Acute altitude illness comprises acute mountain sickness (AMS), high altitude cerebral edema (HACE), and high altitude pulmonary edema (HAPE). Symptoms of AMS, the most common form of altitude illness, include headache, nausea, vomiting, fatigue, dizziness, and insomnia. If not appropriately treated, AMS can progress to life-threatening HACE or HAPE, which can present together or separately. Although HACE presents with similar symptoms as AMS, the cerebral edema can lead to ataxia, confusion, or altered mental status. HAPE is characterized by reduced exercise tolerance, exertional dyspnea, and cough, followed by dyspnea at rest, cyanosis, and productive cough with pink frothy sputum.
Unacclimatized people are at high risk of acute altitude illness when ascending above 8,200 ft (2,500 m), but AMS can occur as low as 6,500 ft (2,000 m). HACE is typically encountered at higher elevations unless presenting with HAPE. The Wilderness Medical Society does not use specific altitude thresholds for diagnosis.