Overview of delirium and dementia

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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, progresses slowly, and is usually irreversible.

Also, the two disorders affect mental function differently:

Delirium affects mainly attention.
Dementia affects mainly memory.

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.