High-pressure nervous syndrome (HPNS – also known as high-pressure neurological syndrome) is a neurological and physiological diving disorder which can result when a diver descends below about 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. Soviet scientist G. L. Zal'tsman first reported on helium tremors in his experiments from 1961. 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 (EEG) changes, and somnolence that appeared during a chamber dive in Marseille.
Symptoms
Symptoms of HPNS include tremors, myoclonic jerking, somnolence, EEG changes, visual disturbance, nausea, dizziness, and decreased mental performance.
Prevention
It is likely that HPNS cannot be entirely prevented but there are effective methods to delay or change the development of the symptoms.
Slow rates of compression, or adding stops to the compression, have been found to prevent large initial decrements in performance. There is circumstantial evidence suggesting that a high chamber temperature can aggravate HPNS, and that the temperature should be maintained at a comfortable level.
Including other gases in the heliox helium–oxygen mixture, such as nitrogen (creating trimix) or hydrogen (producing hydreliox), suppresses the neurological effects.
Alcohol, anesthetics, and anticonvulsant drugs have had varying results in suppressing HPNS in animals.