While many think of decompression sickness secondary to diving, it also occurs after exposure to reduced environmental pressure and can occur in aviation as well. High altitudes can expose flight crews and individuals to the risk of decompression sickness. The general pathophysiology is similar to decompression sickness and involves gas, such as nitrogen, forming bubbles in tissue, which is responsible for the symptoms ranging from relatively minor symptoms to profound neurologic manifestations.
Traditionally, decompression sickness is categorized as type 1, referring to skin changes and milder symptoms such as joint pain, or type 2, referring to more severe symptoms involving the neurologic, cardiac, and pulmonary systems. Some difficulties with that naming system have led to a better system that refers to the patient’s involved organ systems, such as musculoskeletal, cutaneous, neurologic, or cardiopulmonary subtypes.
Decompression sickness that occurs with high-altitude exposure such as in aviation can happen through various mechanisms. The first is by exposure to a high-altitude chamber such as is used by military organizations. The second is through exposure to altitude without proper aircraft or suit pressurization. Commercial aircraft typically have a pressurized cabin; however, failure of that system or noncommercial air travel can lead to decompression sickness. A third mechanism is due to the pressure difference as can occur with an individual who was to scuba dive and then fly a commercial flight in a short period. The sudden exposure to a cabin pressure of around 7000 feet may cause decompression sickness. It is important to note that decompression sickness still can occur in individuals who follow standard decompression procedures before flying.
It is difficult to determine accurate rates of decompression sickness secondary to aviation and altitude exposure, as there have been reports of potential cases that are not characterized as altitude-exposure decompression illnesses. One publication noted 63 cases of individuals who experienced decompression sickness from flying after a dive over a 4-year period. With that noted, in one study involving 2001 subjects, the rate of people experiencing any form of decompression illness following altitude exposure from an altitude chamber was 40%. Of individuals experiencing decompression sickness, the majority of symptoms were joint pain (72.8%) or paresthesias (12.6%). Only 3.4% of people who experienced decompression sickness secondary to altitude in that study experienced symptoms that were classified as neurologic or respiratory. Only 1.3% of individuals experienced recurrent symptoms. The studies revealed a low rate (3.4%) of individuals experiencing neurologic or respiratory symptoms though did not include paresthesias, which have been noted to be a neurologic symptom by others. When considering neurologic symptoms that include a headache and paresthesia, the rates may be closer to 14% to 34% of cases.
Individuals flying a military or private flight may be exposed to high-altitude situations predisposing them to a decompression illness. Approximately 30% of the time flying between 18,000 ft and 30,000 ft results in symptoms with a much higher percentage when flying at altitudes between 30,000 ft to 45,000 feet.
Aside from straightforward factors such as altitude or time from the last dive, other factors have been noted to affect the risk of decompression sickness. Gender has not been found to be a risk factor despite the suggestion that females have a higher risk of developing decompression sickness during menses, with risk decreasing associated with time since last menses. Other risk factors include having a higher body mass index and being less physically fit.
Decompression sickness occurs as a result of gas bubbles, referred to as evolved gas emboli, forming in tissues that can exert pressure on nerves, block blood vessels, and interact with proteins. This can result in a varying severity of symptoms that include pain and skin changes as well as neurologic and cardiopulmonary symptoms. The gas emboli form as a result of inert gas (nitrogen) that is dissolved in tissues at ground level, becomes supersaturated at altitude, and develops into bubbles. One interesting factor is that not all tissue dissolves gas at the same rate, with fat dissolving at least fivefold the nitrogen dissolved in blood.
Secondary effects also can occur, which may in part be responsible for some of those with delayed development of certain symptoms. Bubbles can cause endothelial damage, resulting in a capillary leak, platelet activation; and deposition is possible, as are other processes such as leukocyte-endothelial adhesion.
The history of a patient presenting with decompression sickness will often involve some aspect of recent diving, flight, non-commercial air travel, or altitude chamber use. The important thing to recognize is that not all individuals will understand the importance of these aspects of their history, so they may not volunteer it.
Specific symptoms patients may complain about depend on the location of the bubbles, with the most common being musculoskeletal symptoms presenting as joint pain that can be mild to severe. Patients may also complain of skin changes, headaches, paresthesias, and respiratory issues. A particular skin manifestation of decompression sickness is cutis marmorata, also called livedo reticularis, which is the result of skin tissue damage caused by gas bubbles forming during decompression. More profound presentations, while less common, can occur with altered mentation and severe neurologic or cardiopulmonary presentations possible. If neurologic signs or symptoms occur after exposure to altitude, consider decompression sickness.
Histories of rapid ascent in aviation or recent diving followed by altitude exposure are key components to a patient's history.
Important aspects of the physical exam include complete skin examination, complete neurological examination, careful lung and cardiac examination, and full evaluation of the joints.
Evaluation of a patient with suspected decompression illness relies primarily on history and physical examination since there are no specific tests to make the diagnosis. With that said, diagnostic testing to rule out other causes for a patient’s presentation may be necessary as decompression sickness can mimic many possible disease processes. For instance, in the hypoxic patient following a flight, the need to evaluate for other possible cardiac and pulmonary pathology may be warranted or an in a patient with a neurologic presentation, radiographic imaging of the brain may be indicated.
Treatment and management may vary depending on the grade/form of decompression sickness and the treating facility or organization.
Oxygen will wash inert gas from the lungs. A gradient is formed from tissues to lungs, allowing for the removal of the inert gas by both perfusion and diffusion.
The United States Air Force has published information on using ground-level 100% oxygen therapy for 2 hours following type-1 decompression sickness that occurs at altitude if it resolves upon descent. They used hyperbaric oxygen for more severe cases and for cases of type-1 decompression sickness that did not resolve. They have reported a high success rate with this treatment, with 94% of subjects not requiring further hyperbaric therapy. Further, the majority of individuals (95.6%) experiencing minor symptoms were manageable with ground-level 100% oxygen and had their symptoms resolved before starting any form of therapy, so consider this form of therapy in an individual with minor symptoms that have resolved before starting therapy. It is important to note that those results were from an altitude-chamber study since the high frequency of rapid onset symptoms at altitude is not always expected.
In more severe cases, hyperbaric recompression is accomplished in hyperbaric chambers. The recompression while breathing 100% oxygen will increase the tissue to lung gradient previously mentioned and decrease the bubble volume (Boyle’s Law), leading to the resolution of inert gas tissue bubbles. Standard recompression protocols are modeled after the United States Navy tables.
Decompression sickness in aviation most commonly is seen following flights in nonpressurized aircraft, in flights with cabin pressure fluctuations, or in individuals who fly after diving. Cases also have been reported after the use of altitude chambers. The manifestations are treated as SCUBA diving decompression sickness is treated, primarily with ground level or hyperbaric oxygen. These are relatively rare clinical events, and the clinician must consider this diagnosis in the proper historical setting. Practitioners should know where local hyperbaric chambers are located. The Divers' Alert Network (DAN) is an excellent source of information.