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  • Updated 08.19.2022
  • Released 04.21.1998
  • Expires For CME 08.19.2025

Decompression sickness: neurologic manifestations



Divers who have experienced pressures greater than 2 atmospheres absolute may develop decompression sickness if they ascend too rapidly. Decompression sickness may be mild, with only limb and joint pain ("bends," type I), or serious, with neurologic, cardiac, and pulmonary manifestations (type II). Divers with a patent foramen ovale are more likely to develop severe forms of decompression sickness than divers without a right-to-left shunt. Treatment in a pressure chamber is essential for recovery, and detailed decompression tables are used to prevent and treat decompression sickness.

Key points

• Decompression sickness usually occurs during rapid ascent from depth after diving but may also occur in rapid ascent to high altitudes from sea level.

• Systemic manifestations may involve the nervous system.

• Decompression sickness can be avoided by gradual ascent, but if decompression sickness occurs, it is treated by hyperbaric recompression.

• Hyperbaric oxygen is useful in treating decompression sickness with neurologic manifestations.

Historical note and terminology

Boyle's law. In 1660, Anglo-Irish natural philosopher Robert Boyle (1627-1691) published the first controlled experiments with "rarified air," obtained by reducing the pressure of the air (19; 20; 119).

Boyle found by 1662 that (in modern language) for a fixed mass of an ideal gas kept at a fixed temperature, pressure and volume are inversely proportional (20). For his experiments, Boyle relied on an air pump devised by English polymath Robert Hooke (1635-1703).

Dalton's law of partial pressures. In a mixture of gases, as in air, each constituent gas has a partial pressure that is the pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. This is Dalton's law, named after English chemist and physicist John Dalton (1766-1844), who determined this experimentally in 1802 (30).

At sea level, where atmospheric pressure is 760 mm Hg, the percent of the total composition and the partial pressures of the various gases are approximately as follows: 78.6%, 597 mm Hg for nitrogen; 20.9%, 159 mm Hg for oxygen; 0.04%, 3.0 mm Hg for water; and 0.004%, 0.3 mm Hg for carbon dioxide. For comparison, the percent of the total composition and partial pressures of alveolar air are as follows: nitrogen 74.9%, 569 mm Hg; oxygen 13.7%, 104 mm Hg; and water 6.2%, 40 mm Hg.

Henry’s law. Henry’s law, formulated by English chemist William Henry (1774-1836) in 1803, states that, at a constant temperature, the solubility of a gas is directly proportional to the pressure that the gas exerts on the solution (52).

When equilibrium is reached, the solution is described as "saturated," but if the pressure is then reduced, the tissues become effectively supersaturated, and the gases leave the solution and may form gas bubbles. Due to the metabolic activity of oxygen and carbon dioxide, and the comparative marked inactivity of nitrogen, it is nitrogen that is by far the most problematic. To determine the time necessary for the clearance of supersaturated nitrogen without biological damage, decompression tables have been generated based on biomathematical models (but relying on the experience and fitness of navy divers). Even following protocols established for safe decompression, problems may occur (eg, interindividual variation, flying in a commercial aircraft within 12 to 18 hours after diving, etc.), which has led to the appreciation that safety factors must be built into the estimated "safe" dive times.

The Bert and Smith "effects" of oxygen toxicity. In 1878, French zoologist, physiologist, and politician Paul Bert (1833-1886) was the first to determine the acute toxicity of high oxygen concentrations in "La Pression Barometrique” (13; 14). Bert was a student of French physiologist Claude Bernard (1813-1878).

Bert applied an apparatus of French physiologist Denis Jourdanet (1815-1892) that was intended for therapeutic use of compressed or "expanded" air. Bert experimented on himself with "superoxygenated air," that is air with an increased partial pressure of oxygen).

In 1878, Bert demonstrated convulsions in larks exposed to air at 15 to 20 atmospheres absolute, and the neurotoxic effects of oxygen at increased pressure were subsequently called the "Bert effect" (13). Then in 1899, Scottish pathologist and physiologist J(ames) Lorain Smith (1862-1931), while trying to reproduce the "Bert effect," noticed fatal pneumonia in rats after 4 days of exposure to 73% oxygen at 1 atmosphere absolute, which marked the discovery of pulmonary toxicity of oxygen at increased partial pressure--the "Smith Effect" (100; 47; 51).

Scottish pathologist and physiologist J(ames) Lorain Smith (1862-1931)

Smith discovered of pulmonary toxicity of oxygen at increased partial pressure, the "Smith effect" (Smith JL. The pathological effects due to increase of oxygen tension in the air breathed. J Physiol 1899;24[1]:19-35). (Source:...

Dysbarism. Dysbarism is a general term that encompasses disturbances in the human body resulting from a change in atmospheric pressure. Dysbarism encompasses five subentities: decompression sickness, barotrauma, gas embolism, inert gas narcosis, and oxygen toxicity.

Decompression sickness. Decompression sickness is one of several forms of dysbarism, ie, disturbances in the human body resulting from a change in atmospheric pressure. Divers, miners, tunnel workers, and caisson workers will experience decompression sickness if they progress too quickly to a lower environmental pressure. Rapid ascent to high altitudes in an aircraft with an uncompressed cabin can also produce decompression sickness. The first and least severe symptoms are characterized by limb and joint pain. In more severe cases, with ascent from greater depths, after longer bottom times, or with more rapid ascent or decompression, other nervous system, cardiac, or pulmonary symptoms may occur.

The condition was well known among caisson workers, and it was, therefore, called "caisson disease." Other terms used to describe the condition are "the bends" (limb and joint pain), "the chokes," and "hits."

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