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Delirium is a medical term used to describe a mental state. There are several definitions (including those in the DSM-IV and ICD-10). However, all include some core features.

These include:

  • impairments of consciousness
  • confusion and disorientation
  • cognitive impairments
  • intrusive abnormalities of awareness and affect, such as hallucinations or innappropriate emotional states.

Delirium should be distinguished from psychosis, in which consciousness and cognition may not be impaired, and dementia which describes an acquired intellectual impairment usually resulting from a degenerative brain disease.

Delirium may be caused by severe physical or mental illness. Fever, poisons (including toxic drug reactions), brain injury, surgery, severe lack of food or water, drug and severe alcohol withdrawal are all known to cause delirium.

It is also referred to as 'acute confusional state' or 'acute brain syndrome'.

Table of contents

Impairment of consciousness

A delirious person may have a clouding of awareness and consciousness. This impairment of consciousness typically fluctates, so the person may be aroused and alert for short periods of time before again relapsing into a clouded state. Fluctation may follow a pattern of diurnal variation, where consciousness levels change as the day progresses. Typically, a delirious person may be more consciousness impaired in the evening and at night.

Confusion and disorientation

Confusion may occur in delirium, where the sufferer loses the capacity for clear and coherent thought. It may be apparent in disorganised or incoherent speech, the inability to concentrate or a lack of goal directed thinking.

Disorientation describes the loss of awareness of the surroundings, environment and context in which the person exists. Disorientation may occur in time (not knowing what time of day, day of week, month, season or year it is), place (not know where you are) or person (not knowing who are).

Cognitive Impairments

Impairments to cognition may include reduction in the function of short or long term memory, attention or problem solving.

Abnormalities of Awareness and Affect

Hallucinations (perceived sensory experience with the lack of an external source) or distortions of reality may occur in delirium. Commonly these are visual distortions, and can take the form of masses of small crawling creatures (particularly common in delirium tremens, caused by severe alcohol withdrawal) or distortions in size or intensity of the surrounding environment.

Strange beliefs may also be held during a delirious state, but these are not considered delusions in the clinical sense as they are considered too short lived. Interestingly, in some cases sufferers may be left with false or delusional memories after delirium, basing their memories on the confused thinking or sensory distortion which occurred.

Abnormalities of affect include any distortions to perceived or communicated emotional states. Emotional states may also fluctate, so a person may rapidly change between, for example, terror, sadness and jocularity.


The duration of delirium is typically affected by the underlying cause. If caused by a fever, the delirious state should subside as does the severity of the fever.

Accounts of delirium

Sims (1995, p31) points out a "superb detailed and lengthy description" of delirium in The Stroller's Tale from Charles Dickens' The Pickwick Papers[?].

Further reading

Sims, A. (1995) Symptoms in the mind: An introduction to descriptive psychopathology. Edinburgh: Elsevier Science Ltd. ISBN 0702026271
Dickens, C. (1837) The Pickwick Papers. Available for free on Project Gutenberg

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