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Merck Manual

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Kenneth Maiese

, MD, National Heart, Lung, and Blood Institute

Last full review/revision Sep 2020| Content last modified Sep 2020
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Consciousness has two parts:

  • Whether a person is awake and alert (wakefulness)

  • What people are conscious of (content)

When wakefulness (alertness) is impaired, people do not respond normally to the outside world (for example, when they are touched or spoken to), and they do not acquire information from it. If wakefulness is impaired, people usually appear sluggish, drowsy, unconscious, or asleep. They may be difficult to arouse, as occurs in stupor, or impossible to arouse, as occurs in coma. Impaired wakefulness is often called impaired consciousness.

The content of consciousness depends on mental (cognitive) function and involves understanding and processing what is experienced and encountered. When mental function is impaired, people have problems with memory, thinking, judgment, and learning, as occurs in dementia.

Often, disorders that impair wakefulness also impair mental function, as occurs in delirium.

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