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. * Memory impairment: Memory impairment is linked to inattention, especially reduced formation of new long-term memory where higher degrees of attention is more necessary than for short-term memory. Since older memories are retained without need of concentration, previously formed long-term memories (i.e. those formed before the onset of delirium) are usually preserved in all but the most severe cases of delirium. * Disorientation: As another symptom of confusion, and usually a more severe one, this describes the loss of awareness of the surroundings, environment and context in which the person exists. One may be disoriented to time, place, or self. * Disorganized thinking: Disorganized thinking is usually noticed with speech that makes limited sense with apparent irrelevancies, and can involve poverty of speech, loose associations, perseveration, tangentiality, and other signs of a formal thought disorder. *Language disturbances: Anomic aphasia, paraphasia, impaired comprehension, agraphia, and word-finding difficulties all involve impairment of linguistic information processing. *Sleep changes: Sleep disturbances in delirium reflect disturbed circadian rhythm regulation, typically involving fragmented sleep or even sleep-wake cycle reversal (i.e. active at night, sleeping during the day) and often preceding the onset of a delirium episode *Psychotic symptoms: Symptoms of psychosis include suspiciousness, overvalued ideation and frank delusions. Delusions are typically poorly formed and less stereotyped than in schizophrenia or Alzheimer's disease. They usually relate to persecutory themes of impending danger or threat in the immediate environment (e.g. being poisoned by nurses). *Mood lability: Distortions to perceived or communicated emotional states as well as fluctuating emotional states can manifest in a delirious person (e.g. rapid changes between terror, sadness and joking). *Motor activity changes: Delirium has been commonly classified into psychomotor subtypes of hypoactive, hyperactive, and mixed, though studies are inconsistent as to the prevalence of these subtypes. Hypoactive cases are prone to non-detection or misdiagnosis as depression. A range of studies suggest that motor subtypes differ regarding underlying pathophysiology, treatment needs, and prognosis for function and mortality though inconsistent subtype definitions and poorer detection of hypoactive subtypes impacts interpretation of these findings. Liptzin and Levkoff first described these subtypes in 1992 as following: *Hyperactive symptoms include hyper-vigilance, restlessness, fast or loud speech, irritability, combativeness, impatience, swearing, singing, laughing, uncooperativeness, euphoria, anger, wandering, easy startling, fast motor responses, distractibility, tangentiality, nightmares, and persistent thoughts (hyperactive sub-typing is defined with at least three of the above). *Hypoactive symptoms include unawareness, decreased alertness, sparse or slow speech, lethargy, slowed movements, staring, and apathy (hypoactive sub-typing is defined with at least four of the above).
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