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High-pressure nervous syndrome is a neurological and physiological diving disorder which can result when a diver descends below about 500 feet using a breathing gas containing helium. The effects experienced, and the severity of those effects, depend on the rate of descent, the depth and the percentage of helium.
"Helium tremors" were described in 1965 by Royal Navy physiologist Peter B. Bennett. Russian scientist G. L. Zal'tsman also reported on helium tremors in his experiments from 1961. However, these reports were not available in the West until 1967.
The term high-pressure nervous syndrome was first used by R. W. Brauer in 1968 to describe the combined symptoms of tremor, electroencephalography changes, and somnolence that appeared during a 1,189-foot chamber dive in Marseille.