Altitude Illness

Altitude Illness is a topic covered in the 5-Minute Clinical Consult.

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  • A spectrum of medical problems 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 but self-limiting syndrome that will 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 be suffering from altitude 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.
  • It may be prudent to advise a low-altitude dwelling for any pregnant woman experiencing complications.


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 (2).

Etiology and Pathophysiology

  • Individuals with a prior history of AMS, HACE, or HAPE are at a higher risk of 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.

Risk Factors

  • Failure to 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 (3)
  • Younger age (<46 years)

General Prevention

  • General guidelines
    • Preacclimatization protects against altitude illness.
    • >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 but does increase odds of summiting.
  • Drug 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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