Acute mountain sickness


  • Acute mountain sickness (AMS) refers to the clinical symptoms caused by exposure to hypoxia at high altitude.
  • To prevent the symptoms, recommended ascent rates should not be exceeded.
  • Descent to a lower altitude is always the first choice treatment in AMS.

Acclimatisation (adaptation)

  • As altitude increases, atmospheric pressure decreases. Ascending to high altitudes consequently lowers the partial pressure of oxygen in blood (pO2).
  • Reduced partial pressure of oxygen in arterial blood triggers the process of acclimatisation, characterised by increased ventilation to increase the partial pressure. Urine output also increases.
  • There is no test available to reliably predict individual ability to adapt to high altitudes.
  • Risk factors for poor adaptation
    • A history of AMS
    • Obesity
    • Living at altitudes below 900 metres
    • Higher than recommended ascent rate and sleeping altitude
    • Conditions impairing haemodynamics and lung function, such as coronary artery disease or chronic obstructive pulmonary disease

Assessment of the risk of developing AMS

  • The risk can be considered small if
    • the person has previously ascended to an altitude of 2,800 m without symptoms, or
    • more than two days are spent ascending to an altitude of 2,500–3,000 m.
  • The risk is moderate if
    • a person with a history of AMS ascends to an altitude of 2,500–2,800 m within one day, or
    • a person with no history of AMS ascends to an altitude of more than 2,800 m, or
    • the ascent rate exceeds 500 m/day (difference between the altitudes at which the nights are spent) at an altitude exceeding 3,000 m.
  • The risk is high if
    • a person with a history of AMS ascends to an altitude exceeding 2,800 m within one day, or
    • the person has a history of cerebral or pulmonary edema, or
    • an altitude exceeding 3,500 m is ascended to within one day or the altitude at which the night is spent increases by more than 500 m/day at an altitude exceeding 3,500 m.


Acute mountain sickness (AMS)

  • Twenty-five per cent of climbers will experience AMS at altitudes of 1,850–2,750 m, and 40% at altitudes > 3,000 m.
  • Headache is the principal symptom
  • The diagnosis of AMS can be made when headache is combined with at least one of the following:
    • Fatigue or weakness
    • Lack of appetite or nausea
    • Peripheral edema
    • Dizziness
  • See Lake Louise Scoring System (LLSS)
  • If mild symptoms of AMS develop, the ascent should be stopped and at least one rest day added, during which physical activity should be limited. Controlled hyperventilation may reduce symptoms.
  • If symptoms do not resolve within 1–2 days or worsen during the first 24 hours, immediate descent is necessary.
  • Children should be taken to lower altitudes as soon as suspicion of AMS arises.

AMS with cerebral manifestations (HACE)

  • High altitude cerebral edema (HACE) is a rare symptom usually occurring only at altitudes exceeding 4,000 m.
  • The brain swells and intracranial pressure increases, possibly leading to brain herniation and death within a few hours or days. Patients may lack insight into their state.
  • The symptoms may progress rapidly. The death rate is around 40%.
    • Ataxia (principal symptom)
    • Severe headache resistant to normal medication
    • Nausea and vomiting
    • Dizzy spells
    • Hallucinations
    • Photophobia
    • Visual disturbances, papilloedema
    • Illogical behaviour, nonchalance, apathy or mood changes
    • Neurological findings (nystagmus, positive Babinski sign, hemiparesis, nuchal rigidity, ophthalmoplegia)
    • Fever
    • Disturbances of consciousness
    • Unconsciousness
    • Oliguria (< 0.5 l/24 hours)

AMS with pulmonary manifestations (HAPE)

  • High altitude pulmonary edema (HAPE) usually occurs at altitudes above 2,500 m but it has been described already at altitudes exceeding 1,500 m.
  • At an altitude of 2,700 m the incidence of HAPE is less than 1 per mille, at an altitude of 4,000 m 2%.
  • The mortality rate is 6% when treated and 44% if left untreated.
    • Sudden deterioration of functional capacity (principal symptom)
    • Dyspnoea on exertion and inadequate recovery
    • Later on dyspnoea at rest
    • Increased heart rate (tachycardia)
    • Cyanosis of the skin and/or lips
    • Dry cough, later frothy sputum tinged with blood
    • Auscultation reveals moist crackles (later audible without the use of a stethoscope)
    • Retrosternal pressure and burning
    • Vomiting
    • Fever
    • Lying down worsens symptoms and becomes impossible later on
    • Oliguria (< 0.5 l/24 hours)
  • If treatment is commenced immediately clinical symptom relief is usually evident within a few hours.


  • Initial management
    • Rest, warmth
    • Always a descent to lower altitudes. If symptoms of HAPE or HACE are present, the patient must be transported avoiding exertion, preferably sitting up and protected from cold. The altitude where the last asymptomatic night was spent is usually suitable; whenever practical, a descent of at least 500 m should be achieved.
  • Oxygen supplementation
    • Initially 6–10 l/min until cyanosis is reversed or SaO2 > 90%
    • Later on 2–4 l/min either continuously or intermittently
  • A portable hyperbaric chamber [Evidence Level: C] (Gamow bag) if available (for 1–2 hours, repeated if necessary)
  • Drug therapy for AMS or cerebral edema
    • Acetazolamide [Evidence Level: C] 250 mg twice daily
    • Dexamethasone [Evidence Level: C], initial dose 8 mg, then 4 mg every 6 hours
      • For severe symptoms, dexamethasone is more effective than acetazolamide.
  • Drug therapy for pulmonary edema
    • PDE5 inhibitors (50 mg sildenafil once daily, 10 mg tadalafil once daily)
    • 10–20 mg nifedipine chewed, and subsequently 20–30 mg of a long-acting dosage form every 12 hours


  • If the risk of falling ill is low, there is no need for prophylactic medication.
  • Gradual ascent and spending two nights at an altitude of 2,500–3,000 m before proceeding further are usually sufficient for acclimatisation.
  • At altitudes above 3,000 m the daily ascent rate should not exceed 500 m and there should be one day of rest every 3–4 days.
  • In moderate- to high-risk situations, prophylactic medication can be used in addition to gradual ascent.
  • For moderate- or high-risk patients, the drug of first choice is 125 mg acetazolamide [Evidence Level: B] once or twice daily. If this cannot be used, dexamethasone [Evidence Level: D] can be used as an alternative.
  • PDE5 inhibitors (sildenafil, tadalafil) in similar doses as for treating AMS can be used to prevent pulmonary edema.


1. Luks AM, Auerbach PS, Freer L, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Acute Altitude Illness: 2019 Update. Wilderness Environ Med 2019;30(4S):S3–S18.  [PMID:31248818]

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