Reforming the taxonomy in disorders of consciousness

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In his essay “The Analytical Language of John Wilkins,” Jorge Luis Borges divides all animals into 14 categories, including “Those that belong to the emperor,” “Embalmed ones”; “Those that are trained,” “Stray dogs”, “Those that are included in this classification,” “Those drawn with a very fine camel hair brush,” “Those that have just broken the flower vase,” and “Those that, at a distance, resemble flies.” Borges's parody reminds us that taxonomy matters. A good taxonomy classifies objects on the basis of their underlying nature and in ways that are relevant to our interests and concerns.1 Taxonomic systems can be faulted insofar as they fail to meet these requirements. Here, we make the case for thinking that the current disorders of consciousness (DoC) taxonomy is seriously deficient and should be significantly reformed if not altogether replaced.
The foundations of the current DoC taxonomy were laid over four decades ago, when Jennett and Plum2 introduced the vegetative state (VS) as a distinct postcomatose state. Subsequent developments included the introduction of the minimally conscious state (MCS) and the emerged from minimally conscious state (EMCS).3 Recently, two subspecies of MCS states, MCS+ and MCS–, have been introduced (Bruno 2011).4 Despite the heterogeneity in cognitive, behavioral, and neural profiles that is exhibited by patients, the taxonomy of DoC demands that all patients must be accommodated by one of only four central categories, only one of which includes subcategories.
Although this taxonomic system is well established, a willingness to critically evaluate its taxonomy is an indicator of scientific maturity. Indeed, even well‐established fields are not immune from taxonomic upheaval. For 130 years, dinosaurs have been divided into two distinct clades, Ornithischia and Saurishia. A recent study5 has challenged that consensus, arguing that the morphological relations between dinosaurs is better accounted for by treating Ornithischia as a sister group of a new category, Theropoda, which are united in the new clade Ornithoscelida. This proposed taxonomic revision involves the introduction of new categories, the redefinition of old categories, and a reorganization of the relationships between categories. Moreover, the authors of this study point out that the proposed revisions would not be cosmetic, but would have an impact on debates about such issues as dinosaur diet, locomotion, and point of origin.
In a similar vein, we believe that the time is ripe for a critical evaluation of the taxonomy used in the DoC field. The current taxonomy was devised before the development of techniques, such as those that involve functional magnetic resonance imaging (fMRI) and electroencephalography (EEG), for identifying neural activity and responsiveness, and the data that these techniques have delivered are not easily accommodated within it. We believe that the interests of both patients and research would be best served by the development of a new taxonomy that does justice to patients’ underlying conscious capacities, irrespective of whether these capacities are behaviorally manifest. This article not only shows that such revision is necessary, but it also provides some guidelines as to how such a revised taxonomy might be developed.
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