In older people, delusions -- false beliefs that aren't typical of a person's culture or faith -- can be a common sign of dementia, of delirium, or of both at
the same time. Delusions essentially are caused by the mind not working normally.
Paranoid delusions, such as believing that a spouse is having an affair or that an acquaintance is plotting harm (when those things aren't true), are especially common.
When delusions are likely to be part of delirium:
They come on suddenly, especially with other signs of confusion and an inability to maintain focus. Delirium is a sign of illness or other stress affecting the body.
When delusions are likely to be part of dementia:
They develop slowly over time. Believing that you can see or hear something that isn't there or that something is happening when it isn't are common signs that a person is developing a dementia such as Alzheimer's disease. Other signs of dementia that may develop at the same time include problems with memory, problems with organization, or difficulty with words and language.
When delusions are likely to be part of both delirium and dementia:
A person with dementia is sick and/or hospitalized. Since the person may normally be somewhat confused to begin with, doctors and other providers may not diagnose the delirium unless a concerned caregiver points out that the sick person is more confused than usual.
A sudden worsening in a person's mental abilities should always be considered delirium until proven otherwise. Prompt medical evaluation is necessary, since delirium can be the only outward symptom of serious medical problems including infection, heart attack, or side effects of medication.