Mountain sickness, acute (AMS): Acute mountain sickness (AMS) is the illness that results from being in a high altitude environment. AMS is common at high altitudes, that is above 8,000 feet (2,440 meters). Three-quarters of people have mild symptoms of AMS over 10,000 feet (3,048 meters). The occurrence of AMS depends on the altitude, the rate of ascent, and individual susceptibility.
Mild AMS: Many people experience mild AMS during the acclimatization process (the first 1 to 3 days at a given altitude). Symptoms usually start 12-24 hours after arrival at altitude and include headache, dizziness, fatigue, shortness of breath, loss of appetite, nausea, disturbed sleep, and a general feeling of malaise. These symptoms tend to be worse at night when respiratory drive is decreased.
Mild AMS does not usually interfere with normal activity and symptoms generally subside within 2-4 days as the body acclimatizes. As long as symptoms are mild, and only a nuisance, ascent can continue at a moderate rate. When hiking, it is essential to communicate any symptoms of illness immediately to others on your trip. AMS is considered to be a neurological problem caused by changes in the central nervous system. It is basically a mild form of high altitude cerebral edema.
Mild AMS is usually treated with pain medications for headache and a drug called DIAMOX which allows one to breathe faster and so metabolize more oxygen, thereby minimizing the symptoms caused by poor oxygenation. This is especially helpful at night when respiratory drive is decreased. Since it takes a while for DIAMOX to have an effect, it is advisable to start taking it 24 hours before going to altitude and continue it for at least 5 days at higher altitude.
Moderate AMS: Moderate AMS includes severe headache (not relieved by medication), nausea, vomiting, increasing weakness and fatigue, shortness of breath, and decreased coordination. Normal activity is difficult. At this stage, only advanced medications or descent can reverse the problem. Descending even a few hundred feet (70-100 meters) may help. Definite improvement is usually seen in descents of 1,000-2,000 feet (305-610 meters). Twenty-four hours at the lower altitude usually results in significant improvement. The person should remain at lower altitude until symptoms have subsided (up to 3 days). At that point, the person has become acclimatized to that altitude and can begin ascending again.
The best test for moderate AMS is to have the person "walk a straight line" heel to toe. Just like a sobriety test, a person with AMS will be unable to walk a straight line. This is a clear indication that immediate descent is required. It is important to get the person to descend before the AMS reaches the point where they cannot walk on their own.
Severe AMS: Severe AMS presents as an increase in the severity of the symptoms, including greater shortness of breath at rest, inability to walk, decreasing mental status, and fluid buildup in the lungs. Severe AMS requires immediate descent to lower altitudes: 2,000 - 4,000 feet (610-1,220 meters).
The only cure for AMS is acclimatization or descent. (Based in part on the Princeton University Outdoor Action "Guide to High Altitude: Acclimatization and Illnesses" by Rick Curtis).