HACE

Why does altitude cause headache? Since the air is thinner, there is less oxygen in the blood, so blood flow to the brain increases. The extra blood can cause blood vessels to swell and tissues to press on the sensitive membrane that surrounds the brain, resulting in a headache. But not everyone develops a headache at moderately high altitudes, partly because the low oxygen content of the air causes the climber to breathe more often, forcing carbon dioxide out of the blood. The body reacts to the lowered carbon dioxide content of the blood by decreasing blood flow to the brain. An individual's susceptibility to altitude-induced headache, as well as the severity of the headache, depends on whether the overall blood flow to the brain increases or decreases. At high altitudes, usually over 10,000 feet, an unrelated condition known as high-altitude cerebral edema, or HACE, can develop. HACE occurs when parts of the brain become waterlogged. Unlike altitude-induced headache, which occurs in over ninety percent of the people who ascend to 11,000 feet, HACE is a rare disorder. Characterized by mental confusion, hallucinations, and a drunken stagger, HACE is almost always fatal if descent is not immediate.