DefinitionDelirium is sudden severe confusion and rapid changes in brain function that occur with physical or mental illness.Alternative NamesAcute confusional state; Acute brain syndromeCauses, incidence, and risk factorsDelirium is most often caused by physical or mental illness and is usually temporary and reversible. Many disorders cause delirium, including conditions that deprive the brain of oxygen or other substances.Causes include:Alcohol or sedative drug withdrawalDrug abuseElectrolyte or other body chemical disturbancesInfections such as urinary tract infections or pneumonia (more likely in people who already have brain damage from stroke or dementia)PoisonsSurgerySymptomsDelirium involves a quick change between mental states (for example, from lethargy to agitation and back to l