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

The causes of altitude sickness are not fully understood. The percentage of oxygen in air remains essentially constant with altitude at 21% up until 70,000 feet (21,330 m), but the air pressure (and therefore the number of oxygen molecules) drops as altitude increases — consequently, the available amount of oxygen to sustain mental and physical alertness decreases above 10,000 feet (3,050 m). Altitude sickness usually does not affect persons traveling in aircraft because the cabin altitude in modern passenger aircraft is kept to 8,000 feet (2,440 m) or lower.

A superficially related condition is chronic mountain sickness, also known as Monge's disease, occurring only after prolonged exposure to high altitude.

An unrelated condition, often confused with altitude sickness, is dehydration, due to the higher rate of water vapor lost from the lungs at higher altitudes.


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. 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. 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. Exertion aggravates the symptoms.

High altitude pulmonary edema (HAPE) and cerebral edema (HACE) are the most ominous of these symptoms, while AMS, retinal hemorrhage, and peripheral edema are less severe forms of the disease. 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. 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. [11566]

Signs and 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% 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% 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 McKinleymarker or Mount Kilimanjaromarker)
  • 25,000 ft / 7,620 m = 3-5 minutes;
  • 29,000 ft / 8,840 m = 1-2 minutes; (peak of Mount Everestmarker)
  • 40,000 ft / 12,200 m = 9-15 seconds (represents the oxygen that was in a person's system before the exposure)


As alcohol tends to dehydrate, avoidance in the first 24 hours at a higher altitude is optimal.

Strenuous activity

People with recurrent AMS note that by avoiding strenuous activity such as skiing, hiking, etc. in the first 24 hours at altitude reduces their problems.

Altitude acclimatization

Altitude acclimatization is the process of adjusting to decreasing oxygen levels at higher elevations, in order to avoid altitude sickness. Once above approximately 3,000 metres (10,000 feet = 70 kPa), most climbers and high altitude trekkers follow the "golden rule" - climb high, sleep low. 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), then return to base camp. A subsequent climb to the higher camp would then include 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 . 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 explains why climbers need to spend days (or even weeks at times) acclimatising before attempting to climb a high peak. Simulated altitude equipment that produce hypoxic (reduced oxygen) air can be used to acclimate to altitude, reducing the total time required on the mountain itself.

Altitude acclimatization is necessary for some people who rapidly move from lower altitudes to more moderate altitudes, usually by aircraft and ground transportation over a few hours, such as from sea level to of many Colorado, USA mountain resorts. Stopping at an intermediate altitude overnight can reduce or eliminate a repeat episode of AMS.


Acetazolamide may help some people making a rapid ascent to sleeping altitude above 2750 metres and also be effective if started early in the course of AMS. 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 altittude locations or due to terrain considerations. 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; with 250mg twice daily recommended for treatment of AMS. The Centers for Disease Control and Prevention suggest the lower value for prevention of 125 mg acetazolamide every 12 hours. The CDC advises that Dexamethasone be reserved for treatment of AMS and HACE during descents and notes that Nifedipine may prevent HAPE.

A single randomized controlled trial found that sumatriptan may help prevent altitude sickness.. Although popular, antioxidant treatments have not been found to be effective. Recent interest in phosphodiesterase inhibitors such as sildenafil has been limited by the possibility that these drugs might worsen the headache of mountain sickness.

For centuries, indigenous cultures of the Altiplanomarker, such as the Aymaras, have chewed coca leaves to try to alleviate the symptoms of mild altitude sickness, but its efficacy has not been studied.

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. The most effective source of supplemental oxygen at high altitude are oxygen concentrators that use vacuum swing adsorption (VSA) technology. As opposed to generators that use pressure swing absorption (PSA), VSA technology does not suffer from performance degradation at increased altitude. The lower air density actually facilitates the vacuum step process.

Other methods

Drinking plenty of water will also help in acclimatisation 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 lead to 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 vehicle 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 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.
  • 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. If overdone, this can remove too much carbon dioxide causing hypocapnia.
  • The folk remedy for altitude sickness in Ecuadormarker, Perumarker and Boliviamarker 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.  — High altitude information for laypeople
  2. The High Altitude Medicine Handbook, 3rd Edition, Andrew J. Pollard and David R. Murdoch.
  3. Altitude and Oxygen
  4. 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.
  5. The Mountaineers. Mountaineering: The Freedom of the Hills, 7th Edition. Seattle, WA: Mountaineers Books, 2003
  6. High Altitude Medicine

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