The pathophysiology of cerebral arterial gas embolism (CAGE) and decompression sickness (DCS) is described in Chapters 6 and 10, respectively. Although both CAGE and DCS are bubble-induced disorders and they share some commonality in pathophysiology and manifestations, the traditional approach to terminology (and the one largely used in this book) was to treat them as separate entities. Thus, symptoms presumed to be caused by formation of bubbles from supersaturated inert gas after diving were labelled DCS. Symptoms considered likely the result of the introduction of gas into the pulmonary veins after pulmonary barotrauma were most commonly labelled arterial gas embolism (AGE), or CAGE, given that cerebral involvement usually dominated the clinical picture. DCS was further subdivided into type 1 and type II categories. Type I was initially a designation for cases with musculoskeletal pain as the only symptom, and it is still sometimes referred to as ‘pain only DCS’. However the definition has subsequently been inconsistently modified and may variously include rash, lymphatic symptoms and some mild neurological symptoms. Type II indicated the presence of neurological symptoms, but the unintended consequence of this was to give equal weight to symptoms with disparate prognostic significance such as patchy paraesthesiae and gross motor dysfunction, hence the shifting of the former into the type I definition. The authors of this text consider the type I and type II classification to be ambiguous and largely without value and do not recommend its use.
The separation of CAGE and DCS as clinical diagnoses was predicated on the perceived contrast between the stereotypical ultra-short latency of focal cerebral symptoms in CAGE and the typically longer latency, broader symptom range and tendency to spinal rather than cerebral involvement in DCS. This diagnostic paradigm was challenged after publication (in 1989) of two studies associating patent foramen ovale (PFO) with neurological DCS. As discussed in Chapter 10, the only plausible explanation for an association between PFO and neurological DCS was that the PFO allowed venous gas emboli (VGE) to enter the arterial circulation and to thence be carried to vulnerable organs. Although diving physicians had been aware of the possibility for right-to-left shunting of VGE before this, these studies brought the issue into sharper focus and fuelled speculation on whether right-to-left shunting of VGE could, in at least some cases, be responsible for short-latency focal cerebral symptoms typically labelled ‘CAGE’ and presumed secondary to pulmonary barotrauma. Although (as mentioned in Chapter 10) there is some doubt about whether ‘arterialized’ VGE are large enough to produce the gross focal cerebral lesions that can occur when air enters the circulation after pulmonary barotrauma, these concerns resonated in the diving medicine community. In particular, uncertainty developed about using diagnostic labels such as DCS and CAGE that implied the underlying pathophysiology.
This matter came to a head in 1991 with the hosting of a consensus meeting to debate the matter in the United Kingdom1. The meeting generated a system for characterizing dysbaric disease in descriptive terms as an alternative to diagnostic labels implying a particular underlying pathophysiology. Thus, the clinical syndromes previously labelled as either DCS or CAGE were amalgamated under the ‘umbrella term’ ‘decompression illness’ (DCI), with a descriptive paradigm in which an evolutionary term and an organ system term would be applied to each symptom. A simplified interpretation appears in Table 11.1.
Thus, a diver presenting with worsening musculoskeletal pain after a dive is said to have ‘progressive musculoskeletal DCI’. A diver presenting with sudden loss of consciousness immediately after a rapid ascent and who is subsequently recovering is said to have ‘remitting neurological (cerebral) DCI’. Under the more traditional pathophysiology-based nomenclature, these divers would likely be diagnosed with ‘type I DCS’ and ‘CAGE’, respectively.
Although this system conveys useful information and avoids implying pathophysiological interpretations that may be wrong, there are problems with it. First, it is imprecise when the nature of the pathophysiological insult is known and inadequate when there is intent to imply a specific underlying mechanism. Clearly, the authors of this text encounter this situation in the pathophysiology chapters of this book where the DCS/CAGE terminology is used (while avoiding the terms type I and type II in relation to DCS). Second, the umbrella term ‘decompression illness’ is too similar to ‘decompression sickness’, and the two are often used interchangeably by commentators who do not appreciate the different meanings described here. Finally, the descriptive terminology has become a significant source of conflict in the field, with frequently polarized views on its utility. A compromise proposed in the last edition of another major diving medicine text published in 2003 was to retain the DCS/CAGE terms in discussion of pathophysiological mechanisms, but to use the descriptive terminology in clinical discussions. Albeit sensible, this policy has not been consistently applied. It is fair to say that the classification of decompression disorders remains ‘messy’ and inconsistent in the modern literature.