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Acute hypoxia (an acute lack of oxygen)

An initial symptom of altitude sickness, otherwise known as “Acute Mountain Sickness” (AMS), is dizziness. People feel lightheaded, their vision can become blurred and they may faint.

Acute mountain sickness (AMS)

Susceptibility to acute mountain sickness differs from person to person, but the condition occurs equally in men, women and children. There is no link between physical fitness and the occurrence of altitude sickness. A respiratory infection is a potential risk factor. AMS is caused by reduced air pressure and lower oxygen levels at high altitudes. Up to six hours after a rapid climb to above 2,000 metres a headache can occur which gets much worse when the sufferer bends over. Loss of appetite, nausea and vomiting can also occur, in addition to fatigue and weakness. Some people experience a fever, especially if there is pulmonary oedema (see below). AMS has similar symptoms to a hangover after a night out. Another characteristic is water retention, although this is more common in women. Two potentially fatal complications of AMS are High Altitude Cerebral Oedema and High Altitude Pulmonary Oedema.

High Altitude Cerebral Oedema (HACE)

People who have AMS or pulmonary oedema and then go on to develop neurological symptoms will be diagnosed as having cerebral oedema. This is evidenced by uncoordinated movements, decreased consciousness and coma. Headache, nausea and vomiting may occur but not necessarily. Untreated HACE can lead to death.

High Altitude Pulmonary Oedema (HAPE)

The most dangerous stage of AMS is pulmonary oedema. Initial symptoms are a dry cough, reduced stamina, shortness of breath upon exertion and dry hawking. Later on the cough becomes more productive (with phlegm and sometimes blood); there will be shortness of breath even at rest, and general weakness. If the levels of oxygen in the blood reduce further, the patient may slip into a coma and die. Often strong, fit people are struck down by pulmonary oedema. A worsening of the situation during the night is typical, especially after a second night at high altitude.

Pulmonary oedema is generally fairly easy to treat with oxygen and by descending to a lower altitude. That people die is often the result of failure to recognise the condition quickly, misdiagnosis or inability to descend.

Other risks

A stay at high altitudes carries risks additional to altitude sickness. Other physical symptoms can be triggered such as:

  • Insomnia. This is common in those who travel to high altitudes. Gaps or lapses in breathing can also occur during sleep. A low dose of acetazolamide (Diamox ®) can prevent this. Sleeping pills are not recommended.
  • Peripheral oedema: swelling of the face, arms and legs due to water retention is a common phenomenon. It will disappear by itself after descent.
  • Changes affecting the retina can occur. These typically disappear ten to fourteen days after descent.
  • Thrombosis is more common at high altitudes. Risk factors include dehydration, cold and inactivity.
  • Bronchitis. The dry air can irritate the respiratory system. This triggers the body to produce more mucus.
  • Snow Blindness (ultraviolet keratitis).  This is extremely painful but the symptoms disappear within 24 hours. It is prevented by wearing sunglasses.

What should you do?

People can generally climb to a height of 2500 metres relatively quickly. To allow the body to acclimatise it is wise then to take a day’s rest. After this, do not attempt to climb more than 300 metres per day (sleeping height) and after each day of climbing, spend an extra day at the same altitude. Try to eat a carbohydrate-rich diet and drink plenty of fluids. An additional litre of fluid per day is needed for every 1000 metres of altitude. Tobacco, alcohol, sleeping pills and drugs that affect a person's reactions are strongly discouraged at high altitudes. Excessive physical exertion is also unwise.


Certain medications can help prevent altitude sickness. People who have previously been affected by it may, on medical advice, take acetazolamide (Diamox ®). In exceptional circumstances the same drug may be prescribed by a doctor who knows the medical history of the traveller very well. Acetazolamide can however trigger side effects which are the same as the symptoms of altitude sickness so extreme caution is needed before prescription. Medication is certainly not a license to climb whilst experiencing any of the symptoms of altitude sickness. A descent is always necessary under such circumstances.

Below is what you should do for the best:

Acute mountain sickness (AMS)

Mild form:

  • do not climb any higher
  • acclimatise to the altitude gained or descend
  • take paracetamol for headaches

More serious form:

  • descend
  • take extra oxygen

Cerebral Oedema (HACE)

  • descend or evacuate
  • take extra oxygen
  • use medications if available

Pulmonary Oedema (HAPE)

  • descend or evacuate
  • take extra oxygen
  • avoid getting cold and any unnecessary exertion
  • use medication if available


Children are just as prone to altitude sickness as adults. Their symptoms are sometimes difficult to recognise, however, and that increases the risks. When children are travelling to high altitudes it is particularly important to be alert to signs such as listlessness, refusing food, excessive crying and irritability, and take seriously the possibility that these may be the first symptoms of altitude sickness. The treatment is similar to that in adults (see above).

Persons at special risk

Travel to high altitudes can entail specific risks to persons with certain pre-existing physical conditions. The most important are:

  • Chronic obstructive pulmonary disease (COPD): increased shortness of breath and greater difficulty with any exertion.
  • Coronary heart disease: just staying at high altitude is an effort. In addition, there is an increased risk of decompensation.
  • Hypertension: no specific problems, but blood pressure seems to rise slightly.
  • Sickle cell anaemia: an increased risk of a sickle cell crisis and therefore travel to high altitudes should be avoided. This is not a problem for genetic carriers of sickle cell anaemia (so called Sickle cell trait).
  • Diabetes: high altitudes can alter the body’s requirements for glucose and insulin, which in turn increases the risk of an adverse episodic effect. Some equipment used by persons with diabetes is less effective at high altitude.
  • Pregnancy: sleeping above 3000 metres is not recommended, nor is staying for any length of time above 5000 metres. In remote areas, access to medical care in the case of complications may be difficult.
  • Contraception: there is no known data about an increased risk of thrombosis.
  • Eye operations: previous laser treatments pose no problem. For the possible effects of other prior eye operations/treatments an ophthalmologist should be consulted.