In prospective studies, people hospitalised from any cause appear to be at greater risk of dementia and faster trajectories of cognitive decline, but these studies did not specifically look at delirium. In the only population-based prospective study of delirium, older persons had an eight-fold increase in dementia and faster cognitive decline. The same association is also evident in persons already diagnosed with Alzheimers dementia.Recent long-term studies showed that many patients still meet criteria for delirium for a prolonged period after hospital discharge, with up to 21% of patients showing persistent delirium at 6 months post-discharge. Dementia in ICU survivors
Dementia is supposed to be an entity that continues to decline, such as Alzheimers disease. Another way of looking at dementia, however, is not strictly based on the decline component, but on the degree of memory and executive function problems. It is now known, for example, that between 50% and 70% of ICU patients have tremendous problems with ongoing brain dysfunction similar to those experienced by Alzheimers or TBI (traumatic brain injury) patients, leaving many ICU survivors permanently disabled. This is a distressing personal and public health problem and is getting an increasing amount of scrutiny in ongoing investigations.The implications of such an "acquired dementia-like illness" can profoundly debilitate a persons livelihood level, often dismantling his/her life in practical ways like impairing ones ability to find a car in a parking lot, complete shopping lists, or perform job-related tasks done previously for years.
Several medical definitions of delirium exist (including those in the DSM and ICD-10), but the core features remain the same. In 2013, the American Psychiatric Association released the fifth edition of the DSM (DSM-5) with the following criteria for diagnosis:
A. Disturbance in attention and awareness. This is a required symptom and involves easy distraction, inability to maintain attentional focus, and varying levels of alertness. B. Onset is acute (from hours to days), representing a change from baseline mentation with fluctuations throughout the day
C. At least one additional cognitive disturbance (in memory, orientation, language, visuospatial ability, or perception)
D. The disturbances (criteria A and C) are not better explained by another neurocognitive disorder
E. There is evidence that the disturbances above are a "direct physiological consequence" of another medical condition, substance intoxication or withdrawal, toxin, or various combinations of causes

Signs and symptoms
Delirium exists as a stage of consciousness somewhere in the spectrum between normal awakeness/alertness and coma. While requiring an acute disturbance in consciousness/attention and cognition, delirium is a syndrome encompassing an array of neuropsychiatric symptoms.The range of clinical features include: poor attention/vigilance (100%), memory impairment (64–100%), clouding of consciousness (45–100%), disorientation (43–100%), acute onset (93%), disorganized thinking/thought disorder (59–95%), diffuse cognitive impairment (77%), language disorder (41–93%), sleep disturbance (25–96%), mood lability (43–63%), psychomotor changes (e.g. hyperactive, hypoactive, mixed) (38–55%), delusions (18–68%), and perceptual change/hallucinations (17–55%). These various features of delirium are further described below:

Inattention: As a required symptom to diagnose delirium, this is characterized by distractibility and an inability to shift and/or sustain attention.