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Acute alterations in the level of consciousness always indicate a serious medical problem, which must be comprehensively evaluated and closely monitored. They may arise from both primary processes within the central nervous system or may be caused by the secondary effects of other systemic disorders. In either case, if the disruption is severe enough, central control of respiratory or cardiovascular function can rapidly deteriorate, leading to a life-threatening emergency. The differential diagnosis for altered consciousness or coma is broad, and imaging studies and laboratory tests alone may not identify an etiology. A careful history of the events leading up to the change in mental status and a full multisystem examination are essential and will help guide the choice of diagnostic tests.

Normal consciousness requires both maintenance of arousal and the ability to focus attention and respond to the environment with a full range of cognitive functions. Maintenance of arousal is mediated by a complex network of interactions within the brain, prominently involving the reticular activating system. This is a poorly delineated brain-stem structure extending from the medulla to the rostral midbrain. It receives input from the cerebral cortex and all major sensory systems and projects to local structures within the brain stem and, via ascending pathways, to the thalamus, the hypothalamus, and the cerebral cortex. Stimulation of the reticular activating system results in increased alertness, and destructive lesions produce unresponsiveness.

The cerebral hemispheres direct conscious response to environmental stimuli. In contrast to the brain stem, where even small lesions affecting the reticular activating system may produce coma, extensive bilateral involvement of the cerebral cortex is necessary to cause severe impairment of consciousness. Focal lesions rarely impair consciousness unless they cause compression or edema of the contralateral hemisphere or when multiple bilateral hemispheric lesions are present. However, both focal and diffuse processes may provoke seizures, which can alter consciousness both during an epileptic attack and in the subsequent postictal state.

Altered consciousness usually begins with mild confusion or lethargy. More severe cases progress to obtundation, where the patient is somnolent but arousable, then to stupor, where a patient responds only to vigorous stimuli and immediately becomes unresponsive when stimulation ceases. Coma refers to true unresponsiveness to external stimuli, although there may be reflexive, nonlocalizing motor responses to pain. Delirium describes a state characterized by confusion, disorientation, and difficulty maintaining attention.1

Because terminology is used inconsistently among observers, and because a patient’s exam may fluctuate, scoring systems such as the Glasgow Coma Scale (Table 104-1) are useful in documenting a patient’s level of consciousness and changes over time. Although these scales provide prognostic information when the etiology of altered consciousness is known, they do not assist in establishing a diagnosis and are not a substitute for a comprehensive history and examination.

Table 104-1. Glasgow Coma Scale

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