Table 1.

Definitions and assessment of states of DoC

DoC	Behavioral features used to classify states of disorders of consciousness
Coma	Complete absence of arousal and awareness; No periods of wakefulness; eyes are closed [7]
UWS/VS	Clear periods of sleep and wakefulness; responses are reflexive [8]
MCS	Periods of sleep and wakefulness with evidence of inconsistent, sustained, reproducible purposeful or voluntary behavior [3]; MCS has been divided into MCS− and MCS+ [9] MCS- low-level behavior without language (visual fixation, visual tracking, localization of noxious stimulation, appropriate smiling or crying to emotional stimuli) MCS+ high-level behavior with language (e.g., command following; intelligible verbalizations; intentional communication) [3]
eMCS	Clear and consistent functional object use and/or functional communication (e.g., consistent, accurate yes/no responses)
Covert consciousness
A state of MCS+ or eMCS identified when volitional brain activities is detected by task-based fMRI or EEG in individuals who display behavioral features of coma, VS/UWS, or MCS-, and thereby do not show command following at the bedside [10]. Synonymous terms are covert cognition and cognitive motor dissociation (CMD) [1, 11, 12]. A subtle but key distinction among these terms centers around the possibility of covert awareness and falsely negative task-based test. The limitation of the task-based command designed to illicit activation of the putative supporting network(s) may not be among the remaining network supporting covert consciousness. Thus, CMD is defined by network activation to command. Whereas covert consciousness is broader term encompassing CMD and awareness that is not necessarily detected by an applied task-paradigm, such as in those with language deficits precluding command following but have intact alternate higher-level networks. Less specific is the term covert processing, which is defined by network processing of internal or external stimuli that may or may not be occurring in a state of awareness. Examples include night terrors or severe brain injury with only an isolated set of networks with processing capacity not linked to potential for awareness
LIS
eMCS defined by the process of recovery from brainstem injury with quadriplegia and aphonia with full awareness. In this process there is recovery of cognitive abilities, arousal regulation impairment, and eye movements (classically vertical and blinking) [13]
AM
Not a DoC, but highlighted here to demonstrate this entity’s deficits that bear some overlap with eMCS typically after cerebellar surgery in which there is command following but with significant response delays, functional use of objects but inconsistently, sustained visual pursuit, purposeful but reduced or absent spontaneous movement, present sleep–wake, and partial awareness of the self in this condition [13]

VS/UWS vegetative state/unresponsive wakefulness syndrome, MCS minimally conscious state, fMRI functional magnetic resonance imaging, EEG electroencephalogram, CMD cognitive motor dissociation, LIS Locked-in Syndrome, AM akinetic mutism