The three main disorders of consciousness (DoC) are coma, the vegetative state (VS), and the minimally conscious state (MCS).

It is important to distinguish between arousal and awareness . Arousal refers only to the overall level of wakefulness. By itself, wakefulness is not sufficient for consciousness.

For someone to be conscious, awareness must be present. For clinical purposes, consciousness is defined as the state of awareness of oneself and/or the environment.

Differing degrees of arousal and awareness distinguish the three DoC from each other ( Table 56.1 ).

The clinical features of the three disorders of consciousness (coma, VS, MCS) are summarized in Box 56.1 .

Coma is a self-limited state that rarely lasts more than 4 weeks, after which patients will have either died or progressed into at least the VS. The end of coma is heralded by eye opening and the return of sleep-wake cycles.

Both VS and MCS can last indefinitely. However, the use of temporal adjectives such as persistent or permanent is now discouraged; instead, the term chronic has been recommended as an alternative. In addition, the length of time that a patient has remained in a particular state should be specified (e.g., “in a VS for 10 months”).

Because of the negative connotations of the term vegetative state, alternatives have been proposed. Of these alternatives, the most widely used is unresponsive wakefulness syndrome (UWS).

Emergence from MCS occurs when the patient demonstrates evidence of functional communication and/or functional object use. Functional communication is demonstrated when the patient can provide accurate yes/no responses to basic questions. Functional object use is demonstrated when the patient can demonstrate knowledge of the appropriate use of common objects.

There are an estimated 35,000 people in the United States in the VS and 280,000 in the MCS, but it is likely that these figures underestimate the true prevalence of DoC.

Consciousness is not localized to any single area of the brain but seems to be subserved by large-scale, integrated corticothalamic networks. Patients with higher levels of consciousness demonstrate more widespread activation and a greater degree of integration of these networks.

One proposed hypothetical model of the consciousness network is the “mesocircuit model.”

The general approach to determining a patient’s level of consciousness is to use behavioral (and, in the future, nonbehavioral) assessments to detect the presence of awareness of self and/or environment.

Behaviors that can indicate the presence of consciousness are summarized in Table 56.2 .

Because behaviors that indicate consciousness can be subtle and inconsistent, they can often be missed.

Numerous studies have documented widespread underestimation of consciousness in these patients. One study found that over 40% of patients diagnosed as being in a VS based on qualitative bedside evaluations were actually conscious when assessed with a standardized behavioral measure. In addition, 10% of patients diagnosed as MCS in this study had actually emerged.

Factors contributing to misdiagnosis of DoC include the lack of use of standardized assessment tools to supplement bedside examinations, performing a limited number of evaluations (which may miss subtle and inconsistent evidence of awareness), and lack of knowledge about DoC among clinicians. Other factors contributing to misdiagnosis include motor, sensory, or cognitive impairments that confound assessment and reversible factors that impair consciousness, such as sedating medications or concurrent medical problems.

Before initiating and/or interpreting the results of clinical assessments, the clinician should screen for conditions that can mimic or overlap with a DoC. These include locked-in syndrome, akinetic mutism, and catatonia. In each of these conditions, patients may appear to have a DoC when, in fact, their awareness is likely intact.

In addition, there should be an evaluation of deficits that can confound the assessment of consciousness ( Box 56.2 ). The presence of these deficits can result in an underestimation of the level (or even presence) of consciousness.

Finally, reversible causes of impaired consciousness should be identified and addressed before proceeding with and/or interpreting the results of behavioral assessments of consciousness ( Box 56.3 ).

The current gold standard for evaluation of DoC patients is behavioral assessment, which should include both qualitative assessments (e.g., bedside evaluations) and more structured assessments, such as standardized scales.

Because behavioral evidence of consciousness is subtle and inconsistent, the approach to assessment should involve:

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  Multiple evaluations over time

  
  
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  Different modes of assessment

  
  
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  Assessment by multiple examiners

  
  
• •
  

  Optimal environmental conditions (including optimal patient arousal)

  
  
• •
  

  Assessment at various times of day

For ambiguous or subtle behaviors observed during the bedside qualitative assessment, the likelihood that the behavior suggests consciousness is directly related to the complexity and frequency of the behavior, especially if the behaviors are appropriately related to environmental stimuli.

Qualitative bedside assessments should be supplemented by standardized rating scales. The Coma Recovery Scale-Revised (CRS-R) is currently the most widely used scale in the United States. It is a 23-item scale composed of six subscales assessing auditory, visual, motor, oromotor/verbal, communication, and arousal functions. The total score can range from 0 to 23.

Each subscale has a threshold score that implies consciousness. The patient only needs to score as “conscious” on one subscale to be considered conscious based on CRS-R.

The criteria for emergence from MCS—that is, functional object use and functional communication—are also operationalized by the CRS-R.

The use of the CRS-R may be supplemented by an individualized quantitative behavioral assessment (IQBA). The IQBA uses a single-subject experimental design to assist in answering specific questions about residual cognitive or behavioral capacities. It is especially useful if the behavioral responses are infrequent or ambiguous.

The assessment process should also include observations from family members. They are often able to provide observations about behaviors that occur when the clinical team is not present. Additionally, DoC patients have been found to react more frequently to the voice of a family member than to a treating clinician, and CRS-R scores are often higher when family members actively participate in the administration of the measure. Although there is a concern that family members may overinterpret the patient’s behaviors, there is evidence that families’ beliefs about the patient’s level of consciousness matched the diagnostic assessment of the clinical team 76% of the time.

Although the behavioral assessments described in the previous section currently form the foundation for the assessment of consciousness, new technologies are likely to play an increasing role in the evaluation of these patients. In particular, the use of both electrophysiological measures (e.g., event-related potentials, specialized electroencephalogram measures) and functional neuroimaging (e.g., functional magnetic resonance imaging [fMRI]) have been recommended in select circumstances.

The rationale behind the use of these modalities is the growing recognition that even the most comprehensive behavioral assessment may underestimate (or completely miss) the presence of consciousness in a subset of patients. Patients whose awareness can only be detected through electrophysiological measures or functional neuroimaging have been characterized as possessing covert consciousness.

One metaanalysis estimated that at least 15% of patients thought to be in a VS after behavioral assessments are in fact able to follow commands by modifying their brain activity.

The medical and neurological issues faced by patients with DoC overlap with those faced by other patients with significant traumatic brain injury but are often more severe in their manifestations. Patients with a DoC in rehabilitation settings have been found to have a high burden of medical complications and higher rates of transfers to the acute care setting. Common complications include infections, paroxysmal sympathetic hyperactivity, hydrocephalus, metabolic disturbances, and seizures. The rate of new complications seems to diminish as a function of time in an inpatient rehabilitation program and not as a function of time since injury, suggesting that the close monitoring and management in rehabilitation units can reduce the rate of these complications. ^,

Particular attention should be paid to medical and neurological conditions that can impair consciousness ( Box 56.3 ).

Neuromuscular issues such as weakness, spasticity, and contractures are also more severe in the DoC population. In the DoC patient, these issues have the added significance of limiting motor output, potentially confounding the assessment of the level of consciousness. Frequent use of nerve blocks, chemodenervation, intrathecal baclofen, and orthopedic tendon lengthening is warranted. Clinicians should also routinely screen patients for other neuromusculoskeletal conditions such as occult fractures, heterotopic ossification, peripheral nerve injuries, and the presence of critical illness polyneuropathy/myopathy.

It is especially important to monitor for and address pain in these patients. Patients with a DoC are at high risk for painful conditions from neuromusculoskeletal issues, skin breakdown, constipation, instrumentation, etc. Functional neuroimaging studies show that patients in MCS are capable of feeling pain. It is currently not thought that patients in a VS are capable of feeling pain, but the high rate of misdiagnosis of VS and the phenomenon of covert awareness would suggest that adequate analgesic control be the goal for all patients, regardless of the presumed level of consciousness.

Treatments that aim to enhance the level of consciousness include both medical and nonmedical interventions, especially pharmacological agents.

Likely to be helpful are standard rehabilitation interventions such as environmental enrichment (increased sensory stimulation and more frequent interpersonal interaction) and aggressive mobilization (including standing programs and body weight–supported gait activities).

Of the medications available, neurostimulants targeting catecholaminergic pathways are most often used. Of these, amantadine has the strongest evidence base supporting its use, primarily because of a clinical trial that found benefit in posttraumatic DoC. ^, Other agents in this category (e.g., methylphenidate, amphetamines, modafinil, and bromocriptine) are reasonable options, although evidence for their benefit in DoC is limited and inconsistent.

In addition to stimulants, GABA agonists have shown efficacy in a subset of patients with a DoC. In particular, zolpidem has been shown in one trial to improve the complexity and consistency of behavioral responses in approximately 5% of patients. Although the response rate seems to be low, it is reasonable to consider a trial of zolpidem for all patients with a DoC (unless contraindicated) given the relatively low risks associated with its use.

Other medications such as selective serotonin receptor inhibitors (SSRIs), lamotrigine, and donepezil may have a role, but there is not enough evidence to make specific recommendations.

In addition to medications, there is growing evidence that certain forms of electrical stimulation may be efficacious in this setting, although none of these modalities are currently in routine clinical use. These modalities include deep brain stimulation (DBS), transcranial direct current stimulation (tDCS), and repetitive transcranial magnetic stimulation (rTMS).

Most of the care described ideally is delivered in programs that specialize in the care of patients with DoC. National practice guidelines state that clinicians should refer patients with DoC to such programs. Sample goals of a DoC program are described in Box 56.4 .

• BOX 56.4

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