High Altitude Cerebral Edema
High altitude cerebral edema (HACE) is the most deadly of all high altitude induced illnesses.
HACE happens when the blood vessels in the brain begin to leak, causing it to swell.
Symptoms are loss of coordination, headache, nausea and vomiting, and lack of energy. More advanced symptoms may include delirium of some degree and loss of muscular control.
Rapid descent to low altitude is imperative for the survival of the victim, and usually some professional medical help is needed.
Although there is nothing you can do to become completely immune to HACE, following a well-thought-out acclimatization plan helps most people keep from getting it.
As always with mountain climbing, go with a competent guide.