It usually develops 24 to 96 hours after ascent and is uncommon below 9,000 feet. A cough is common, dry and tickling at first but often becoming loose and bubbling. A large amount of phlegm, often pink or even bloody, may be coughed up. The person may have a slight fever. In high-altitude cerebral edema, fluid accumulates within the brain.
Headaches are more severe than in acute mountain sickness.
Hallucinations appear somewhat later but often are not recognized as such. The higher the altitude, the greater the impairment of judgment and perception. Symptoms may resemble the effects of drinking alcohol. A person thought to have high-altitude cerebral edema should immediately descend to a lower altitude. In part, it's caused by the changing distribution of salts that occurs in the body at high altitudes, but strenuous exertion causes changes in salt and water distribution even at sea level.