Altitude Illness



  • A spectrum of cerebral and pulmonary syndromes ranging from mild discomfort to fatal illness that occur on ascent to higher altitudes as a direct result of inadequate acclimatization
  • Categories of altitude: intermediate, 1,520 to 2,440 m; high, 2,440 to 4,270 m; very high, 4,270 to 5,490 m; and extreme, >5,490 m
  • Altitude illness can affect anyone, including experienced and fit individuals. For most, it is an unpleasant (self-limited) syndrome that does not require medical intervention (1).
  • Acute mountain sickness (AMS): symptoms associated with a physiologic response to a hypobaric, hypoxic environment. Onset usually occurs within 6 to 12 hours after ascending >2,500 m. Neurologic symptoms predominate, ranging from mild/moderate headache and malaise to severe impairment.
  • High-altitude pulmonary edema (HAPE): noncardiogenic pulmonary edema; typically after 2 or more days at altitudes >3,000 m, rare between 2,500 and 3,000 m
  • High-altitude cerebral edema (HACE): a potentially fatal neurologic syndrome considered to be the end stage of AMS; onset after at least 2 days at altitudes >4,000 m
  • System(s) affected: nervous/pulmonary (2)
  • Synonym(s): mountain sickness

Geriatric Considerations

  • Risk does not increase with age.
  • Age alone should not preclude travel to high altitude; allow extra time to acclimate.
  • Worsening of preexisting medical problems referred to as altitude-exacerbated conditions

Pediatric Considerations

  • Altitude illness seems to have the same incidence in children as in adults; diagnosis may be delayed in younger children.
  • Any child who experiences behavioral symptoms after recent ascent should be presumed to have an altitude-related illness.

Pregnancy Considerations

  • The risk during pregnancy is unknown.
  • No evidence suggests that exposure to high altitudes (1,500 to 3,500 m) poses a risk to a pregnancy.


Most epidemiologic studies are limited to relatively homogeneous male populations.


  • AMS: 10–25% of unacclimatized persons who ascend to 2,500 m; 50–85% at altitudes of 4,500 to 5,500 m
  • HAPE/HACE: 0.5–1.0% of unacclimatized persons with 2 or more days of exposure at altitudes exceeding 3,000 m. Risk increases with rate of ascent.
  • Above 2,500 m (8,200 feet), for every 1,000-m increase in altitude, there is a 13% increase in the AMS (3).

Etiology and Pathophysiology

  • Individuals with a prior history of AMS, HACE, or HAPE are at a higher risk for recurrent AMS.
  • Hypobaric hypoxia and hypoxemia are the pathophysiologic precursors to altitude illness.
  • Symptoms of AMS may be the result of cerebral swelling, either through vasodilatation induced by hypoxia or through cerebral edema.
  • Other mechanisms include impaired cerebral autoregulation, release of vasogenic mediators, and alteration of the blood–brain barrier.
  • HAPE is a noncardiogenic pulmonary edema characterized by exaggerated pulmonary hypertension leading to vascular leakage through overperfusion, stress failure, or both.

Genetic factors involved in predisposition to developing AMS are poorly understood.

Risk Factors

  • Failure to properly acclimatize at a lower altitude
  • Ascent rate >300 to 500 m/day
  • Extreme altitude
  • Increased duration at high altitude
  • Higher altitude during sleep cycle
  • Prior history of altitude illness
  • Cardiac congenital abnormalities
  • Female gender
  • History of migraines (4)
  • Younger age (<46 years)
  • History of anxiety (5)

General Prevention

  • General guidelines
    • Preacclimatization (exposure of hypoxia prior to ascent) protects against altitude illness.
    • Staged ascent (spending 6 to 7 days) at 2,200 to 3,000 m can also prevent altitude illness (6).
    • >2,500 m, do not ascend faster than 500 m/day; rest every 3 to 4 days (2).
    • Lower sleeping elevation: “Climb high and sleep low” for anyone going >3,500 m.
    • Avoid heavy exertion for the first 1 to 3 days at altitude.
    • Avoid respiratory depressants (alcohol and sedatives).
    • Preascent physical conditioning is not preventive.
  • Pharmacologic prophylaxis
    • Acetazolamide, dexamethasone, and ibuprofen (see “Treatment”)
    • For prevention of HAPE only (if at risk):
      • Consider nifedipine, β-agonists, and tadalafil (see “Treatment”).

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