Acute Mountain Sickness (AMS) is the most common form of altitude illness, affecting individuals who ascend too quickly to high elevations. It typically occurs after rapid exposure to altitudes above 8,000 feet (2,400 meters). While often mild and self-limiting, AMS represents the body’s difficulty adjusting to the atmospheric changes caused by reduced oxygen availability. If not recognized and managed early, AMS can progress to more serious, life-threatening conditions.
Understanding How Altitude Affects the Body
The primary cause of Acute Mountain Sickness is hypobaric hypoxia, a significant reduction in the partial pressure of inspired oxygen. At higher altitudes, the percentage of oxygen in the air remains constant at about 21%, but the barometric pressure decreases substantially. This drop in atmospheric pressure means that with every breath, fewer oxygen molecules are pushed into the lungs and subsequently into the bloodstream.
The body’s immediate physiological response to this oxygen deprivation, or hypoxia, is to increase ventilation, known as hyperventilation. Specialized sensors detect the decrease in blood oxygen levels and signal the brain to increase both the rate and depth of breathing. This compensatory mechanism aims to draw more oxygen into the lungs and exhale more carbon dioxide, partially offsetting the effects of the lower barometric pressure.
This rapid increase in breathing leads to a temporary state called respiratory alkalosis, where the blood becomes slightly more alkaline due to the excessive removal of carbon dioxide. The kidneys begin to excrete bicarbonate in an attempt to normalize the blood’s pH balance, a gradual process that allows for continued, sustained hyperventilation and is a key part of acclimatization. When the body’s initial response is insufficient or the ascent is too fast, the resulting cerebral changes are thought to lead to the symptoms of AMS.
Identifying the Signs of Acute Mountain Sickness
AMS is diagnosed by the presence of a headache combined with at least one other symptom, following an ascent above 8,000 feet. Symptoms commonly begin within 6 to 12 hours after arrival at the new elevation, though this timing can vary widely between individuals.
The characteristic signs of mild to moderate AMS often resemble a bad hangover or the flu. Common symptoms include:
- A persistent headache that is not relieved by simple pain medication.
- Loss of appetite.
- Nausea, which may sometimes progress to vomiting.
- Fatigue or lassitude.
- Dizziness or lightheadedness.
- Difficulty sleeping.
It is important to recognize that AMS exists on a spectrum of altitude illness, and these symptoms can rapidly worsen. If symptoms like severe confusion, inability to walk in a straight line (ataxia), or shortness of breath while resting develop, it may indicate a progression to the more severe conditions: High Altitude Cerebral Edema (HACE) or High Altitude Pulmonary Edema (HAPE). Both HACE and HAPE are considered medical emergencies that can be fatal without immediate intervention.
Proactive Steps for Prevention and Acclimatization
The most effective strategy for preventing AMS is allowing the body sufficient time to acclimatize to the reduced oxygen environment through a gradual ascent rate. This process requires a gradual ascent rate, which is the single most important factor in prevention. Above an altitude of 9,850 feet (3,000 meters), the increase in sleeping elevation should not exceed 1,000 to 1,600 feet (300 to 500 meters) per night.
Travelers should incorporate rest days into their itinerary, spending two consecutive nights at the same elevation every 3 to 4 days, especially after significant altitude gains. A beneficial strategy is the “climb high, sleep low” principle, where a climber ascends to a higher point during the day but returns to a lower altitude to sleep. This allows for greater physiological adaptation while minimizing the risk of developing symptoms overnight.
Maintaining adequate hydration is important for altitude preparation, as the increased respiratory rate causes a greater loss of water vapor from the lungs. Travelers should avoid alcohol and excessive caffeine intake upon arrival, as these substances can interfere with acclimatization and potentially worsen dehydration. A diet high in carbohydrates is recommended, as the body uses oxygen more efficiently when metabolizing carbohydrates compared to fats or proteins.
For individuals with a history of AMS or those planning a rapid ascent to very high altitudes, prophylactic medication may be considered. Acetazolamide is the most commonly used pharmacologic agent, working as a carbonic anhydrase inhibitor to stimulate ventilation and speed up the acclimatization process. This medication should be started before the ascent and continued during the initial days at high altitude, but its use requires consultation with a healthcare provider.
Immediate Management and Treatment
If symptoms of mild AMS begin to appear, the immediate rule is to halt further ascent. The individual should rest at the current altitude and avoid strenuous physical activity, which can exacerbate the symptoms. For the headache, non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen can be used for symptomatic relief.
If symptoms persist or worsen despite resting and taking simple analgesics, the definitive treatment for any form of altitude illness is descent to a lower elevation. Even a drop of 1,600 to 3,300 feet (500 to 1,000 meters) can lead to a rapid improvement in symptoms. No one experiencing AMS symptoms should ascend further until their symptoms have completely resolved.
For moderate to severe cases where descent is not immediately possible, supplemental oxygen can provide temporary relief by increasing the inspired oxygen pressure. In a remote setting, a portable hyperbaric bag may be used to simulate a lower altitude environment, but this is a temporary measure to stabilize the person for eventual descent. Any sign of progression, such as worsening headache, confusion, or difficulty breathing at rest, demands an immediate descent, as this suggests the onset of HACE or HAPE.