Why Climbers Hallucinate at High Altitudes

Mountaineering in Canada
A climber walking along a mountain ridge Christopher Kimmel

An ascent like Mount Everest can be crippling. The low levels of oxygen and barometric pressure at its peak—29,029 feet above sea level—trigger altitude sickness in even the most experienced climbers.

Headaches, dizziness, and muscle aches are expected, but some extreme climbers also suffer from hallucinations, going temporarily mad on the mountain, according to research from Cambridge University. And it’s not a side effect of altitude sickness.

In the analysis, published in Psychological Medicine, researchers looked at 83 different psychotic episodes from German literature. They noted symptoms, like hearing voices,  hallucinations, delusions, disorganized speech, impaired cognition, depression, and mania. Researchers call it “isolated high-altitude psychosis.”

These episodes weren’t linked to physical symptoms of altitude sickness or mental illness. The men and women were completely healthy. And while researchers aren’t exactly sure what the causes are, they do believe it’s similar to what triggers altitude sickness: oxygen deficiency or early stages of swelling in parts of the brain, which accompanies heights exceeding 22,965 feet above sea-level.

“It is also known that complete deprivation of social contact and loneliness for a long time can provoke hallucinations,” study co-author Hermann Brugger told Live Science.

Thinking about attempting Everest? Use our three-year game plan to prep properly.