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 move up (ascend) to higher altitudes, air pressure reduces. Air still contains the same proportion of oxygen (21%) at high altitude. However, because of the lower air pressure, there is actually less oxygen available 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 adapt (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 increased activity (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 climb or travel up (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.

Altitude sickness is due to the fact that your body is not adapted (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 move down (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 is 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 climb (ascend) to altitude too quickly.
• The higher the altitude to which you climb or travel.
• The higher the altitude at which you sleep.
• 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 will give mild symptoms. These can include headache, loss of appetite, and feeling sick (nausea). Being sick (vomiting), feeling tired, dizziness or feeling light-headed and difficulty sleeping may also occur.

These symptoms tend to come on 6 to 12 hours after arrival at altitude and usually get better after 1 to 3 days, provided that you do not climb (ascend) to a higher altitude. Symptoms can vary from mild to severe. Mild symptoms can be quite vague. It is best to assume 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 mild AMS is to stop your ascent and to rest at the same altitude. For most people, symptoms will improve within 24-48 hours with no specific treatment. Adapting to conditions (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 medicines are sometimes used. The most common is a medicine called acetazolamide. It is thought that acetazolamide helps to 'speed up' your acclimatisation. 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 move down (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 (AMS). 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. Fits (seizures) can also occur.

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

What is the treatment of high-altitude cerebral oedema?

This is a move down (a descent) to a lower altitude immediately. If this does not happen, or is delayed, death can occur. Treatment with oxygen and the medicine 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 provide the same effect as (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 outlook (prognosis) 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?

'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 high-altitude pulmonary oedema (HAPE) can develop are unknown. 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 escape 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 liquid (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 acute mountain sickness (AMS) or high-altitude cerebral oedema (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 move down (make a descent) 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 medicine nifedipine may also help symptoms but does 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 outlook (prognosis) 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. The best way to try to prevent altitude sickness is to climb (ascend) to higher altitudes slowly. This gives time for your body to adapt to conditions (acclimatise). It allows your body to adapt to lower oxygen levels. Different people will acclimatise at different rates.

For further details see separate leaflet called Preventing Acute Mountain Sickness.