High-pressure nervous syndrome

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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 500 feet (150 m) using a breathing gas containing helium. The effects experienced, and the severity of those effects, depend on the rate of descent, the depth and percentage of helium. [1]

Diving disorders, or diving related medical conditions, are conditions associated with underwater diving, and include both conditions unique to underwater diving, and those that also occur during other activities. This second group further divides into conditions caused by exposure to ambient pressures significantly different from surface atmospheric pressure, and a range of conditions caused by general environment and equipment associated with diving activities.

Underwater diving Descending below the surface of the water to interact with the environment

Underwater diving, as a human activity, is the practice of descending below the water's surface to interact with the environment. Immersion in water and exposure to high ambient pressure have physiological effects that limit the depths and duration possible in ambient pressure diving. Humans are not physiologically and anatomically well adapted to the environmental conditions of diving, and various equipment has been developed to extend the depth and duration of human dives, and allow different types of work to be done.


"Helium tremors" were first widely described in 1965 by Royal Navy physiologist Peter B. Bennett, who also founded the Divers Alert Network. [1] [2] 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. [3]

Royal Navy Maritime warfare branch of the United Kingdoms military

The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by the English kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against the Kingdom of France. The modern Royal Navy traces its origins to the early 16th century; the oldest of the UK's armed services, it is known as the Senior Service.

Peter B. Bennett is the founder and former president and CEO of the Divers Alert Network (DAN), a non-profit organization devoted to assisting scuba divers in need. He is a professor of anesthesiology at Duke University Medical Center, and is currently the Senior Director of the Center for Hyperbaric Medicine and Environmental Physiology at Duke. Bennett is recognized as a leading authority on the effects of high pressure on human physiology.

Divers Alert Network (DAN) is a group of not-for-profit organizations dedicated to improving diving safety for all divers. It was founded in Durham, North Carolina, United States, in 1980 at Duke University providing 24/7 telephonic hot-line diving medical assistance. Since then the organization has expanded globally and now has independent regional organizations in North America, Europe, Japan, Asia-Pacific and Southern Africa.

The term high-pressure nervous syndrome was first used by Brauer in 1968 to describe the combined symptoms of tremor, electroencephalography (EEG) changes, and somnolence that appeared during a 1,189-foot (362 m) chamber dive in Marseille. [4]

Electroencephalography electrophysiological monitoring method

Electroencephalography (EEG) is an electrophysiological monitoring method to record electrical activity of the brain. It is typically noninvasive, with the electrodes placed along the scalp, although invasive electrodes are sometimes used, as in electrocorticography. EEG measures voltage fluctuations resulting from ionic current within the neurons of the brain. Clinically, EEG refers to the recording of the brain's spontaneous electrical activity over a period of time, as recorded from multiple electrodes placed on the scalp. Diagnostic applications generally focus either on event-related potentials or on the spectral content of EEG. The former investigates potential fluctuations time locked to an event, such as 'stimulus onset' or 'button press'. The latter analyses the type of neural oscillations that can be observed in EEG signals in the frequency domain.

Somnolence is a state of strong desire for sleep, or sleeping for unusually long periods. It has distinct meanings and causes. It can refer to the usual state preceding falling asleep, the condition of being in a drowsy state due to circadian rhythm disorders, or a symptom of other health problems. It can be accompanied by lethargy, weakness, and lack of mental agility.

Marseille Second-largest city of France and prefecture of Provence-Alpes-Côte dAzur

Marseille is the second largest city in France after Paris. The main city of the historical province of Provence, it is the prefecture of the department of Bouches-du-Rhône and region of Provence-Alpes-Côte d'Azur. It is located on the Mediterranean coast near the mouth of the Rhône. The city covers an area of 241 km2 (93 sq mi) and had a population of 869,815 in 2016. Its metropolitan area, which extends over 3,173 km2 (1,225 sq mi) is the third-largest in France after those of Paris and Lyon, with a population of 1,831,500 as of 2010.


Symptoms of HPNS include tremors, myoclonic jerking, somnolence, EEG changes, [5] visual disturbance, nausea, dizziness, and decreased mental performance. [1] [2]

Tremor involuntary muscle contraction

A tremor is an involuntary, somewhat rhythmic, muscle contraction and relaxation involving oscillations or twitching movements of one or more body parts. It is the most common of all involuntary movements and can affect the hands, arms, eyes, face, head, vocal folds, trunk, and legs. Most tremors occur in the hands. In some people, a tremor is a symptom of another neurological disorder. A very common tremor is the teeth chattering, usually induced by cold temperatures or by fear.

Nausea medical symptom or condition

Nausea is an unpleasant, diffuse sensation of unease and discomfort, often perceived as an urge to vomit. While not painful, it can be a debilitating symptom if prolonged, and has been described as placing discomfort on the chest, upper abdomen, or back of the throat.

Dizziness general term for a number of types of impairment in spatial perception and stability

Dizziness is an impairment in spatial perception and stability. The term dizziness is imprecise: it can refer to vertigo, presyncope, disequilibrium, or a non-specific feeling such as giddiness or foolishness.


HPNS has two components, one resulting from the speed of compression and the other from the absolute pressure. The compression effects may occur when descending below 500 feet (150 m) at rates greater than a few metres per minute, but reduce within a few hours once the pressure has stabilised. The effects from depth become significant at depths exceeding 1,000 feet (300 m) and remain regardless of the time spent at that depth. [1]

The susceptibility of divers and animals to HPNS varies over a wide range depending on the individual, but has little variation between different dives by the same diver. [1]

The effect of dissolved helium on an embedded trans-membrane channel has also been studied by molecular modeling tools. Those suggest that helium might cause substantial lipid membrane distortion. The high hydrostatic pressure itself has a less damaging influence on the membrane, reducing molecular volumes, but leaving the molecular boundary intact. [6]


It is likely that HPNS cannot be entirely prevented but there are effective methods to delay or change the development of the symptoms. [1] [7]

Utilizing slow rates of compression or adding stops to the compression have been found to prevent large initial decrements in performance. [1] [8]

Including other gases in the helium–oxygen mixture, such as nitrogen (creating trimix) or hydrogen (producing hydreliox) suppresses the neurological effects. [9] [10] [11]

Alcohol, anesthetics and anticonvulsant drugs have had varying results in suppressing HPNS with animals. [1] None are currently in use for humans.[ citation needed ]

HPNS is a plot point in the 1989 James Cameron film The Abyss.

See also

Related Research Articles

Nitrogen narcosis Reversible narcotic effects of respiratory nitrogen at elevated partial pressures

Narcosis while diving is a reversible alteration in consciousness that occurs while diving at depth. It is caused by the anesthetic effect of certain gases at high pressure. The Greek word ναρκωσις (narcosis) is derived from narke, "temporary decline or loss of senses and movement, numbness", a term used by Homer and Hippocrates. Narcosis produces a state similar to drunkenness, or nitrous oxide inhalation. It can occur during shallow dives, but does not usually become noticeable at depths less than 30 meters (100 ft).

Trimix (breathing gas) Breathing gas consisting of oxygen, helium and nitrogen

Trimix is a breathing gas consisting of oxygen, helium and nitrogen and is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.

Deep diving Underwater diving to a depth beyond the norm accepted by the associated community

Deep diving is underwater diving to a depth beyond the norm accepted by the associated community. In some cases this is a prescribed limit established by an authority, and in others it is associated with a level of certification or training, and it may vary depending on whether the diving is recreational, technical or commercial. Nitrogen narcosis becomes a hazard below 30 metres (98 ft) and hypoxic breathing gas is required below 60 metres (200 ft) to lessen the risk of oxygen toxicity.

Altitude diving Underwater diving at altitudes above 300 m

Altitude diving is underwater diving using scuba or surface supplied diving equipment where the surface is 300 metres (980 ft) or more above sea level. The U.S. Navy tables recommend that no alteration be made for dives at altitudes lower than 91 metres (299 ft) and for dives between 91 and 300 metres correction is required for dives deeper than 44 metres (144 ft) of sea water. Altitude is significant in diving because the depths and decompression used for dives at altitude are different from those used for the same dive profile at sea level.

Breathing gas Gas used for human respiration

A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common, and only natural, breathing gas. But other mixtures of gases, or pure gases, are also used in breathing equipment and enclosed habitats such as scuba equipment, surface supplied diving equipment, recompression chambers, high-altitude mountaineering, high-flying aircraft, submarines, space suits, spacecraft, medical life support and first aid equipment, and anaesthetic machines.

Gas blending for scuba diving Mixing and filling cylinders with breathing gases for use when scuba diving

Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.

Diving medicine Diagnosis, treatment and prevention of disorders caused by underwater diving

Diving medicine, also called undersea and hyperbaric medicine (UHB), is the diagnosis, treatment and prevention of conditions caused by humans entering the undersea environment. It includes the effects on the body of pressure on gases, the diagnosis and treatment of conditions caused by marine hazards and how relationships of a diver's fitness to dive affect a diver's safety.

Hydreliox is an exotic breathing gas mixture of helium, oxygen and hydrogen. For the Hydra VIII mission at 50 atmospheres of ambient pressure, the mixture used was 49% hydrogen, 50.2% helium, and 0.8% oxygen.

In diving, the oxygen window is the difference between the partial pressure of oxygen (ppO2) in arterial blood and the ppO2 in body tissues. It is caused by metabolic consumption of oxygen.

Edward D. Thalmann American hyperbaric medicine specialist and decompression researcher

Capt. Edward Deforest Thalmann, USN (ret.) was an American hyperbaric medicine specialist who was principally responsible for developing the current United States Navy dive tables for mixed-gas diving, which are based on his eponymous Thalmann Algorithm (VVAL18). At the time of his death, Thalmann was serving as Assistant Medical Director of the Divers Alert Network (DAN) and an Assistant Clinical Professor in Anesthesiology at Duke University's Center for Hyperbaric Medicine and Environmental Physiology.

Hydrox, a gas mixture of hydrogen and oxygen, was used as a breathing gas in very deep diving. It allows divers to descend several hundred metres.

Equivalent narcotic depth (END) is used in technical diving as a way of estimating the narcotic effect of a breathing gas mixture, such as heliox and trimix. The method is, for a given mix and depth, to calculate the depth which would produce the same narcotic effect when breathing air.

Hydrogen narcosis is the psychotropic state induced by breathing hydrogen at high pressures. Hydrogen narcosis produces symptoms such as hallucinations, disorientation, and confusion, which are similar to hallucinogenic drugs. It can be experienced by deep-sea divers who dive to 300 m (1,000 ft) below sea level breathing hydrogen mixtures. However, hydrogen has far less narcotic effect than nitrogen and is very rarely used in diving. In tests of the effect of hydrogen narcosis, where divers dived to 500 m (1,600 ft) with a hydrogen–helium–oxygen (Hydreliox) mixture containing 49% hydrogen, it was found that while the narcotic effect of hydrogen was detectable, the neurological symptoms of high-pressure nervous syndrome were only moderate.

Professor Albert A. Bühlmann was a Swiss physician who was principally responsible for a number of important contributions to decompression science at the Laboratory of Hyperbaric Physiology at the University Hospital in Zürich, Switzerland. His impact on diving ranged from complex commercial and military diving to the occasional recreational diver. He is held in high regard for his professional ethics and attention to his research subjects.

Compression arthralgia is pain in the joints caused by exposure to high ambient pressure at a relatively high rate of compression, experienced by underwater divers. Also referred to in the US Navy diving Manual as compression pains.

Physiology of underwater diving is the physiological influences of the underwater environment on the physiology of air-breathing animals, and the adaptations to operating underwater, both during breath-hold dives and while breathing at ambient pressure from a suitable breathing gas supply. It, therefore, includes both the physiology of breath-hold diving in humans and other air-breathing animals, and the range of physiological effects generally limited to human ambient pressure divers either freediving or using underwater breathing apparatus. Several factors influence the diver, including immersion, exposure to the water, the limitations of breath-hold endurance, variations in ambient pressure, the effects of breathing gases at raised ambient pressure, effects caused by the use of breathing apparatus, and sensory impairment. All of these may affect diver performance and safety.

Diving hazards are the agents or situations that pose a threat to the underwater diver or their equipment. Divers operate in an environment for which the human body is not well suited. They face special physical and health risks when they go underwater or use high pressure breathing gas. The consequences of diving incidents range from merely annoying to rapidly fatal, and the result often depends on the equipment, skill, response and fitness of the diver and diving team. The hazards include the aquatic environment, the use of breathing equipment in an underwater environment, exposure to a pressurised environment and pressure changes, particularly pressure changes during descent and ascent, and breathing gases at high ambient pressure. Diving equipment other than breathing apparatus is usually reliable, but has been known to fail, and loss of buoyancy control or thermal protection can be a major burden which may lead to more serious problems. There are also hazards of the specific diving environment, and hazards related to access to and egress from the water, which vary from place to place, and may also vary with time. Hazards inherent in the diver include pre-existing physiological and psychological conditions and the personal behaviour and competence of the individual. For those pursuing other activities while diving, there are additional hazards of task loading, of the dive task and of special equipment associated with the task.


  1. 1 2 3 4 5 6 7 8 Bennett, Peter B; Rostain, Jean Claude (2003). "The High Pressure Nervous Syndrome". In Brubakk, Alf O; Neuman, Tom S (eds.). Bennett and Elliott's physiology and medicine of diving, 5th Rev ed. United States: Saunders. pp. 323–57. ISBN   0-7020-2571-2.
  2. 1 2 Bennett, P. B. (1965). "Psychometric impairment in men breathing oxygen-helium at increased pressures". Royal Navy Personnel Research Committee, Underwater Physiology Subcommittee Report No. 251. London.
  3. Zal'tsman, G. L. (1967). "Psychological principles of a sojourn of a human in conditions of raised pressure of the gaseous medium (in Russian, 1961)". English translation, Foreign Technology Division. AD655 360. Wright Patterson Air Force Base, Ohio.
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  6. Bliznyuk, Alice; Grossman, Yoram; Moskovitz, Yevgeny (25 July 2019). "The effect of high pressure on the NMDA receptor: molecular dynamics simulations". Scientific Reports. Nature Research. 9 (1). doi:10.1038/s41598-019-47102-x.
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  8. Bennett, P. B.; R. Coggin; M. McLeod. (1982). "Effect of compression rate on use of trimix to ameliorate HPNS in man to 686 m (2250 ft)". Undersea Biomed. Res. 9 (4): 335–51. ISSN   0093-5387. OCLC   2068005. PMID   7168098 . Retrieved 2008-04-07.
  9. Vigreux, J. (1970). "Contribution to the study of the neurological and mental reactions of the organism of the higher mammal to gaseous mixtures under pressure". MD Thesis. Toulouse University.
  10. Fife, W. P. (1979). "The use of Non-Explosive mixtures of hydrogen and oxygen for diving". Texas A&M University Sea Grant. TAMU-SG-79-201.
  11. Rostain, J. C.; Gardette-Chauffour, M. C.; Lemaire, C.; Naquet, R. (1988). "Effects of a H2-He-O2 mixture on the HPNS up to 450 msw". Undersea Biomedical Research. 15 (4): 257–70. ISSN   0093-5387. OCLC   2068005. PMID   3212843 . Retrieved 2008-04-07.