Clinical disorders of consciousness have attracted extensive scientific and media attention. The vegetative state (VS) or, preferably, the unresponsive wakefulness syndrome (UWS), and the minimally conscious state (MCS) were originally described in 1972 (VS/UWS) and 2002 (MCS). However, functional neuroimaging and electrophysiological studies suggest that some degree of consciousness or awareness that has not been or could not be determined by behavioral evaluations alone may be present in some of these patients who, hence, have covert consciousness. This is known as cognitive-motor dissociation (CMD).
This raises multiple therapeutic and ethical questions:
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(1) Do patients with disorders of consciousness appreciate pain?
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(2) Should the usual duration of aggressive rehabilitation therapies be extended in patients in minimally conscious state and cognitive-motor dissociation?
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(3) Should end-of-life or right-to-life issues be adjusted in minimally conscious state and cognitive-motor dissociation patients?
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• Vegetative state/unresponsive wakefulness syndrome state (VS/UWS) requires the presence of a sleep-wake cycle; eyes are open during wakefulness, but there is no evidence of conscious behavior.
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• The minimally conscious state (MCS) requires intermittent evidence of behavioral awareness.
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• Emergence from the MCS requires that the patient shows functional interactive communication, or the ability to use objects appropriately, or both.
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• Functional neuroimaging and electrophysiological technologies may indicate consciousness through neural correlates, surrogates, or proxies in patients without behavioral evidence of consciousness owing to loss of appropriate motor function. This is known as cognitive-motor dissociation (CMD).
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• Prolonged recovery in patients with disorders of consciousness is not infrequent, in particular in minimally conscious state patients, and most patients may benefit from continued aggressive physical therapies.
Historical note and terminology
The advent of modern intensive care in the 1960s allowed for continuing and prolonged cardiorespiratory support of critically ill patients, one consequence of which was recognition of various clinical disorders of consciousness. These include brain death (1968), the vegetative state/unresponsive wakefulness syndrome (1972), and, later, the minimally conscious state (2002), in addition to the previously recognized state of unarousable-unresponsive-unconsciousness known as coma. Locked-in syndrome (1965), although not actually a disorder of consciousness, can be confused with these other disorders of consciousness. Previous names for the vegetative state/ unresponsive wakefulness syndrome include “apallic syndrome,” “neocortical death,” and “coma vigil.” Several authors have suggested that the term “vegetative” is pejorative, outdated, and has negative social and ethical connotations (93). Hence, authors have advocated changing the name of “vegetative state” to “unresponsive wakefulness syndrome” (103).
Patients with VS/UWS and MCS have severe brain damage who survive initial periods of coma (usually 1 to 3 weeks), can maintain brainstem functions, do not require mechanical respiratory support, and redevelop sleep-wake cycles, including prolonged periods of eye-opening, but have no (vegetative state) or intermittent but definite clinical evidence of consciousness (MCS). These disorders of consciousness are often temporary evolutionary syndromes that exist on a continuum, with some patients transitioning sequentially from VS/UWS to MCS and then, possibly, to higher states of consciousness.
The causes of disorders of consciousness include traumatic brain injuries, diffuse cerebral ischemia or hypoxia as can follow cardiac arrest, or cerebral infarction or hemorrhage. These disorders of consciousness are currently diagnosed on the basis of behavioral features, although imaging technologies such as PET and fMRI and varied electrophysiological studies have the potential to significantly enhance our understanding of disorders of consciousness.
Jennett and Plum described the "persistent vegetative state" (ie, VS/UWS) as a chronic condition following severe brain injury that resulted in the absence of cognitive function but with the persistence of sleep-wake cycles (86). Individuals could open their eyes to auditory stimuli (unlike in coma) and were autonomically stable with the maintenance of respiratory and hemodynamic functions.
Jennett and Plum chose the term “persistent” because of the unreliability of any clinical or laboratory criteria that would be prognostic (86). Empirically, “persistent” has been diagnosed if VS/UWS exists for more than 1 month and “permanent” after 3 months following nontraumatic (eg, anoxic) events, or after 12 months with traumatic head injuries. However, because studies have demonstrated that small but significant numbers (perhaps up to 20%) of such patients will recover consciousness beyond these time frames, new guidelines suggest that “permanent” be replaced by “chronic” (62; 71; 70).
The presence of sleep-wake cycles in VS/UWS and MCS suggests integrity of the reticular activating system, perhaps, more specifically, a small region of the left rostral dorsolateral pontine tegmentum near the medial parabrachial nucleus (64).
In recognition that some patients with severe alterations in consciousness can, nonetheless, demonstrate discernible and reproducible but intermittent behavioral evidence of consciousness, a committee, the Aspen Neurobehavioral Conference Workgroup, proposed diagnostic criteria for MCS (68).
Bruno and colleagues divided the minimally conscious state (MCS) into MCS- and MCS+. MCS- patients are able to demonstrate simple nonreflexive behavioral responses, such as visual tracking and localization of noxious stimuli; MCS+ patients additionally show more complex behaviors, such as following commands (eg, “Look up, look down; stick out your tongue.”), or and producing some appropriate verbalizations, or both (26; 27; Bruno et al 2013). It is not uncommon for patients with diffuse brain injury to progress from coma to VS/UWS and then to MCS (14). The minimally conscious state is often a transitional condition as patients who were previously comatose or vegetative state/unresponsive wakefulness syndrome improve, or in case of secondary brain injury worsen again (73). Progression from minimally conscious state to higher states of consciousness or better is evident when patients demonstrate functional interactive communication, or the ability to use 2 different objects appropriately, or both (68).
This taxonomy, therefore, characterizes disorders of consciousness in a hierarchal manner: coma is the most profound (unresponsive, no awareness or wakefulness); vegetative state/unresponsive wakefulness syndrome (VS/UWS) is somewhat less profound (unresponsive and unaware, but with wakefulness); minimally conscious state (MCS) is even less profound (wakefulness, some awareness and responsivity); and emergence from minimally conscious state (eMCS) has the highest levels of consciousness. This taxonomy assumes that levels of consciousness exist along a continuum (73; 16), similar to time and temperature, although some authors dispute this characterization of a scalable unidimensional linearity to consciousness (09; 10; 181). There are a number of possible revisions of this taxonomy (10), but none are in use as yet. However, this is likely to change soon as major international initiatives to reassess coma and disorders of consciousness are underway (130).