Merck Manual

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Overview of Delirium and Dementia


Juebin Huang

, MD, PhD, Department of Neurology, University of Mississippi Medical Center

Reviewed/Revised Feb 2023
Topic Resources

Delirium Delirium Delirium is an acute, transient, usually reversible, fluctuating disturbance in attention, cognition, and consciousness level. Causes include almost any disorder or drug. Diagnosis is clinical... read more (sometimes called acute confusional state) and dementia Dementia Dementia is chronic, global, usually irreversible deterioration of cognition. Diagnosis is clinical; laboratory and imaging tests are usually used to identify treatable causes. Treatment is... read more 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 and awareness.

  • Dementia affects mainly memory and other cognitive function.

Other specific characteristics also help distinguish the two disorders (see table ):

  • Delirium is typically caused by acute illness or a medication or recreational 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.

Mistaking delirium for dementia in an older patient is a common clinical error.

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.


Although delirium and dementia are considered distinct disorders, they have a complex interrelationship. Delirium often develops in patients with dementia;it is called delirium superimposed on dementia (DSD). DSD can occur in up to 49% of patients with dementia during hospitalization. Also, patients with delirium have a higher risk of developing dementia (1 General reference 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... read more ).

General reference

  • 1. Fong TG, Inouye SK:The inter-relationship between delirium and dementia: The importance of delirium prevention. Nat Rev Neurol 18 (10):579–596, 2022. doi: 10.1038/s41582-022-00698-7

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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