High-altitude illness
High-altitude illness is a spectrum of hypobaric-hypoxia syndromes ranging from acute mountain sickness through the life-threatening cerebral and pulmonary edemas, triggered by ascent above roughly 2000-2500 m that outpaces acclimatization. Acute mountain sickness and high-altitude cerebral edema are clinical diagnoses treated with descent, oxygen, acetazolamide, and dexamethasone, while high-altitude pulmonary edema adds nifedipine and demands the same immediate priority: stop ascending and get down.
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What this article covers
- Overview
A hypobaric-hypoxia spectrum, not one disease: high-altitude illness comprises acute mountain sickness (AMS), high-altitude cerebral edema (HACE), and high-altitude pulmonary edema (HAPE), all…
- Epidemiology
Extremely common at modest elevation: AMS affects up to 25% of visitors to 2000 m, the elevation of most major western U.S.
- Risk factors
Rapid ascent: more than 3500 m gained in under 2 days, or more than 500 m/day of sleeping elevation gain above 3000 m.
- Pathophysiology
Diminished ambient oxygen (hypobaric hypoxia) is the initiating insult: as barometric pressure falls with ascent, the inspired partial pressure of oxygen drops, reducing oxygen available for gas exchange.
- Clinical features
Acute mountain sickness (the most common form): nonspecific symptoms within hours of ascent, headache the hallmark, plus fatigue, malaise, anorexia, nausea, and vomiting; disturbed sleep from high-altitude periodic breathing is common.
- Differential diagnosis
Dehydration/heat illness: headache, fatigue, and nausea from volume depletion or exertion, but no ascent-timing pattern and resolves with rehydration alone rather than descent (see heat stroke for the hyperthermic mimic).
- Diagnosis
All three syndromes are clinical diagnoses; no confirmatory test is required.
- Management
First, stop further ascent as soon as any symptom of altitude illness appears, regardless of severity; this alone often halts progression of mild AMS.
- Complications
High-altitude cerebral edema progressing to coma and death if descent is delayed once ataxia or altered mentation appears.
- Prognosis
Excellent for uncomplicated AMS: symptoms typically resolve within 24-48 hours with no further ascent, with or without treatment.
- Prevention
Ascend gradually: limit sleeping-altitude gain to no more than 500 m/day once above 3000 m, and build in an extra acclimatization (rest) day for every 3-4 days of ascent, or every 1000 m gained.
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