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  • Updated 07.11.2022
  • Released 12.29.1997
  • Expires For CME 07.11.2025

High-pressure neurologic syndrome

Introduction

Overview

High-pressure neurologic syndrome is a condition encountered in deep-sea diving beyond a depth of 100 meters and is characterized by several neurologic and psychiatric symptoms and signs. The effects are primarily due to excessive atmospheric pressure on different CNS structures. This syndrome should be differentiated from inert gas narcosis, decompression sickness, and oxygen poisoning. No definite treatment exists, but barbiturates, anticonvulsant drugs, and anesthetics have been used in the management of this syndrome.

Key points

• Clinical features of high-pressure neurologic syndrome vary according to the depth (beyond 100 meters) and the gas mixture used.

• There is no evidence of permanent neurologic sequelae or histopathologic changes in the brain resulting from high-pressure neurologic syndrome.

• Because of different approaches to management, high-pressure neurologic syndrome needs to be differentiated from inert gas narcosis, decompression sickness, and oxygen toxicity.

• The pharmacological approach is based on the resemblance of high-pressure neurologic syndrome to serotonin syndrome, and 5-HT1A receptor antagonists may provide a preventive approach.

Historical note and terminology

High-pressure neurologic syndrome is a condition encountered in deep-sea diving beyond a depth of 100 meters, a feat that is made possible by the breathing of special gas mixtures such as helium and oxygen (heliox). Divers have worked at depths of 500 meters in the sea (500 msw) and have reached 700 meters in simulated chamber dives (40).

In 1926, American physician and industrial hygienist Royd Ray Sayers (1885-1965) and William Parks Yant (1893-1963) of the U.S. Bureau of Mines, Pittsburgh Experiment Station, found that humans could breathe helium-oxygen mixtures without discomfort, and both humans and animals could breathe this mixture without demonstrable ill effects (46).

Royd R Sayers MD (1885-1965), c. 1940

Sayers was an American physician and industrial hygienist. In 1926, Sayers and William Parks Yant (1893–1963) of the U.S. Bureau of Mines found that humans could breathe helium-oxygen mixtures without discomfort, and both human...

To prevent oxygen toxicity at extreme depths, it was necessary to employ a mixture of oxygen with some inert gas rather than using pure oxygen. Helium was an excellent choice to mix with oxygen to prevent oxygen toxicity and to prevent nitrogen narcosis ("narks"; "rapture of the deep"), a change in consciousness, neuromuscular function, and behavior brought on by breathing compressed inert gasses (typically nitrogen, but also neon, argon, krypton, and xenon). Because of its reduced solubility, little helium is taken into cell membranes at high pressures, so helium does not have the narcotic effect that nitrogen does (24), nor does it have the same risk of later forming bubbles during decompression and the development of "the bends" or decompression sickness. Heliox is also easier to breathe at high pressures than compressed air (oxygen plus nitrogen).

High-pressure neurologic syndrome is characterized by neurologic, psychological, and electroencephalographic abnormalities (11; 01; 02; 13; 32; 40; 44). In 1967, Royal Navy physiologist Peter B Bennett (b 1931) described a decline of mental performance and tremors following dives to depths between 200 and 250 meters and termed this condition "helium tremors." In 1980, Bennett founded the Divers Alert Network, or DAN, a nonprofit organization devoted to assisting scuba divers in need (11; 13). Bennett was unaware that the same phenomenon had been observed in Russia earlier by Zaltsman, who used the same term to describe it, as Zaltsman's report was unavailable in the West until 1967 (54).

High-pressure neurologic syndrome, characterized by tremors and EEG changes, was first observed in divers who dove deeper than 300 meters (22; 13). Most of the basic research in this area has been done on experimental animals. The term "neurologic" is preferred to the term "nervous" (original translation of nerveux, as described in French).

High-pressure neurologic syndrome, along with decompression sickness, nitrogen narcosis, and barotrauma, is included in the broad category of dysbarism, which is defined as any adverse medical condition that results from changes in ambient pressure, which must occur either at a rate or duration exceeding the capacity of the body to adapt safely (08).

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