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 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 (see Effects of Aging on the Nervous System).
Comparing Delirium and Dementia
Sudden, sometimes with a definite beginning point
Slow, with an uncertain beginning point
Almost always another condition, such as an infection, dehydration, or use or stopping of certain drugs
Usually a brain disorder, such as Alzheimer disease, vascular dementia, or Lewy body dementia
Main early symptom
Inability to pay attention
Loss of memory, especially for recent events
Effect at night
Almost always worse
Level of alertness (consciousness)
Impaired to varying degrees, can vary from being hyperalert to sluggish
Normal until late stages
Orientation to surroundings
Effect on language
Slowed speech, often with incoherent and inappropriate language
Sometimes difficulty finding the right word
Lost, especially for recent events
Causes variations in mental function—people are alert one moment and sluggish and drowsy the next
Slowly progresses, gradually but eventually greatly impairing all mental functions
Days to weeks, sometimes longer
Almost always permanent
Need for treatment
Needed but less urgently
Effect of treatment
Usually reverses the losses
May slow progression but cannot reverse or cure the disorder
Last full review/revision July 2013 by Juebin Huang, MD, PhD
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