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Kyle Wu, MD, & Timothy R. Smith, MD, PhD

Consciousness, the complex interplay between wakefulness and awareness, exists on a spectrum from “normal” consciousness to coma, and various states in between. Acute confusional state (delirium) and encephalopathy are among the most common disorders of consciousness encountered in acutely ill patients, especially those recovering from surgery or anesthetic medications or adapting to the hospital environment during the postoperative period. The prevalence of altered mental status in hospitalized patients is high, with reported rates up to 55%, and is associated with increased rates of mortality ranging from 10%-70%. Given the high incidence of altered mental states and their similarity at presentation to various states of primary neurologic disease, an understanding of the pathophysiology, diagnosis, and management of common etiologies is warranted for all medical practitioners.


Consciousness is generally defined as a combination between the experience of the environment and self. It is composed of two components: arousal, a measure of wakefulness, and awareness, a state of perception. This distinction is useful, since the two processes are dissociable. For example, a vegetative state is characterized by a patient who is awake but not necessarily aware of external stimuli or his or her environment (and thus not interactive). On the other hand, the unfortunate patient suffering from locked-in syndrome (due to a structural lesion of the brainstem) is both awake and aware; however, he or she is incapable of movement.

Arousal is generated by activity of the ascending reticular activating system, which is composed of neurons within the central mesencephalic brainstem, the lateral hypothalamus, and portions of the thalamus. Widespread projections of these nuclei synapse with neurons in the cerebral cortex to generate an arousal response. The arousal response defines the level of consciousness (eg, being awake vs asleep vs comatose). Awareness is thought to be generated through networks involving the thalamus and association cortices of the frontal, parietal, temporal, and occipital lobes. Processes related to awareness define the content of consciousness (eg, seeing a blue circle vs a red triangle). It can be thought of as the way in which one experiences his or her own state of being and relates this to incoming sensory stimuli.

Many terms are used to describe levels of consciousness ranging from alert to comatose. Here we will describe some of the more commonly used terminology. An alert patient is awake and immediately responsive to all stimuli. Stupor is a condition in which the patient is less alert but still responds appropriately to stimulation. An obtunded patient appears to be asleep much of the time but responds, albeit less briskly than expected, to noxious stimuli. A vegetative state is a state of arousal without awareness in which the patient may open his or her eyes, track objects, chew, and swallow, but not respond to auditory ...

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