Disorders of consciousness

Steven H Horowitz MD (

Dr. Horowitz of Tufts University School of Medicine has no relevant financial relationships to disclose.

)
James G Greene MD PhD, editor. (Dr. Greene of Emory University School of Medicine has no relevant financial relationships to disclose.)
Originally released October 4, 1993; last updated March 4, 2018; expires March 4, 2021

This article includes discussion of disorders of consciousness, persistent vegetative state, unresponsive wakefulness syndrome, and minimally conscious state. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.

Overview

Clinical disorders of consciousness have attracted extensive scientific and media attention. The persistent vegetative state (also known as unresponsive wakefulness syndrome) and the minimally conscious state are being reconsidered and redefined since their original descriptions in 1972 (persistent vegetative state) and 2002 (minimally conscious state). The results of 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. This raises multiple therapeutic and ethical questions, such as:

 

(1) Do the findings of these research evaluations truly represent consciousness, and if they are present on fMRI, PET, or EEG in a patient who cannot otherwise demonstrate conscious behavior, are they adequate and appropriate neural correlates?

 

(2) Do these patients appreciate pain?

 

(3) Should the usual duration of aggressive rehabilitation therapies be extended given the multiple reports of continued improvement of patients in persistent vegetative state well past the standard estimates of permanency of 3 months for nontraumatic and 12 months for traumatic etiologies?

 

(4) Should end-of-life or right-to-life issues be reconsidered given the aforementioned concerns?

Key points

 

• Persistent vegetative state or unresponsive wakefulness syndrome requires the presence of sleep-wake cycles but no evidence of conscious behavior.

 

• The minimally conscious state requires intermittent evidence of behavioral awareness.

 

• Emergence from the minimally conscious state requires functional interactive communication and the ability to use 2 different objects appropriately.

 

• Modern neuroimaging and electrophysiological technologies may indicate nonbehavioral consciousness through neural correlates, surrogates, or proxies in patients without behavioral evidence of consciousness.

 

• Prolonged recovery from both states is not infrequent, and 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 persistent vegetative state (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 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. Additionally, because of frequent misdiagnosis and confusion between patients who are vegetative and those who are minimally conscious, authors have advocated changing the name of persistent vegetative state to “unresponsive wakefulness syndrome” (Laureys et al 2010; Gosseries et al 2011).

Patients with persistent vegetative state (aka, unresponsive wakefulness syndrome) and minimally conscious state are those with severe brain damage who survive initial periods of coma (usually 2 to 4 weeks), can maintain brainstem functions, do not require mechanical respiratory support, and redevelop sleep-wake cycles but have no (persistent vegetative state) or only intermittent but definite clinical evidence of consciousness (minimally conscious state). Whether these disorders of consciousness are temporary evolutionary syndromes that exist on a continuum, with some patients transitioning sequentially from persistent vegetative state to minimally conscious state and then, possibly, to higher states of consciousness, is under debate (Giacino et al 2014; Fins 2015).

The chief causes of these disorders of consciousness are traumatic brain injuries, diffuse cerebral ischemia or hypoxia as can follow cardiac arrest, or cerebral infarction or hemorrhage (Bernat 2009; Bernat 2010). 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, all of which are presently research modalities, have the potential to significantly enhance our understanding of disorders of consciousness and could provide diagnostic criteria in the future.

Jennett and Plum described and named the persistent vegetative state in 1972 (Jennett and Plum 1972). They proposed the term persistent vegetative state for several reasons: “It describes behaviour, and it is only data about behaviour, which will always be available, and in every patient, because such observations are independent of special procedures such as EEG [electroencephalography] and measurements of cerebral blood-flow or cerebral metabolism. This term presumes neither a particular physio-anatomical abnormality nor a specific pathological lesion…. [It] invites further clinical and pathological investigation…. The word vegetative itself is not obscure: vegetate is defined in the Oxford English Dictionary as “to live a merely physical life, devoid of intellectual activity or social intercourse” (1740), and vegetative is used to describe “an organic body capable of growth and development but devoid of sensation and thought” (1764). Modern definitions of these terms are identical.

Jennett and Plum described the "vegetative state" as a chronic condition following severe brain injury that resulted in the absence of cognitive function but with the persistence of sleep-wake cycles (Jennett and Plum 1972). Individuals could open their eyes to auditory stimuli (unlike in coma) and were autonomically stable with the maintenance of respiratory and hemodynamic functions. “What is common to all patients in this vegetative, mindless state is that, as best can be judged behaviourally, the cerebral cortex is not functioning, whether the lesion be in the cerebral cortex itself, in subcortical structures, the brainstem, or in all these sites.” Presently, from the neurologic perspective, consciousness consists of 2 dimensions: wakefulness, indicative of arousal and level of consciousness, and awareness, indicative of responsivity to outside stimuli from the environment and first-person subjective experiences (Evers 2016). Fingelkurts and colleagues take issue with this binary characterization of consciousness, preferring to assign wakefulness to a vigilance domain, which, in turn, limits the amount of information available for conscious access (Fingelkurts 2014).

Jennett and Plum chose the term “persistent” because of the unreliability of any clinical or laboratory criteria that would be prognostic (Jennett and Plum 1972). However, empirically, it appears that “persistent” can be diagnosed if the vegetative state exists for more than 1 month and “permanent” after 3 months following nontraumatic (eg, anoxic) events, or after 12 months with traumatic head injuries. “Persistent vegetative state is a diagnosis; permanent vegetative state is a prognosis” (The Multi-Society Task Force on PVS 1994).

Vegetative state differs from brain death in that brain death involves death of the brainstem and cortex whereas the vegetative state results from loss of function of the cortex with a functioning brainstem (Young et al 1989; Jennett 2003). The presence of sleep-wake cycles in persistent vegetative state and minimally conscious state 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 (Fischer et al 2016).

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 minimally conscious state (Giacino et al 2002). Minimally conscious state involves functional recovery of verbal or nonverbal communication (Noirhomme et al 2008).

More recently, 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 can evince more complex behaviors, such as following commands and producing some appropriate verbalizations (Bruno et al 2011; Bruno et al 2012; Bruno et al 2013). It is not uncommon for patients with diffuse brain injury to progress from coma to persistent vegetative state and then to minimally conscious state (Bernat 2010a). The minimally conscious state is often a transitional condition as patients who were previously comatose or vegetative improve, or as others decline secondary to a neurodegenerative disease (eg, Alzheimer disease) (Giacino et al 2014). Fins does not view disorders of consciousness as reified static diagnoses but, rather, as temporary evolutionary syndromes (Fins 2015). Progression from minimally conscious state to higher states of consciousness would be evident when activities that demonstrate functional interactive communication and the ability to use 2 different objects appropriately are present (Giacino et al 2002).

This taxonomy, therefore, characterizes disorders of consciousness in a hierarchal manner: coma is the most profound (unresponsive, no awareness or wakefulness); persistent vegetative state is somewhat less profound (unresponsive and unaware, but with wakefulness); minimally conscious state is even less profound (wakefulness, some awareness and responsivity); and emergence from minimally conscious state has the highest levels of consciousness. This taxonomy assumes that levels of consciousness exist along a continuum (Giacino et al 2014; Bernat 2017), similar to time and temperature. Several authors dispute this characterization of a scalable unidimensional linearity to consciousness, arguing that consciousness is composed of multiple factors, such as arousal, speech, attention, volition, executive control, etc., that may vary independently or may modulate each other, thereby rendering the current taxonomy and the transitions from one state to another problematic in assessing patient functioning (Bayne et al 2016; Bayne et al 2017; Xie et al 2017). Ropper raises the possibility that “ensembles of modular brain functions will add up to consciousness of different degrees and varieties, and the line between consciousness and unconsciousness will be blurred” (Ropper 2010). Fingelkurts and colleagues view consciousness as “not merely a quantitative matter of a degree but in fact a qualitative matter of absence or presence of a particular state … a categorical (all-or-none) phenomenon rather than a continuous (gradual) one,” with a threshold level at the border between persistent vegetative state and minimally conscious state (Fingelkurts et al 2014). There are a number of possible revisions of this taxonomy (Bayne et al 2017), but none are in use as yet, and none are entirely satisfactory given current knowledge of consciousness.

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