• Impairment of consciousness is the key feature that separates delirium from most other psychiatric disorders. There is a continuum between mild impairment of consciousness and near unconsciousness. There is fluctuation in intensity, and symptoms are often worse at night. The patient may be unmistakably drowsy, but milder states are easy to miss, especially by those who are unfamiliar with the patient's normal intellectual performance. They may be apparent only in reduced or slowed performance on bedside cognitive testing. There is disorientation in time, place, and the identity of other people ( Table 1).
• Appearance and behaviour: the patient looks unwell and behaviour may be marked by agitation or hypoactivity, by a fluctuation between these states, or by a mixture of them—for example, a drowsy patient plucking aimlessly at the bedclothes.
• Mood is frequently labile, with perplexity, intermittent periods of anxiety or depression, or occasionally of other mood states such as elation and irritability. Usually, the mood states have an empty and transitory quality.
• Speech: the patient may mumble and be incoherent.
• Perception: visual perception is the modality most often affected. Illusions and misinterpretations are frequent. For example, a patient may become agitated and fearful, believing that a shadow in a dark room is actually an attacker. Visual hallucinations also occur. The small living creatures which may be seen in delirium tremens are the best-known example. Auditory and tactile hallucinations also occur. Complex sensory distortions, such as colours being experienced as tastes, would suggest intoxication with hallucinogens.
• Cognition: there are abnormalities in all areas of cognitive function. Memory registration, retention, and recall are all affected. Mild cases may show their most pronounced abnormalities in slow performance on tasks or in the wandering of attention away from the task at hand.
• Orientation: in obvious cases, orientation in person, time, and place will all be disturbed. Milder degrees of disorientation will need to be interpreted in the context of the individual patient. For example, it may be considered not abnormal for a person who has been seriously ill in hospital for a long time to be unaware of the day of the month.
• Concentration is impaired, for example, on tests such as 'serial sevens' or 'days of the week backwards'.
• Memory disturbances are seen, with impaired registration (e.g. digit span), short-term recall (e.g. name and address), and long-term recall (e.g. current news items). After recovery from the illness there is usually (but not always) amnesia for the illness.
• Insight is usually impaired. The patient will have no understanding of why a psychiatric assessment has been requested.
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