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Altitude Sickness

Atmospheric pressure decreases linearly with altitude. Although the percentage of oxygen in air remains constant at altitude the reduced barometric pressure reduces the absolute level of oxygen in the lungs and hence in the blood. At 5500m (Everest Base Camp in Nepal) the available oxygen is about half that at sea level; this falls to roughly one third at the summit of Everest. It is the body's response to this hypoxia that results in altitude sickness although the exact mechanism for this remains unclear.

There are three forms of altitude sickness:

  • Acute Mountain Sickness (AMS)
  • High Altitude Pulmonary Oedema (HAPE)
  • High Altitude Cerebral Oedema (HACE)
  • Acute Mountain Sickness


    Who gets AMS?

    AMS is common. More than 50% of travellers develop it in some form at 3500m. Almost all do if they ascend rapidly to 5000m. There is no simple way of predicting who will be affected by AMS; age, gender, physical condition or genetic makeup do not correlate with susceptibility to AMS. Being at altitude in the past without problems does not guarantee that future trips will be problem free. Those with medical conditions associated with/worsened by hypoxia are more likely to be affected by that condition at altitude.

    For example:

    • Chronic obstructive airways disease
    • Chronic heart failure
    • Major cardiac arrhythmias
    • Pregnancy
    • Pulmonary hypertension
    • Serious angina
    • Sickle cell disease
    • Transient cerebral ischaemic attacks

    However, these medical conditions do not predict susceptibility to AMS. Early symptoms There is a wide variation in the rate of onset and severity of symptoms and the height at which they occur. Most people experience some symptoms if they ascend rapidly (drive, fly or travel by train) from sea level to 3500m.

    Common symptoms include:

    • Dizziness
    • Fatigue, flu like symptoms
    • Headache
    • Irregular breathing during sleep
    • Loss of appetite
    • Nausea and/or vomiting
    • Palpitations
    • Poor quality of sleep
    • Swelling of the face (particularly around the eyes), hands and feet
    • Undue breathlessness on exertion

    These symptoms usually do not develop immediately upon arrival but occur during the first 36 hours at altitude.

    Note: Some antimalarial tablets cause side effects similar to symptoms of AMS (e.g. dizziness with mefloquine) and lead to confusion over the cause. Taking the antimalarial as a trial prior to departure usually identifies those travellers with side effects and suitable alternatives can be sought. The final decision on which antimalarial individual travellers should take is a complex one and needs careful consideration with an expert.

    Prevention and acclimatisation

    There is no guaranteed way to prevent AMS but acclimatisation and slow ascent are strongly recommended.

    • Gradual ascent - it is unwise to travel rapidly from sea level to heights above 3500m. Over 3500m the body needs more time to adjust to changes in atmospheric pressure and the lower availability of oxygen to the tissues. Acclimatisation and ascent should be gradual, a maximum of 300m per day. o The major cause of altitude sickness is going too high too quickly. The higher the traveller goes the more severe or dangerous AMS becomes. If a traveller experiences any symptoms suggestive of AMS they should stop their ascent and see if the symptoms resolve. If symptoms persist or worsen they should descend promptly to a lower altitude.
    • Recognition of symptoms - it is essential that travellers recognise the importance of AMS and are not tempted to continue to climb despite symptoms. This is a particular concern in the young, fit and healthy who may find it difficult to believe that they can be affected by this potentially dangerous illness and feel admission of symptoms indicates failure. Those travelling in a group may be apprehensive or worried about slowing down others if they need longer to acclimatise. Tour organisers should ensure that they allow adequate time for acclimatisation.
    • Adequate hydration - air is dryer at altitude than at sea level. Moisture is lost in the breath, contributing to dehydration which may worsen symptoms of AMS. Drinking at least 4-5 litres of water a day can help prevent this. A practical way to monitor hydration is by ensuring that urine is clear and copious. Avoidance of smoking and alcohol is recommended.
    • Light activity - light activity is recommended, strenuous activity will worsen symptoms.
    • Simple analgesics - ibuprofen or paracetamol may provide relief from the headache associated with AMS but they do not treat the underlying cause.
    • Acetazolamide (Diamox®) (unlicensed) - acetazolamide reduces the headache of AMS and increases blood and tissue oxygenation at high altitude by altering the body's acid-base balance and promoting fluid loss.[1] Taking acetazolamide is a matter of personal choice and travel to high altitude is quite possible without it.
      • Acetazolamide is not usually recommended as a routine, though there is variation of opinion on this.
      • It should not be used as an alternative to a slow ascent.
      • Acetazolamide 125mg to 250mg twice daily can be taken as a trial at sea level for two days before a visit to high altitude. It should be taken 24 hours before ascent and discontinued after 2 days at maximum sleeping altitude. It is a diuretic so urine output will be increased.[1]
      • Side effects (common) include tingling of fingers, face and feet (not a reason to stop), carbonated drinks taste strange, facial flushing.
      • Side effects (uncommon) include dizziness, vomiting, drowsiness, confusion and rashes.
      • Exceptionally the drug has caused serious haematological/renal problems.
      • Those who are allergic to sulphonamide antibiotics may also be allergic to acetazolamide.[2][3]
    • Coca Tea - in the Andes, Coca tea or Mate de Coca, is made from the natural leaf of the coca plant and has been used for centuries to prevent AMS. It is brewed in tea and drunk often throughout the day. Whilst there is no harm in drinking coca tea, there is no scientific evidence to support the claim that it prevents AMS. It should never be relied upon for treatment.

    HAPE and HACE


    Acute mountain sickness occurs in more serious forms (<2% of travellers) at around 4000-5000m and occasionally at lower altitude. Without treatment these forms are rapidly fatal but complete recovery is usual if treated early and appropriately. The most important treatment is prompt descent. High Altitude Pulmonary Oedema Fluid accumulates in the lungs, restricting the uptake of oxygen by the blood and causing severe illness. Signs and symptoms: • Breathlessness at rest (suspect early pulmonary oedema). • Marked fatigue. • Tachycardia. • A bubbling sound in the chest/persistent cough with a white watery, frothy sputum are very worrying signs. • Cyanosis.

    High Altitude Cerebral Oedema


    Swelling of the brain caused by increased permeability of cerebral blood vessels.

    Signs and symptoms (occur over a period of hours)

    • Drowsiness.
    • Irrational/confused.
    • Ataxia.
    • Double vision may occur.
    • Travellers with pulmonary/cerbral oedema are dangerously ill and should be evacuated to a lower altitude as an emergency.
    • Frequently a descent of only 500m is sufficient to improve the situation dramatically.
    • Oxygen by mask should be given if available.
    • Pressure Bag (portable compression chamber) may be used but is not a substitute for descent.
    • Nifedipine (20mg stat, then 6 hourly for 24 hours).
    • Steroid drugs i.e. dexamethasone (Decadron®) may be carried as emergency standby treatment, can be life saving in an emergency. Steroids should be used under medical supervision.

    Useful Contacts

    British Mountaineering Council Information sheets for Doctors/Climbers/Trekkers. British Mountaineering Council Address:
    177-179 Burton Road,


    M20 2BB

    Tel: 0161 445 4747

    Fax: 0161 445 4500

    Website: is very comprehensive and informative with additional records e.g. prevention of mountain sickness.

    This website is produced by UK doctors and aims to provide information about high altitude and its effect on the body. It includes several useful interactive altitude calculators and detailed tutorials about altitude sickness and also allows individuals who may have had HAPE to register on a HAPE database.

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