High pressure nervous syndrome is a dangerous diving disorder that can arise when a diver spends too much time breathing a high-pressure mixture of helium and oxygen (heliox) at depths in excess of 130 metres / 429 feet. Its symptoms include dizziness, nausea, vomiting, termors, myoclonic jerking, and decreased intellectual performance. It can be avoided by including small amounts of other gases in the mix, such as nitrogen (trimix) or hydrogen (hydreliox ).
For HPNS in fiction, see The Abyss.
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