MSD Manual

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Overview of Delirium and Dementia


Juebin Huang

, MD, PhD, Memory Impairment and Neurodegenerative Dementia (MIND) Center, University of Mississippi Medical Center

Last full review/revision Dec 2019| Content last modified Dec 2019
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Delirium (sometimes called acute confusional state) and dementia are the most common causes of cognitive impairment, although affective disorders (eg, depression) can also disrupt cognition. Delirium and dementia are separate disorders but are sometimes difficult to distinguish. In both, cognition is disordered; however, the following helps distinguish them:

  • Delirium affects mainly attention.

  • Dementia affects mainly memory.

Other specific characteristics also help distinguish the 2 disorders (see table Differences Between Delirium and Dementia):

  • Delirium is typically caused by acute illness or drug toxicity (sometimes life threatening) and is often reversible.

  • Dementia is typically caused by anatomic changes in the brain, has slower onset, and is generally irreversible.

Delirium often develops in patients with dementia. Mistaking delirium for dementia in an older patient—a common clinical error—must be avoided, particularly when delirium is superimposed on chronic dementia. No laboratory test can definitively establish the cause of cognitive impairment; a thorough history and physical examination as well as knowledge of baseline function are essential.


Differences Between Delirium and Dementia*





Sudden, with a definite beginning point

Slow and gradual, with an uncertain beginning point


Days to weeks, although it may be longer

Usually permanent


Almost always another condition (eg, infection, dehydration, use or withdrawal of certain drugs)

Usually a chronic brain disorder (eg, Alzheimer disease, dementia with Lewy bodies, vascular dementia)


Usually reversible

Slowly progressive

Effect at night

Almost always worse

Often worse


Greatly impaired

Unimpaired until dementia has become severe

Level of consciousness

Variably impaired

Unimpaired until dementia has become severe

Orientation to time and place



Use of language

Slow, often incoherent, and inappropriate

Sometimes difficulty finding the right word



Lost, especially for recent events

Need for medical attention


Required but less urgently

Effect of treatment

Usually reverses symptoms

May slow progression but cannot reverse or cure the disorder

* Differences are generally true and helpful diagnostically, but exceptions are not rare. For example, traumatic brain injury occurs suddenly but may result in severe, permanent dementia; hypothyroidism may produce the slowly progressive picture of dementia but be completely reversible with treatment.

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