Acute mountain sickness is caused by the reduced air pressure and lower oxygen levels at high altitudes, which causes the brain's blood vessels to dilate to try to deliver more oxygen. This process can trigger the trigeminal nerve to release chemicals called neuropeptides. These chemicals cause inflammation and dilation of blood vessels in the protective layers around the brain (the meninges), which can lead to headaches and nausea.

This happens to everyone when they climb in high altitudes, but in people who get acute mountain sickness (AMS), the reaction is stronger. The faster you climb to a high altitude, the more likely you will get acute mountain sickness.

The best way to prevent altitude illness is to ascend gradually. It is a good idea to spend a few days ascending to 9850 ft (3000 m). Above this point, ascend very slowly so that the elevation at which you sleep does not increase more than 990 to 1640 ft (300 to 500 m) per night.

You are at higher risk for acute mountain sickness if:

• You live at or near sea level and travel to a high altitude.
• You have had the illness before.
• You ascend quickly.
• You have not acclimatized appropriately to the altitude.
• Alcohol or other substances have interfered with acclimatization.
• You have medical problems involving the heart, nervous system, or lungs.
• If you have anemia

Your symptoms will also depend on the speed of your climb and how hard you push (exert) yourself. Symptoms range from mild to life-threatening. They can affect the nervous system, lungs, muscles, and heart.

In most cases, symptoms are mild. Symptoms of mild to moderate acute mountain sickness may include:

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

Symptoms that may occur with more severe acute mountain sickness include the following and may progress to high altitude pulmonary edema or high altitude cerebral edema:

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

Your health care provider will examine you and listen to your chest with a stethoscope. This may reveal sounds called crackles (rales) in the lung. Rales may be a sign of fluid in the lungs.

Tests that may be done include:

• Blood tests
• Brain CT scan
• Chest x-ray
• Electrocardiogram (ECG)

Early diagnosis is important. Acute mountain sickness is easier to treat in the early stages.

The main treatment for all forms of mountain sickness is to descend to a lower altitude as rapidly and safely as possible. You should not continue moving up if you develop symptoms.

Extra oxygen should be given, if available.

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

A medicine called acetazolamide (Diamox) may be given to help you breathe better. It can help reduce symptoms. This medicine can make you urinate more often. Make sure you drink plenty of fluids and avoid alcohol when taking this medicine. This medicine works best when taken before reaching a high altitude.

If you have fluid in your lungs (pulmonary edema), treatment may include:

• Oxygen
• A high blood pressure medicine called nifedipine
• Beta agonist inhalers to open the airways
• Breathing machine in severe cases
• Medicine to increase blood flow to the lungs called phosphodiesterase inhibitor (such as sildenafil)

Dexamethasone (Decadron) may help reduce acute mountain sickness symptoms and swelling in the brain (cerebral edema).

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

Most cases are mild. Symptoms improve quickly when you climb down the mountain to a lower altitude.

Severe cases may result in death due to lung problems (pulmonary edema) or brain swelling (cerebral edema).

In remote locations, emergency evacuation may not be possible, or treatment may be delayed. This can have a negative effect on the outcome.

The outlook depends on how quickly the affected person can get to a lower altitude once symptoms begin. Some people are more prone to developing altitude-related sickness and may not respond as well as others.

Complications may include:

• Coma (unresponsiveness)
• Fluid in the lungs (pulmonary edema)
• Swelling of the brain (cerebral edema), which can lead to seizures, mental changes, or permanent damage to the nervous system
• Death

Contact your provider if you have or had symptoms of acute mountain sickness, even if you felt better when you returned to a lower altitude.

Call 911 or the local emergency number if you or another climber have any of the following symptoms:

• Altered level of alertness
• Coughing up blood
• Severe breathing problems

Climb down the mountain right away and as safely as possible.

Keys to preventing acute mountain sickness include:

• Climb the mountain gradually. Gradual ascent is the most important factor in preventing acute mountain sickness.
• Stop for a day or two of rest for every 2000 ft (600 m) of climb above 8000 ft (2400 m).
• Sleep at a lower altitude when possible.
• Make sure that you have the ability to rapidly descend if needed.
• Learn how to recognize early symptoms of mountain sickness.

If you plan on climbing quickly, or climbing to a high altitude, ask your provider about medicines that may help prevent acute mountain sickness, such as acetazolamide or dexamethasone.

If you are at risk for a low red blood cell count (anemia), ask your provider if your planned trip is safe. Also ask if an iron supplement is right for you. Anemia lowers the amount of oxygen in your blood. This makes you more likely to have mountain sickness.

While climbing:

• Do not drink alcohol
• Drink plenty of fluids
• Eat regular meals that are high in carbohydrates

You should avoid high altitudes if you have heart or lung disease.

High altitude cerebral edema; Altitude anoxia; Altitude sickness; Mountain sickness; High altitude pulmonary edema

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Updated by: Jesse Borke, MD, CPE, FAAEM, FACEP, Attending Physician at Kaiser Permanente, Orange County, CA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.