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Acute Mountain Sickness (AMS) Fact Sheet

Chris Imray

AMS is a potentially serious condition that may affect all travellers to altitudes over about 2,500m (8000ft).

Atmospheric pressure falls with increasing altitude, and although the percentage of oxygen in the air is constant, the partial pressure of oxygen falls (at 5,500m or 18,000ft, the amount of oxygen available in each breath is roughly half that at sea level).

Diagnosis and Symptoms of AMS

Unlike many medical conditions, which require complex tests, the diagnosis of AMS is based upon the subject’s symptoms. The Lake Louise Self Assessment Scoring is a simple questionaire that can be used by anyone to diagnose AMS.

Symptoms include headache, lethargy, shortness of breath, sleep disturbance, loss of appetite, nausea and vomiting and may lead to ataxia (unsteadiness), coma and death.

Risk factors for AMS

There is no absolute way to predict an individual’s susceptibility to AMS.
Risk factors include: Rate of ascent, altitude achieved, previous recent exposure (reduces risk), past experience of susceptibility.

Natural course of AMS

Mild cases of AMS will resolve over 24-72 hours if no further ascent is made; descent of 500m will treat all mild cases.

Further ascent or failure to descend may precipitate a significant deterioration in the subject’s condition, which can lead to fluid on the lung (high altitude pulmonary oedema [HAPE]) or fluid on the brain (high altitude cerebral oedema [HACE]).

Prevention of AMS
  • 1. Rate of ascent

    AMS can be avoided by a slow rate of ascent allowing the body to acclimatise to the lack of oxygen. Above 3000m, the average rate of ascent should not exceed 300m per 24hrs. Rest days should be used to allow this graded rate of ascent to be achieved. Tight inflexible schedules are potentially dangerous. The altitude at which the individual sleeps is important, the 300m/day ascent rule can be exceeded by day if the person sleeps no higher than 300m above the previous night's altitude: ‘Climb high, sleep low’.

  • 2. Drug prophylaxis

    Acetazolamide is the drug of choice in the prevention of AMS.
    250 mg bd (twice daily) or 500mg slow release od (once daily) starting 48-24hrs prior to ascent. Recently it has been suggested that a dose of 750mg per day (250mg tds) is more effective. Acetazolamide should be continued for about a week (or as long as the subject is at altitude if this is shorter)
    Side affects of acetazolamide are mild and most commonly include paraesthesia (pins & needles) and diuresis (increased urine output). Acetazolamide must be avoided in people allergic to sulphonamide drugs. Rashes may occur; the drug should be stopped if this occurs and medical advice should be sought.

Treatment of AMS
  • 1. Descent

    If symptoms are mild, it is usually safe to not to ascend further and merely to rest. Simple analgesics such as aspirin or paracetamol may be used to treat the headache.
    Moderate or severe symptoms, particularly if there are neurological signs or pulmonary oedema, necessitate urgent descent of at least 500m.
    Sometimes the weather conditions or local terrain may preclude descent in which case the following treatments have been shown to be beneficial.

  • 2. Oxygen

    Administration of oxygen at about 6 litres/minute.

  • 3. Acetazolamide

    Oral acetazolamide 250 bd (twice daily) or 500mg slow release od (once daily) starting immediately.

  • 4. Dexamethasone

    Dexamethasone 8 mg immediately then 4mg qds (four times per day) oral, intravenously or IM.

  • 5. Hyperbaric chamber

    Portable hyperbaric chambers (Gamow or Certec bags) can be used to treat individuals with AMS. The subject is placed inside the chamber and the bag pumped up until a pressure of 200mBar is achieved. The effect of this is equivalent to about 2000m of descent. Treatment should be continued for at least 2 hours. The aim is to improve the subject’s condition sufficiently so that he/she can then descend.

Lake Louise Self Assessment Scoring for AMS

This simple self-assessment scoring system can be used to determine whether or not an individual has AMS. A score of 3 or more (with a headache) after recent gain in altitude is consistent with a diagnosis of AMS.

none at all
mild headache
moderate headache
severe incapacitating headache
Gastrointestinal symptoms
good appetite
poor appetite or nausea
moderate nausea or vomiting
severe nausea or vomiting
Not tired or weak
Mild fatigue/weakness
Moderate fatigue/weakness
Severe fatigue/weakness
Dizziness / lightheadedness
Difficulty sleeping
slept as well as usual
did not sleep as well as usual
woke many times, poor nights sleep
could not sleep at all
Overall, did these symptoms affect your activities?
not at all
mild reduction
moderate reduction
severe reduction (bedrest)

Further reading:

The High Altitude Medicine Handbook. Radcliffe Medical Press, Inc. ISBN 81 7303 128 2
Andrew Pollard & David Murdoch
High Altitude Medicine and Physiology. 3rd Edition. 2000 ISBN 0 340 75980
Arnold 338 Euston Road, London NW1 3BH Michael Ward, James Milledge & John West

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