Heat Illness & Heat Stroke

Heat-related illness is a spectrum, from heat cramps and heat exhaustion through to heat stroke — a medical emergency that can be rapidly fatal from multiorgan failure. See Hypothermia for the opposite end of thermoregulatory failure.

Clinical presentation

• Heat cramps and heat exhaustion: cramping, fatigue, dizziness, nausea, vomiting, headache. Core temperature is usually normal or only mildly elevated. Responds to rest, fluids and cooling.
• Heat injury: heat exhaustion plus evidence of end-organ damage (renal, hepatic, muscle) without central nervous system dysfunction.
• Heat stroke: central nervous system dysfunction — confusion, ataxia, delirium or seizures — is what distinguishes heat stroke from heat injury, brought on by strenuous exertion or environmental heat exposure. Core temperature is typically but not always above 40°C.

Classic vs exertional

Two distinct patterns, with different typical patients and physiology:

• Classic heat stroke: older adults with chronic medical conditions; often presents with hot, dry skin from failure of the sweating response (anhidrosis); respiratory alkalosis predominates. Medication review is crucial — diuretics, beta-blockers and anticholinergic drugs all impair heat tolerance.
• Exertional heat stroke: otherwise healthy people exercising in hot or humid conditions; sweating often continues after exercise stops (anhidrosis is uncommon); concomitant lactic acidosis and more pronounced rhabdomyolysis are typical.

Assessment

Frequent vital signs and rectal (not tympanic or oral) temperature. Vital sign abnormalities are typical: elevated core temperature, sinus tachycardia, tachypnoea, widened pulse pressure; around a quarter of patients are hypotensive. Investigations to consider based on clinical judgement: full blood count, electrolytes and renal function, coagulation studies, blood gas, creatine kinase and urine myoglobin (rhabdomyolysis), and an ECG (ischaemic-pattern ST/T changes and QT prolongation can occur from electrolyte derangement).

Differentials: neuroleptic malignant syndrome, serotonin syndrome, sepsis, meningitis, toxic ingestion, and — with a relevant travel history — malaria. Muscle rigidity and clonus point away from heat stroke and toward the two toxidromes; malaria, sepsis and meningitis don’t typically produce the same degree of temperature elevation as heat stroke.

Management

Airway, breathing and circulation first, then rapid cooling is the single most effective intervention — it is not adjunctive, it is the treatment. Intubation for reduced consciousness is rarely needed, since cooling itself usually improves conscious state.

• Cooling method: no single method has been shown superior. Ice-water immersion cools fastest but can be impractical (cardiac monitoring difficulty, agitation) in older or unstable patients. Evaporative cooling (misting with fans) plus ice packs to the groin, axillae and neck is the practical default in most settings and is what a GP/urgent- care setting can actually deliver while arranging transfer.
• Monitor core temperature continuously (rectal or oesophageal probe) and stop active cooling once it reaches 38–39°C to avoid overshoot into hypothermia.
• Rehydrate, but correct sodium abnormalities gradually — rapid correction risks cerebral injury.
• No specific drug treatment. Dantrolene does not improve outcomes and is not recommended. Antipyretics have no role (this isn’t a cytokine-driven fever) and may add hepatotoxicity risk. Benzodiazepines are not used routinely, though considered for significant agitation or shivering.
• Exertional heat stroke: rest from exercise for at least 7 days after treatment, with follow-up at 1 week to check for end-organ damage.

Referral

Any suspected heat stroke needs urgent transfer once cooling and initial resuscitation are underway — definitive management is beyond a GP or urgent-care setting.

Heat exhaustion without red flags (altered mental state, high temperature, end-organ injury) can usually be managed with rest, oral fluids and cooling. Safety-net to return if symptoms don’t settle within a few hours, or worsen.

Prevention

Everyone is at risk during a heatwave, but some patients are more vulnerable and become symptomatic at lower temperatures: older age, pregnancy, infants, chronic cardiovascular/renal/respiratory disease, socioeconomic disadvantage (poorly insulated housing, no air conditioning), outdoor workers, and people who are homeless. Diabetes is a specific case — a HbA1c above 8.5% is linked to reduced sweating and vasodilation capacity, so these patients can be advised to be proactive with cooling once forecast temperatures reach the high 20s (°C).

Review medications before summer, not just at presentation. Diuretics, ACE inhibitors, antipsychotics and beta-blockers all impair heat tolerance or fluid handling, and patients on multiple medications or fluid restriction are particularly at risk. A health assessment consultation (45–49 years, or 75+) heading into summer is a reasonable place to do it.

Practical cooling advice for patients: avoid outdoor activity in the hottest part of the day; light clothing; hydration; a fan can make a room feel around 4°C cooler for a fraction of an air conditioner’s running cost (though evaporative cooling from sweat or a fan stops being effective above about 40°C ambient); self-dousing, wet clothing, ice towels and foot immersion are accessible options for patients who can’t afford air conditioning. Discuss a backup plan (a cool public space, a relative’s home) for power outages during a heatwave.