Acute mountain sickness

Revision as of 21:38, 30 May 2022 by Beren85 (talk | contribs) (→‎Prevention)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)


  • Also referred to as AMS
  • Usually only occurs with altitude >7,000-8,000ft (2,000 m)
    • May occur at lower altitudes in patients who are particularly susceptible (COPD, CHF, obesity, history of AMS)
  • Development of symptoms based on: rate of ascent, sleeping altitude, strength of hypoxic ventilatory response, alcohol intake, obesity
    • NOT based on physical fitness, age, sex, smoking, previous high-altitude experience
  • Tend to have recurrence of symptoms whenever they return to the symptomatic altitude
  • Low partial pressure of O2 leads to decreased diffusion of O2 across alveolar spaces -> hypoxemia results in tachypnea and cerebral vasodilation (↓ CO2) -> increased ICP

Clinical Features

  • Lake Louise Consensus Definition [1]
    • An Individual is at or above 2500 m (~8000 ft) above sea level and
    • A headache is present and:
    • An Individual has any one of the following:
  • Ataxia and confusion heralds onset of headache


  • Symptoms usually develop 1-6hr after arrival at elevation
    • May be delayed for 1-2d
  • Especially common after the 1st or 2nd night's sleep


  • Average duration of symptoms at 10,000ft = 15hr
  • At higher elevations symptoms may last weeks / more likely to progress to headache

Differential Diagnosis

High Altitude Illnesses


  • Clinical diagnosis
  • Mental status, cerebellar exam - rule out HACE
  • Lung exam - rule out HAPE
  • Lake Louise Scoring System: Sum of symptoms 0-3 (none, mild, moderate, severe/incapacitating). Mild corresponds to score 3-4, Moderate-severe corresponds to score ≥5
    • Headache
    • Gastrointestinal symptoms
    • Fatigue/weakness
    • Lightheadedness
    • Difficulty Sleeping

Expected SpO2 and PaO2 levels at altitude[2]

Altitude SpO2 PaO2 (mm Hg)
1,500 to 3,500 m (4,900 to 11,500 ft) about 90% 55-75
3,500 to 5,500 m (11,500 to 18,000 ft) 75-85% 40-60
5,500 to 8,850 m (18,000 to 29,000 ft) 58-75% 28-40


High altitude management algorithm.


  • Terminate ascent and/or descend [3]
    • Descend to lower altitude of 300-500m (1000-3000ft) OR acclimatize for 12-36hr at same altitude
  • Acetazolamide
    • Mechanism: speeds acclimatization by promoting bicarb diuresis -> metabolic acidosis -> ↑ ventilation
    • Indications:
      • History of altitude illness
    • Abrupt ascent to >9800ft
      • AMS requiring treatment
      • Bothersome periodic breathing during sleep
    • 125-250mg PO BID until symptoms resolve
    • Side-effects
      • Allergic reaction (if patient allergic to sulfa), paresthesias, polyuria, carbonated beverages taste bitter
      • Can also worsen dehydration by promoting bicarb diuresis
  • Symptomatic treatment as necessary with analgesics (NSAIDs) and antiemetics (ondansetron)
  • Sleep-agents


  • Immediate descent of 500 m (~ 1500 ft) for worsening symptoms
  • Low-flow 0.5-1 L/min O2 if available (especially nocturnal administration)
  • Acetazolamide 250mg PO BID
  • Dexamethasone 4mg PO q6hr
    • Symptom-improvement only; unlike acetazolamide does not aid acclimatization
  • Hyperbaric therapy


  • Graded ascent with adequate time for acclimatization is the best prevention
  • Acetazolamide prophylaxis
    • Indicated for patients with history of altitude illness or forced rapid ascent to altitude
    • Start 125 mg PO BID 24hr before ascent and continue for the first 48hrs at peak altitude
    • Can be restarted if illness develops
    • Reduces symptoms of AMS by 75% in patients ascending rapidly to altitudes >8200ft
  • Dexamethasone[4]
    • Start day of ascent and continue for first 2 days at altitude
    • 4mg PO q12hr
    • Prevents and treats cerebral edema
  • Combination acetazolamide and dexamethasone
  • Ginkgo biloba
    • Controversial if effective; safe
  • Ibuprofen
    • Prevention - less effective than acetazolamide, better than placebo
    • Treatment of headache (however, important to consider that taking ibuprofen may mask symptoms)


  • Most patients are treated symptomatically and managed as outpatients
  • If other high altitude illnesses are considered, consider further work-up and/or observation

See Also


  1. Maggiorini M, Müller A, Hofstetter D, Bärtsch P, Oelz O. Assessment of acute mountain sickness by different score protocols in the Swiss Alps. Aviat Space Environ Med. 1998;69(12):1186-1192.
  2. Gallagher, MD, Scott A.; Hackett, MD, Peter (August 28, 2018). "High altitude pulmonary edema". UpToDate. Retrieved May 2, 2019.
  3. Luks A, McIntosh S, Grissom C, Auerbach P, Rodway G, Schoene R, Zafren K, Hackett P. Wilderness Medical Society Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness: 2014 Update. Wilderness Environ Med. 2014; S4-S14.
  4. Luks AM, McIntosh SE, Grissom CK, et al. Wilderness Medical Society Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness: 2014 Update. Wilderness & Environmental Medicine. 2014(25): S4–S14)
  5. Bernhard WN, Schalick LM, Delaney PA, Bernhard TM, Barnas GM. Acetazolamide plus low-dose dexamethasone is better than acetazolamide alone to ameliorate symptoms of acute mountain sickness. Aviat Space Environ Med. 1998;69(9):883-886.