Heat stroke is defined as a systemic inflammatory response with a core temperature that is greater than 40.6°C accompanied by a change in mental state and varying levels of organ dysfunction.

There are two forms of heat stroke:

• Classic non-exertional heat stroke – which occurs during high environmental temperatures (this form typically affects elderly patients during heat waves)
• Exertional heat stroke – which occurs during strenuous physical exercise in high environmental temperatures (for example, endurance athletes competing in hot conditions).

Epidemiology

Heat stroke is relatively uncommon in the UK but still causes around 2,000 deaths per year. These mainly occur in the elderly. However, the incidence of heat-related deaths has been predicted to rise by over 250% by the 2050s due to a combination of climate change, an increase in the number of older people and population growth.

The incidence is higher in warmer countries, and individual heatwaves can cause thousands of deaths; for example, the 2015 summer heatwave in India was responsible for over 2,500 deaths.

Risk factors

Heat stroke occurs when thermoregulation is overwhelmed by a combination of excessive environmental heat, excessive metabolic production of heat (usually from exertion), and insufficient or impaired heat loss.

Risk factors for developing heat stroke include the following:

• Hot and humid environmental conditions
• Age (the elderly and infants are at particularly high risk)
• Physical factors, e.g. obesity, excessive exertion, dehydration
• Medical co-morbidities, e.g. anorexia, cardiovascular disease, skin conditions (e.g. psoriasis and eczema), poorly controlled diabetes, Parkinson’s disease, thyrotoxicosis
• Drugs, e.g. alcohol, amphetamine, anticholinergics, beta-blockers, cocaine, diuretics, phenothiazines, SSRIs, sympathomimetics.

Heat cramps and heat exhaustion

Heat-related illnesses occur on a continuum of increasing severity. Heat cramps and heat exhaustion often precede heat stroke.

Heat cramps usually present with intense thirst and muscle cramps. Body temperature is often elevated but usually remains below 40°C.

Sweating, heat dissipation mechanisms and cognition are preserved, and there is no neurological impairment.

Heat exhaustion usually precedes heat stroke. If left untreated, heat exhaustion will often progress to heat stroke. Heat dissipation is still functioning, and the body temperature is usually <41°C. Clinical features include nausea, oliguria, weakness, headache, thirst and sinus tachycardia. Central nervous system functioning is usually largely preserved. Patients typically complain of feeling hot and appear flushed and sweaty.

Clinical presentation of heat stroke

The typical clinical features of heat stroke include:

• Core temperature that is greater than 40.6°C
• Early features include extreme fatigue, headache, syncope, facial flushing, vomiting and diarrhoea
• The skin is usually hot and dry
• Sweating can occur (present in around 50% of cases of exertional heat stroke)
• The loss of the ability to sweat occurs late and is an ominous sign
• Hyperventilation occurs almost invariably
• Cardiovascular dysfunction, including arrhythmias, hypotension and shock
• Respiratory dysfunction, including acute respiratory distress syndrome (ARDS)
• Central nervous system dysfunction, including seizures and coma
• If the temperature rises above 41.5°C, multi-organ failure, coagulopathy and rhabdomyolysis can occur.

Differential diagnosis

Other clinical conditions that can cause an increased core temperature should be considered:

• Sepsis can present similarly and should be excluded
• In patients taking phenothiazines or other antipsychotics, neuroleptic malignant syndrome should be excluded
• In patients taking serotonergic medications such as SSRIs, serotonin syndrome should be excluded
• If there is a recent history of a general anaesthetic, then malignant hyperthermia should be considered
• Patients should be screened for recreational drug use, particular for cocaine, amphetamines and ecstasy

Basic management principles

The mainstay of treatment of heat stroke is supportive measures. The core temperature should be rapidly reduced to approximately 39.0°C. Patients with severe heat stroke should be managed in a critical care setting.

The patient should be managed using an ABCDE approach with a focus on cooling the patient:

• A – a definitive airway should be obtained if the patient is obtunded
• B – ventilate the patient is needed
• C – use haemodynamic monitoring to guide fluid therapy, large fluid volumes may be required, and electrolyte disturbances may need to be corrected
• D – manage blood glucose levels to restore normoglycaemia is necessary
• E – remove clothes, eliminate the cause of hyperthermia and monitor core and skin temperatures

Pharmacological therapies

Antipyretics (e.g. paracetamol, aspirin and NSAIDs) have no role in the treatment of heat stroke. Antipyretics interrupt the change in the hypothalamic set point caused by pyrogens and, therefore, do not work in heat stroke where the hypothalamus is heathy but overloaded. Antipyretics may actually be harmful in patients who develop hepatic, haematological, and renal complications as they can aggravate bleeding tendencies.

Benzodiazepines (e.g. diazepam) can be helpful in patients with agitation and/or shivering. In these circumstances, they help to stop excessive heat production as well as reducing agitation. Severely agitated patients may require paralysis.

Dantrolene is often used, but there is currently no high-level evidence to support its use. Neuroleptics, such as chlorpromazine, which were once mainstays of therapy, are best avoided because of their potential adverse effects, including lowering of the seizure threshold, interference with thermoregulation, anticholinergic side effects, hypotension, and hepatotoxicity.

Cooling techniques

A number of different cooling techniques are advocated, but at present, there is little conclusive evidence on which is the best approach. Possible methods include the following:

• Simple measures, e.g. cold drinks, fanning, ice water packs and spraying tepid water
• Cold water immersion therapy can be helpful, but requires the patient to be stable and co-operative and is not practical in very sick patients
• Advanced cooling techniques, e.g. cold IV fluids, surface cooling devices (SCD), intravascular cooling devices (ICD) and extracorporeal circuits, for sicker patients.

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Thank you to the joint editorial team of www.mrcemexamprep.net for this article.