Altitude Sickness Fast Facts

Altitude sickness, sometimes called mountain sickness or high-altitude sickness, is a disorder that affects people who spend time at high altitudes without first becoming accustomed to the high-altitude environment.

The mildest form of the disorder, acute mountain sickness (AMS), affects approximately 40-50% of people who live at a low altitude and spend at least a day at elevations over 10,000 feet (3,048 meters).

AMS symptoms may begin in as little as one hour, but they typically set in after 6-12 hours at a high elevation.

The most severe form of altitude sickness, high-altitude cerebral edema (HACE), results when a buildup of fluid causes swelling of the brain. This condition is life-threatening.

HACE typically occurs after a person has spent 1-3 days at an altitude above 9,800 feet (2,743 meters).

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Altitude sickness can be relatively mild, or it can be severe and life-threatening.

What is Altitude Sickness?

Altitude sickness is the term for a collection of symptoms that affect people who spend time at high elevations without first becoming accustomed to the low-oxygen environment. The disorder is also sometimes called mountain sickness or high-altitude sickness.

The symptoms of altitude sickness occur because the air’s oxygen content at high altitudes is relatively less than it is at low altitudes. The lack of oxygen puts stress on the body and causes a variety of symptoms. A person can become accustomed to the low-oxygen environment by climbing slowly and spending an adequate amount of time at each level before climbing higher. Altitude sickness happens when you climb too high too fast.

Altitude sickness can be relatively mild, or it can be severe and life-threatening. The term is generally applied to three different disorders with varying degrees of severity:

  • Acute mountain sickness (AMS) is the least severe form of the disorder. It typically sets in after 12-24 hours at high altitude. Symptoms usually resolve within 1-2 days at altitude as the body adjusts to the environment.
  • High-altitude cerebral edema (HACE) is swelling of the brain caused by an accumulation of fluid. It usually occurs after the symptoms of AMS. If not treated immediately, HACE can be fatal.
  • High-altitude pulmonary edema (HAPE) is an accumulation of fluid in the lungs. It can occur in conjunction with AMS and/or HACE, but HAPE can also occur independently. Without treatment, HAPE can also be life-threatening.

Symptoms of Acute Mountain Sickness

Symptoms of AMS include:

  • Headache
  • Shortness of breath
  • Nausea and/or vomiting
  • Weakness
  • Fatigue
  • Dizziness
  • Insomnia
  • Loss of appetite

Symptoms of High-Altitude Cerebral Edema

HACE affects the sufferer’s thought processes, so symptoms might only be noticed by others around them.

Symptoms of HACE include:

  • Extreme headache
  • Vomiting
  • Difficulty walking in a straight line
  • Extreme fatigue
  • Confusion
  • Difficulty concentrating or forming thoughts
  • Changes in behavior (sometimes irrational)
  • Hallucinations

If left untreated, HACE can quickly cause coma or death.

Symptoms of High-Altitude Pulmonary Edema

HAPE symptoms may follow AMS symptoms, or they may occur when there have been no prior symptoms.

Symptoms of HAPE include:

  • Tightness in the chest
  • Shortness of breath even without exertion
  • Rapid heart rate
  • Fatigue
  • Blue or gray tint to the lips and/or fingernails
  • Wheezing, rattling, or gurgling breath sounds
  • Cough, sometimes producing a pink foamy liquid
  • Fever

Symptoms can be severe and may linger for weeks, even with treatment.

What Causes Altitude Sickness?

AMS, HACE, and HAPE occur as the body tries to adjust to suddenly decreased access to oxygen. The body’s initial reaction to the oxygen deficit is to increase its breathing and heart rates to take in as much oxygen as possible and transport that oxygen more effectively around the body. As the oxygen deprivation continues, the body responds by making changes to its circulatory system and other processes to protect vital organs and functions.

The body’s rapid adjustments to the new environment put stress on many organs and tissues, resulting in AMS symptoms. As the brain and lungs contend with circulatory function changes, the normal flow of fluid in their tissues is disrupted, and fluid may accumulate abnormally in the organs, causing HACE and HAPE.

Is Altitude Sickness Hereditary?

Scientists are currently unsure about the heritability of altitude sickness. Some people are more susceptible than others to altitude sickness, leading some scientists to suspect an inherited component that increases the risk for the disorders. Studies have attempted to find specific genetic traits that are associated with AMS, HACE, and HAPE.

Recent studies have been unable to find a genetic link to either AMS or HACE. The studies’ results have been inconclusive, partly because AMS symptoms are challenging to measure objectively, and HACE is relatively very rare.

Studies have been able to identify gene variations that appear to increase an individual’s risk of developing HAPE and some other altitude-related disorders.

How Is Altitude Sickness Detected?

The first symptoms of AMS are commonly rapid breathing, rapid heart rate, and headache. These symptoms may not be dangerous on their own, but in rare cases, AMS can progress to HACE and/or HAPE, so even mild symptoms shouldn’t be ignored.

HACE progresses rapidly and can only be stopped with shift, decisive intervention. The symptoms often begin overnight while the sufferer is sleeping, and because HACE adversely affects thought processes, the sufferer might not recognize the signs on their own. It is often up to the people around the sufferer to identify the symptoms and take action.

Early signs of HACE include:

  • Difficulty walking in a straight line
  • Changes in behavior
  • Hallucinations

The first sign of HAPE is often extreme fatigue, but other early symptoms may include a dry cough, rapid heart rate, and rapid breathing.

How Is Altitude Sickness Diagnosed?

AMS can be diagnosed if the patient has a headache and at least one other symptom within 1-2 days of climbing to a high altitude.

HACE should be suspected when someone at high altitude shows symptoms such as behavior changes, hallucinations, or difficulty walking in a straight line. Magnetic resonance imaging (MRI) scans can detect brain swelling consistent with HACE.

If a doctor suspects HAPE, they will use a stethoscope to listen for a crackling sound in the patient’s chest. X-rays may be required to confirm the accumulation of fluid in the lungs.


How Is Altitude Sickness Treated?

Mild cases of AMS can be treated by resting and not moving any higher in elevation. If any HACE or HAPE symptoms are present in cases of more severe symptoms, moving to a lower altitude is the first step in treatment.

The medication acetazolamide may also be recommended to treat AMS symptoms. This drug stimulates breathing and helps the body counter the effects of oxygen deprivation.

Treatment for HACE typically includes:

  • Moving to a lower altitude as quickly and safely as possible
  • Administration of supplemental oxygen
  • Administration of the steroid dexamethasone to reduce brain swelling
  • Use of a hyperbaric chamber, a high-pressure environment that delivers oxygen more effectively to the patient

Treatment of HAPE can include:

  • Movement to a lower altitude
  • Supplemental oxygen
  • Medications to regulate blood flow to the lungs
  • Hyperbaric therapy
  • Use of a mechanical respirator in severe cases

How Does Altitude Sickness Progress?

HACE is the most immediately life-threatening type of altitude sickness, and without immediate treatment, brain swelling can quickly result in severe complications and death. Complications of severe, untreated HACE can include:

  • Permanent cognitive impairment
  • Seizures
  • Vision disruption
  • Coma
  • Death

How Is Altitude Sickness Prevented?

Most of the time, altitude sickness can be prevented by following recommended climbing practices. It’s important to remember that altitude sickness does not only affect mountain climbers. It can affect anyone who spends time at a high altitude without first becoming accustomed to the elevation change.

To reduce your risk of developing altitude sickness:

  • Move slowly to a high altitude if possible. Do not drive or fly directly to a high elevation without stopping.
  • Do not exert yourself physically for the first few days after moving to a higher elevation.
  • Drink plenty of water as you move up in elevation.
  • Do not climb more than 1,000 feet per day when climbing above 10,000 feet.
  • Spend an extra night each time you ascend 3,000 feet.
  • Move to a lower altitude whenever you feel symptoms of altitude sickness.

Altitude Sickness Caregiver Tips

If you’re spending time at a high altitude with other people, you may be responsible for keeping your companions safe and healthy.

  • Know the symptoms of altitude sickness. Educate yourself so that you can spot the early signs of altitude-related illnesses. Learn how to distinguish between common symptoms of minor acute mountain sickness and the more severe signs of cerebral and pulmonary edemas.
  • Be prepared to act quickly. Whenever you’re at a high altitude, have a plan for what you’ll do if you or your companions begin to suffer from altitude sickness. Know the safest, quickest way to move to a lower altitude. Don’t hesitate to descend and seek medical help at the first signs of severe altitude sickness.

Many people with exposure to high altitudes also suffer from other brain and mental health-related issues, a situation called co-morbidity. Here are a few of the disorders commonly associated with high-altitude exposure:

Altitude Sickness Brain Science

Scientists are working to understand how high-altitude cerebral edema (HACE) develops and how this severe condition is related to less severe acute mountain sickness (AMS). Specifically, researchers want to know what causes fluid accumulation in the brain and whether it is directly related to AMS’s early symptoms.

Cerebral edema is generally caused either by the accumulation of fluid within the brain cells themselves or by the leakage of water and proteins from blood vessels into the brain. Recent studies have suggested that HACE is likely caused by fluid that leaks from blood vessels, although it’s not clear how that fluid passes through the barrier that usually keeps contaminants from entering the brain. It’s possible, too, that after fluid accumulates from that source, additional fluid might accumulate within the cells as well.

Another question is whether the headaches characteristic of early AMS are related to brain swelling. Recent studies have shown that the brain tends to swell at high altitudes. Some research suggests that people who have relatively little space inside their skulls to accommodate that swelling might be more susceptible to AMS.

Altitude Sickness Research

Title: Breathing Training to Improve Human Performance at High Altitude

Stage: Recruiting

Contact: Robert Roach, PhD

Altitude Research Center

Aurora, CO 

Individuals traveling to altitudes above 8,000 feet may suffer from impaired exercise and cognitive performance and acute mountain sickness (AMS). Decreased barometric pressure, which leads to low blood oxygen levels, is the primary cause of these disorders. AMS symptoms are characterized by headache, nausea, vomiting, dizziness, fatigue, and difficulty sleeping. This research aims to identify whether Respiratory Muscle Training will improve physical and cognitive performance and reduce the symptoms of AMS at simulated high altitudes.


Title: Physiological Adaptations to Simulated Intermittent Altitude on Human Health and Performance

Stage: Recruiting

Principal investigator:  Courtney Wheatley-Guy, PhD  

Mayo Clinic in Arizona

Scottsdale, AZ

The investigators expect to find that different intensity and altitude exposure levels will show what kind of intermittent exposure protocol is more beneficial to athletes and healthy individuals that experience acute exposure to altitude during exercise. This may furthermore be related to acute altitude exposure for recreational exercise use as well.


Title: Gut-microbiota Targeted Nutritional Intervention for Gut Barrier Integrity at High Altitude

Stage: Recruiting

Contact: J. Philip Karl, PhD

United States Army Research Institute of Environmental Medicine

Natick, MA 

This randomized, crossover clinical trial aims to determine the efficacy of a gut microbiota-targeted nutritional intervention containing a blend of fermentable fibers and polyphenols (FP) for mitigating increases in GI permeability, and decrements in immune function and neuropsychologic performance following rapid ascent to simulated high altitude. Fifteen healthy young adults will participate in each of three study phases that include a 14-day supplementation period in which participants will consume 1 of 2 supplement bars: placebo (PL, will be consumed during 2 phases) and FP supplementation (will be consumed during one phase only). During the final 2-d of each phase, participants will live in a hypobaric chamber under sea level or high altitude conditions.

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