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Altitude sickness
Classification and external resources
ICD-10 T70.2
ICD-9 E902.0
DiseasesDB 8375 29615
MedlinePlus zal
eMedicine med/3225
MeSH D000532

Altitude sickness, also known as acute mountain sickness (AMS), altitude illness, hypobaropathy, or soroche, is a pathological effect of high altitude on humans, caused by acute exposure to low partial pressure of oxygen at high altitude. It commonly occurs above 2,400 metres (8,000 feet).[1][2] Acute mountain sickness can progress to high altitude pulmonary edema (HAPE) or high altitude cerebral edema (HACE).[1][3]

The causes of altitude sickness are not fully understood.[1][4] The percentage of oxygen in air, at 21%, remains almost unchanged up to 70,000 feet (21,000 m). The RMS velocities of diatomic nitrogen and oxygen are very similar and thus no change occurs in the ratio of oxygen to nitrogen. However, it is the air pressure itself, the number of molecules (of both oxygen and nitrogen) per given volume, which drops as altitude increases. Consequently, the available amount of oxygen to sustain mental and physical alertness decreases above 10,000 feet (3,000 m).[5] Although the cabin altitude in modern passenger aircraft is kept to 8,000 feet (2,400 m) or lower, a large proportion of passengers on long-haul flights may experience some symptoms of altitude sickness.[6]

Chronic mountain sickness, also known as Monge's disease, is a different condition that only occurs after very prolonged exposure to high altitude.[7]

Dehydration due to the higher rate of water vapor lost from the lungs at higher altitudes may contribute to the symptoms of altitude sickness.[8]



High altitude or mountain sickness is defined as a collection of nonspecific symptoms that can resemble a case of flu, carbon monoxide poisoning or a hangover caused by high altitudes[9]. It is hard to determine who will be affected by altitude-sickness, as there are no specific factors that compare with this susceptibility to altitude sickness. However, most people can climb up to 2500 meters (8000 ft) normally.

Generally, different people have different susceptibilities to altitude sickness. For some otherwise healthy people, acute mountain sickness (AMS) can begin to appear at around 2000 meters (6,500 ft) above sea level, such as at many mountain ski resorts, equivalent to a pressure of 80 kPa[10]. AMS is the most frequent type of altitude sickness encountered. Symptoms often manifest themselves six to ten hours after ascent and generally subside in one to two days, but they occasionally develop into the more serious conditions. Symptoms include headache, fatigue, stomach illness, dizziness, and sleep disturbance[3]. Exertion aggravates the symptoms.

High altitude pulmonary edema (HAPE) and cerebral edema (HACE) are potentially fatal problems that may occur at high altitude. AMS, retinal hemorrhage, and peripheral edema are less severe high-altitude ailments. The rate of ascent, altitude attained, amount of physical activity at high altitude, as well as individual susceptibility, are contributing factors to the onset and severity of high-altitude illness.

Altitude sickness usually occurs following a rapid ascent and can usually be prevented by ascending slowly.[3] In most of these cases, the symptoms are temporary and usually abate as altitude acclimatisation occurs. However, in extreme cases, altitude sickness can be fatal.

The word "soroche" came from South America and originally meant "ore", because of an old, incorrect belief that it was caused by toxic emanations of ores in the Andes mountains. [1]

Signs and symptoms

This sign near the peak of Mount Evans (elev. 14264 ft or 4,350 meters) in Colorado, USA, warns of altitude sickness symptoms.

Headaches are a primary symptom used to diagnose altitude sickness, although a headache is also a symptom of dehydration. A headache occurring at an altitude above 2,400 meters (8000 feet = 76 kPa), combined with any one or more of the following symptoms, can indicate altitude sickness:

Symptoms that may indicate life-threatening altitude sickness include:

  • pulmonary edema (fluid in the lungs):
    • persistent dry cough
    • fever
    • shortness of breath even when resting
  • cerebral edema (swelling of the brain):
    • headache that does not respond to analgesics
    • unsteady gait
    • increased vomiting
    • gradual loss of consciousness.

Severe cases

The most serious symptoms of altitude sickness are due to edema (fluid accumulation in the tissues of the body). At very high altitude, humans can get either high altitude pulmonary edema (HAPE), or high altitude cerebral edema (HACE). The physiological cause of altitude-induced edema is not conclusively established. It is currently believed, however, that HACE is caused by local vasodilation of cerebral blood vessels in response to hypoxia, resulting in greater blood flow and, consequently, greater capillary pressures. On the other hand, HAPE may be due to general vasoconstriction in the pulmonary circulation (normally a response to regional ventilation-perfusion mismatches) which, with constant or increased cardiac output, also leads to increases in capillary pressures. For those suffering HACE, dexamethasone may provide temporary relief from symptoms in order to keep descending under their own power.

HAPE occurs in about 2%[citation needed] of those who are adjusting to altitudes of about 3000 m (10,000 feet = 70 kPa) or more. It can progress rapidly and is often fatal. Symptoms include fatigue, severe dyspnea at rest, and cough that is initially dry but may progress to produce pink, frothy sputum. Descent to lower altitudes alleviates the symptoms of HAPE.

HACE is a life threatening condition that can lead to coma or death. It occurs in about 1%[citation needed] of people adjusting to altitudes above 2700 m (9,000 feet = 73 kPa). Symptoms include headache, fatigue, visual impairment, bladder dysfunction, bowel dysfunction, loss of coordination, paralysis on one side of the body, and confusion. Descent to lower altitudes may save those afflicted with HACE.

A person suffering from serious symptoms of altitude sickness has a relatively short period of time of useful consciousness in which corrective action can be taken. The following is a correlation of approximate altitude to the amount of time that a person will have useful consciousness:

  • 20,000 ft / 6,100 m = 5-12 minutes (peak of Mount McKinley or Mount Kilimanjaro)
  • 25,000 ft / 7,620 m = 3-5 minutes;
  • 29,000 ft / 8,840 m = 1-2 minutes; (peak of Mount Everest)
  • 40,000 ft / 12,200 m = 9-15 seconds (represents the oxygen that was in a person's system before the exposure)


Ascending slowly is the best way to avoid altitude sickness.[3] Avoiding strenuous activity such as skiing, hiking, etc. in the first 24 hours at high altitude reduces the symptoms of AMS. As alcohol tends to cause dehydration, which exacerbates AMS, avoiding alcohol consumption in the first 24-hours at a higher altitude is optimal.


Altitude acclimatization

Altitude acclimatization is the process of adjusting to decreasing oxygen levels at higher elevations, in order to avoid altitude sickness.[11] Once above approximately 3,000 metres (10,000 feet = 70 kPa), most climbers and high-altitude trekkers take the "climb-high, sleep-low" approach. For high-altitude climbers, a typical acclimatization regime might be to stay a few days at a base camp, climb up to a higher camp (slowly), and then return to base camp. A subsequent climb to the higher camp then includes an overnight stay. This process is then repeated a few times, each time extending the time spent at higher altitudes to let the body adjust to the oxygen level there, a process that involves the production of additional red blood cells[citation needed]. Once the climber has acclimatised to a given altitude, the process is repeated with camps placed at progressively higher elevations. The general rule of thumb is to not ascend more than 300 metres (1,000 ft) per day to sleep. That is, one can climb from 3,000 (10,000 feet = 70 kPa) to 4,500 metres(15,000 feet = 58 kPa) in one day, but one should then descend back to 3,300 metres (11,000 feet = 67.5 kPa) to sleep. This process cannot safely be rushed, and this is why climbers need to spend days (or even weeks at times) acclimatising before attempting to climb a high peak. Simulated altitude equipment that produces hypoxic (reduced oxygen) air can be used to acclimate to high altitude, reducing the total time required on the mountain itself.

Altitude acclimatization is necessary for some people who move rapidly from lower altitudes to intermediate altitudes, e.g. by aircraft and ground transportation over a few hours, such as from sea level to 8,000 feet (2,400 m) of many Colorado, USA mountain resorts. Stopping at an intermediate altitude overnight can reduce or eliminate a reoccurrence of AMS.

Medical treatment

The drug acetazolamide may help some people making a rapid ascent to sleeping altitude above 2750 metres, and it may also be effective if started early in the course of AMS.[12] The Everest Base Camp Medical Centre cautions against its routine use as a substitute for a reasonable ascent schedule, except where rapid ascent is forced by flying into high altitude locations or due to terrain considerations.[13] The Centre suggests a dosage of 125-250 mg twice daily for prophylaxis, starting from 24 hours before ascending until a few days at the highest altitude or on descending;[13] with 250 mg twice daily recommended for treatment of AMS.[14] The Centers for Disease Control and Prevention suggest a lower dose for prevention of 125 mg acetazolamide every 12 hours.[15] The CDC advises that Dexamethasone be reserved for treatment of AMS and HACE during descents, and notes that Nifedipine may prevent HAPE.[15]

A single randomized controlled trial found that sumatriptan may help prevent altitude sickness.[16]. Despite their popularity, antioxidant treatments have not been found to be effective medications for prevention of AMS.[17] Interest in phosphodiesterase inhibitors such as sildenafil has been limited by the possibility that these drugs might worsen the headache of mountain sickness.[18]

For centuries, indigenous peoples of the Americas such as the Aymaras of the Altiplano, have chewed coca leaves to try to alleviate the symptoms of mild altitude sickness, and its efficacy has been studied.[19]

Oxygen enrichment

In high-altitude conditions, oxygen enrichment can counteract the effects of altitude sickness, or hypoxia. A small amount of supplemental oxygen reduces the equivalent altitude in climate-controlled rooms. At 3,400 m (67 kPa), raising the oxygen concentration level by 5 percent via an oxygen concentrator and an existing ventilation system provides an effective altitude of 3,000 m (70 kPa), which is more tolerable for surface-dwellers.[20].

Other methods

Drinking plenty of water will also help in acclimatisation[21] to replace the fluids lost through heavier breathing in the thin, dry air found at altitude, although consuming excessive quantities ("over-hydration") has no benefits and may cause dangerous hyponatremia.

Oxygen from gas bottles or liquid containers can be applied directly via a nasal cannula or mask. Oxygen concentrators based upon pressure swing adsorption (PSA), VSA, or vacuum-pressure swing adsorption (VPSA) can be used to generate the oxygen if electricity is available. Stationary oxygen concentrators typically use PSA technology, which has performance degradations at the lower barometric pressures at high altitudes. One way to compensate for the performance degradation is to utilize a concentrator with more flow capacity. There are also portable oxygen concentrators that can be used on vehicular DC power or on internal batteries, and at least one system commercially available measures and compensates for the altitude effect on its performance up to 4,000 meters (13,000 ft). The application of high-purity oxygen from one of these methods increases the partial pressure of oxygen by raising the FIO2 (fraction of inspired oxygen).


The only reliable treatment and in many cases the only option available is to descend. Attempts to treat or stabilise the patient in situ at altitude is dangerous unless highly controlled and with good medical facilities. However, the following treatments have been used when the patient's location and circumstances permit:

  • Oxygen may be used for mild to moderate AMS below 12,000 feet (3,700 m) and is commonly provided by physicians at mountain resorts. Symptoms abate in 12-36 hours without the need to descend.
  • For more serious cases of AMS, or where rapid descent is impractical, a Gamow bag, a portable plastic hyperbaric chamber inflated with a foot pump, can be used to reduce the effective altitude by as much as 1,500 meters (5,000 ft). A Gamow bag is generally used only as an aid to evacuate severe AMS patients, not to treat them at altitude.
  • Acetazolamide may assist in altitude aclimatisation but is not a reliable treatment for established cases of even mild altitude sickness.[22][23]
  • Some claim that mild altitude sickness can be controlled by consciously taking 10-12 large, rapid breaths every 5 minutes, (hyperventilation) but this claim lacks both empirical evidence and a plausible medical reason as to why this should be effective.[citation needed] If overdone, this can remove too much carbon dioxide causing hypocapnia.
  • The folk remedy for altitude sickness in Ecuador, Peru and Bolivia is a tea made from the coca plant. See mate de coca.
  • Other treatments include injectable steroids to reduce pulmonary edema, this may buy time to descend but treats a symptom, it does not treat the underlying AMS.

See also


  1. ^ a b c Roach, Robert; Stepanek, Jan; and Hackett, Peter. (2002). "24". Acute Mountain Sickness and High-Altitude Cerebral Edema. In: Medical Aspects of Harsh Environments. 2. Washington, DC. Retrieved 2009-01-05. 
  2. ^ Baillie, Kenneth; Simpson, Alistair. "Altitude Tutorials - Altitude Sickness". Apex (Altitude Physiology Expeditions). Retrieved 2010-01-26. 
  3. ^ a b c d A.A.R. Thompson. "Altitude Sickness". Apex. Retrieved 2007-05-08. 
  4. ^ The High Altitude Medicine Handbook, 3rd Edition, Andrew J. Pollard and David R. Murdoch.
  5. ^ J.K. Baillie. "Living in Thin Air". Apex. Retrieved 2007-12-17. 
  6. ^ Muhm, J. Michael; Paul B. Rock, Dianne L. McMullin, Stephen P. Jones, I.L. Lu, Kyle D. Eilers, David R. Space, Aleksandra McMullen (2007-07-05). "Effect of Aircraft-Cabin Altitude on Passenger Discomfort". N Engl J Med 357 (1): 18–27. doi:10.1056/NEJMoa062770. PMID 17611205. Retrieved 2009-12-23. 
  7. ^ A.J. Giannini, H.R. Black, R.L. Goettsche. The Psychiatric, Psychogenic and Somatopsychic Disorders Handbook. New Hyde Park, NY. Medical Examination Publishing Co.,1978. pp.190,192. ISBN 0-87488-596-5.
  8. ^ Hackett, P H; R C Roach (2001-07-12). "High-altitude illness". The New England Journal of Medicine 345 (2): 107–14. doi:10.1056/NEJM200107123450206. ISSN 0028-4793. PMID 11450659. Retrieved 2009-03-25. 
  9. ^ The Mountaineers. Mountaineering: The Freedom of the Hills, 7th Edition. Seattle, WA: Mountaineers Books, 2003
  10. ^ K. Baillie and A. Simpson. "Acute mountain sickness". Apex (Altitude Physiology Expeditions). Retrieved 2007-08-08.  — High altitude information for laypeople
  11. ^ Muza, S.R.; Fulco, C.S.; Cymerman, A. (2004). "Altitude Acclimatization Guide.". U.S. Army Research Inst. of Environmental Medicine Thermal and Mountain Medicine Division Technical Report (USARIEM-TN-04-05). Retrieved 2009-03-05. 
  12. ^ World Health Organization (1 January 2007). "CHAPTER 3 Environmental health risks" (PDF). International travel and health. p. 31. Retrieved 2009-11-21. 
  13. ^ a b Himalayan Rescue Association - Everest Medicial Clinic. "Prophylaxis". ExplorersWeb. Retrieved 2009-11-21. 
  14. ^ Himalayan Rescue Association - Everest Medicial Clinic. "Treating AMS". ExplorersWeb. Retrieved 2009-11-21. 
  15. ^ a b Hackett P, Shlim D (2009). "Chapter 2 The Pre-Travel Consultation - Self-Treatable Diseases - Altitude Illness". in Turell D, Brunette G, Kozarsky P, Lefor A. CDC Health Information for International Travel 2010 "The Yellow Book". St. Louis: Mosby. ISBN 0-7020-3481-9. Retrieved 2009-11-21. 
  16. ^ Jafarian S., Gorouhi F., Salimi S., Lotfi J. (2007). "Sumatriptan for prevention of acute mountain sickness: randomized clinical trial". Ann. Neurol. 62 (3): 273–7. doi:10.1002/ana.21162. PMID 17557349. 
  17. ^ Baillie JK, Thompson AA, Irving JB, Bates MG, Sutherland AI, Macnee W, Maxwell SR, Webb DJ (May 2009). "Oral antioxidant supplementation does not prevent acute mountain sickness: double blind, randomized placebo-controlled trial". QJM 102 (5): 341–8. doi:10.1093/qjmed/hcp026. PMID 19273551. 
  18. ^ Bates MG, Thompson AA, Baillie JK (March 2007). "Phosphodiesterase type 5 inhibitors in the treatment and prevention of high altitude pulmonary edema". Curr Opin Investig Drugs 8 (3): 226–31. PMID 17408118. 
  19. ^
  20. ^ West JB (February 1995). "Oxygen enrichment of room air to relieve the hypoxia of high altitude". Respir Physiol 99 (2): 225–32. doi:10.1016/0034-5687(94)00094-G. PMID 7777705. 
  21. ^ Dannen, Kent; Dannen, Donna (2002). Rocky Mountain National Park. anywere: Globe Pequot. pp. 9. ISBN 0762722452. "Visitors unaccustomed to high elevations may experience symptoms of Acute Mountain Sickness (AMS)[...s]uggestions for alleviating symptoms include drinking plenty of water[.]" 
  22. ^ Cain SM, Dunn JE (July 1966). "Low doses of acetazolamide to aid accommodation of men to altitude". J Appl Physiol 21 (4): 1195–200. PMID 5916650. 
  23. ^ Grissom CK, Roach RC, Sarnquist FH, Hackett PH (March 1992). "Acetazolamide in the treatment of acute mountain sickness: clinical efficacy and effect on gas exchange". Ann. Intern. Med. 116 (6): 461–5. PMID 1739236. 

External links


Travel guide

Up to date as of January 14, 2010

From Wikitravel

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Altitude Sickness Warning Sign on Mount Evans, Colorado
Altitude Sickness Warning Sign on Mount Evans, Colorado

Altitude sickness is a reaction to the lower amounts of oxygen available at high altitudes (due to the lower air pressure). Your body will respond in various ways to this: some are normal, some are illnesses. The illnesses are a serious health hazard and can result in death if ignored or left untreated.


The further you move away from sea level up into higher altitudes, the lower the air pressure is. The body has two main problems with high altitude and the corresponding lower air pressure:

  • Air at lower pressure has less oxygen per lungful. Your body adjusts to this by making more red blood cells to carry oxygen more efficiently. Most of the cell-building happens while you sleep; however, the process can take days and in the meanwhile, you may be ill.
  • At lower air pressure, water evaporates faster. This can lead to dehydration.

The changes to your body at altitude are complex and can be quite dramatic. Some changes are normal, some are signs of illness. The difficulty your body has maintaining a good oxygen supply and keeping related problems under control is directly related to how high up you are, and also to recent changes in your altitude. These are the two major factors that cause altitude sickness.

Hence this article talks a lot about ascent and descent. Ascending further away from sea level is the risky activity and the time you must be alert. Conversely, descending towards sea level is the single major factor that helps with moderate to severe altitude sickness.

Altitude sickness is very dangerous for four reasons: its rapid progression; its deadliness; the fact sufferers are usually some distance from medical help and are difficult to evacuate swiftly; and the fact that in many cases sufferers are reliant on their health because they're doing a lot of physical activity in dangerous environments.

How high is high?

A minority of people, about 20%, have some symptoms of altitude sickness if they ascend to about 2500 meters (8000 feet) above sea level and sleep there. However, most people will acclimatize to 3000 meters (10,000 feet) with relative ease, perhaps having symptoms after the first night.

Acclimatizing to heights of 3000–5000 meters (10,000–16,000 feet) is much more difficult, and it is here that it is absolutely necessary to ascend slowly and return to a lower altitude to sleep if you have been travelling around at a higher altitude during the day. Over 50% of people will become ill if they ascend rapidly from sea level to 3500 meters (11,000 feet) without acclimatization, and everyone will if they ascend rapidly to 5000 meters (16,000 feet).

It is thought to be impossible to permanently acclimatize to heights above 5500 meters (18,000 feet). It is possible to spend several weeks sleeping as high as 6000 meters (20,000 feet) once acclimatized, but gradual deterioration of physical well-being will still occur.

Regions above 7500 meters (25,000 feet) are referred to as the death zone: you will deteriorate noticeably while you remain at such high altitudes, some of your body's major systems will shut down and climbers will only remain there for two or three days. Death rates from altitude sickness above 7000 meters (23,000 feet) are estimated at 4% of all people who venture that high.

If your home is significantly above sea level, you gain a definite leg up on ascending to higher elevations, but that doesn't make you immune to altitude problems; it just pushes the threshold for their onset higher. Most otherwise healthy people who live at elevations of 1500 meters (5000 feet) to 2500 meters (8000 feet), an elevation range containing quite a few major cities, experience little trouble going to 3000 meters (10,000 feet) or a bit higher, but even they will be at risk of altitude problems at 5000 meters (16,000 feet).

Risk factors

Altitude sickness tends to affect men more than women, especially men between the ages of 16 and 25, for unknown reasons. It is important to remember that just because you are young and healthy, and haven't experienced altitude sickness in the past doesn't mean you are immune to it on future climbs. Physical fitness is not necessarily a good indicator, and neither are strength or good health. You may react badly to altitude despite being fit, young and healthy. In fact, the fit, young and healthy have a hidden risk: their general physical capacity leads them to believe that they should handle altitude just fine, which is not always true.

Bad health, on the other hand, is a risk factor: particularly cardiac or respiratory problems. Healthy hearts and lungs have a hard enough time getting oxygen to your tissues at high altitudes. Naturally, if you have physical problems that make exertion difficult for you, you have reason to think carefully about exertion at high altitude, where it is much harder!

Scuba diving increases risk of decompression sickness. If you have recently been diving and have not fully gotten rid of the nitrogen in your blood, you should not ascend to a higher altitude (or travel in a plane). See scuba diving for recommendations on how long to wait.

Effects of altitude

Altitude has some physiological effects on all people who are at high altitudes. These effects are not in and of themselves symptoms of illness, although they are signs of the increased difficulty that the body has getting at altitude.


You will naturally breathe faster at higher altitudes to compensate for the lower air pressure. It's possible you won't notice this: a similar effect happens during air travel.

Altitude diuresis

Your body will try and concentrate your blood to compensate for the lack of oxygen. This will make you urinate a lot at altitude. If you aren't urinating much more than you usually would, then you might actually be dehydrated.

Periodic breathing

Because of the disruption to oxygen and carbon dioxide levels in your blood due to the body chemistry changes and hyperventilation that occurs at altitude, your body's "when to breathe" chemical signals become confused. While you're awake you will remember to breathe, but when you sleep it is common to have interrupted breathing: holding your breath for up to fifteen seconds and then breathing very rapidly when you start breathing again.

This can be very alarming when you wake up knowing that you weren't breathing or were short of breath; or when you notice someone else has stopped breathing. But it is a normal response to altitude, and happens to almost everyone. Acclimatisation only improves it a little.

Illnesses at altitude

As well as the less dangerous physiological effects, altitude makes you susceptible to actual illnesses, several of them very dangerous. While not all the effects of altitude can be avoided, you should take sensible steps to avoid actual illness, and take it very seriously if it does occur.


You need to drink a lot of water at high altitudes. The loss of appetite, a precursor to nausea, can lead you in to a dehydration headache. Unfortunately, it is easy to mistake dehydration headaches for AMS (below) headaches and vice versa. If a headache does not improve after drinking a liter of water it should be considered as an AMS effect. Dehydration headache can also be recognised by comparing pulse rates: if your pulse rate goes up more than 20% when standing up after lying down for five minutes, you need more fluids.


Acute mountain sickness (AMS) is the most common unhealthy response to altitude: it's a collection of signs that your body is becoming ill and has not adapted successfully to a higher altitude.

For your own safety, assume any illness at altitude is AMS. The most common reasons that people fail to descend as soon as they should are bad assumptions. They assume that having AMS is a sign of weakness; that their level of fitness means they can't have AMS; or mistake their symptoms for the flu or another illness. Assume AMS first: it happens to healthy strong people, and if it turns out you are indeed sick with something else, descending to a lower altitude will make it easier for your body to heal anyway.

In particular, if you've recently ascended, and you have a headache and any other symptom, you have AMS. The other signs of AMS vary for different people, but include:

  • fatigue
  • dizziness
  • loss of appetite
  • nausea or vomiting
  • confusion
  • difficulty walking (called gait ataxia)
  • rattling breath
  • feeling generally extremely ill

The last three signs in particular are signs that you are becoming quite ill, but you should not wait for the onset of these symptoms before acknowledging you have AMS: they're fairly reliable indicators of the onset of HACE or HAPE.

You and your party should keep an eye on each other for signs of AMS, and if you have AMS, for signs of it worsening. Very sick people can become confused and not realise how ill they are. Loss of appetite is a particularly good sign: anyone who has been walking or climbing at altitude for a day should be hungry for a good meal in the evening.

If you have symptoms of AMS, do not ascend further. Consider descending.

If you have signs of HACE or HAPE, descend immediately. Your life may depend on it.


High altitude cerebral edema (HACE) is the end-stage of AMS (conversely AMS can be thought of as the mild form of HACE). When you have HACE, your brain swells and stops working properly.

HACE symptoms include a number of signs of mental functions failing: confusion, fatigue and weird behaviour. But the most reliable one is gait ataxia, and you can test it by walking heel to toe along a straight line on the ground. Healthy people can pass this test easily, anyone who has difficulty balancing while they do it is showing signs of HACE.

HACE is extremely serious, and you may only have a few hours to help someone with HACE. The main treatment for this is descent, but a person experiencing these symptoms will need significant help. Dexamethasone is one drug that can be used to relieve symptoms, but it is just a temporary bridge to give more time for descent.

A 2008 medical study Why Climbers Die On Mount Everest shows HACE as the leading cause of death.


High altitude pulmonary edema (HAPE) is another severe altitude illness. It sometimes occurs in conjunction with AMS or HACE, but sometimes doesn't — it's thought to have different causes. When you have HAPE, your lungs fill with fluid. Signs include extreme fatigue; breathlessness (when not due to periodic breathing — give yourself 30 seconds to recover upon waking); a cough, especially if it is wet and has blood in it; rattling or gurgling breath; chest congestion; very fast heart rate; very fast breathing; and blue extremities. A fever is sometimes present. It most commonly sets in at night.

HAPE is another extremely serious illness, and like HACE should be treated as a critical emergency. Nifedipine is the drug of choice for the treatment of HAPE, but it can only provide temporary relief and rapid descent is very important.

Cheyne Stokes Breathing

Above 10,000 feet (3000 meters), some people experience a periodic breathing during sleep known as Cheyne-Stokes Respirations. The pattern begins with a few shallow breaths and increases to deep sighing respirations then falls off rapidly. Respirations may cease entirely for a few seconds and then the shallow breaths begin again. During the period when breathing stops the person often becomes restless and may wake with a sudden feeling of suffocation. This can disturb sleeping patterns, exhausting the climber.

Acetazolamide is helpful in relieving the periodic breathing. This type of breathing is not considered abnormal at high altitudes. However, if it occurs first during an illness (other than altitude illnesses) or after an injury (particularly a head injury) it may be a sign of a serious disorder.


Decompression sickness (DCS, also known as the bends or caisson disease) is a severe illness in which bubbles of nitrogen forms in your blood, blocking blood supply to parts of your body. Symptoms include persistent tingling or joint pain, fatigue, itching, rashes, confusion and collapse. Decompression sickness is brought on by extremely sudden changes in air pressure (effectively an increase in altitude), such as loss of cabin pressure in a plane you are flying in. Even a fast ascent to most altitudes (such as by plane) would not normally cause decompression sickness. The exception is for anyone who has recently been scuba diving, who should avoid ascents above the altitude that their dive took place at for between 12 and 24 hours depending on dive activity. See the Scuba diving article for more information.


Keep hydrated

Remember to drink a lot. At 3500 meters (12,000 feet), make sure you drink at least 3 litres of water every day. That means, two large bottles of water. Do not leave anything in the bottom, drink it all, even if it feels bad. A headache is the punishment for those who do not heed this advice, and that feels worse.

Acclimatize to altitude gradually

Acclimatization is the process of getting your body to adapt to the lower oxygen levels by ascending slowly into higher altitudes, spending some time at each one to adapt. The most important factor is to increase your sleeping elevation (the altitude where you're spending the night) slowly. If you're on a hiking or climbing holiday, a typical strategy is to spend a day (or initially part of a day) at a higher altitude and return to a lower altitude to sleep. This also works for people doing winter sports at high altitudes: ski at the top of the resort and sleep at the bottom.

Here are the recommended maximum increases in your sleeping elevation will stop most people from proceeding to AMS:

  • Go no higher than 2400 meters (8000 feet) the first night
  • Increase your sleeping elevation by 300 meters (1000 feet) per night after 3000 meters (10,000 feet).
  • Every 1000 meters (3000 feet), you should spend a second night at the same altitude. This will be every fourth night if you have been ascending at the maximum pace recommended above.

You can, of course, ascend more gradually than these rates. Many people ascending from sea level choose to spend several nights at 2500 meters (8000 feet) to 3000 meters (10,000 feet) before beginning acclimatization to higher altitude.

During acclimatisation, drink a lot of non-alcoholic drinks. Some people find vegetarian food slightly accelerates the acclimatisation. Local recipes like mate de coca in Peru (tea from coca leafs) are fine, but its effect is doubtful.

Avoid rapid ascents

Rapid ascents are the opposite of acclimatization; you make a rapid ascent when you're gaining altitude faster than recommended. This may mean climbing and camping higher than recommended, but you can also make an even more rapid ascent by driving to a high altitude location, and flying from low altitude to high altitude is an even more rapid ascent. For example, flying from sea level to Lhasa, Tibet, which is 3700 meters (12,000 feet) high, is distinctly unwise. Consider spending a week or so at an intermediate altitude; see Overland to Tibet for some possibilities. If you are going to travel around Tibet — where some inhabited areas are over 5000 meters (16,000 feet) and some mountains over 8000 meters (26,000 feet), so do not set out until you are thoroughly acclimatised in Lhasa.

Where possible, avoid ascents more rapid than recommended above, particularly any sudden ascent to 3000 meters (10,000 feet) or higher. Even if you are taking Acetazolamide (below) a rapid ascent makes it more likely you'll get AMS and makes AMS progress to serious illness faster, so you will have less time to respond and descend.

Be particularly wary with oxygen equipment: some tourists have died at altitude when their equipment failed and they were utterly unacclimatized.

Consider road or rail travel rather than flying directly to somewhere with a very high altitude — but remember that the surface option often involves a lot higher altitudes : the the Manali-Leh road for example will take you from below 2000 meters (7000 feet) to 5000 meters (16,000 feet) in less than a day. Or fly in stages, stopping somewhere at moderate altitude in between. If you must fly to any destination about 3000 meters (10,000 feet) at least spend a few days at some intermediate destination en route. If flying to a more moderate altitude above 2500 meters (8000 feet), you will still want to spend several nights at that altitude before setting off into higher country.

Refrain from smoking and alcohol when you arrive by plane in a high altitude area from lower altitudes.


As soon as the symptoms of AMS appear, your first priority is recovering. You must not ascend any further until the symptoms have disappeared. This may take up to 48 hours, if it takes longer, descend. You could also descend on the onset of symptoms, this will make them disappear much faster, probably within hours.

If you are getting sicker or showing signs of HACE or HAPE, you must descend to a lower altitude as quickly as possible. If it is nighttime, do not wait for morning if you have a choice at all. You should descend at least as far as you were the last night you had no AMS symptoms. You may need to seek hospital care.

People with HACE and HAPE are frequently confused or exhausted, and are likely to need help with the descent. Help them down!

There is some equipment available to treat people with HACE or HAPE at high altitudes, including hyperbaric bags in which the sufferer can lie in a higher pressure atmosphere. Likewise, because the main cause of these illnesses is a lack of oxygen, breathing oxygen from a tank will slow their onset and may provide some temporary relief of symptoms. Either treatment buys some time if it is too dangerous to descend, but they are not a substitute for descent.

Sufferers of DCS needed to be hospitalised and treated in a recompression chamber: descent to sea level is not sufficient to alleviate DCS symptoms. As with HACE and HAPE, breathing oxygen may provide some temporary relief of symptoms allowing for rescue. Scuba diving organisations can advise further.


This drug (sold as Diamox) stimulates your breathing. The drug was originally designed as a treatment for glaucoma, but a side effect of increased breathing rates and depth have proven useful to climbers. It has the effect of increasing acclimatisation rates; improving periodic breathing; and helping people recover from AMS more quickly. There are some side effects. The drug acts as a diuretic and can cause easy dehydration, so drinking plenty of water is important. This drug can be useful for people who have had AMS in the past; people on a forced ascent (for example, flying into Tibet); and anyone who has AMS, particularly if they are choosing not to descend. Many climbers also take it as a prophylaxis.

Acetazolamide is not an absolute preventative measure, particularly in the case of forced ascents. A prescription is necessary, and a doctor should be consulted about proper dosages.

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