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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
The Bottom Line
- Spectrum: heat cramps → heat exhaustion (core <40°C, intact CNS) → heat stroke (core ≥40°C + CNS dysfunction = EMERGENCY)
- Two types of heat stroke: exertional (young, exercising in heat) and classical/non-exertional (elderly, heatwave, comorbid)
- Heat stroke = core temp ≥40°C + altered mental status (confusion, seizures, coma). Multi-organ failure if untreated
- Treatment: RAPID COOLING is the priority. Cold water immersion (ice bath) is the most effective method — target core <39°C within 30 min
- Alternatives: evaporative cooling (tepid water spray + fan), ice packs to groin/axillae/neck, cold IV fluids
- Dantrolene is NOT effective (unlike malignant hyperthermia). Antipyretics (paracetamol) are NOT effective (thermoregulatory failure, not pyrogen-mediated)
Overview
Heat-related illness encompasses a spectrum of conditions caused by the body's failure to maintain normal core temperature in the face of excessive environmental heat or metabolic heat production. Heat exhaustion is characterised by heavy sweating, fatigue, nausea, headache, and dizziness with core temperature typically below 40°C and preserved neurological function. Heat stroke is defined as a core temperature ≥40°C with central nervous system dysfunction (confusion, delirium, seizures, coma) — it is a life-threatening emergency with mortality rates of 10–50% depending on speed of treatment. Heat stroke is classified as exertional (occurring during strenuous exercise in heat — typically young, fit individuals) or classical/non-exertional (during heatwaves in vulnerable populations — elderly, very young, comorbid, medications that impair thermoregulation). The pathophysiology involves thermoregulatory failure, systemic inflammatory response, endothelial damage, and multi-organ dysfunction (DIC, rhabdomyolysis, AKI, hepatic failure, ARDS).
Epidemiology
Heat-related illness is an increasing concern in the UK due to climate change and more frequent heatwaves. The 2022 UK heatwave caused an estimated 2,800 excess deaths. Exertional heat stroke occurs in military recruits, athletes, and outdoor workers. Classical heat stroke predominantly affects the elderly (>75 years), those with chronic conditions (cardiovascular, respiratory, renal, diabetes, dementia), people taking medications that impair thermoregulation (anticholinergics, diuretics, beta-blockers, antipsychotics), and those living alone without air conditioning. The UK Heatwave Plan provides public health guidance.
Clinical Features
Symptoms
Heat exhaustion: heavy sweating, fatigue, headache, nausea, dizziness, thirst, muscle cramps. Core temp usually <40°C
Heat stroke: confusion, agitation, slurred speech, irritability, hallucinations, seizures, loss of consciousness
May be preceded by sudden cessation of sweating (ominous sign in classical heat stroke — anhidrosis)
Exertional: rapid onset during exercise. Classical: gradual onset over hours to days during heatwave
Signs
Heat stroke: core temperature ≥40°C (rectal preferred — tympanic/oral unreliable)
Altered mental status: confusion, delirium, seizures, coma — the defining feature that distinguishes heat stroke from heat exhaustion
Tachycardia, hypotension (distributive shock from vasodilation)
Hot skin — may be dry (classical) or sweaty (exertional)
Tachypnoea
Signs of DIC: petechiae, prolonged bleeding from venepuncture sites
Investigations
First-line
Core temperatureRectal temperature most accurate. ≥40°C + CNS dysfunction = heat stroke. Tympanic is unreliable in extremes
FBC, U&Es, LFTs, CK, clottingCK elevated (rhabdomyolysis), AKI, hepatic injury, DIC (low platelets, raised INR/APTT, low fibrinogen)
VBG/ABGMetabolic acidosis, raised lactate
Second-line
Blood glucoseHypoglycaemia common (glycogen depletion)
Urine myoglobinDark urine — rhabdomyolysis
ECGArrhythmias, electrolyte-related changes
Specialist
Serial bloodsMonitor for evolving DIC, worsening hepatic/renal injury over 24–72 h. LFTs may peak at 48–72 h
1
Heat exhaustion
- Move to cool/shaded environment
- Remove excess clothing
- Oral rehydration: cool water, sports drinks (if able to swallow)
- Cool water spray and fanning
- Lie flat with legs elevated if dizzy
- Monitor: if no improvement within 30 min or deterioration → treat as heat stroke
2
Heat stroke — RAPID COOLING (the priority)
- ABCDE approach. This is a medical emergency — call for help
- Cold water immersion (CWI): immersion in ice bath/cold water (1–10°C) is GOLD STANDARD for cooling
- Target: reduce core temp to <39°C within 30 minutes of recognition
- If CWI unavailable: evaporative cooling (continuous tepid water spray + fan), ice packs to groin, axillae, neck; cold IV fluids (not a primary cooling method)
- Remove clothing. Continuous core temperature monitoring during cooling
- STOP cooling at 38–39°C (risk of overshoot hypothermia)
- Paracetamol/NSAIDs are NOT effective — this is thermoregulatory failure, not fever from pyrogens
- Dantrolene is NOT effective for heat stroke (unlike malignant hyperthermia which is a different condition)
3
Supportive ICU care
- IV fluid resuscitation — but cautious (risk of pulmonary oedema in multi-organ failure)
- Treat seizures: IV benzodiazepines (lorazepam). Avoid excessive shivering (generates more heat) — if shivering interferes with cooling, benzodiazepines may help
- Correct hypoglycaemia, electrolyte abnormalities
- Monitor and treat: DIC (blood products), rhabdomyolysis (aggressive IV fluids, target UO >1 mL/kg/h), AKI (may need RRT), hepatic failure
- No role for antipyretics or active warming
Complications
- Multi-organ failure: DIC, rhabdomyolysis → AKI, hepatic failure, ARDS, cardiovascular collapse — develops over 24–72 h
- Rhabdomyolysis: Particularly in exertional heat stroke — CK may exceed 100,000 IU/L. Aggressive IV fluids essential
- Permanent neurological damage: Cerebellar damage is particularly common — ataxia, dysarthria may persist
- Death: Mortality 10–50% depending on duration of hyperthermia and speed of cooling. >2 h at core ≥40°C = very poor prognosis
UKMLA Exam Tips
- 1Heat EXHAUSTION = hot, sweaty, core <40°C, INTACT neurology. Heat STROKE = core ≥40°C + ALTERED MENTAL STATUS. Know the distinction
- 2Rapid cooling is THE treatment. Cold water immersion = gold standard. Target <39°C within 30 min
- 3Paracetamol does NOT work for heat stroke — it is thermoregulatory failure, not pyrogen-mediated fever. Do NOT give antipyretics
- 4Dantrolene does NOT work for heat stroke — only for malignant hyperthermia (which is a different condition)
- 5Medications that impair thermoregulation: anticholinergics (stop sweating), diuretics (dehydration), beta-blockers (impair cardiac output response), antipsychotics
- 6Classical heat stroke: elderly + heatwave + comorbidities + medications → gradual onset. Exertional: young + exercise + heat → acute onset
- 7DIC, rhabdomyolysis, hepatic failure may WORSEN over 24–72 h after initial presentation — serial monitoring essential
practicetest your knowledge on heat-related illnessApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — emergency medicine and beyond.
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