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Delirium and dementia are the most common causes of mental (cognitive) dysfunction—the inability to acquire, retain, and use knowledge normally. Although delirium and dementia may occur together, they are quite different. Delirium begins suddenly, causes fluctuations in mental function, and is usually reversible. Dementia begins gradually, is slowly progressive, and is usually irreversible. Also, the two disorders affect mental function differently. Delirium impairs the ability to pay attention and to think clearly. Dementia causes loss of memory and a severe decline in all aspects of mental function. Both delirium and dementia may occur at any age but are much more common among older people, because of age-related changes in the brain (see Biology of the Nervous System: Effects of Aging).
Last full review/revision February 2003
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