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Altitude sickness, also known as acute mountain sickness (AMS) or altitude illness is a pathological condition that is caused by acute exposure to low air pressure (usually outdoors at high altitudes). It commonly occurs above 2,400 metres (approximately 8,000 feet)^[1]. Acute mountain sickness can progress to high altitude pulmonary edema (HAPE) or high altitude cerebral edema (HACE).^[2]

Altitude sickness does not typically affect persons traveling in aircraft, as the cabins of modern airplanes are pressurized.

Another rarer type of altitude sickness caused by prolonged exposure to high altitude is chronic mountain sickness, also known as Monge's disease.

This condition is caused by reduced partial pressure of oxygen, while the percentage of oxygen in air remains essentially constant with altitude at 21 percent. ^[3]

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 feet) 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 6 to 10 hours after ascent and generally subside in 1 to 2 days, but they occasionally develop into the more serious conditions. Symptoms are described as headache with fatigue, stomach sickness, dizziness, and sleep disturbance as additional possible symptoms. Exertion aggravates the symptoms.

High altitude pulmonary edema (HAPE) and cerebral edema (HACE) are the most ominous of these symptoms, while acute mountain sickness, retinal haemorrhages, and peripheral edema are the less severe forms of the disease. The rate of ascent, the altitude attained, the amount of physical activity at high altitude, as well as individual susceptibility, are contributing factors to the incidence and severity of high-altitude illness.

Altitude sickness usually occurs following a rapid ascent and can usually be prevented by ascending slowly ^[4]. In most of these cases, the symptoms are only temporary and usually abate with time as altitude acclimatisation occurs. However, in more extreme cases symptoms can be fatal.

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

Headache is a primary symptom used to diagnose altitude sickness, although 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, could be an indication of altitude sickness.

• Lack of appetite, nausea, or vomiting
• Fatigue or weakness
• Dizziness or light-headedness
• Insomnia
• Pins and needles

Additional early indications of altitude sickness may include shortness of breath upon exertion, persistent rapid pulse, drowsiness, general malaise, and peripheral edema (swelling of hands, feet, and face).

Symptoms of life-threatening conditions resulting from extreme altitude sickness include:

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

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 ~2% of those who are adjusting to altitudes of ~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.

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 "get used" 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 feet) 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.

Acetazolamide may help some people in speeding up the acclimatisation process and can treat mild cases of altitude sickness.

A single randomized controlled trial found that sumatriptan may be able to prevent altitude sickness.^[5]

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.^[6] The most effective source of supplemental oxygen at high altitude are oxygen concentrators that use vacuum swing adsorption (VSA) technology.^{[neutrality disputed]} As opposed to generators that use pressure swing adsorption (PSA), VSA technology does not suffer from performance degradation at increased altitude. The lower air density actually facilitates the vacuum step process.

For centuries, indigenous cultures of the Altiplano, such as the Aymaras, have used coca leaves to treat mild altitude sickness. 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.

The only real cure once symptoms appear is for the sufferer to move to a lower altitude as quickly as possible. For serious cases of AMS, a Gamow bag can be used to reduce the effective altitude by as much as 1,500 meters (5,000 feet). A Gamow bag is a portable plastic pressure bag inflated with a foot pump.

Acetazolamide may treat mild cases of altitude sickness.^{[citation needed]}

Patients can sometimes control mild altitude sickness by consciously taking ten to twelve large, rapid breaths every five minutes.^{[citation needed]} If overdone, this can remove too much carbon dioxide and cause tingling in the extremities of the body. Other treatments include injectable steroids to reduce pulmonary edema, and inflatable pressure vessels to relieve and evacuate severe mountain-sick persons.

In Peru hotels on the shore of Lake Titicaca at 3,812 m (12,507 feet = 63.1 kPa) offer oxygenated bedrooms at a premium charge. The same is true at the Monasterio hotel in Cuzco at the lower elevation of 3,500 m (11,500 feet = 65.7 kPa)^[7].

The folk remedy for altitude sickness in Peru and Bolivia is a tea made from the coca plant.

• Mountain climbing
• Cabin pressurization
• Secondary Polycythemia
• Altitude training
• High Altitude Pulmonary Edema
• High altitude cerebral edema

• The tutorial on altitude illness from the International Society for Mountain Medicine
• Merck Manual entry on altitude sickness
• High Altitude Pathology Institute
• University of Buffalo Reporter article on research into the cause of altitude sickness
• Mountain sickness
• Base Camp MD: Guide To High Altitude Medicine
• Altitude Illness Clinical Guide for Physicians
• General information about Altitude sickness by the Prince Leopold Institute of Tropical Medicine
• An online calculator to show the effects of high altitude on oxygen delivery
• An online calculator to compute altitude from air pressure