Epidemiology

Incidence

Relatively low in the UK, estimated at around 40 cases of heat-related mortality per million population annually.^[1] A heat wave in France in 2003 was estimated to have caused over 11,000 deaths, particularly amongst the elderly in residential care.^[2] The incidence can rise when customs or beliefs cause groups of people to be exposed to the heat for long periods of time such as Hajj, the annual Muslim pilgrimage in Saudi Arabia.

Risk factors

• Environmental - hot and humid.
• Age - infants and elderly (particularly if bed-bound/unable to self-care).
• Physical - obesity, dehydration, unacclimatised, unusual exertion, inappropriate clothing, sleep deprivation, sunburn, sweat gland dysfunction.
• Medical conditions - alcoholism, anorexia, cardiac illness, cystic fibrosis, dehydration, delirium tremens, dermatological conditions with decreased sweating, diabetes insipidus, epilepsy, poorly controlled diabetes mellitus, febrile illness, gastroenteritis, previous heat-related illness, hypokalaemia, Parkinson's disease, spinal injuries, and thyrotoxicosis.
• Drugs - alcohol, anticholinergics, alpha-adrenergics, antihistamines, tricyclic antidepressants, selective serotonin reuptake inhibitors (SSRIs), diuretics, phenothiazines,^[3] betablockers, calcium-channel blockers, lysergic acid diethylamide (LSD), phencyclidine (PCP), cocaine, amfetamines, ecstasy, aspirin, and lithium.

Presentation

Heat cramps

• Body temperature often elevated, usually <40?C.
• Intense thirst with muscle cramps and tachycardia.
• Sweating and heat dissipation mechanisms preserved.
• Normal alertness and higher functions with no neurological problems.

Heat exhaustion

Signs of heat-related illness in an individual often begin with heat exhaustion, which, if left untreated, might progress to heatstroke.

• Central nervous system (CNS) function is usually largely preserved, but sufferers may experience mild confusion, irritability and poor co-ordination.
• Heat dissipation is still functioning, and temperature is usually <41?C.
• Patients may experience nausea, oliguria, weakness, headache, thirst, occasionally syncope, sinus tachycardia, and orthostatic hypotension.
• They often complain of being hot and appear flushed and sweaty.

Heat stroke

• This is a combination of hyperthermia (classically defined as a core body temperature of at least 40.6°C)^[4] and often with loss of the capacity to dissipate heat (may not be sweating) and with CNS impairment.
• Loss of ability to sweat is often a late and ominous sign.^[5]
• Hyperventilation is almost invariable, with hypotension and shock occurring commonly.
• If the condition progresses to a more severe form (core temperature >41.5°C) it can cause widespread damage, in particular to the brain, liver, kidney and muscle.
• The thermoregulatory centre may fail so that the patient actually feels cold with dry, vasoconstricted skin leading to a vicious cycle.

Differential diagnosis

• The history of exposure to adverse environment ± physical exertion usually clinches the diagnosis, but sepsis and alternative causes of fever (particularly malaria if relevant area/travel history) should be considered as a cause or precipitant.
• In patients taking phenothiazines or other antipsychotics, consider neuroleptic malignant syndrome. Those on SSRIs or other serotonergic medication may be suffering from serotonin syndrome.
• If there is a history of recent inhalational anaesthetic, consider malignant hyperpyrexia.
• Recreational drug toxicity, particularly involving cocaine, amfetamines and ecstasy, is an increasingly common cause of hyperthermia.

Investigations

• Monitor temperature regularly.
• U&E.
• Glucose.
• Urate - may predict acute renal failure.
• Liver function tests.
• Creatine kinase - may indicate rhabdomyolysis.
• Full blood count.
• Arterial blood gases.
• Urinalysis - may show myoglobinuria.
• ECG.
• Chest X-ray - to check for aspiration/pulmonary oedema.

Management

Begin (as always) with resuscitation ABC.

• A irway protection may be necessary, as coma, fits and vomiting are common. Intubate, but avoid using suxamethonium.
• Breathing should be checked and supported, as necessary.
• C irculatory support is given with IV fluids as 0.9% NaCl or 5% dextrose.
  • Avoid K+ containing fluids.
  • Gradually reduce concentration of Na+ if hypernatraemic.
  • If inotropes are required, try to use those with less alpha activity, eg dopamine.
• Rapid cooling - aiming for <40?C as soon as possible.^[6]
  • Strip the patient, spray with tepid water and use gentle fanning (cools at ~ 0.3°C/minute).
  • Apply ice packs to the patient's neck, axillae and groins (cools at ~ 0.1°C/minute).
  • Ice-bath immersion has been shown to be the most effective cooling method, although it is often technically impractical.^[7][8]
  • Gastric/rectal/peritoneal lavage and cooled cardiopulmonary bypass/haemodialysis can also be used in expert hands.^[5][9]
  • Modify or discontinue cooling methods once the temperature is <38.5?C, to avoid overshooting. Antipyretics are ineffective, as is dantrolene.^[7][10]
• Benzodiazepines and non-depolarising muscle relaxants should be used to control shivering and fits.
  • Neuroleptics may be used to treat excessive shivering associated with cooling.
• Catheterisation should be considered to monitor urine output.
• Complications should be treated as they arise - see below.

Complications

• Hyperkalaemia.
• Hypocalcaemia.
• Acidosis.
• Rhabdomyolysis.
• Disseminated intravascular coagulation.
• Hepatic/renal failure.
• Ventricular fibrillation (often fatal).
• Initial successful resuscitation and 'lucid interval' leading to deterioration due to irreversible thermal shock to the CNS.^[11]

Prognosis

With rapid cooling, sufficient rehydration and careful management of complications survival rates for heatstroke approach 85-90% in most modern centres.^[4][5] Poor prognostic indicators include:

• Coagulopathy.
• Lactic acidosis (in the absence of severe physical exertion).
• Core temperature >42.2?C.
• Coma lasting >4 hours.
• Acute renal failure.
• Hyperkalaemia.
• Very high transaminase level.
• Prolonged period of hyperthermia.

Prevention

Education of the public about the risks of heat is important. Adverse effects can be avoided by:

• Adequate hydration.
• Avoiding exercising in the heat.
• Acclimatisation, ie the process of repeated or increasing exposure (for example, over 1-2 weeks and with daily exercise in heat) of initially 30-60 minutes increasing to about 100 minutes at a time. During acclimatisation the body becomes more efficient in work production as well as heat dissipation through various mechanisms, including a number of changes to sweat rate, volume and composition.