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Unexpected Decompression Illness

DAN Offers Its Flying-After-Diving Perspective On a Diving Dilemma

NOTE: This article describes what DAN Research refers to as "unexpected DCI," an occurrence of decompression illness in spite of the fact that none of the established rules were violated. Similar terms have been used in describing this type of DCI, e.g. "undeserved DCI" and "unpredictable DCI" (see "Mind & Body," July/August 1995). DAN Research has chosen the term "unexpected DCI" because it has neither statistical nor stigmatising implications.??

In your initial scuba diving course, you were instructed to follow the rules. For example, you learned to follow the no-decompression table limits; monitor your depth and pressure gauges; monitor your bottom time; and ascend slowly. Plus, you learned how important it is to keep yourself in shape and be prepared for anything strenuous that might happen during a dive.

By following the rules and by using common sense, it seems logical that you should be able to avoid the problems frequently associated with scuba diving such as the bends, a form of decompression illness (DCI). Unfortunately however, even when we follow all the rules for avoiding decompression illness there are no guarantees that it won’t occur. And what is even more perplexing is when you know you haven’t violated any of the established rules – and yet find that you’re bent. When signs and symptoms of DCI occur after a dive, series of dives or days of diving, and the diver has not violated any safety guidelines, it is sometimes referred to as "unexpected DCI." It is these unexpected cases that have stimulated research into methods of preventing them. Such cases of mild DCI – these unexpected hits – occasionally have occurred during DAN’s Flying After Diving (FAD) research project, during or after the "dive" section of the tests. This article describes safety measures for DAN’s FAD tests. Volunteer divers undergo medical screening to minimize the chances of serious injury. Mild DCI occasionally occurs during or after the flights. As with actual day-to-day dives, DCI can occur after chamber dives that are within the accepted exposure limits of many tables. All FAD dives are dry and at rest. The results for immersed, exercising dives might be different, due to such factors as exercise level and water temperature. These conditions will be tested in the future.


When divers express an interest to participate in Flying After Diving research, they begin a rather rigorous screening and control process, to ensure they are physically qualified; then they are carefully monitored every step of the way. The first control present in this process is the pre-screening. DAN’s Research team sends a detailed questionnaires to each potential subject. These questionnaires are returned to DAN, where they are screened. Any problems are brought to the attention of a Diving Medical Officer (DMO).

This permits obvious health problems to be eliminated early, and saves a potentially disappointing trip to the Duke Hyperbaric Center. During the pre-screening, the questionnaire is reviewed for health problems, including height / weight standards; any gas-trapping disorders, such as asthma; head trauma; recent injuries; or ear-clearing problems. Although a "yes" to one of these questions does not automatically disqualify a potential subject from participation; further clarification is required to determine the nature and extent of the illness or injury. Approximately 2 percent of the prospective volunteers are excluded. Most people who pass the pre-screen are able to participate as subjects, but they must be interviewed by a DMO and undergo a thorough neurological baseline exam before they’re allowed to participate. In this step, the DMO will once again refer to the medical history to establish a baseline for an individual. The DMO will analyse "skills" that could be affected by decompression illness, including strength and coordination testing. Additionally, the DMO performs "routine" tests such as listening to lungs and examining the ears. By establishing a baseline examination, the DMO can often identify very subtle signs which could later be attributed to DCI but which are actually already a part of that person’s make-up. An example of such subtle attributes might be small areas of numbness on a diver’s skin that may or may not have been previously noticed by the diver.


During actual "dive" and "altitude" exposures, the pressure within the chamber is painstakingly controlled. The rates of compression and decompression are maintained at a steady 30 feet / 9.1 meters per minute unless an individual has difficulty equalizing the ears. The rates can be slowed to accommodate any ear-clearing problems. Once at pressure, the depth control is a lot tighter than any diver can maintain – within an inch of the desired depth (No "dolphining" allowed here!). The result of this rigid dive profile is truly a "square" dive, rather than the "sawtooth" dive often experienced in open-water diving. At the end of each dive or flight, the FAD research team asks participants to assess themselves and report any changes to the DMO. If any symptoms or problems are reported, the research team response is quick and decisive, ensuring symptoms are treated immediately. "By following this practice," notes DAN Research Director Dr. Richard Vann, "we are certain that all participants are symptom-free prior to undertaking further pressure changes."

Four hours post-flight, all participants again see the DMO. This is yet another physical to verify that everyone remains at their baseline status established during the first physical. At this point, individuals are "released" from the study, and allowed to travel home if they live within two hours. Otherwise, they can leave the next morning. If they are not at their baseline status, a neurological exam is performed to determine why and what further action is necessary. However, the observation period does not end here. All participants are contacted by telephone both 24 and 48 hours post-flight. According to the 1997 edition of DAN’s Annual Report on Decompression Illness and Diving Fatalities, 95 percent of all DCI symptoms occur within 33 hours of the dive exposure. By following each individual so closely, we continue to collect data even on those obscure cases that occur outside normal time windows.

If a diver has no complaints after this 48-hour call, participants are considered to be symptom-free.


Why are we so careful? First, and foremost, it’s for protection of the divers. The Flying After Diving project is carefully designed to minimize the chance of serious injury and these controls are necessary to accomplish this. We must have cases of decompression sickness, however: if we didn’t, we wouldn’t be able to estimate the relationship of pre-flight surface interval to DCI risk. But if DCI does occur, the safety of the subjects must be paramount. The second reason for rigid controls is for uniformity of the data. Most of the symptoms we have encountered in this study have been minor, the kind that might be overlooked on a dive site. By monitoring our subjects so closely, we can recognize these minor symptoms as potentially DCI-related. Additionally, we know our dive profiles, and we’re performing the data analysis on a known set of conditions. This ensures reliability of the information we release to the diving community.


DAN began its FAD studies in 1993 with initial support from PADI. Dive profiles were selected using the DSAT Recreational Dive Planner. The first trials examined one dive to 60 feet / 18.2 meters. The study has continued with a two- and three-dive series, to 60 feet, 100 feet / 30.4 meters, 40 feet / 12.1 meters and currently a multidive profile. Overall, 580 dive profiles have been completed. Of this number, a total of 22 individuals have had signs and symptoms of DCI and were treated in the hyperbaric chamber. Five individuals suffered DCI after the dive and prior to the flight. These cases of unexpected DCI were within the table limits. They are summarized below.


Case 1 – A Two-Dive Profile to 60 feet for 55 minutes, with a one-hour surface interval, then 60 feet for 30 minutes. Approximately 15 minutes after the second dive, the subject complained of tingling in the fingertips. The tingling progressed through the hand and up the left arm to the shoulder. There was no pain or weakness associated with the tingling. Diagnosed with DCS II, he was recompressed to 60 feet. After treatment, he reported that all symptoms had resolved.

Case 2 – A Single-Dive Profile to 100 feet for 20 minutes. During decompression from this single dive, the subject experienced hip pain. Approximately 10 minutes post-dive, she experienced some relief, without treatment. Diagnosed with DCS I, she was recompressed to 60 feet and experienced 90 percent relief almost immediately, with complete relief after the treatment.

Case 3 – A Single-Dive Profile to 100 feet for 20 minutes. Approximately one hour after this dive, the subject noticed an aching sensation in her left hip and lower leg. It lasted for five to 10 minutes. One and a half hours later, she had a burning sensation in her left hip and ankle, with a general aching in her left leg. That evening, she was very restless and had trouble sleeping. Upon rising the next morning, however, she was symptomatic upon waking up. She did not report pain to us until it returned during the flight. Her pain disappeared with descent from altitude, but it returned again approximately five hours after the flight. She was diagnosed with DCS I and recompressed to 60 feet. Her pain was gone prior to the end of the treatment.

Case 4 – A Single-Dive Profile to 100 feet for 15 minutes. Ten minutes after surfacing from this dive, the participant complained of numbness in the left hand. He was diagnosed with DCS II and recompressed to 60 feet, with complete resolution of symptoms by end of the treatment.

Case 5 – A Single-Dive Profile to 100 feet for 15 minutes. While ascending, the subject noticed a transient sharp pain in the middle of her back. When she surfaced, she noticed bilateral neck pain and crackling sounds in ears when she turned her head. When she awoke the next morning, she noticed a return of the neck pain, as well as pain in her right calf. She was diagnosed with DCS I and recompressed to 60 feet. She experienced complete relief of all symptoms within the first 30 minutes of the treatment.


These divers were well rested, well-hydrated and physically fit; they stayed within the limits, made a controlled ascent in the chamber and had a non-stressful, non-demanding dive at rest. They followed all the rules, but they still experienced decompression illness. Why?

DCI is still a mystery in many ways: we do not yet understand why some individuals follow the same dive profile and get DCI one day but not the next. The cases involving individuals who do not follow the "rules" – those divers who overstay their limits, ascend too rapidly, have other health problems or party too hard the night before – are easier to understand. Unfortunately, many of the DCI cases DAN deals with are unexpected DCI – they’re not easy to explain because they occur in individuals who follow all the rules.

This is why divers should be familiar with the symptoms of DCS and know exactly what to do if they occur (calling the DAN Hotline is a good first step). The DAN Research program continues to move our knowledge of DCI forward, gathering more data with each set of trials and each new set of participants. With a little luck, and a lot more hard work on our part – and that of our participants – we’ll hopefully draw nearer our goal. In the near future we hope we will be able to offer more concrete knowledge about the risks of DCI.

What are DCS I and DCS II?

U.S. Navy recompression treatment protocols for decompression illness are based on the classification of DCI as either DCS I or DCS II.?? DCS I means that the diver has joint and/or muscle pain and that the examining physician has found no indication of any symptoms of DCS II prior to beginning treatment. DCS II symptoms include neurological symptoms, such as numbness, tingling, muscle weakness or bladder problems. Sometimes with DCS II, cardio respiratory problems can also occur. This can arise from intravscular bubbling and includes symptoms like chest pain and an irritating (to the throat and chest) cough. DCS II symptoms can range from mild to serious and life threatening.