Altitude illness occurs when a person ascends to a higher elevation more rapidly than the body can adjust ("acclimatize") to the decreased oxygen at the higher elevation. Some people adjust very easily (possibly genetic factors), whereas others cannot go above even moderate elevations without experiencing symptoms. Only past experience can predict whether one will be a good acclimatizer; if symptoms occurred before, they are likely to occur again at the same elevation.
Altitude illness is generally divided into 3 syndromes: acute mountain sickness (AMS), high-altitude cerebral edema (HACE), and high-altitude pulmonary edema (HAPE). Symptoms can range from mild to life-threatening and most can be prevented or minimized by proper acclimatization and/or preventive medications. Risk and prevention strategies vary depending on the type of travel planned, for example, travel to typical tourist destinations at relatively moderate elevations versus trekking in extremely high elevation situations.
Risk of Altitude Illness
Personal Risk Factors
Persons are at low risk if they:
- Have no prior history of altitude illness and are ascending to less than 2,800 m (9,200 ft)
- Allow 2 or more days to arrive at 2,500 to 3,000 m (8,000-9,800 ft) and keep subsequent increases in sleeping elevation to less than 500 m (1,600 ft) per day
Persons are at moderate risk if they:
- Have a prior history of AMS and are ascending to 2,500 to 3,000 m in 1 day
- Have no prior history of AMS and are ascending to more than 2,800 m in 1 day
- Are ascending more than 500 m per day (increase in sleeping elevation) at elevations more than 3,000 m
Persons are at high risk if they:
- Have a history of AMS and are ascending to 2,800 m or higher in 1 day
- Have a prior history of HAPE or HACE
- Are ascending to more than 3,500 m (11,500 ft) in 1 day
- Are ascending more than 500 m per day (increase in sleeping elevation) above 3,500 m
- Make rapid ascents (e.g., < 7-day ascent of Mount Kilimanjaro)
Risk by Type of Travel
Typical Tourist Destinations
Travelers going to typical tourist destinations at elevations of 3,000 m or less rarely experience the more severe forms of altitude illness, such as HACE or HAPE, unless they are genetically predisposed. Mountain resorts are usually located, by design, at elevations ranging from 1,200 to 3,000 m (3,900-9,800 ft). Mild symptoms of altitude illness have been documented at these elevations, and HAPE occurs infrequently at 2,500 to 3,000 m. Daytime activities (e.g., skiing, hiking, sightseeing) may take travelers to higher elevations, but risk is reduced by descending to the lower resort elevation overnight.
Risk increases for those who rapidly ascend (hike vigorously) to destinations higher than 3,000 m and for those who fly (or who are otherwise transported) directly to these relatively higher destinations because these modes preclude gradual acclimatization. Examples of destinations that allow access, without hiking, to relatively high elevations include: La Paz, Bolivia; Lhasa, Tibet; and Cuzco, Peru.
Trekkers are at higher risk of HAPE and HACE at high elevations, although the risk is lower compared to that of AMS. Most trekking itineraries take a "one-size fits all" approach toward the pace of the trek and thus cannot guarantee that altitude illness will not occur. Altitude illness affects 50% or more trekkers on popular high-elevation routes. Complications of altitude sickness result in 2 to 3 deaths per year in Nepal. Trekking agencies also feel pressure to offer shorter expeditions for busy people who cannot take long holidays. For example, Mount Kilimanjaro treks that summit in 5 days are offered, even when a 7-day ascent already yields elevation gains more rapid than typical Himalaya treks.
AMS symptoms include headache (which can be mild to extremely painful), loss of appetite (which can progress to nausea and vomiting), and extreme fatigue.
HACE begins like AMS but the symptoms become more severe, including changes in consciousness and loss of coordination; HACE can progress rapidly to coma and death and can occur alone or in combination with HAPE.
HAPE symptoms begin as decreased exercise tolerance progressing to severe breathlessness upon exertion and, eventually, even while at rest; substantial chest fullness; and cough (although cough at high elevations is common from other causes). Descent is mandatory as soon as HAPE is suspected because the symptoms can progress rapidly, and death can occur within hours. Unfortunately, exertion considerably worsens HAPE, so exertion by the sick person should be minimized during descent, if at all possible.
Commonly, individuals who sleep above 3,000 m will have an alteration of their breathing pattern during sleep. The result is a form of periodic breathing in which increasingly deeper then shallower breaths are followed by a brief (5-30 seconds) period of not breathing. If periodic breathing at a high elevation is disturbing to the trekker, acetazolamide (125 mg) taken before bed can relieve the problem.
High-altitude retinopathy (retinal bleeding) is a rare development at high elevations.
Travelers should know the early symptoms of altitude illness and be willing to acknowledge them if they occur. Deaths from altitude illness almost invariably result because symptoms were ignored or not recognized. General acclimatization recommendations:
- Ascend gradually to allow time to acclimatize.
- Do not ascend directly to elevations higher than 3,000 m, if possible.
- If an abrupt ascent is unavoidable (e.g., flying directly to the destination), acetazolamide might be needed.
- Avoid alcohol and only participate in mild exercise for the first 48 hours.
- "Climb high, sleep low." If participating in activities at elevations higher than 3,000 m during the day, return to a lower elevation to sleep. Once at 3,000 m, increase the sleeping elevation by no more than 300 to 500 m (1000-1,600 ft) per day.
- Never ascend to sleep at a higher elevation with any symptoms of altitude illness.
- In organized trekking groups, a great deal of pressure exists to keep up with the group schedule so as not to be left behind. If symptoms occur, do not allow group pressure to decide what actions to take.
Acetazolamide (Diamox) is most commonly prescribed to prevent altitude illness, but several different medications may be prescribed in certain circumstances.
For prevention of AMS: Start taking the drug the day before ascending, take each day during ascent, and continue to take for 24 to 48 hours after arrival at highest elevation.
- Adult dose: 125 mg every 12 hours; 250 mg every 12 hrs if weight is more than 100 kg (220 lb)
- Pediatric dose: 2.5 mg/kg/dose every 12 hours
- Persons with multiple drug allergies or a history of a life-threatening reaction to sulfa drugs should have a test dose of acetazolamide administered in a controlled environment before the trip.
- Persons with an isolated allergy to sulfa antibiotics can take acetazolamide safely.
- Almost always causes numbness of fingers and toes (and occasionally around the mouth)
- Gives carbonated beverages a metallic taste
- Occasionally causes nausea and sensitivity to sunlight
For prevention of AMS (in rare situations): 2 to 4 mg every 6 to 12 hours
Precautions: does not prevent HAPE
Side effects: euphoria; can increase the need for insulin or oral agents in diabetics
Ibuprofen (Advil, Motrin)
For prevention of headache: 600 mg every 8 hours, starting a few hours before ascent
Precautions: Risk of gastrointestinal bleeding may be increased at high elevations. Do not use for more than 1 to 2 days.
Side effects: gastrointestinal irritation and bleeding
Nifedipine, salmeterol, sildenafil, or tadalafil may be used for prevention of HAPE in known susceptible individuals, but should be prescribed by a clinician well versed in high-altitude medicine.
Gingko biloba is not recommended for prevention or treatment of altitude illness.
Treatment and Self-Treatment
Three options are available for treatment. Descent, oxygen, and pressurization bags (portable hyperbaric chambers).
Descent is the treatment of choice for both tourists and trekkers.
- Descent invariably improves altitude illness.
- Descend if symptoms are getting worse while resting at the same elevation.
- Descent until all symptoms are gone is unnecessary because symptoms can take 48 to 72 hours to clear.
- In severe cases, however, descent must continue until clear signs of improvement are recognized or until the person is below the elevation at which symptoms started.
Oxygen is the second treatment choice for both tourists and trekkers.
- Oxygen is available at many tourist locations, often from the front desk of the hotel, and is helpful in treating mild altitude illness.
- Bottled oxygen is carried by many trekking expeditions but is expensive and heavy to carry; therefore, sufficient oxygen is usually not available.
Pressurization bags are another option for trekkers.
- Groups on long treks or climbs to very high elevations (where rapid descent might not be possible) should consider carrying a pressurization bag (e.g., Gamow Bag, Hyperlite, etc.), which can effectively mimic descent.
- A 1-hour treatment in a portable pressurization bag is usually enough to dramatically improve mild to moderate AMS. In more severe cases, several hours in the bag may be necessary.
Discuss with the health care provider what drugs might be used for treatment and when to use them. For side effects and precautions, see drugs listed under Prevention.
Diamox is used for treatment of AMS or periodic breathing and sleep apnea at higher elevations.
Dexamethasone is effective in treating mild to moderate AMS and in improving HACE.
Ibuprofen and aspirin are effective in treating headache associated with high elevation.
Effect of High Elevations on Preexisting Medical Conditions
In general, the more severely limited one's exercise tolerance is at sea level, the worse he or she will do at higher elevations. Additionally, many high-elevation destinations are far from the nearest medical help. Some travelers with preexisting conditions should avoid trekking if medical care is not readily available.
Persons with ongoing angina should avoid trekking. Persons with congestive heart failure that is well controlled at home should limit themselves to moderate activity and stay in areas with readily available medical care. Poorly controlled heart failure is a contraindication to travel to high elevations.
Travelers who are well controlled on antihypertensive medications and are going for a short tourist trip to moderate elevations do not need to adjust dosage. Expatriates and long-stay travelers at elevations above 2,500 m (8,200 ft) may require dose adjustment after arrival. Persons with unstable blood pressure need close monitoring at elevation and access to a medical setting where drugs can be quickly adjusted or blood pressure rapidly stabilized.
Persons with chronic obstructive pulmonary disease or primary pulmonary hypertension should avoid trekking. Persons with mild, well-controlled asthma have generally done well at higher elevations, possibly due to the greatly decreased presence of allergens at high elevations. However, persons with asthma should carry their medications with them at all times. Persons with moderate to severe persistent asthma should exercise extreme caution if going to high elevations.
Persons with uncontrolled or poorly controlled seizures should avoid high elevations. A history of migraines increases risk of headaches at higher elevations.
Even moderate altitudes, such as those encountered in airplane travel, can trigger a sickle cell crisis in a person with sickle cell trait or sickle cell disease. Typical tourist elevations will often cause a crisis, even without physical exertion. High elevations are contraindicated in this population. Dark-skinned persons born outside the U.S., especially in developing countries, may never have been tested for sickle cell trait as children and should be tested prior to any high-elevation travel.
Persons with low red cell counts could experience difficulty adjusting to high elevations because their oxygen-carrying capacity would already be low; they should proceed with caution. Persons with polycythemia could have a risk of blood clots.
Persons with stable diabetes can travel safely to high elevations if they are comfortable with self-monitoring and pay very close attention to their glucose balance. Glucose meters may provide inaccurate glucose readings and insulin pumps may administer incorrect doses at high elevations. Additionally, it may be difficult to keep insulin supplies at close hand and unfrozen during a long, cold, backcountry journey.
Pregnant women should not take part in high-elevation trekking because medical care (in the event of early labor or other complications) is not readily available. Pregnant women should avoid elevations above 3,000 m, due to the possibility of lower oxygen levels. Acetazolamide is contraindicated in pregnancy.
Infants and Children
Travel to elevations up to 2,500 m is low risk for healthy children. Children are at risk for AMS just as adults are, but symptoms may be more difficult to assess in young children, especially those who are not yet talking. For prevention, acetazolamide may be advised.
HAPE and HACE are not well reported in traveling children due to the infrequency of children traveling to high elevations, but HAPE may be more likely to occur when the child also has a viral illness. For treatment of AMS and HACE in children, dexamethasone might be advised.
Concerns exists that the increased risk of blood clots in women taking oral contraceptives at sea level might be compounded by high elevations. Women taking oral contraceptives who will not spend much time at elevations above 4,200 m (13,800 ft) can continue to take oral contraceptives. Women climbing at extremely high elevations on expeditions (above 6,000 m; 19,700 ft) should consider discontinuing oral contraceptives.