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Heat Related Illness

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An increase in body temperature can cause symptomatic illness. This usually comes about as a result of conditions of:

  • High ambient temperature
  • High relative humidity
  • Physical exertion

There is a spectrum of heat-related illness ranging from heat cramps, through heat exhaustion to heatstroke.
Temperature alone does not define the type of heat-related illness, but does so in combination with the associated symptoms and signs.

Epidemiology

Incidence

Relatively low in 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 & humid
  • Age - infants and elderly
  • 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, thyrotoxicosis
  • Drugs - alcohol, anticholinergics, alpha-adrenergics, antihistamines, tricyclic antidepressants, selective serotonin re-uptake inhibitors, diuretics, phenothiazines,3 beta blockers, Ca-channel blockers, LSD, PCP, cocaine, amphetamines, ecstasy, aspirin, lithium
Presentation

Heat cramps

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

Heat exhaustion

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

Heat stroke

  • This is a combination of hyperthermia (>41°C classically, though can present with lower temperatures), often with loss of the capacity to dissipate heat (may not be sweating) and CNS impairment.
  • Loss of ability to sweat is often a late and ominous sign.4
  • Hyperventilation is almost invariable, with hypotension and shock occurring commonly.
  • If the condition progresses to more severe form (core temperature >41.5°C) it can cause widespread damage in particular to brain, liver, kidney and muscle.
  • The thermoregulatory centre may fail so that 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 anti-psychotics, 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, amphetamines and ecstasy is an increasingly common cause of hyperthermia.
Investigations
Management

Begin (as always) with resuscitation ABC.

  • Airway protection may be necessary as coma, fits and vomiting are common. Intubate, but avoid using suxamethonium.
  • Breathing should be checked and supported, as necessary.
  • Circulatory support is given with IV fluids as 0.9% NaCl or 5% Dextrose.
    • Avoid K+ containing fluids.
    • Gradually reduce of Na+ if hypernatremic.
    • If inotropes are required, try to use those with less alpha activity e.g. dopamine.
  • Rapid cooling - aiming for <40ºC a.s.a.p.5
    • Strip patient, spray with tepid water & use gentle fanning (cools at ~0.3°C/min).
    • Apply ice packs to neck, axillae & groins (cools at ~0.1°C/min).
    • Ice-bath immersion has been shown to be the most effective cooling method, though it is often technically impractical.6,7
    • Gastric/rectal/peritoneal lavage & cooled cardiopulmonary bypass/haemodialysis can also be used in expert hands.4,8
    • Modify or discontinue cooling methods once temp. <38.5ºC to avoid overshooting. Antipyretics are ineffective, as is dantrolene.6,9
  • 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
Prognosis

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

  • Coagulopathy
  • Lactic acidosis (in absence of severe physical exertion)
  • Rectal temperature > 42.2ºC
  • Coma lasting > 4 hrs
  • Acute renal failure
  • Hyperkalaemia
  • Very high transaminase level
  • Prolonged period of hyperthermia
Prevention

Adverse effects can be avoided by:

  • Adequate hydration
  • Avoiding exercising in heat
  • Acclimatisation


Document references
  1. Keatinge WR, Donaldson GC, Cordioli E, et al; Heat related mortality in warm and cold regions of Europe: observational study. BMJ. 2000 Sep 16;321(7262):670-3. [abstract]
  2. Grynszpan D; Lessons from the French heatwave. Lancet. 2003 Oct 11;362(9391):1169-70.
  3. Kwok JS, Chan TY; Recurrent heat-related illnesses during antipsychotic treatment. Ann Pharmacother. 2005 Nov;39(11):1940-2. Epub 2005 Sep 20. [abstract]
  4. Hoppe J, Sinert R; Heat exhaustion and Heatstroke. eMedicine, March 2006.
  5. Glazer JL; Management of heatstroke and heat exhaustion. Am Fam Physician. 2005 Jun 1;71(11):2133-40. [abstract]
  6. Smith JE; Cooling methods used in the treatment of exertional heat illness. Br J Sports Med. 2005 Aug;39(8):503-7; discussion 507. [abstract]
  7. Casa DJ, McDermott BP, Lee EC, et al; Cold water immersion: the gold standard for exertional heatstroke treatment. Exerc Sport Sci Rev. 2007 Jul;35(3):141-9. [abstract]
  8. Wakino S, Hori S, Mimura T, et al; Heat stroke with multiple organ failure treated with cold hemodialysis and cold continuous hemodiafiltration: a case report. Ther Apher Dial. 2005 Oct;9(5):423-8. [abstract]
  9. Channa AB, Seraj MA, Saddique AA, et al; Is dantrolene effective in heat stroke patients? Crit Care Med. 1990 Mar;18(3):290-2. [abstract]
  10. Casa DJ, Armstrong LE, Ganio MS, et al; Exertional heat stroke in competitive athletes. Curr Sports Med Rep. 2005 Dec;4(6):309-17. [abstract]

Internet and further reading
  • Bouchama A, Knochel JP; Heat stroke. N Engl J Med. 2002 Jun 20;346(25):1978-88.
  • Armstrong LE, Casa DJ, Millard-Stafford M, et al; American College of Sports Medicine position stand. Exertional heat illness during training and competition. Med Sci Sports Exerc. 2007 Mar;39(3):556-72. [abstract]
  • Dunn R. The Emergency Medicine Manual. 2nd Ed. 2000.
  • Stroud MA. Oxford Textbook of Medicine. 4th Edition. Eds; Warrell DA et al. OUP 2003.
Acknowledgements EMIS is grateful to Dr Hayley Willacy for writing this article and to Dr Adrian Bonsall and Dr Sean Kavanagh for earlier versions. The final copy has passed scrutiny by the independent Mentor GP reviewing team. ©EMIS 2009.
DocID: 2244
Document Version: 21
DocRef: bgp1105
Last Updated: 16 Mar 2008
Review Date: 16 Mar 2010

The authors and editors of this article are employed to create accurate and up to date content reflecting reliable research evidence, guidance and best clinical practice. They are free from any commercial conflicts of interest. Find out more about updating.

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