Delirium can be confused with dementia, a disease characterized by forgetfulness. Differentiating dementia and delirium can be particularly challenging, and both can occur in one person. In fact, delirium is frequently seen in individuals with dementia. However, experiencing episodes of delirium does not always mean a person has dementia. A dementia assessment should not be performed during an episode of delirium, as the results may be misleading.

Dementia is a progressive decline in memory and other thinking abilities due to gradual impairment and loss of brain cells. The most common cause of dementia is Alzheimer's disease. Key distinctions between dementia and delirium are:

Dementia is a neurological disease in itself. Delirium is a clinical syndrome caused by medical illnesses.
Delirium has an acute onset of mental changes; dementia has a slow, insidious onset.
Delirium symptoms often resolve within two weeks with treatment of the underlying cause; dementia symptoms have a more chronic course.
In delirium, altered consciousness, disorientation (time, place, person), and impaired attention are prominent; these symptoms appear in later stages of dementia.
The ability to focus or sustain attention, the level of consciousness, is significantly impaired with delirium. A person in the early stages of dementia usually does not experience altered consciousness.
The appearance of delirium symptoms can fluctuate significantly and frequently throughout the day. While people with dementia may have better and worse times of day, their memory and thinking abilities remain at a fairly consistent level throughout the day.