Decompression illness is caused by bubbles that appear in either the bloodstream or in other tissues in the body by ascending too fast from either underwater depth to the surface or from the surface to an altitude such as in high altitude fighter pilots. When ambient pressure increases, such as during diving, so will the pressure of any gas that the person is breathing. This gas will get distributed throughout the body at different speeds, where highly vascularized tissues equilibrate at a faster rate. Bubbles are formed when the ambient pressure is reduced faster than the ability of the blood or tissue to transport the gas which is in solution (usually nitrogen) to the lungs to get exhaled. These bubbles can have various and complex effects on the body, from direct obstruction of blood flow, obstruction of lymph flow, to nerve injury either by direct irritation or through inflammatory mediators. These effects can be mild, resulting in pain or paresthesias, or severe, leading to paralysis and death. One of the most widely used classifications of decompression illness is Decompression Sickness Type 1 (DCS1) with mild pain only or (mild) skin symptoms, Decompression Sickness Type 2 (DCS2) with neurological involvement and Arterial Gas Embolism (AGE), resulting from pulmonary barotrauma.  The primary treatment of decompression illness is recompression in a hyperbaric chamber, which will increase the ambient pressure again, reducing the speed of new bubble formation and reducing existing bubble size as well as creating a large diffusion gradient for the offending gas by breathing 100% oxygen. The appropriate treatment of decompression illness is usually very effective, especially when started early after the injury, leaving either minimal or no residual effects. Most residual effects will improve even further with time as the body is trying to heal, however, some residual effects may be permanent. In addition, a diver may also suffer from Barotrauma to air-filled spaces such as ears, sinuses and the lung during a dive.
After an episode of decompression illness or other diving-related injuries, many divers want to go diving again either for recreation, work or military service. A thorough understanding of the factors that led to the injury and careful clinical exam will help the diving medicine specialist to decide if additional tests are indicated and to advise the diver of his future risk with a return to diving. Guidelines do exist for military, government, scientific and commercial divers. However, no guidelines have been published in the United States for recreational divers, including dive masters and dive instructors.
Divers who have suffered from decompression illness, especially those without any physiologic explanation are at higher risk of experiencing decompression illness again and possibly aggravating any pre-existing residual injury. The odds ratio of experiencing neurologic DCS has been reported to be as high as 8.4 in divers who have suffered previously from DCS, depending on experience, large right to left shunt and not modifying their diving practice.  There are 2 general concerns for a diver who may be at higher risk of developing decompression illness. One is the risk of re-injury, but the other is the risk incurred by the fellow divers, coming to the rescue of an injured dive buddy in an unphysiological environment for the human body.
In general, returning to diving after an episode of decompression illness should only occur after a certain time has elapsed, which depends on the severity of symptoms and on any residual injury. Recommendations as to the time delay vary a bit according to the different diving organizations and it should be remembered that those are recommendations that are specific to their diving populations and may contain special rules and conditions such as supervised dives, full face mask, recompression chamber on site, specified evacuation plan, among others. Applying those recommendations to the recreational scuba diver may not be possible nor in the best interest of the patient and an individual approach tailored to their health and anticipated diving, conditions should be used.
There are only a few tests that are indicated in determining either etiology or risk of future damage to the central nervous system. An echocardiogram can be used to determine the presence or absence of a patent foramen ovale (PFO). This is indicated after episodes (usually more than one) of decompression illness after non-provocative dive profiles or severe neurological or severe skin symptoms. Divers with PFOs have been reported to have a higher incidence of DCI,  however PFOs are very prevalent in the normal population of close to 30%, so there are many divers without decompression illness who will have PFOs. If a PFO is discovered after episodes of decompression illness, the reported (low) risk of any closure attempts in divers  should be weight carefully against the risk of decompression illness with risk modification such as shallow dives or diving with higher oxygen-enriched gas mix.  After the closure of a PFO, the diver should undergo a repeat echocardiogram with a Valsalva maneuver to demonstrate closure of the PFO.
A magnetic resonance imaging (MRI) is useful to assess damage to the central nervous system in severe neurologic decompression illness. It is usually not helpful in the acute event, other than to possibly rule out other reasons for the diver’s symptoms when the symptoms either do not fit or do not respond to treatment as expected. It may discover unanticipated significant lesions in the brain or spinal cord, providing information about future fitness to dive. A subject of current medical debate is the appearance of hyperintense white matter lesions on MRI in divers with cerebral decompression illness. While there is not sufficient scientific evidence as to the etiology or prognosis of those lesions in divers, caution with a return to diving may be recommended by some organizations as they represent abnormal MRI findings.
Return to diving after decompression illness of the inner ear, deserves special consideration and specialized testing, as divers can become asymptomatic after some time, which may not be related to healing of the injured inner ear, but rather due to compensation of the other inner ear.  A subsequent insult to the uninjured ear may lead to severe disability of the diver. A consultation with otolaryngology who is knowledgeable in diving medicine, who can provide cochlear and vestibular testing is needed to understand the potential risk of returning to diving
For an organization to permit a diver to go back to diving, the implications and liabilities are different than for an individual who takes responsibility for his own actions (and can mitigate the risk of the dive) ideally after a thorough discussion with a physician specialized in diving medicine. 
The United States Navy (USN) will permit a diver who had mild DCS1 symptoms, which resolved on initial treatment to return to diving after 7 days, whereas after a DCS2 episode where symptoms resolved on initial treatment a 30-day delay is mandated. If there are persistent neurologic symptoms beyond the initial treatment the diver is disqualified from further diving unless a medical waiver is granted after a thorough medical evaluation including MRI and depending on the finding and a repeat MRI after one month and a consultation with a neurologist. Again these are recommendations for their otherwise young and healthy fit divers performing Navy dives.
The Association of Diving Contractors International (ADCI) has also published return to diving guidelines after decompression illness, where they do not recommend diving with residual symptoms or in divers with an abnormal MRI of the brain after an episode of DCS2. However, with symptoms resolved, they do permit a return to diving depending on the severity of the initial symptoms and the speed of resolution after single or serial treatments, ranging from 72 hours for pain-only resolved after one treatment to 7 days after serial treatments. After neurological sensory symptoms resolved after one treatment, divers should delay for seven days whereas any motor symptoms resolved with one treatment the time delay extends to 4 weeks. DCS2 symptoms requiring serial treatments to resolve should delay 4 to 6 months and then be reevaluated. Any vestibular symptoms should also wait 4 to 6 months after treatment, after which a thorough examination including caloric and rotational/tilt table evaluation by an otolaryngologist has to confirm an intact vestibular system, as the patient can compensate with one side after the loss of function of the other. Any subsequent injury to the functioning side may leave the diver without balance for life.
In Europe, the Diving Medical Advisory Committee (DMAC) has issued its guidance for returning to diving after decompression illness, which is similar to the USN and ADCI. After limb pain or non-specific symptoms with uncomplicated recovery, divers should delay diving for seven days and after a relapse that requires further treatment for 14 days. They differ in their recommendations regarding skin or lymphatic manifestations without neurological signs, which others may treat as milder DCS1, and also recommend a four-week lapse of diving activities. They further recommend for more severe neurologic manifestations or pulmonary barotrauma a delay of 3 months. Any diving deferral of greater than two weeks should be reviewed by a diving medicine specialist before the commercial diver can return to diving. Divers who suffered pulmonary barotrauma or arterial gas embolism should undergo a high-resolution computed tomography scan (HRCT) to assess the structural lung integrity. According to DMAC, any diver with residual symptoms “should be considered unfit for occupational diving.”
As previously mentioned, companies and organizations can make rules and regulations for their divers and also enforce them, protecting the divers and limiting the organization’s liabilities. However, for recreational divers, who in most jurisdictions are not subject to the same regulations, buy-in from the diver is needed to optimize his/her safety and those who dive with them. The above-mentioned waiting periods for uncomplicated decompression illness episodes with complete resolution of symptoms are reasonable to most recreational divers. The issue of returning to diving with residual symptoms, however, comes up frequently and is more controversial. Ideally, a diving medicine specialist should guide these decisions because they are familiar not only with various forms of decompression illness but also with the various stresses on the body during diving and possible mitigation strategies, such as shallow dives, using oxygen-enriched air, prolonging safety stops and adding extra conservative factors into their dive computers. Consultations from other specialties, some limited tests and medical imaging, such as MRI, HRCT, echocardiography and vestibular testing can help in advising the diver as to the future risk.
Another population to consider are the numerous dive masters and dive instructors who are compensated for their diving and taking students underwater who are dependent on them. While they really should be considered occupational divers, many of them do not have the occupational diver insurance and safety net, covering health and disability benefits and do see the need to continue to make a living. They are often independent contractors or working for small businesses. Since they are often not employed by large corporations, they are not protected by the conservative strategies used in commercial diving that protect the companies from liability while protecting the divers from injury. They may not be able to modify their risk profile, other than modifying their breathing gas, to the same extent than a recreational scuba diver can. In those instances the diver-doctor relationship, with an understanding that the goal is to get them back to what they love to do, if at all possible, goes a long way in maximizing their safety and that of their potential students.
Apart from decompression illness, there are other diving-related injuries that usually relate to the pressure changes that occur during a dive, such as problems with ears, sinuses, and barotrauma to the lung. Patients with previous lung trauma should refrain from diving for at least 6 months and then have a normal lung CT.
Existing literature does provide some guidance for other pressure related issues and injuries to ears  and sinuses.
Most of the recommendations to return to diving after decompression illness are based on expert opinion and knowledge of injury and healing of various tissues, rather than on randomized controlled trials or rigorous clinical research. In large surveys undertaken by diving research organizations, such as the Divers Alert Network, Diving Diseases Research Centre, the various navies, commercial organizations, and others, risks are observed and/or modeled, but quantifying the individual risk of a particular dive, on a particular day in a particular diver, who may have done the same dives many times before, still eludes us today. 
The diving medicine specialist should be the coordinating physician and lead the discussions on the individual risk as well as the risk for the organization of returning to dive after a diving-related injury. However, he/she often needs input from various other specialties such as cardiologists, neurologists, radiologists, and otolaryngologists. In addition, on an organizational level, input from the dive supervisor on what will be expected on working dives will be helpful in determining the future of an occupational diver. Unfortunately, for the dive masters and dive instructors in the recreational dive industry, there is no dive supervisor who can predict the dive conditions, dive location and workload, which will make recommendations to the divers who want to return to diving after an incident more conservative.
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