Definition of Acute mountain sickness (AMS)
Acute mountain sickness (AMS) definition - medical term Acute mountain sickness (AMS) is t he effect
on the body of 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).
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