Altitude/Mountain Sickness

What is a high altitude?

  • High altitude is an altitude between 1500-3500 metres (5,000-11,500 feet).
  • Very high altitude is an altitude between 3500-5500 metres (11,500-18,000 feet).
  • Extreme altitude is an altitude above 5500 metres (18,000 feet).

What are the normal responses of the body to altitude?

As you ascend to higher altitudes, air pressure reduces. Air still contains the same proportion of oxygen (21%). However, because of the lower air pressure, even though the proportion of oxygen is the same, there is actually less oxygen because the air is thinner (it contains less of all of the gases) in a given volume. So, at high altitude, each breath that you take will contain fewer oxygen molecules. This means that you have to breathe faster and deeper to get oxygen into your body.

After a few days, your body starts to acclimatise to the higher level of altitude that you are at. You will notice that your breathing rate will start to slow down. Your body also makes more red blood cells to help with oxygen transport around your body.

Because of these changes in your body, there are some ‘normal’ symptoms that you will notice at higher altitudes while your body is acclimatising and adjusting to the reduced availability of oxygen. They include:

  • An increased breathing rate or feeling that you are breathing more deeply.
  • Shortness of breath on exercise or exertion.
  • A change in your breathing pattern at night.
  • Disturbed sleep.
  • Passing more urine than usual.

What is altitude sickness?

Altitude sickness is something that can affect people who climb or travel to more than 2500 metres altitude, particularly if they ascend too quickly. It is rare at altitudes of less than 2500 metres and is more common at altitudes of 3500 metres or more. Just to give an idea, you can commonly ski at an altitude of 2500 metres in a ski resort.

Basically, altitude sickness is due to the fact that your body is not acclimatised to the altitude that you are at. For most people, it just produces mild symptoms that improve with rest and time spent at altitude. However, in some people, it can lead to more serious symptoms which can become life threatening, particularly if they are not recognised and the person does not descend to a lower altitude.

There are three main ways that altitude sickness can affect you. You may develop one or more of the following problems:

  • Acute mountain sickness (AMS)
  • High altitude cerebral oedema (HACE)
  • High altitude pulmonary oedema (HAPE)

Each of these are explained in more detail below.

How common is altitude sickness?

It is thought that about half of trekkers in Nepal who walk to altitudes above 4000 metres for five or more days develop AMS. One study also showed that about 8 in 10 people who flew directly to 3860 metres were affected by AMS. HACE and HAPE are much less common.

Who is at risk of altitude sickness?

It is difficult to predict who will be affected by altitude sickness. However, your risk is higher:

  • If you ascend to altitude too quickly.
  • The higher the altitude that you climb or travel to.
  • The higher the altitude that you sleep at.
  • Depending on your individual susceptibility to developing altitude sickness – genetics may play a part in your susceptibility.
  • Depending on your level of exertion (or how active you are) at altitude – the more active you are, the greater the risk.

Note that your level of physical fitness does not seem to play a part in your chances of developing altitude sickness. Someone who has experienced altitude sickness in the past may be more likely to develop it again.

Acute mountain sickness

What causes acute mountain sickness?

The exact cause of acute mountain sickness (AMS) is not known but it is thought to be a response of the brain to lower oxygen levels in the blood at higher altitudes which produces some swelling of the brain.

What are the symptoms of acute mountain sickness?

For most people, AMS just produces mild symptoms. These can include headache, loss of appetite and nausea (feeling sick). Vomiting, feeling tired, dizziness or lightheadedness and difficulty sleeping may also occur.

These symptoms tend to come on between 6 to 12 hours after arrival at altitude and tend to get better after 1 to 3 days, provided that you do not ascend to a higher altitude. Symptoms can vary from mild to severe. Mild symptoms can be quite vague. It is best to think that, if you are at altitude and you feel unwell, you have AMS unless there is another obvious cause.

What is the treatment for acute mountain sickness?

The most important treatment if you start to develop symptoms of acute mountain sickness is stop your ascent and rest at the same altitude. For most people, symptoms will improve within 24-48 hours with no specific treatment. Acclimatisation usually occurs after 1 to 3 days at a given altitude.

Simple painkillers such as paracetamol and anti-sickness medication can help headache and sickness. Other drugs are sometimes used. The most common is a drug called acetazolamide. It is thought that acetazolamide helps to ‘speed up’ your acclimatisation. (Acetazolamide is discussed further below.) You should also make sure that you drink plenty of fluids.

However, if your symptoms are severe, they do not improve after 24 hours, or they are getting worse, you need to descend to a lower altitude. You also need to descend urgently if you develop any symptoms or signs of HACE or HAPE (see below).

High altitude cerebral oedema

What causes high altitude cerebral oedema?

High altitude cerebral oedema (HACE) usually develops in someone who already has acute mountain sickness. The swelling of the brain that has led to AMS gets worse and starts to interfere with the function of the brain. So, HACE is really a severe form of AMS.

What are the symptoms of high altitude cerebral oedema?

Symptoms of HACE include headache, nausea, vomiting, hallucinations (for example seeing or hearing things that are not actually there), feeling disorientated and confusion. Often these symptoms are not noticed by the person who is developing HACE, or by their companions. An affected person can lose their co-ordination and become unsteady on their feet. As the cerebral oedema progresses, their consciousness level will drop and coma and death can occur if treatment is not started. Seizures (fits) can also occur.

HACE can develop quickly, over a few hours. Someone may also have symptoms of high altitude pulmonary oedema (see below).

What is the treatment of high altitude cerebral oedema?

This is descent to a lower altitude immediately. If this does not happen, or is delayed, death can occur. Treatment with oxygen and the drug dexamethasone (a steroid medicine) can help to relieve symptoms and can mean that getting someone down to a lower altitude becomes easier. However, these treatments do not remove the need for descent. The descent should be at least to the last altitude at which the person woke up feeling well.

A device has been developed called a portable hyperbaric chamber. It is, essentially, an airtight bag that is pressurised by a pump. The person with HACE is placed inside it and it can simulate descent. They will be breathing air equivalent to that at much lower altitude. This can be life-saving when descent is not possible and oxygen is unavailable.

What is the prognosis (outlook) for high altitude cerebral oedema?

People with HACE usually do well if they descend to a lower altitude soon enough and far enough. They will usually have a complete recovery.

High altitude pulmonary oedema

What causes high altitude pulmonary oedema (HAPE)?

‘Pulmonary’ refers to the lungs and ‘oedema’ means that there is a build up of fluid. So, pulmonary oedema is a build-up of fluid within the lungs. The exact reasons why HAPE can develop are unknown. However, it is thought that the high altitude causes an increase in pressure in the blood vessels around the lungs which leads to smaller blood vessels becoming ‘leaky’, allowing fluid to leak from the blood vessels into the lungs.

What are the symptoms of high altitude pulmonary oedema?

If someone develops HAPE, the symptoms usually start to appear a few days after arrival at altitude. They start with shortness of breath on exertion and then worsen so that there is shortness of breath even when resting. Someone can also develop a cough and feel generally weak and tired. They may start to cough up pink/frothy sputum and complain of chest tightness. They may have swelling of their ankles or legs and their lips or fingernails may be blue or grey. In severe cases they become extremely short of breath at rest and drowsy. Coma and death can occur if HAPE is not treated quickly.

HAPE can happen in someone who also has AMS or HACE, or they may have no obvious symptoms of these other problems.

What is the treatment of high altitude pulmonary oedema?

Again, someone with HAPE needs to descend to a lower altitude immediately. Even a descent of a few hundred metres can make a difference but ideally descent should be at least to the last altitude at which the person woke up feeling well. Treatment with oxygen and the drug nifedipine may also help symptoms but do not replace the need for descent.

A hyperbaric chamber (as explained above) can be used if descent is not possible and/or oxygen and other treatment are not available.

What is the prognosis (outlook) for high altitude pulmonary oedema?

HAPE tends gets better quickly on descent and there is usually complete recovery.

Can altitude sickness be prevented?

There are a number of things that may help to prevent altitude sickness.

Ascend slowly

The best way to try to prevent altitude sickness is to ascend to higher altitudes slowly. This gives time for your body to acclimatise. It allows your body to adapt to lower oxygen levels. Note that different people will acclimatise at different rates.

There are recommended rates of ascent to altitude to help acclimatisation:

  • If possible, spend at least one night at an ‘intermediate’ elevation below 3000 metres.
  • Above 3000 metres increase your sleeping altitude by only 300-500 metres per day.
  • Above 3000 metres take a rest day for every 1000 metres of elevation gained (ie spend a second night at the same altitude).
  • If possible, don’t fly or drive directly to high altitude.
  • If you do go directly to high altitude by car or plane, do not overexert yourself or move higher for the first 24 hours.
  • Always try to sleep at a lower altitude. Climbers commonly use the phrase, “climb high, sleep low”.
  • If symptoms are not improving, delay further ascent.
  • If symptoms get worse, descend as soon as possible.


The drug acetazolamide can help to prevent altitude sickness. It may be helpful for people who seem to be susceptible to altitude sickness or for people who unavoidably have to ascend more quickly than recommended. However, acetazolamide isn’t an alternative to adequate acclimatisation and gradual ascent. The routine use of acetazolamide before ascent should be avoided.

Treatment should be started at least one day before you ascend to altitude and continued until you are thought to have fully acclimatised (usually after the second or third night at maximum altitude). Common side-effects can include pins and needles and passing lots of urine; however, most people can tolerate this. Allergic reactions can also sometimes occur. You should not take acetazolamide if you are allergic to sulphonamides.

The drug dexamethasone is sometimes used as an alternative to acetazolamide. Some recent large studies have shown that ginkgo biloba extract does not seem to be helpful in preventing altitude sickness.

Other measures

There are certain things that can slow down your breathing rate and so may increase your chances of developing altitude sickness. These include alcohol, taking sleeping tablets and taking strong painkillers (those that contain opiate drugs). So, you should avoid these when you are at altitude.



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