Hallucinatory experiences in extreme-altitude climbers

Objective: This study attempted a systematic investigation of incidence, ty pe, and circumstances of anomalous perceptual experiences in a highly speci alized group of healthy subjects, extreme-altitude climbers. Background: Th ere is anecdotal evidence for a high incidence of anomalous perceptual expe riences during mountain climbing at high altitudes. Method: In a structured interview, we asked eight world-class climbers, each of whom has reached a ltitudes above 8500 m without supplementary oxygen, about hallucinatory exp eriences during mountain climbing at various altitudes. A comprehensive neu ropsychological, electroencephalographic, and magnetic resonance imaging ev aluation was performed with a week of the interview (8). Results: All but o ne subject reported somesthetic illusions (distortions of body scheme) as w ell as visual and auditory pseudohallucinations (in this order of frequency of occurrence). A disproportionately Large number of experiences above 600 0 m as compared to below 6000 m were reported (relative to the total time s pent at these different altitudes). Solo climbing and (in the case of somes thetic illusions) life-threatening danger were identified as probable trigg ers for anomalous perceptual experiences. No relationship between the numbe r of reported experiences and neuropsychological impairment was found. Abno rmalities in electroencephalographic (3 climbers) and magnetic resonance im aging (2 climbers) findings were likewise unrelated to the frequency of rep orted hallucinatory experiences. Conclusions: The results confirm earlier a necdotal evidence for a considerable incidence of hallucinatory experiences during climbing at high altitudes. Apart from hypoxia, social deprivation and acute stress seem to play a role in the genesis of these experiences.