Nosologic considerations in disorders of consciousness
Historically, the nosology of DOC encompasses several clinical syndromes. First, coma was defined as a pathologic state of eyes‐closed unresponsiveness from which patients could not be aroused to wakefulness or awareness.4 In 1972, Jennett and Plum defined the vegetative state (VS) as a coma sequel in which unresponsive patients became wakeful with spontaneous eye opening, eye movements, and sleep–wake cycles, but to the fullest extent possible to test, their awareness of self and environment remained absent.5 In 2002, a committee defined the minimally conscious state (MCS) as a syndrome of profound unresponsiveness in which patients showed only limited or intermittent responses to stimuli.6 Subsequently, degrees of severity of MCS (MCS + and MCS−) were distinguished.7
Each DOC state was delineated as a syndrome into which patients were grouped who shared certain clinical features. Experts devised clinical diagnostic criteria for inclusion into or exclusion from each syndrome. Although syndromic classification is the usual initial step in nosology, once schemes based on pathophysiology become available, they supplant the syndromic approach. The most familiar example of this phenomenon is the many neurogenetic diseases that have been reclassified from a phenotypic to a genotypic nosology. This experience has shown the major limitation of syndromic nosology: that conditions differing pathologically and genetically can exhibit similar clinical signs and symptoms. The most scientifically sound basis for classification is by pathophysiology and genetics.8
Syndromic classification of DOC has several additional shortcomings. Patients who qualify for the diagnosis of VS or MCS may have reached the state by completely different mechanisms, such as by traumatic brain injury (TBI), stroke, or diffuse hypoxic–ischemic neuronal damage. Furthermore, patients classified within each syndrome often vary in severity. For example, some VS patients have isoelectric electroencephalograms (EEGs), although most do not.9 The breadth of severity within each syndrome has led to the need to subclassify each syndrome, for example, by dividing MCS into MCS + and MCS−. Syndromic classification also has confounded prognosis, because accurate prognosis ultimately is a function of the pathophysiology producing the state, not membership in a syndrome. Thus, in early studies of VS, it became clear that VS resulting from TBI had a more favorable prognosis than similar‐appearing VS resulting from diffuse neuronal hypoxia and ischemia.10
One limitation results from the choice of syndromic criteria. VS clinical criteria were delineated in the negative, that is, by stating those behaviors that VS patients could not perform. Irrespective of its accuracy, delineating clinical criteria in the negative has the unintended consequence of amplifying the frequency of false‐positive diagnoses. Some VS patients may not be able to perform certain behavioral tasks (thereby qualifying for the diagnosis of VS) for reasons not directly resulting from their brain damage, such as from the sedating effect of anticonvulsant drugs. The problem of frequent false‐positive diagnoses of VS remains frustratingly persistent.11
The most serious shortcoming of syndromic classification of DOC is that behavioral responses count as the sole evidence of awareness. However, unresponsiveness in the setting of diffuse or multifocal brain damage does not necessarily imply unawareness. The bedside assessment of awareness is purely inferential, because it is impossible for any person to directly assess the conscious experience of another.