High altitude cerebral edema (HACE) is a potentially fatal fluid buildup in the brain, considered a severe form of acute mountain sickness. Urgent treatment is essential. Symptoms include severe headache, vomiting, lethargy, ataxia (unsteadiness), confusion, drowsiness, and possible coma. A useful diagnostic test is the inability to walk heel-to-toe in a straight line. Irrational or bizarre behavior may also indicate HACE. It can lead to death within hours, necessitating immediate hospitalization. Administering oxygen can prevent HACE.