Ascending to great heights can give you an exhilarating feeling -- or a very ill one.

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High Altitude Cerebral Edema (HACE)

The third and most serious form of altitude sickness is High Altitude Cerebral Edema (HACE). Acute Mountain Sickness is actually a mild case of HACE.

Much like High Altitude Pulmonary Edema (HAPE), HACE occurs when a combination of higher altitudes and the resulting lower air pressure causes fluid to leak from blood vessels [source: Curtis]. But instead of the fluid collecting in the lungs, as it does with HAPE, it collects in the brain. While the brain is naturally surrounded by cranial fluid, HACE introduces fluid inside the brain. CT scans of HACE patients have shown fluid in pockets of their brains. HACE is a very dangerous condition and needs immediate medical attention. A person with this condition could become comatose or die if not soon treated.

The most obvious symptom of HACE is ataxia, which we've already discussed. To determine if someone has this symptom, you can test his or her coordination. Stretch a rope along the ground or draw a line in the dirt or snow and ask him or her to walk in a straight line beside it. Someone with ataxia will be unable to walk a straight line. (Interestingly, HACE doesn't affect the performance of a finger-to-nose test.) As we learned earlier, loss of coordination is also a sign of severe Acute Mountain Sickness. A mild amount of trouble walking or moving could very well be severe AMS, but it's safest to assume HACE, especially if other symptoms -- like difficulty thinking -- are present.

A person with HACE should immediately descend at least to the last elevation where he or she woke up symptom-free. A person can die from HACE in as little as six to 24 hours after showing signs of ataxia [source: CDC].

Medical attention should be sought immediately. Once a person with HACE is taken to a safe, lower elevation, his or her mental confusion usually clears up quickly. It may, however, take up to several weeks for physical coordination to be fully restored. Lasting neurological damage is rare but can occur. This damage is specific to the patient and the severity of the case, but possible effects are impaired memory, motor skills and cognitive functioning.

Once fully recovered at a lower elevation, a person can safely reascend to (and beyond) the elevation that previously caused him or her so much trouble, so long as he or she properly acclimates on the trip.

If you're suffering from altitude sickness and have the choice of being stuffed into a pressurized bag or consuming ginkgo, what should you do? Find out in the next section.