Delirium is a functional brain disorder, characterized by disturbances of consciousness, attention and cognition. The term ‘acute confusional state’ is often used synonymously with delirium, although delirium is preferred, as confusion (uncertainty about what is happening, intended, or required) is not specific to delirium and its medical definition is imprecise.

It can reflect a primary neurological disorder, substance intoxication or withdrawal, an adverse effect of drugs (especially those with an anticholinergic effect), or a systemic disorder such as sepsis (Appendix ).

In sepsis, delirium may be caused by cytokines which enter the brain and activate microglia. Microglia then release molecules which cause neuronal dysfunction and thus delirium. Failure of cholinergic inhibition of microglia (due to neurodegenerative disease, or drugs with an anticholinergic effect) may amplify the microglial response to inflammatory cytokines.

It is distinguished from dementia (with which it may coexist, as dementia is a major risk factor for delirium) by its speed of onset (over hours or days) and reversibility with correction of the underlying cause.

Delirium is clinical diagnosis; criteria are given in Table 4.1.