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Every year, people die of altitude sickness. All of these deaths are preventable. If you are travelling above 2500m (8000ft), read this information and tell your companions about it - it could save your life.

What is altitude sickness? 

Altitude sickness has three forms. Mild altitude sickness is called acute mountain sickness (AMS) and is quite similar to a hangover - it causes headache, nausea, and fatigue. This is very common: some people are only slightly affected, others feel awful. However, if you have AMS, you should take this as a warning sign that you are at risk of the serious forms of altitude sickness: HAPE and HACE. Both HAPE and HACE can be fatal within hours.

Avoiding Altitude Sickness: Complete Beginner’s Guide (2020)

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Find out more about the cause, symptoms and treatment of acute mountain sickness.

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HAPE stands for high altitude pulmonary oedema. HAPE is excess fluid on the lungs, and causes breathlessness. HAPE can also cause a fever (a high temperature) and coughing up frothy spit. HAPE and HACE often occur together.

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HACE stands for high altitude cerebral oedema. HACE is fluid on the brain. It causes confusion, clumsiness, and stumbling. The first signs may be uncharacteristic behaviour such as laziness, excessive emotion or violence. Drowsiness and loss of consciousness may occur shortly before death. HACE is a rare but serious condition.




What causes altitude sickness? 

Two things are certain to make altitude sickness very likely - ascending faster than 500m per day, and exercising vigourously. Physically fit individuals are not protected - even Olympic athletes get altitude sickness. Altitude sickness happens because there is less oxygen in the air that you breathe at high altitudes.

The Golden Rules
  1. If you feel unwell, you have altitude sickness until proven otherwise

  2. Do not ascend further if you have symptoms of altitude sickness

  3. If you are getting worse then descend immediately

Altitude sickness prevention 

Go up slowly, take it easy, and give your body time to get used to the altitude. The body has an amazing ability to acclimatise to altitude, but it needs time. For instance, it takes about a week to adapt to an altitude of 5000m. 

Can I take drugs to prevent altitude sickness?

Only one drug is currently known to prevent AMS and to be safe for this purpose: acetazolamide (diamox). It causes some minor side effects, such as tingling fingers and a funny taste in the mouth. As with everything, many alternative treatments and untested herbal remedies are claimed to prevent mountain sickness. These treatments can make AMS worse or have other dangerous side effects - many herbs are poisonous.


Please help us to spread this information as widely as possible. Everyone who travels to high altitude should know this.

Following these simple rules could prevent many deaths in the mountains each year.

Altitude sickness


Mild altitude sickness is called acute mountain sickness (AMS) and is quite similar to a hangover - it causes headache, nausea, and fatigue.

Where does acute mountain sickness happen?

Most people remain well at altitudes of up to 2500m, the equivalent barometric pressure to which aeroplane cabins are pressurised. However, even at around 1500m above sea level you may notice more breathlessness than normal on exercise and night vision may be impaired. Above 2500m, the symptoms of altitude sickness become more noticeable.

What are the other names for acute mountain sickness?

Acute mountain sickness is sometimes colloquially referred to as altitude sickness or mountain sickness and in South America it is called soroche.

How are the symptoms of altitude sickness measured?

The most prominent symptom is usually headache, and most people also experience nausea and even vomiting, lethargy, dizziness and poor sleep. Symptoms are very similar to a really bad hangover. Acute mountain sickness can be diagnosed using a self-assessment score sheet. If you have recently ascended to over 2500m, have a headache and your total score is 3 points or more on the score sheet, then you have acute mountain sickness.

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Lake Louise Consensus on Acute Mountain Sickness 2018

The original Lake Louise Score (LLS) was devised by a consensus process and originally published in 1993. Since then it has been an invaluable tool for research into acute mountain sickness (AMS). In recent years, however, research has suggested that sleep disturbance, a diagnostic criterion in the original LLS, is, in fact, a separate entity from AMS. As such in 2018 a newly revised Lake Louise Acute Mountain Sickness Score was agreed by consensus and published. This revised score removed sleep disturbance and also recommended the use of an optional AMS clinical functional score, where the study design allowed.

This new score represents a significant change in the field of high altitude research and should be the standard assessment of AMS in studies involving the condition.


  1. Hall, D. P. et al. Network Analysis Reveals Distinct Clinical Syndromes Underlying Acute Mountain Sickness. PLoS ONE 9, e81229 (2014).
    [pre-print version published online at arXiv is available here]

  2. Macinnis, M. J., Lanting, S. C., Rupert, J. L., Koehle, M. S. Is poor sleep quality at high altitude separate from acute mountain sickness? Factor structure and internal consistency of the Lake Louise Score Questionnaire. High Alt. Med. Biol. 14, 334–337 (2013).

  3. Roach RC, Hackett PH, Oelz O, Bärtsch P, Luks AM, MacInnis MJ, Baillie JK, and the Lake Louise AMS Score Consensus Committee, The 2018 Lake Louise Acute Mountain Sickness Score. High Alt Med Biol. 2018;19: 4–6. doi:10.1089/ham.2017.0164

Lake Louise Score
Who gets acute mountain sickness?

Anyone who travels to altitudes of over 2500m is at risk of acute mountain sickness. Normally it does't become noticeable until you have been at that altitude for a few hours. Part of the mystery of acute mountain sickness is that it is difficult to predict who will be affected. There are many stories of fit and healthy people being badly limited by symptoms of acute mountain sickness, while their older companions have felt fine.

There are a number of factors that are linked to a higher risk of developing the condition. The higher the altitude you reach and the faster your rate of ascent, the more likely you are to get acute mountain sickness. If you have a previous history of suffering from acute mountain sickness, then you are probably more likely to get it again. Older people tend to get less acute mountain sickness.

What causes altitude sickness?

There is so much less oxygen in the high mountains that it is not surprising that travelling to high altitude causes people to feel unwell, but how this shortage of oxygen actually leads to altitude sickness is still not fully understood. Some scientists believe that it is due to swelling of the brain but the evidence for this hypothesis is not conclusive. The theory is that in susceptible individuals, swelling could cause a small increase in the pressure inside the skull and lead to symptoms of acute mountain sickness. The swelling may be due to increased blood flow to the brain or leakiness of blood vessels in the brain.

What are the treatments for altitude sickness (mountain sickness)?

It is better to prevent acute mountain sickness than to try to treat it. Following the golden rules should mean that your body can acclimatise as you ascend and so you will be less likely to develop acute mountain sickness. However, if you need to go up more quickly, you could consider taking a drug called acetazolamide (also known as Diamox). There is now good evidence [BMJ. 2004;328:797] that acetazolamide reduces symptoms of acute mountain sickness in trekkers, although it does have some unusual side-effects: it makes your hands and feet tingle, and it makes fizzy drinks taste funny.

As with any form of altitude sickness, if you do have acute mountain sickness, the best treatment is descent. Painkillers may ease the headache, but they don’t treat the condition. Acetazolamide may be helpful, especially if you need to stay at the same altitude, and resting for a day or two might give your body time to recover. It is essential that you should NEVER go up higher if you have acute mountain sickness.

If a travelling companion has symptoms of acute mountain sickness and becomes confused or unsteady, or develops an extremely severe headache or vomiting, they may have a life-threatening condition called high altitude cerebral oedema (HACE). 

There are many other remedies touted as treatments or 'cures' for altitude sickness, but there is no evidence to support any of them. On our  research expeditions we have conducted drug trials of antioxidants, which did not prevent altitude sickness, and sildenafil (viagra) (Baillie JK et al, QJM 2009 102(5):341-348.


HAPE is a dangerous build-up of fluid in the lungs that prevents the air spaces from opening up and filling with fresh air with each breath. When this happens, the sufferer becomes progressively more short of oxygen, which in turn worsens the build-up of fluid in the lungs. In this way, HAPE can be fatal within hours.

If you have had HAPE, please register with the HAPE database and tell us about your experience. 



HAPE usually develops after 2 or 3 days at altitudes above 2500 m. Typically the sufferer will be more breathless compared to those around them, especially on exertion. Most will have symptoms of acute mountain sickness. Often, they will have a cough and this may produce white or pink frothy sputum. The breathlessness will progress and soon they will be breathless even at rest. Heart rate may be fast, the lips may turn blue and body temperature may be elevated. It is easy to confuse symptoms of HAPE with a chest infection, but at altitude HAPE must be suspected and the affected individual must be evacuated to a lower altitude.


If you think you have had HAPE, register on the HAPE database.

Unfortunately, it is currently impossible to predict who will get HAPE. This is one of the reasons why we have established the HAPE database. People who have had HAPE before are much more likely to get it again. Therefore, there must be some factor that puts certain individuals at high risk of the condition. However, just like acute mountain sickness, there are some known risk factors. A fast rate of ascent and the altitude attained will make HAPE more likely. Vigorous exercise is also thought to make HAPE more likely and anecdotal evidence suggests that people with chest infections or symptoms of the common cold before ascent may be at higher risk.

The most important treatment for HAPE is descent. Providing extra oxygen and/or raising the air pressure around a victim with a Gamow bag can reverse the underlying process, lack of oxygen, but these measures are really no substitute however for rapid descent down the mountain.


Some drugs can be helpful, but should only be used by trained doctors. Nifedipine is a drug that helps to open up the blood vessels in the lungs. By doing so, it reduces the high pressure in those vessels that is forcing fluid out into the lungs. Sildenafil (Viagra®), by a different mechanism, also opens up the blood vessels in the lung and may be a useful treatment for HAPE. Following recent research, medics may also give the steroid, dexamethasone. Drug treatment should only ever be used as a temporary measure; the best treatment is descent.

What causes HAPE?

Despite years of careful research the exact causes of HAPE remain poorly understood. Fluid has been shown to fill up the air pockets in the lungs preventing oxygen getting into the blood and causing the vicious circle of events that can kill people with HAPE. As with many biological processes many factors play a role in the disease and there is good evidence to support a number of theories about how this fluid gets there.

Normally, oxygen gets into your blood and is supplied to the body from your lungs. Each time you take a breath in, air rushes into the tiny air pockets at the end of all the airway branches in your lungs. At the same time, blood from your heart is brought close to these thin-walled air pockets, so that oxygen can move into your blood while waste products move out. Oxygen-rich blood then returns to the heart and is supplied to the body. If, by accident, you inhaled a small object into your lungs, it would become stuck in one of the airways branches. Little oxygen would get to the downstream air pockets. To prevent this area of lung supplying blood starved of oxygen back to the heart (and therefore the rest of body), blood vessels in the area closed down or constrict. This is normally a very good thing and is an example of the body protecting itself.

At altitude however, this same process is a cause of the disease HAPE. Because the whole lung is starved of oxygen, the whole lung reacts in the same way – blood vessels constricting all over the place and not just in small areas. The blood in these vessels is squeezed and the pressure goes up forcing fluid out of blood and into air pockets. 

Very dangerous and reactive substances are formed in your blood when you are starved of oxygen and these can directly damage the special membrane between air and blood in your lungs causing further fluid leak and worsening HAPE.



HACE is a build-up of fluid in the brain. HACE is life-threatening and requires urgent action.

HACE is thought to be a severe form of acute mountain sickness. A severe headache, vomiting and lethargy will progress to unsteadiness, confusion, drowsiness and ultimately coma. HACE can kill in only a few hours. A person with HACE will find it difficult to walk heel-to-toe in a straight line – this is a useful test to perform in someone with severe symptoms of acute mountain sickness. HACE should also be suspected if a companion starts to behave irrationally or bizarrely.

About 1% of people of ascend to above 3000m get HACE. The lowest altitude at which a case of HACE has been reported was 2100m. HACE can also occur in people with HAPE and vice versa. Factors that increase the risk of HACE are similar to those for acute mountain sickness and HAPE. The faster the rate of ascent and the higher the altitude, the more likely it is that HACE will develop. HACE is thought to occur mainly in trekkers or climbers who have ignored symptoms of acute mountain sickness and climbed higher rather than staying at the same altitude or descending.

Descent is the most effective treatment of HACE and should not be delayed if HACE is suspected. A Gamow bag, or portable altitude chamber, can be used as a temporary measure and, if available, oxygen and a drug called dexamethasone should be given.

What causes HACE?

The cause of HACE remains unknown. Several factors may play a role including increased blood flow to the brain. An increase in blood flow is a normal response to low oxygen levels as the body needs to maintain a constant supply of oxygen to the brain. However, if the blood vessels in the brain are damaged, fluid may leak out and result in HACE. Although we know that reactive chemicals are released when oxygen levels are low and that these chemicals can damage blood vessel walls, it still hasn’t been proven that the blood vessels in the brain are actually more leaky.

Refs: Hackett P and Roach RC. High altitude cerebral oedema. HAMB 2004; 5(2):136-146

by Kenneth BaillieA.A. Roger ThompsonMatthew Bates



Poor sleep is common at high altitude.

Most people don’t sleep well at altitude. Climbers commonly report vivid dreams, feelings of being suffocated and wake feeling unrefreshed. The cold, the wind, noisy or smelly tent companions and long distance travel can all disturb a restful night’s sleep. However, there are important changes in the way we sleep at altitude that makes sleep quality poor.

Sleep research at altitude during APEX2
Sleep Stages

Sleep can be divided into stages that are defined by the pattern of electrical activity in the brain and eye movement. Deeper stages of sleep and rapid eye movement (REM) sleep are reduced at altitude, therefore more of the night will be spent as light sleep and sleep quality will not be as good as at sea level.


Periodic Breathing

Periodic breathing (Cheyne Stokes breathing, or PB) is common at high altitude and becomes more frequent with increasing altitude. Periodic breathing involves alternating periods of deep breathing and shallow breathing. Typically, three to five deep breaths will be followed by a couple of very shallow breaths or even a complete pause in breathing. A pause in breathing like this usually lasts around 5 to 15 seconds and is called an apnoea. Apnoeas may end with a gasp that sometimes wakes the individual or their sleeping companions! People may breathe this way for most of the night.

During apnoeic phases, oxygen levels drop and heart rate slows. Oxygen levels and heart rate rise again when breathing resumes resulting in cyclical variations in heart rate and the amount of oxygen in the blood.

Low oxygen levels overnight are likely to disturb sleep but PB may also contribute to arousals: periods when you almost or completely wake up. Arousals are more frequent at altitude, but they can occur even in the absence of periodic breathing. Perhaps surprisingly, although PB may disturb sleep, it doesn’t seem to make the other symptoms of acute mountain sickness worse.

Why does periodic breathing happen?

At sea level the build up of the waste gas, carbon dioxide, in the blood controls breathing. If you hold your breath, carbon dioxide levels rise and create the urge to breathe. At high altitude, the body senses low oxygen levels and this becomes the main drive to breathe. Breathing faster and deeper at high altitude leads to a profound reduction in the carbon dioxide levels in the blood. You can read more about the effects of breathing harder at altitude here. During sleep at high altitude, the levels of carbon dioxide in the blood can drop very low and this can switch off the drive to breathe. Only after the body senses a further drop in oxygen levels do you start breathing again. During the apnoea carbon dioxide levels rise but levels fall again when ventilation resumes, continuing the cycle.

by A.A. Roger Thompson

Sleep at altitude


A dry, debilitating cough at high altitude is common

Many people who travel to high altitude complain of cough. It is a dry debilitating cough and its consequences, aside from interfering with climbing and sleeping can be severe. On the 1971 International Himalayan Expedition no less than four members suffered rib fractures because of their cough.

Altitude cough may be due to bronchoconstriction (the narrowing of the airways that commonly occurs in asthma) or infections, but research has shown that the cough can occur without any evidence of infection or airway narrowing.

Cough could be caused by breathing cold dry air on the mountains, but studies of cough in hypobaric chambers that controlled the ambient temperature and humidity suggest that the receptors in the airways that provoke cough are actually more sensitive at altitude.

What might make cough receptors more sensitive? Three possible theories exist. Firstly, inflammation in the airways at high altitude may increase the receptor sensitivity. Secondly, changes in the brain caused by acclimatisation could sensitise the receptors that cause cough or thirdly, there could be a build up of fluid in the lungs. Some scientists think that a small amount fluid leaks out into the air spaces in many people who go to high altitude without actually causing the symptoms and signs of HAPE. Nevertheless, a severe cough and breathlessness could represent HAPE and if suspected, urgent descent is necessary.


by by A.A. Roger Thompson

High altitude cough

NHS Fit for Travel Information

Click the link below to be taken to the NHS Fit For Travel Webpage on Travelling to High Altitudes. This site contains lots of relevant information on AMS, HAPE and HACE.

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