Acute Mountain Sickness (AMS) is the most common form of altitude illness, affecting individuals who travel too quickly to high elevations, typically above 8,000 feet (2,500 meters). This condition is a response to the thinner air encountered during travel, such as hiking, climbing, or even driving to mountain resorts. While AMS is generally mild and self-limiting, it serves as a warning sign that the body is struggling to adjust to the environment. Ignoring the initial symptoms and continuing to ascend can lead to severe, potentially life-threatening conditions. Recognizing this illness early is important for a safe experience in mountainous regions.
Understanding the Body’s Reaction to High Altitude
The root cause of AMS is the phenomenon known as hypobaric hypoxia, which is a reduced availability of oxygen caused by the drop in atmospheric pressure at higher elevations. Although the air still contains 21% oxygen, the pressure pushing that oxygen into the lungs and blood decreases significantly, meaning each breath delivers fewer oxygen molecules. This lack of oxygen, or hypoxia, triggers a cascade of physiological responses as the body attempts to compensate, a process called acclimatization.
The body’s immediate response involves increasing the rate and depth of breathing, known as hyperventilation. This increased ventilation helps raise the oxygen level in the blood and lowers the carbon dioxide level, which in turn causes the blood to become more alkaline. The kidneys work to excrete bicarbonate to rebalance the blood’s pH, a slow process that contributes to successful acclimatization.
Cardiovascular changes also occur initially, including an increase in heart rate and cardiac output to move oxygenated blood more rapidly throughout the body. Over time, the body reduces the volume of fluid in the blood, concentrating red blood cells and improving the oxygen-carrying capacity. When this natural acclimatization process fails to keep pace with a rapid ascent, the resulting strain manifests as the symptoms of AMS.
Recognizing the Signs of AMS
The diagnosis of AMS is based on the recent ascent to an altitude above 8,000 feet (2,500 meters) and the presence of a headache accompanied by at least one other specific symptom. Symptoms usually appear within six to twelve hours of arrival at the new altitude and can resemble a severe hangover or a flu-like illness. The most common sign is a headache, which is typically described as throbbing and worsens with exertion or bending over.
Other primary indicators include gastrointestinal distress, such as loss of appetite, nausea, and sometimes vomiting. Individuals often report fatigue, weakness, or a general feeling of being unwell, which is disproportionate to the level of physical activity. Dizziness or lightheadedness, along with difficulty sleeping, are also frequently reported.
Physical fitness does not confer protection against AMS, and symptoms can affect anyone who ascends too quickly. The severity of symptoms can range from mild discomfort, which resolves with rest, to a progression toward severe, life-threatening conditions. Any worsening of these initial signs, particularly changes in coordination or confusion, demands immediate attention and signals potential progression to more severe forms of altitude illness.
Strategies for Safe Ascent
The most effective way to prevent AMS is to allow the body sufficient time to acclimatize to the change in elevation. A gradual ascent rate is the most important prophylactic measure when planning any high-altitude trip. Once above 8,000 feet (2,400 meters), limit the increase in sleeping altitude to no more than 1,000 to 1,600 feet (300 to 500 meters) per day.
Travelers should follow several key strategies:
- Plan for mandatory rest days, including an extra day for acclimatization for every 3,300 feet (1,000 meters) gained above 9,800 feet (3,000 meters).
- Practice the principle of “climb high, sleep low,” ascending during the day but returning to a lower elevation to sleep.
- Stay well-hydrated, as fluid loss is accelerated at altitude.
- Avoid alcohol and sedatives, especially during the first 48 hours, because they can suppress breathing.
Prophylactic medication can be used to accelerate the acclimatization process when a gradual ascent is not possible. Acetazolamide is commonly prescribed, as it works by increasing ventilation and correcting the blood’s pH balance, speeding up the natural acclimatization process. Taking this medication typically begins 24 hours before ascending to the higher elevation.
Immediate Steps for Management and Recovery
If symptoms of AMS begin to appear, the immediate step is to stop ascending and rest at the current altitude. Continuing to climb with symptoms can be extremely dangerous, as it increases the risk of the condition progressing. For mild to moderate AMS, supportive care includes resting, maintaining hydration, and using over-the-counter pain relievers like ibuprofen or acetaminophen for headache relief.
If symptoms worsen despite rest, or if they do not improve within a day, immediate descent is the most effective treatment for all forms of altitude sickness. A descent of at least 1,000 to 3,300 feet (300 to 1,000 meters) is often sufficient to produce rapid improvement in symptoms. Supplemental oxygen, if available, can also provide immediate relief by counteracting the hypoxia, but it is not a substitute for descent if symptoms are severe.
In cases of severe AMS, or if signs of the severe complications, High Altitude Cerebral Edema (HACE) or High Altitude Pulmonary Edema (HAPE), develop, immediate emergency descent is necessary. HACE involves brain swelling and is indicated by severe confusion, loss of coordination, or an inability to walk a straight line. HAPE involves fluid accumulation in the lungs, marked by severe shortness of breath even at rest and a persistent cough. A corticosteroid medication, such as dexamethasone, is often used alongside immediate descent to treat HACE and severe AMS, as it helps reduce swelling.