High Altitude Cerebral Edema (HACE) is less common than High Altitude Pulmonary Edema (HAPE), but it tends to be even more deadly. A baffling ailment, HACE occurs when fluid leaks from oxygen-starved cerebral blood vessels, causing severe swelling of the brain, and it can strike with little or no warning. As pressure builds inside the skull, motor and mental skills deteriorate with alarming speed - typically within a few hours or less - and often without the victim even noticing the change. The next step is coma, and then, unless the afflicted party is quickly evacuated to lower altitude, death.

Mainly a problem for high altitude mountaineers, they represent the most severe incarnations of Acute Mountain Sickness (AMS) which, in its moderate form, causes headache, nausea, disturbed sleep, fatigue, shortness of breath, malaise, dizziness, loss of appetite and poor sleep patterns.