HACE is the most severe altitude illness, caused by hypoxia-induced cerebral swelling. Disruption of the blood-brain barrier leads to fluid leakage into brain tissue, increasing intracranial pressure. Symptoms include severe headache, confusion, ataxia, and coma. Immediate descent, oxygen, and corticosteroids are essential treatments, and prevention relies on gradual ascent, acclimatization, and avoiding alcohol or sedatives.