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, dementia affects mainly memory, and delirium affects mainly attention.

Other specific characteristics help distinguish the 2 disorders (see Table 1: Delirium and Dementia: 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 elderly 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.

Table 1
PrintOpen table in new window
Differences Between Delirium and Dementia* Feature Delirium Dementia Onset Sudden, with a definite beginning point Slow and gradual, with an uncertain beginning point Duration Days to weeks, although it may be longer Usually permanent Cause Almost always another condition (eg, infection, dehydration, use or withdrawal of certain drugs) Usually a chronic brain disorder (eg, Alzheimer disease, Lewy body dementia, vascular dementia) Course Usually reversible Slowly progressive Effect at night Almost always worse Often worse Attention Greatly impaired Unimpaired until dementia has become severe Level of consciousness Variably impaired Unimpaired until dementia has become severe Orientation to time and place Varies Impaired Use of language Slow, often incoherent, and inappropriate Sometimes difficulty finding the right word Memory Varies Lost, especially for recent events Need for medical attention Immediate Required but less urgently *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. Differences Between Delirium and Dementia* Feature Delirium Dementia Onset Sudden, with a definite beginning point Slow and gradual, with an uncertain beginning point Duration Days to weeks, although it may be longer Usually permanent Cause Almost always another condition (eg, infection, dehydration, use or withdrawal of certain drugs) Usually a chronic brain disorder (eg, Alzheimer disease, Lewy body dementia, vascular dementia) Course Usually reversible Slowly progressive Effect at night Almost always worse Often worse Attention Greatly impaired Unimpaired until dementia has become severe Level of consciousness Variably impaired Unimpaired until dementia has become severe Orientation to time and place Varies Impaired Use of language Slow, often incoherent, and inappropriate Sometimes difficulty finding the right word Memory Varies Lost, especially for recent events Need for medical attention Immediate Required but less urgently *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.

Last full review/revision April 2013 by Juebin Huang, MD, PhD