Acute mountain sickness (AMS)
Acute mountain sickness (AMS), or altitude sickness, is a hypoxia‑related clinical syndrome that occurs after ascent to high altitude. Symptoms usually begin within 6-12 hours of ascent above approximately 2,500 metres (≈8,000 ft).
Typical features are headache with one or more of anorexia, nausea, sleep disturbance, dizziness, fatigue/malaise and shortness of breath. The risk rises with both the altitude reached and the speed of ascent (≈10-25% incidence at ~2,500 m in unacclimatised travellers; >50% at ~4,500 m with rapid ascent).
Pathophysiology
At altitude the lower barometric pressure reduces the inspired partial pressure of oxygen (hypobaric hypoxia). Compensation includes increased ventilation and sympathetic activation. In susceptible people hypoxia causes cerebral blood‑flow changes, increased blood-brain barrier permeability and mild vasogenic oedema, producing the symptoms of AMS. Rapid ascent and higher altitude increase risk.
Risk factors
- Rapid ascent, especially by air to a high sleeping altitude without prior acclimatisation
- Prior history of AMS
- High sleeping altitude and strenuous exertion on the first day at altitude
- Individual susceptibility (not reliably predictable)
Clinical features Core symptoms:
- Headache (almost universal and often required for the diagnosis)
- Nausea, anorexia, vomiting
- Sleep disturbance
- Dizziness or light‑headedness