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Decompression sickness (DCS), the diver’s disease, the bends, or caisson disease is the name given to a variety of symptoms suffered by a person exposed to a reduction in the pressure surrounding their body. It is a type of diving hazard and dysbarism.
Decompression sickness can happen in these situations:
- A diver ascends from a dive.
- An unpressurized aircraft flies upwards.
- The cabin pressurization system of an aircraft fails.
- Divers flying in any aircraft shortly after diving. Pressurized aircraft are not risk-free, since the cabin pressure is not maintained at sea-level pressure.
- A worker comes out of a pressurized caisson, or out of a mine, which has been pressurized to keep water out.
- An astronaut exits a space vehicle to perform an extra-vehicular activity because the pressure in the space suit is lower than the pressure in the vehicle.
This causes inert gases (mainly nitrogen), normally dissolved in body fluids and tissues, to come out of physical solution and form gaseous bubbles.
According to Henry’s Law, when the pressure of a gas over a liquid is decreased, the amount of gas dissolved in that liquid will also decrease. One of the best practical demonstrations of this law is offered by opening a soft drink. When the cap is removed from the bottle, gas is heard escaping, and bubbles can be seen forming in the soda. This is carbon dioxide gas coming out of solution as a result of the pressure inside the container reducing to atmospheric pressure. Similarly, nitrogen is an inert gas normally stored throughout the human body, such as tissues and fluids, in physical solution. When the body is exposed to decreased pressures, such as when flying an un-pressurised aircraft to altitude, or during a scuba ascent through water, the nitrogen dissolved in the body comes out of solution. If nitrogen is forced to come out of solution too quickly, bubbles form in parts of the body, causing signs and symptoms ranging from itching and rashes, to joint pain, which is known as "the bends," to sensory system failure, paralysis and death.
Air embolism, caused by other processes, can have many of the same symptoms as DCS. The two conditions are grouped together under the name decompression illness or DCI.
The first documented cases of DCS were reported in 1841 by a mining engineer who observed the occurrence of pain and muscle cramps among coal miners exposed to air-pressurized mine shafts designed to keep water out. The submarine pioneer Julius H. Kroehl died of decompression sickness during experimental dives with the Sub Marine Explorer in 1867. Another early case resulting from diving activities while wearing an air-pumped helmet was reported in 1869.
- Magnitude of the pressure reduction: A large pressure reduction is more likely to cause DCS than a small one. For example, the ambient pressure halves by ascending during a dive from 10 metres / 33 feet (2 bar) to the surface (1 bar) or by flying from sea level (1 bar) to an altitude of 16,000 feet / 5,000 metres (0.5 bar) in an un-pressurised aircraft. Diving and then flying shortly afterwards increases the pressure reduction as does diving at high altitude.
- Repetitive Exposures: Repetitive dives or ascents to altitudes above 18,000 feet within a short period of time (a few hours) also increase the risk of developing altitude DCS.
- Rate of Ascent: The faster the rate of ascent to altitude, the greater the risk of developing altitude DCS. An individual exposed to a rapid decompression (high rate of ascent) above 18,000 feet has a greater risk of altitude DCS than being exposed to the same altitude but at a lower rate of ascent.
- Time at Altitude: The longer the duration of the flight to altitudes of 18,000 feet and above, the greater the risk of altitude DCS.
- Age: There are some reports indicating a higher risk of altitude DCS with increasing age.
- Previous Injury: There is some indication that recent joint or limb injuries may predispose individuals to developing "the bends."
- Ambient Temperature: There is some evidence suggesting that individual exposure to very cold ambient temperatures may increase the risk of altitude DCS.
- Body Type: Typically, a person who has a high body fat content is at greater risk of altitude DCS. Due to poor blood supply, nitrogen is stored in greater amounts in fat tissues. Although fat represents only 15% of a normal adult body, it stores over half of the total amount of nitrogen (about 1 litre) normally dissolved in the body.
- Exercise: When a person is physically active while flying at altitudes above 18,000 ft., before a dive or after a dive, there is greater risk of altitude DCS.
- Alcohol Consumption: The after-effects of alcohol consumption increase the susceptibility to DCS (as well as to the usual results of operating equipment or machinery while intoxicated).
- Patent foramen ovale: A hole between the atrial chambers of the heart in the fetus is normally closed by a flap with the first breaths at birth. In up to 20% of adults the flap does not seal, however, allowing blood through the hole with coughing or other activities which raise chest pressure. In diving, this can allow blood with microbubbles in the venous blood from the body to return directly to the body's arteries (including arteries to the brain, spinal cord and heart) rather than pass through the lungs, where the bubbles would otherwise be filtered out by the lung capillary system. In the arterial system, bubbles (arterial gas embolism) are far more dangerous, because they block circulation and cause infarction (tissue death, due to local loss of blood flow). In the brain, infarction results in stroke, in the spinal cord it may result in paralysis, and in the heart it results in myocardial infarction (heart attack).
Signs and symptoms
Bubbles can form anywhere in the body, but symptomatic sensation is most frequently observed in the shoulders, elbows, knees, and ankles.
This table gives symptoms for the different DCS types. "The bends" (joint pain) accounts for about 60 to 70% of all altitude DCS cases, with the shoulder being the most common site. These types are classifed medically as DCS I. Neurological symptoms are present in 10% to 15% of all DCS cases with headache and visual disturbances the most common. DCS cases with neurological symptoms are generally classified as DCS II. "The chokes" are rare and occur in less than 2% of all DCS cases. Skin manifestations are present in about 10 to 15% of all DCS cases.
Recompression is the only effective treatment for severe DCS, although rest and oxygen (increasing the percentage of oxygen in the air being breathed via a tight fitting oxygen mask) applied to lighter cases can be effective. Recompression is normally carried out in a recompression chamber. In diving, a high-risk alternative is in-water recompression.
Oxygen first aid treatment is useful for suspected DCS casualties or divers who have made fast ascents or missed decompression stops. Most fully closed-circuit rebreathers can deliver sustained high concentrations of oxygen-rich breathing gas and could be used as an alternative to pure open-circuit oxygen resuscitators.
Common pressure reductions that cause DCS
The main cause of DCS is a reduction in the pressure surrounding the body. Common ways in which the required reduction in pressure occur are:
- leaving a high atmospheric pressure environment
- ascent through water during a dive
- ascent to altitude while flying
Leaving a high pressure environment
The original name for DCS was caisson disease; this term was used in the 19th century, when large engineering excavations below the water table, such as with the piers of bridges and with tunnels, had to be done in caissons under pressure to keep water from flooding the excavations. Workers who spend time in high pressure atmospheric pressure conditions are at risk if they leave that environment and reduce the pressure surrounding them.
DCS was a major factor during construction of Eads Bridge, when 13 workers died from what was then a mysterious illness, and later during construction of the Brooklyn Bridge, where it incapacitated the project leader Washington Roebling.
Ascent during a dive
DCS is best known as an injury that affects scuba divers. The pressure of the surrounding water increases as the diver descends and reduces as the diver ascends. The risk of DCS increases by diving long or deep without slowly ascending and making the decompression stops needed to eliminate the inert gases normally, although the specific risk factors are not well understood. Some divers seem more susceptible than others under identical conditions.
There have been known cases of bends in snorkellers who have made many deep dives in succession. DCS may be the cause of the disease taravana which affects South Pacific island natives who for centuries have dived without equipment for food and pearls.
Two linked factors contribute to divers' DCS, although the complete relationship of causes is not fully understood:
- deep or long dives: inert gases in breathing gases, such as nitrogen and helium, are absorbed into the tissues of the body in higher concentrations than normal (Henry's Law) when breathed at high pressure.
- fast ascents: reducing the ambient pressure, as happens during the ascent, causes the absorbed gases to come back out of solution, and form "micro bubbles" in the blood. Those bubbles will safely leave the body through the lungs if the ascent is slow enough that the volume of bubbles does not rise too high.
The physiologist John Haldane studied this problem in the early 20th century, eventually devising the method of staged, gradual decompression, whereby the pressure on the diver is released slowly enough that the nitrogen comes gradually out of solution without leading to DCS. Bubbles form after every dive: slow ascent and decompression stops simply reduce the volume and number of the bubbles to a level at which there is no injury to the diver.
Severe cases of decompression sickness can lead to death. Large bubbles of gas impede the flow of oxygen-rich blood to the brain, central nervous system and other vital organs.
Even when the change in pressure causes no immediate symptoms, rapid pressure change can cause permanent bone injury called dysbaric osteonecrosis (DON) "bone cell death from bad pressure". DON can develop from a single exposure to rapid decompression. DON is diagnosed from lesions visible in X-ray images of the bones. Unfortunately, X-rays appear normal for at least 3 months after the permanent damage has occurred; it may take 4 years after the damage has occurred for its effects to become visible in the X-ray images. 
Decompression tables and dive computers have been developed that help the diver choose depth and duration of decompression stops for a particular dive profile at depth.
Avoiding decompression sickness is not an exact science. Accidents can occur after relatively shallow and short dives. To reduce the risks, divers should avoid long and deep dives and should ascend slowly. Also, dives requiring decompression stops and dives with less than a 16 hour interval since the previous dive increase the risk of DCS. There are many additional risk factors, such as age, obesity, fatigue, use of alcohol, dehydration and a patent foramen ovale. In addition, flying at high altitude less than 24 hours after a deep dive can be a precipitating factor for decompression illness.
Astronauts aboard the International Space Station preparing for Extra-vehicular activity "camp out" at low atmospheric pressure (approximately 10 psi = 700 mbar) spending 8 sleeping hours in the airlock chamber before their spacewalk. Their spacesuits can operate at 4.7 psi = 330 mbar for maximum flexibility.
Nitrogen is not the only breathing gas that causes DCS. Gas mixtures such as trimix and heliox include helium, which can also be implicated in decompression sickness.
Helium both enters and leaves the body faster than nitrogen and for long dives, of around 3 hours or more, the body almost reaches saturation of helium. For such dives, the decompression is shorter than for nitrogen based breathing gases such as air.
There is some debate as to the decompression effects of helium for shorter dives. Most divers do longer decompressions, whereas some groups like the WKPP have been pioneering the use of shorter decompression times by including deep stops.
Decompression time can be significantly shortened by breathing rich nitrox (or pure oxygen if in very shallow water), during the decompression phase of the dive. The reason is that the nitrogen comes out of solution at a rate proportional to difference between the ppN2 (partial pressure of nitrogen) in the diver's body and the ppN2 in the gas that he is breathing; but the likelihood of bubbles is proportionate to the difference between the ppN2 in the diver's body and the total surrounding air or water pressure.
Ascent to altitude
People flying in un-pressurised aircraft at high altitude, such as stowaways, passengers after explosive decompression of the cabin pressure vessel or pilots in an open cockpit, can suffer from decompression sickness. Divers who dive and then travel in aircraft are at risk even in pressurised aircraft because the cabin air pressure is less than the air pressure at sea level. The same applies to going onto very high land after diving.
Altitude DCS became a commonly observed problem associated with high-altitude balloon and aircraft flights in the 1930s. In present-day aviation, technology allows civilian aircraft (commercial and private) to fly higher and faster than ever before. Though modern aircraft are safer and more reliable, occupants are still subject to the stresses of high altitude flight and the unique problems that go with these lofty heights. A century and a half after the first DCS case was described, our understanding of DCS has improved, and a body of knowledge has accumulated; however, this problem is far from being solved. Altitude DCS is still a risk to the occupants of modern aircraft.
There is no specific altitude that can be considered an absolute altitude exposure threshold, below which it can be assured that no one will develop altitude DCS. However, there is very little evidence of altitude DCS occurring among healthy individuals at pressure altitudes below 18,000 feet who have not been scuba diving. Individual exposures to pressure altitudes between 18,000 feet and 25,000 feet have shown a low occurrence of altitude DCS. Most cases of altitude DCS occur among individuals exposed to pressure altitudes of 25,000 feet or higher. A US Air Force study of altitude DCS cases reported that only 13% occurred below 25,000 feet The higher the altitude of exposure, the greater the risk of developing altitude DCS. It is important to clarify that although exposures to incremental altitudes above 18,000 feet show an incremental risk of altitude DCS, they do not show a direct relationship with the severity of the various types of DCS (see Table 1).
Arterial gas embolism and DCS have very similar treatment because they are both the result of gas bubbles in the body. Their spectra of symptoms also overlap, although those from arterial gas embolism are more severe because they often cause infarction and tissue death as noted above. In a diving context, the two are joined under the general term of decompression illness. Another term, dysbarism, encompasses decompression sickness, arterial gas embolism, and barotrauma.
Ascent to altitude can happen without flying, in places such as the Ethiopia and Eritrea highland (8000 feet = about 1.5 miles above sea level) and the Peru and Bolivia altiplano and Tibet (2 to 3 miles above sea level).
Mild cases of "the bends" and skin bends (excluding mottled or marbled skin appearance) may disappear during descent from high altitude, but still require medical evaluation. If the signs and symptoms persist during descent or reappear at ground level, it is necessary to provide hyperbaric oxygen treatment immediately (100% oxygen delivered in a high-pressure chamber). Neurological DCS, "the chokes," and skin bends with mottled or marbled skin lesions (see Table 1) should always be treated with hyperbaric oxygenation. These conditions are very serious and potentially fatal if untreated.
Effects of breathing pure oxygen
One of the most significant breakthroughs in altitude DCS research was oxygen pre-breathing. Breathing pure oxygen before exposure to a low barometric pressure decreases the risk of developing altitude DCS. Oxygen pre-breathing promotes the elimination or washout of nitrogen from body tissues. Pre-breathing pure oxygen for 30 minutes before starting ascent to altitude reduces the risk of altitude DCS for short exposures (10-30 minutes only) to altitudes between 18,000 and 43,000 feet. However, oxygen pre-breathing has to be continued without interruption with in-flight, pure oxygen to provide effective protection against altitude DCS. Furthermore, it is very important to understand that breathing pure oxygen only during flight (ascent, en route, descent) does not decrease the risk of altitude DCS, and should not be used instead of oxygen pre-breathing.
Although pure oxygen pre-breathing is an effective method to protect against altitude DCS, it is logistically complicated and expensive for the protection of civil aviation flyers, either commercial or private. Therefore, it is only used now by military flight crews and astronauts for their protection during high altitude and space operations.
Scuba diving before flying
The rule about decompression sickness risk on ascending to lower surrounding pressure, does not stop at sea level (even though decompression tables stop at sea level), but continues when a diver soon after diving goes into air pressure much less than at sea level. Altitude DCS can occur during exposure to altitudes as low as 5,000 feet or less. This can happen:-
- In an airliner at high altitude the cabin pressure is often not at full sea level pressure, but like the air pressure at say 8000 feet altitude.
- At high altitudes on land: e.g. if you scuba dive in Eritrea, and then go onto the Asmara plateau (where Eritrea's main airport is), which is about 8000 feet or 1.5 miles or 2400 meters above sea level.
- Occasionally in cave diving, "Torricellian chambers" are found; they are full of water at less than amospheric pressure. They arise when the water level drops and there is no way for air to get into the chamber.
What to do if altitude DCS occurs
- Put on your oxygen mask immediately and switch the regulator to 100% oxygen.
- Begin an emergency descent and land as soon as possible. Even if the symptoms disappear during descent, you should still land and seek medical evaluation while continuing to breathe oxygen.
- If one of your symptoms is joint pain, keep the affected area still; do not try to work pain out by moving the joint around.
- Upon landing seek medical assistance from an aviation authority medical officer, aviation medical examiner (AME), military flight surgeon, or a hyperbaric medicine specialist. Be aware that a physician not specialized in aviation or hypobaric medicine may not be familiar with this type of medical problem. Therefore, be your own advocate.
- Definitive medical treatment may involve the use of a hyperbaric chamber operated by specially trained personnel.
- Delayed signs and symptoms of altitude DCS can occur after return to ground level whether or not they were present during flight.
Things to remember
- Altitude DCS is a risk every time you fly in an un-pressurized aircraft above 18,000 feet (or at lower altitude if you scuba dive prior to the flight).
- Be familiar with the signs and symptoms of altitude DCS (see Table 1). Monitor all aircraft occupants, including yourself, any time you fly an un-pressurized aircraft above 18,000 feet.
- Avoid unnecessary strenuous physical activity prior to flying an un-pressurized aircraft above 18,000 feet, and for 24 h after the flight.
- Even if you are flying a pressurized aircraft, altitude DCS can occur as a result of sudden loss of cabin pressure (in-flight rapid decompression).
- After exposure to an in-flight rapid decompression, do not fly for at least 24 h. In the meantime, stay vigilant for the possible onset of delayed symptoms or signs of altitude DCS. If you present delayed symptoms or signs of altitude DCS, seek medical attention at once.
- Keep in mind that breathing 100% oxygen during flight (ascent, en route, descent) without oxygen pre-breathing before take off does not prevent altitude DCS.
- Do not ignore any symptoms or signs that go away during the descent. This could confirm that you are suffering altitude DCS. You should be medically evaluated as soon as possible.
- If there is any indication that you may have experienced altitude DCS, do not fly again until you are cleared to do so by an aviation authority medical officer, an aviation medical examiner, a military flight surgeon, or a hyperbaric medicine specialist.
- Allow at least 24 hours to elapse between scuba diving and flying.
- Be prepared for a future emergency by finding where hyperbaric chambers are available in your area of operations. However, keep in mind that not all of the available hyperbaric treatment facilities have personnel qualified to handle altitude DCS emergencies. To obtain information on the locations of hyperbaric treatment facilities capable of handling altitude DCS emergencies, call the Diver's Alert Network at (USA phone number) (919) 684-8111.
- Undersea and Hyperbaric Medical Society Scientific publications about Decompression Sickness
- Decompression Sickness: Prevention, Risks, Exercise, PFO, References, Links
- What causes the bends?
- Whales Suffer From Bends
- UK Sport Diving Medical Committee: Bone Necrosis
- Divers Alert Network: diving medicine articles
- Caissons disease. Photos of tunnel project with decompresson chamber