al., Prehosp Emerg Care 2018, doi:10.1080/10903127.2017.1392666
Racinais et al., Br J Sports Med 2015, doi:10.1136/bjsports-2015-094915

Medical disclaimer: this article is general information, not personal medical advice. Always consult a qualified doctor about your own situation.