Decompression sickness

Decompression sickness is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompression. DCS most commonly occurs during or soon after a decompression ascent from underwater diving, but can also result from other causes of depressurization, such as emerging from a caisson, decompression from saturation, flying in an unpressurised aircraft at high altitude, and extravehicular activity from spacecraft. DCS and arterial gas embolism are collectively referred to as decompression illness.
Since bubbles can form in or migrate to any part of the body, DCS can produce many symptoms, and its effects may vary from joint pain and rashes to paralysis and death. DCS often causes air bubbles to settle in major joints like knees or elbows, causing individuals to bend over in excruciating pain, hence its common name, the bends. Individual susceptibility can vary from day to day, and different individuals under the same conditions may be affected differently or not at all. The classification of types of DCS according to symptoms has evolved since its original description in the 19th century. The severity of symptoms varies from barely noticeable to rapidly fatal.
Decompression sickness can occur after an exposure to increased pressure while breathing a gas with a metabolically inert component, then decompressing too fast for it to be harmlessly eliminated through respiration, or by decompression by an upward excursion from a condition of saturation by the inert breathing gas components, or by a combination of these routes. Theoretical decompression risk is controlled by the tissue compartment with the highest inert gas concentration, which for decompression from saturation, is the slowest tissue to outgas.
The risk of DCS can be managed through proper decompression procedures, and contracting the condition has become uncommon. Its potential severity has driven much research to prevent it, and divers almost universally use decompression schedules or dive computers to limit their exposure and to monitor their ascent speed. If DCS is suspected, it is treated by hyperbaric oxygen therapy in a recompression chamber. Where a chamber is not accessible within a reasonable time frame, in-water recompression may be indicated for a narrow range of presentations, if there are suitably skilled personnel and appropriate equipment available on site. Diagnosis is confirmed by a positive response to the treatment. Early treatment results in a significantly higher chance of successful recovery.

Classification

DCS is classified by symptoms. The earliest descriptions of DCS used the terms: "bends" for joint or skeletal pain; "chokes" for breathing problems; and "staggers" for neurological problems. In 1960, Golding et al. introduced a simpler classification using the term "Type I " for symptoms involving only the skin, musculoskeletal system, or lymphatic system, and "Type II " for symptoms where other organs are involved. Type II DCS is considered more serious and usually has worse outcomes. This system, with minor modifications, may still be used today. Following changes to treatment methods, this classification is now much less useful in diagnosis, since neurological symptoms may develop after the initial presentation, and both Type I and Type II DCS have the same initial management.

Decompression illness and dysbarism

The term dysbarism encompasses decompression sickness, arterial gas embolism, and barotrauma, whereas decompression sickness and arterial gas embolism are commonly classified together as decompression illness when a precise diagnosis cannot be made. DCS and arterial gas embolism are treated very similarly because they are both the result of gas bubbles in the body. The U.S. Navy prescribes identical treatment for Type II DCS and arterial gas embolism. Their spectra of symptoms also overlap, although the symptoms from arterial gas embolism are generally more severe because they often arise from an infarction.

Signs and symptoms

While bubbles can form anywhere in the body, DCS is most frequently observed in the shoulders, elbows, knees, and ankles. Joint pain accounts for about 60% to 70% of all altitude DCS cases, with the shoulder being the most common site for altitude and bounce diving, and the knees and hip joints for saturation and compressed air work. Neurological symptoms are present in 10% to 15% of DCS cases with headache and visual disturbances being the most common symptom. Skin manifestations are present in about 10% to 15% of cases. Pulmonary DCS is very rare in divers and has been observed much less frequently in aviators since the introduction of oxygen pre-breathing protocols. The table below shows symptoms for different DCS types.

DCS type	Bubble location	Signs & symptoms
Musculoskeletal	Mostly large joints of the limbs	Localized deep pain, ranging from mild to excruciating. Sometimes a dull ache, more rarely a sharp pain. Active and passive motion of the joint may aggravate the pain. The pain may be reduced by bending the joint to find a more comfortable position. If caused by altitude, pain can occur immediately or up to many hours later.
Cutaneous	Skin	Itching, usually around the ears, face, neck, arms, and upper torso Sensation of tiny insects crawling over the skin Mottled or marbled skin usually around the shoulders, upper chest and abdomen, with itching Swelling of the skin, accompanied by tiny scar-like skin depressions
Neurologic	Brain	Altered sensation, tingling or numbness, increased sensitivity Confusion or memory loss Visual abnormalities Unexplained mood or behaviour changes Seizures, unconsciousness
Neurologic	Spinal cord	Ascending weakness or paralysis in the legs Urinary incontinence and fecal incontinence Girdling around the abdominal region and/or chest
Constitutional	Whole body	Headache Unexplained fatigue Generalised malaise, poorly localised aches
Audiovestibular	Inner ear	Loss of balance Dizziness, vertigo, nausea, vomiting Hearing loss
Pulmonary	Lungs	Dry persistent cough Burning chest pain under the sternum, aggravated by breathing Shortness of breath

Frequency

The relative frequencies of different symptoms of DCS observed by the U.S. Navy are as follows:

Symptoms	Frequency
local joint pain	89%
arm symptoms	70%
leg symptoms	30%
dizziness	5.3%
paralysis	2.3%
shortness of breath	1.6%
extreme fatigue	1.3%
collapse/unconsciousness	0.5%

Onset

Although the onset of DCS can occur rapidly after a dive, in more than half of all cases, symptoms do not begin to appear for at least an hour. In extreme cases, symptoms may occur before the dive has been completed. The U.S. Navy and Technical Diving International, a leading technical diver training organization, have published a table that documents the time to onset of first symptoms. The table does not differentiate between types of DCS, or types of symptom.

Time to onset	Percentage of cases
within 1 hour	42%
within 3 hours	60%
within 8 hours	83%
within 24 hours	98%
within 48 hours	100%

Causes

DCS is caused by a reduction in ambient pressure that results in the formation of bubbles of inert gases within tissues of the body. It may happen when leaving a high-pressure environment, ascending from depth, or ascending to altitude. A closely related condition of bubble formation in body tissues due to isobaric counterdiffusion can occur with no change of pressure.

Ascent from depth

DCS is best known as a diving disorder that affects divers who have breathed gas that is at a higher pressure than the surface pressure, owing to the pressure of the surrounding water. The risk of DCS increases when diving for extended periods or at greater depth, without ascending gradually and making the decompression stops needed to slowly reduce the excess pressure of inert gases dissolved in the body. The specific risk factors are not well understood and some divers may be more susceptible than others under identical conditions. DCS has been confirmed in rare cases of breath-holding divers who have made a sequence of many deep dives with short surface intervals, and may be the cause of the disease called taravana by South Pacific island natives who for centuries have dived by breath-holding for food and pearls.
Two principal factors control the risk of a diver developing DCS:

the rate and duration of gas absorption under pressure – the deeper or longer the dive, the more gas is absorbed into body tissue in higher concentrations than normal ;
the rate and duration of outgassing on depressurization – the faster the ascent and the shorter the interval between dives the less time there is for absorbed gas to be offloaded safely through the lungs, causing these gases to come out of solution and form "micro bubbles" in the blood.

Even when the change in pressure causes no immediate symptoms, rapid pressure change can cause permanent bone injury called dysbaric osteonecrosis. DON can develop from a single exposure to rapid decompression.

Leaving a high-pressure environment

When workers leave a pressurized caisson or a mine that has been pressurized to keep water out, they will experience a significant reduction in ambient pressure. A similar pressure reduction occurs when astronauts exit a space vehicle to perform a space-walk or extra-vehicular activity, where the pressure in their spacesuit is lower than the pressure in the vehicle.
The original name for DCS was "caisson disease". This term was introduced in the 19th century, when caissons under pressure were used to keep water from flooding large engineering excavations below the water table, such as bridge supports and tunnels. Workers spending time in high ambient pressure conditions are at risk when they return to the lower pressure outside the caisson if the pressure is not reduced slowly. DCS was a major factor during the construction of Eads Bridge, when 15 workers died from what was then a mysterious illness, and later during the construction of the Brooklyn Bridge, where it incapacitated the project leader Washington Roebling. On the other side of the Manhattan island during construction of the Hudson River Tunnel, contractor's agent Ernest William Moir noted in 1889 that workers were dying due to decompression sickness; Moir pioneered the use of an airlock chamber for treatment.