Acute mountain sickness is an illness that can affect mountain climbers, hikers, skiers, or travelers who climb too fast. It usually occurs when people rapidly reach a high altitude (typically above 8,000 feet or 2,400 meters). 

Acute mountain sickness occurs from the combination of reduced air pressure and a lower concentration of oxygen at high altitude. Symptoms can range from mild to life-threatening, and can affect the nervous system, lungs, muscles, and heart.

In most cases the symptoms are mild. In severe cases fluid collects in the lungs (pulmonary edema) causing extreme shortness of breath, which further reduces how much oxygen a person gets. Brain swelling may also occur (cerebral edema). This can cause confusion, coma, and, if untreated, death.

The chance of getting acute mountain sickness increases the faster a person climbs to a high altitude. The severity of the symptoms also depend on this factor, as well as how hard the person pushes (exerts) himself or herself. People who normally live at or near sea level are more prone to acute mountain sickness.

Approximately 20% of people will develop mild symptoms at altitudes between 6,300 to 9,700 feet, but pulmonary and cerebral edema are extremely rare at these heights. However, above 14,000 feet, a majority of people will experience at least mild symptoms. Some people who stay at this height can develop pulmonary or cerebral edema.

Symptoms generally associated with mild to moderate altitude illness include:

• Difficulty sleeping
• Dizziness or light-headedness
• Fatigue
• Headache
• Loss of appetite
• Nausea or vomiting
• Rapid pulse (heart rate)
• Shortness of breath with exertion

Symptoms generally associated with more severe altitude illness include:

• Bluish discoloration of the skin
• Chest tightness or congestion
• Confusion
• Cough
• Coughing up blood
• Decreased consciousness or withdrawal from social interaction
• Gray or pale complexion (cerebral edema)
• Inability to walk in a straight line, or to walk at all
• Shortness of breath at rest

Listening to the chest with a stethoscope (auscultation) reveals sounds called crackles in the lung, which can mean pulmonary edema.

A chest x-ray may be performed.

The main form of treatment for all forms of mountain sickness is to climb down (descend) to a lower altitude as rapidly and safely as possible. Supplemental oxygen should also be given, if available.

People with severe mountain sickness may be admitted to a hospital.

Acetazolamide (Diamox) is a drug used to stimulate breathing and reduce mild symptoms of mountain sickness. This drug can cause increased urination. Make sure you drink plenty of fluids. Do not drink alcohol while taking this drug.

Pulmonary edema, the build-up of fluids in the lungs, is treated with oxygen, the high blood pressure medicine nifedipine or phosphodiesterase inhibitors (sildenafil), and, in severe cases, a breathing machine (respirator).

The steroid drug dexamethasone (Decadron) may help reduce swelling in the brain (cerebral edema).

Portable hyperbaric chambers have been developed to allow hikers to simulate their conditions at lower altitudes without moving from their location on the mountain. These new devices are very important if bad weather or other factors make climbing down the mountain impossible.

Most cases are mild, and symptoms improve promptly with a return to lower altitude. Severe cases may result in death due to respiratory distress or brain swelling (cerebral edema).

In remote locations, emergency evacuation may not be possible, or treatment may be delayed. These conditions could adversely affect the outcome.

• Coma
• High altitude cerebral edema (brain swelling)
• Pulmonary edema

Call your health care provider if symptoms of acute mountain sickness develop, even if symptoms resolved when returning to a lower altitude.

Call 911 or your local emergency number, or seek emergency medical assistance if severe difficulty breathing develops, or if you notice a lower level of consciousness, coughing up of blood, or other severe symptoms. If unable to contact emergency help, descend immediately, as rapidly as is safely possible.

Education of mountain travelers before ascent is the key to prevention. Basic principles include: gradual ascent, stopping for a day or two of rest for each 2,000 feet (600 meters) above 8,000 feet (2,400 meters); sleeping at a lower altitude when possible; and learning how to recognize early symptoms so you can return to lower altitude before worsening symptoms occur.

Mountaineering parties traveling above 9,840 feet (3,000 meters) should carry an oxygen supply sufficient for several days.

Acetazolamide (Diamox) helps speed the process of getting used to higher altitude, and reduces minor symptoms. Therapy should start one day before the ascent and continue one to two days into the excursion. This measure is recommended for those making a rapid ascent to high altitudes.

Those who may be prone to anemia (particularly women) should consult a doctor regarding an iron supplement to correct the condition before traveling in high altitudes. Anemic people have a reduced red blood cell count, and therefore a lower amount of oxygen carried in the blood.

Drink enough fluids, avoid alcohol, and eat regularly. Foods should be relatively high in carbohydrates.

People with underlying cardiac or pulmonary (lung) diseases should avoid high altitudes.

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