- 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
- 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
- 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.
- 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.
- 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 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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