Differentiating MCS from VS/UWS can be challenging since voluntary and reflexive behaviors can be difficult to distinguish and subtle signs of consciousness may be missed. The development of diagnostic criteria for MCS [11] would reasonably be expected to reduce the incidence of misdiagnosis relative to the rates reported before these criteria were established [15, 16]. However, recent studies found that around 40 % of patients believed to be in VS/UWS were still misdiagnosed [17, 18] (Fig. 3.1).

The high rate of misdiagnosis likely reflects different sources of variance. Variance in diagnostic accuracy may result from biases contributed by the examiner, the environment, and/or the patient. First, examiner errors may arise when the range of behaviors sampled is too narrow, response-time windows are over- or under-inclusive, criteria for judging purposeful responses are poorly defined, and examinations are conducted too infrequently to capture the full range of behavioral fluctuation. The use of standardized rating scales offers some protection from these errors, although failure to adhere to specific administration and scoring guidelines may jeopardize diagnostic accuracy. Second, the environment in which the patient is evaluated may bias assessment findings. Paralytic and sedative medications, restricted range of movement stemming from restraints and immobilization techniques, poor positioning, and excessive ambient noise/heat/light can decrease or distort voluntary behavioral responses. The last source of variance concerns the patient. Fluctuations in arousal level, fatigue, subclinical seizure activity, occult illness, pain, cortical sensory deficits (e.g., cortical blindness/deafness), motor impairment (e.g., generalized hypotonus, spasticity, or paralysis), or cognitive deficits (e.g., aphasia, apraxia, agnosia) constitute a bias to the behavioral assessment and therefore decrease the probability to observe signs of consciousness.

Some sources of error can be avoided, but this is not always possible or within the examiner’s control. It is, however, particularly crucial to optimize the way consciousness assessments are performed as clinical management, from treatment of pain to end-of-life decision-making, often depends on behavioral observations. For this reason, the use of standardized and sensitive behavioral scales can substantially help clinicians to detect subtle signs of consciousness.

Behavioral assessment is based on two main components: wakefulness and awareness. Wakefulness refers to the patient’s level of arousal and is assessed by observing eye opening. Awareness is related to subjective experiences and can be subdivided into awareness of the external world (i.e., perception of the environment or “consciousness”) and awareness of the internal world (i.e., stimulus-independent thoughts such as mental imagery and inner speech or “self-awareness”). Raters assessing patients with severe brain injury will mainly assess consciousness of the environment, since self-awareness is difficult to evaluate when only based on bedside observations and not on patients’ report. The assessment of consciousness can be done through repeated examinations revealing reproducible, oriented, or voluntary behavioral responses to various stimuli (the most common being auditory, verbal, and motor stimuli). The first scale widely used and known for assessing severely brain-injured patients recovering from coma is the Glasgow coma scale (GCS) [19]. This scale is short and can easily be incorporated into routine clinical care. Despite its widespread use, the GCS has been criticized for fluctuant inter-rater reliability and problems of scoring in patients with ocular trauma, tracheostomy, or ventilatory support [20]. The Full Outline of UnResponsiveness (FOUR) has been developed to replace the GCS for assessing severely brain-injured patients in intensive care [21]. The scale includes four subscales assessing motor and ocular responses, brainstem reflexes, and breathing. The total score ranges from 0 to 16. Unlike the GCS, the FOUR does not assess verbal functions to accommodate the high number of intubated patients in intensive care. It also assesses brainstem reflexes and breathing and, therefore, helps to better monitor comatose and VS/UWS patients. The FOUR also tracks emergence from VS/UWS since it includes the assessment of early signs of consciousness such as visual pursuit. The scale is globally more sensitive than the GCS for diagnosing MCS but like the GCS is not adapted to a rehabilitation setting.

Since the 1970s, a high number of scales have been validated for being used in subacute and chronic patients with severe brain injury (Table 3.2). Recently, the American Congress of Rehabilitation Medicine (ACRM) has conducted a systematic evidence-based review of the available scales to provide recommendations for use according to validity, reliability, outcome prediction, and diagnostic sensitivity [22]. Among the scales evaluated, the Wessex Head Injury Matrix (WHIM) has been recommended with moderate reservations. The WHIM was developed to capture changes in patients in VS/UWS through emergence from post-traumatic amnesia [23]. This tool is particularly sensitive to detect changes in patients in MCS not captured by other scales such as the GCS. The WHIM has been structured according to the sequence of recovery observed in 88 patients recovering from traumatic brain injury. The scale assesses arousal level and concentration, visual pursuit, communication, cognition (i.e., memory and spatiotemporal orientation), and social behaviors. The WHIM score represents the rank of the most complex behavior observed. Despite a good validity, its reliability is still unproven, and, even though superior to the GCS, its diagnostic sensitivity is lower than other standardized scales such as the Coma Recovery Scale-Revised (CRS-R) [24]. In fact, according to the ACRM, the CRS-R is the most reliable tool for differentiating disorders of consciousness and received the strongest recommendation with minor reservations [22]. This scale was developed in 1991 and revised in 2004. Its primary purpose is to differentiate VS/UWS from MCS and MCS from EMCS. It measures auditory, visual, motor, and verbal functions as well as communication and arousal. Each of these subscales is hierarchically structured; the lowest scores reflect reflexive behaviors, while the highest scores indicate cognitively mediated behaviors. This scale has clear definitions for both the administration and the scoring of each item. The CRS-R can be administered reliably by trained examiners and produces reasonably stable scores over repeated assessments. Validity analyses have shown that the CRS-R is capable of discriminating patients in MCS from those in VS/UWS better than the GCS, the FOUR, and the WHIM [24].