The most common form of altitude sickness is acute mountain sickness (AMS), which can begin at elevations as low as 2,000 metres (6,500 feet). If AMS develops at any elevation, it causes tiredness and nausea. It can also cause swelling of the face and hands, difficulty sleeping, and severe headaches. Exercise tends to make symptoms worse; however, some experts believe moderate exercise on the day of arrival will help the body acclimatize more quickly. AMS is rarely fatal. If someone with AMS remains at a high altitude, it will often disappear after a few days when the body adjusts to the higher elevation.
However, AMS can be a warning sign of high altitude cerebral edema (HACE). HACE causes swelling in the brain that can result in coma and death within hours. Early signs of HACE include severe headache, confusion, and hallucinations, as well as staggering or stumbling. A person with HACE must descend to a lower altitude immediately to prevent serious consequences.
High-altitude pulmonary edema (HAPE) develops one to three days after a rapid ascent above 2,500 metres (8,000 feet). When most people climb this high, fluid begins to accumulate in their lungs. With HAPE, the fluid builds up more quickly than it can drain. People become so breathless they remain unable to breathe normally even after long rests. Their skin turns blue, and they may cough up frothy, pink sputum and eventually fall into a coma. If not treated immediately and moved to a lower elevation, people with HAPE can die from the illness. Both HACE and HAPE can develop without AMS occurring first.
Altitude sickness can be complicated by any chronic illness. It can also be complicated by other high-altitude health problems such as frostbite, blood clots in the legs and lungs, dehydration, and swollen feet and ankles.