DCS is precipitated by a decrease in environmental pressure. Although seemingly obvious, this means that any symptoms arising during descent or during the period at depth (before ascent) will not be caused by DCS (unless the symptoms have been ‘carried over’ from a previous decompression). Most cases arise after arrival of the diver at the surface, although there may rarely be onset of symptoms during the ascent; particularly on a dive with long decompression stops. This should be considered an ominous sign that more serious symptoms are likely after surfacing. If cases across the entire range of severity are considered, more than 50 per cent of patients develop symptoms within 1 hour of surfacing, and 90 per cent have symptoms within 6 hours. If severe manifestations such as motor weakness are considered alone, most will begin to manifest within the first hour.
Notwithstanding these estimates, there is frequently a significant latency between symptom onset and the victim’s reporting the problem, particularly when mild symptoms are involved. Sometimes there are obvious reasons why this may be so, such as an asymptomatic diver retiring for the night several hours after the last dive and waking with symptoms in the morning. However, ‘denial’ and seeking alternative explanations for symptoms on the part of the diver are often contributory. This stems in part from a long-standing stigma about the diagnosis that has its roots in the entrenched notion that if someone has DCS then he or she must have done something incompetent or wrong. The way in which this belief can affect the behaviour of divers needs to be understood by any physician assessing a possible DCS case. Thus, not only do divers have a tendency to denial of symptoms and consequent late presentation, they may also then misrepresent timing of onset to avoid criticism for late reporting. This needs to be borne in mind when applying the typical symptom latencies described earlier to a diagnostic paradigm. Another reason for symptom denial among divers is the understanding that early reporting of symptoms may result in termination of a dive trip and/or a logistically difficult (and potentially expensive) evacuation for recompression treatment. Given such motivation to downplay problems, it is hardly surprising that physicians who eventually see evacuated divers at receiving units frequently find more serious symptoms than were reported in initial discussions by telephone.
Divers learn that for any depth, a period of ‘bottom time’ can be spent there that, when not exceeded and when followed by direct ascent to the surface at the correct rate, is associated with an acceptably small level of tissue nitrogen supersaturation (see Chapter 10) and a correspondingly small risk of DCS. These acceptable bottom times are often referred to as ‘no decompression limits’. More advanced exponents (see Chapter 62) learn how to stage their ascent appropriately using ‘decompression stops’ to maintain a small level of risk if the no decompression limit is exceeded. Some divers inappropriately come to see adherence (or not) to no decompression limits or to decompression stop prescriptions as the threshold for a binary outcome. Thus, they may believe ‘exceed the limits and you will get DCS’ and ‘stay within the limits and you can’t get DCS’. Such beliefs are clearly false, but they can contribute to strange conduct such as symptom denial if a dive was within the limits or, at the other extreme, high anxiety and illness behaviour because a no decompression limit was slightly exceeded.
An appreciation of no decompression limits and how they are calculated or monitored by divers is important for the diving physician evaluating a potential DCS case because, notwithstanding the previous comments, it remains true that a dive is likely to carry a higher risk if there are events such as exceeding a no decompression limit, omitting recommended decompression stops or ascending too rapidly. Such events are often characterized as representing a ‘provocation’ for DCS. Similarly, but at the opposite end of the risk spectrum, a dive well inside the no decompression limit with a perfect controlled ascent would be considered ‘unprovocative’, and this could influence the way a physician interprets mild non-specific symptoms arising after the dive. Dive planning algorithms are discussed further in Chapter 12.