Since the 1950s, delirium has been known to be associated with slowing of resting-state EEG rhythms, with abnormally decreased background alpha power and increased theta and delta frequency activity.From such evidence, a 2018 systematic review proposed a conceptual model that delirium results when insults/stressors trigger a breakdown of brain network dynamics in individuals with low brain resilience (i.e. people who already have underlying problems of low neural connectivity and/or low neuroplasticity like those with Alzheimers disease). Neuropathology
Only a handful of studies exist where there has been an attempt to correlate delirium with pathological findings at autopsy. One research study has been reported on 7 patients who died during ICU admission. Each case was admitted with a range of primary pathologies, but all had acute respiratory distress syndrome and/or septic shock contributing to the delirium, 6 showed evidence of low brain perfusion and diffuse vascular injury, and 5 showed hippocampal involvement. A case-control study showed that 9 delirium cases showed higher expression of HLA-DR and CD68 (markers of microglial activation), IL-6 (cytokines pro-inflammatory and anti-inflammatory activities) and GFAP (marker of astrocyte activity) than age-matched controls; this supports a neuroinflammatory cause to delirium, but the conclusions are limited by methodological issues.A 2017 retrospective study correlating autopsy data with MMSE scores from 987 brain donors found that delirium combined with a pathological process of dementia accelerated MMSE score decline more than either individual process.
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.