Altitude sickness can strike as low as 6,000 feet but more commonly occurs above 8,000 feet, usually in people who've reached that elevation rapidly. The first phase is Acute Mountain Sickness (AMS), which always involves mild brain swelling that, according to the Spanish study, can cause lasting damage. Symptoms include headache, nausea, and malaise; if these start, climbers should descend until they disappear. If the swelling continues to worsen, it can become High-Altitude Cerebral Edema (HACE), a very serious condition. Delusions, confusion, and emotional instability are early symptoms; it can progress to cause lack of coordination, unconsciousness, and death.
Here's what happens: At altitude, the lack of oxygen causes your heart and respiration rates to increase. This causes you to exhale too much carbon dioxide, which upsets the water and electrolyte balance in the blood. That, in turn, damages the walls of brain (and lung) capillaries, causing them to leak fluid into surrounding tissue and make the brain swell. The blood also becomes thicker as more red blood cells are produced to transport oxygen and as water is pulled out by dehydration. In the most serious cases, clots develop in the thickened blood, causing minor strokes.