Disorders of Consciousness

Brian D. Greenwald and Phalgun Nori

BACKGROUND AND GENERAL PRINCIPLES

Definition

States of altered consciousness, referred to as disorders of consciousness (DOC), can be categorized as follows: coma, vegetative state (VS), and minimally conscious state (MCS). This classification is based on the recommendations of an Aspen neurobehavioral conference workgroup in 1995 [1,2].

Pathophysiology

The etiology of DOC can be broadly categorized into traumatic versus nontraumatic brain injuries. Coma results from severe diffuse dysfunction of cerebral cortices, underlying white matter, or brainstem structures. The most common acute causes of VS are traumatic brain injury (TBI) and hypoxic-ischemic encephalopathy. After an initial severe TBI, a patient may enter the comatose stage, which can last from several days to weeks. Thereafter, the brainstem and lower diencephalon resume function and the patient enters the VS. In a minority of patients, the VS occurs immediately after the insult, without an initial period of coma. The two most common neuropathologic changes noted in patients in VS are diffuse laminar cortical necrosis and diffuse axonal injury (DAI).

Differential Diagnosis

Other causes of altered mental status, including subclinical seizures, toxic, metabolic, and infectious encephalopathies, and structural changes such as hydrocephalus should be considered and ruled out.

Evaluation of the Patient

A thorough bedside neurologic examination should be performed to evaluate a patient with altered consciousness. The examination must be repeated to avoid misdiagnosis. The neurologic examination should evaluate the integrity of the brainstem and presence of higher cortical functions. Findings on physical exam should be correlated with radiologic findings.

SPECIFIC STATES OF ALTERED CONSCIOUSNESS

Coma

Coma is a state of pathologic unconsciousness in which eyes remain closed and patient cannot be aroused. The defining feature is absence of sleep–wake cycles.

Evaluation

• Glasgow Coma Scale (GCS)—measures the best eye, motor, and verbal responses, and is a widely used and accepted severity score for TBI. A score of 13 to 15 is considered mild TBI, whereas a score of 9 to 12 is considered moderate TBI and a score of 3 to 8 is classified as severe TBI. The lowest total score (i.e., 3 out of 15) indicates likely fatal damage, especially if both pupils fail to respond to light and oculovestibular responses are absent. Higher initial scores tend to predict better recovery [3]. By convention, the severity of brain injury is initially defined by the GCS.

• JFK Coma Recovery Scale-Revised (CRS-R)—CRS-R is considered the most accurate objective clinical evaluation measure of DOC [4]. CRS-R was developed to help characterize and monitor patients with DOC, and has been used widely in both clinical and research settings within the United States and Europe. The CRS-R assesses auditory, visual, verbal, and motor functions as well as communication and arousal level.

Vegetative State

VS is characterized by the absence of behavioral evidence of awareness of self or the environment in the context of evidence of functional restoration of the reticular activating system (e.g., eye opening or wakefulness). This diagnosis is made when there is no evidence of sustained or reproducible purposeful behavioral response to visual, auditory, tactile, or noxious stimuli, and no evidence of language comprehension or expression. VS is usually preceded by a period of coma [1],

• The terms Persistent VS (PVS) and Permanent VS (PNS) have been used by some, but these terms are discouraged because they imply not only level of consciousness but also prognosis, and may be misleading. The Aspen group recommends simply using the term VS accompanied by the cause of injury along with the specific length of time since onset [1].

• Prognosis—The Multi-Society Task Force on PVS concluded that patients in a VS due to a TBI for greater than 1 year had a low probability of recovering awareness [5]. Based on a study by Estraneo [6], however, 20% of patients who were in a VS greater than 12 months from onset of injury (14–28 months) either progressed to the MCS or regained consciousness. Outcome is generally more favorable in younger patients and patients with a traumatic rather than nontraumatic etiology. For patients who remain in the VS, the Task Force summarized the duration of survival time as follows: “Life expectancy ranges from 2 to 5 years, survival after 10 years is unusual” [5].

• Neuroimaging—Functional neuroimaging has been shown to aid in identification of covert cognitive function in patients in the VS. Activation studies have the potential to demonstrate distinct and specific physiological responses to environmental stimuli, such as changes in regional blood flow or changes in regional cerebral hemodynamics [7,8]. Functional magnetic resonance imaging (fMRI) detects brain activity by detecting the blood oxygen level dependent (BOLD) signal. In a recent study by Vogel et al. [9], 22 patients were enrolled in a pilot study with 10 patients in VS and 12 patients in MCS. Participants performed a mental imagery fMRI paradigm in which they were asked to alternatively imagine playing tennis and navigating through their homes. In 14 of the 22 examined patients (VS, n = 5; MCS, n = 9), a significant activation of the regions of interest (ROIs) of the mental imagery paradigm could be found. All five patients with activation of a significant BOLD signal, who were in a VS at the time of the fMRI examination, reached at least an MCS at the end of the observation period. In contrast, five participants in a VS who failed to show activation in ROIs, did not. Six of nine patients in an MCS with activation in ROIs emerged from an MCS. Imagery fMRI has not been adequately studied at this point to be used as a prognostic tool.

Only gold members can continue reading. Log In or Register to continue