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Decompression sickness occurs when rapid pressure reduction (eg, during ascent from a dive, exit from a caisson or hyperbaric chamber, or ascent to altitude) causes gas previously dissolved in blood or tissues to form bubbles in blood vessels. Symptoms typically include pain, neurologic symptoms, or both. Severe cases can be fatal. Diagnosis is clinical. Definitive treatment is recompression therapy. Proper diving techniques are essential for prevention.

Henry's law states that the solubility of a gas in a liquid is directly proportional to the pressure exerted on the gas and liquid. Thus, the amount of inert gases (eg, N₂, helium) taken up in the blood and tissues increases at higher pressure. During ascent, when the surrounding pressure decreases, bubbles may form. The liberated gas bubbles can arise in any tissue and cause local symptoms, or they can travel via the blood to distant organs. Bubbles cause symptoms by blocking vessels, rupturing or compressing tissue, or activating clotting and inflammatory cascades. Because N₂ dissolves readily in fat, tissues with a high lipid content (eg, in the CNS) are particularly susceptible.

Decompression sickness occurs in about 2 to 4/10,000 dives. Risk factors include cold-temperature dives, stress, fatigue, asthma, dehydration, obesity, older age, exercise, flying after diving, rapid ascents, and prolonged or deep dives. Because excess N₂ remains dissolved in body tissues for at least 12 h after each dive, repeated dives within 1 day require special techniques to determine proper decompression procedures and are most likely to cause decompression sickness.

Classification: Generally, there are 2 types. Type I, which involves muscles, skin, and lymphatics, is milder and not typically life threatening. Type II is serious, is sometimes life threatening, and affects various organ systems. The spinal cord is especially vulnerable; other vulnerable areas include the brain, respiratory system (eg, pulmonary emboli), and circulatory system (eg, heart failure, cardiogenic shock). “The bends” refers to local joint or muscle pain due to decompression sickness but is often used as a synonym for any component of the disorder.

Symptoms and Signs

Severe symptoms may manifest within minutes of surfacing, but in most patients, symptoms begin gradually, sometimes with a prodrome of malaise, fatigue, anorexia, and headache. Symptoms occur within 1 h of surfacing in about 50% of patients and by 6 h in 90%. Rarely, symptoms can manifest 24 to 48 h after surfacing, particularly after exposure to altitude after diving.

Type I decompression sickness typically causes progressively worsening pain in the joints (typically elbows and shoulders), back, and muscles; the pain intensifies during movement and is described as “deep” and “boring.” Other symptoms include lymphadenopathy, skin mottling, itching, and rash.

Type II decompression sickness typically manifests with paresis, numbness and tingling, neuropraxia, difficulty urinating, and loss of bowel or bladder control. Headache and fatigue may be present but are nonspecific. Dizziness, tinnitus, and hearing loss may result if the inner ear is affected. Severe symptoms include seizures, slurred speech, vision loss, confusion, and coma. Death can occur. The chokes (respiratory decompression sickness) is a rare but grave manifestation; symptoms include shortness of breath, chest pain, and cough. Massive bubble embolization of the pulmonary vascular tree can result in rapid circulatory collapse and death.

Dysbaric osteonecrosis is a late manifestation of decompression sickness. It is an insidious form of aseptic bone necrosis caused by prolonged or closely repeated exposures to pressurized areas (typically in people working in compressed air and in deep commercial rather than recreational divers). Deterioration of shoulder and hip articular surfaces can cause chronic pain and severe disability.

Diagnosis

Diagnosis is clinical. CT and MRI may show brain or spinal cord abnormalities but are not sensitive, and treatment should usually begin based on clinical suspicion. Sometimes arterial gas embolism is similar (for a comparison of features, see Table 1: Injury During Diving or Work in Compressed Air: Comparison of Gas Embolism and Decompression Sickness).

For dysbaric osteonecrosis, plain x-rays may show joint degeneration, which cannot be distinguished from that caused by other joint disorders; MRI is usually diagnostic.

Treatment

About 80% of patients recover completely.

Initially, high-flow 100% O₂ enhances N₂ washout by widening the N₂ pressure gradient between the lungs and the circulation, thus accelerating reabsorption of embolic bubbles.

Recompression therapy (see Injury During Diving or Work in Compressed Air: Recompression Therapy) is indicated for all patients except perhaps those whose symptoms are limited to itching, skin mottling, and fatigue; they should be observed for deterioration. Other patients are transported to a suitable recompression facility. Because time to treatment is a main determinant of outcome, transport should not be delayed even in cases that appear mild or for performance of nonessential procedures. If air evacuation is required, low altitude (eg, ground-level cabin pressure or < 2000 ft [< 609 m] in unpressurized aircraft) is preferred. Commercial airlines typically have a cabin pressure equivalent to 8000 ft (2438 m), which may exacerbate symptoms. Flying in commercial aircraft shortly after a dive can precipitate symptoms.

Prevention

Significant bubble formation can usually be avoided by limiting the depth and duration of dives to a range that does not need decompression stops during ascent (called no-stop limits) or by ascending with decompression stops as specified in published guidelines (eg, the decompression table in the US Navy Diving Manual). Many divers now wear a portable dive computer that continually tracks depth and time at depth and calculates a decompression schedule. Also, many divers make a safety stop for a few minutes at about 15 ft (4.6 m) below the surface.

However, about 50% of cases develop after appropriately identified no-stop dives, and the incidence of decompression sickness has not decreased despite widespread use of dive computers. The reason may be that published tables and computer programs do not completely account for the variation in risk factors among divers or that people do not obey the recommendations precisely.

Last full review/revision November 2005

Content last modified November 2005