Clinical features

Delirium usually develops over hours to days. Typically, the symptoms fluctuate and are worse at night. The fluctuation can be a diagnostic trap, with nurses or relatives reporting that patients had disturbed behaviour at night whereas doctors find patients lucid the next day.

Impaired cognitive functioning is central and affects memory, orientation, attention, and planning skills. Impaired consciousness, with a marked variability in alertness and in awareness of the environment is invariably present. A mistaken idea of the time of day, date, place, and identity of other people (disorientation) is common. Poor attention, and disturbed thought processes may be reflected in incoherent speech. This can make assessment difficult and highlights the need to obtain a history from a third party. Relatives or other informants may report a rapid and drastic decline from premorbid functioning that is useful in distinguishing delirium from dementia.

Disturbed perception is common and includes illusions (misperceptions) and hallucinations (false perceptions). Visual hallucinations are characteristic and strongly suggest delirium. However, hallucinations in auditory and other sensory modalities can also occur.