Delirium is an impairment of cognitive function that is not progressive, but is reversible. The impairment of consciousness varies, often being worse at night. It may be described as a transient organic brain syndrome characterized by concurrent disorders of attention, perception, thinking, memory, psychomotor behaviour and the sleep-waking cycle.

This condition is present in about 10% of all admissions aged over 65 years (Hodkinson, 1973) and up to 40% of those referred to traditional geriatric services (Bedford, 1959). The cause can be established within a few hours of admission in over 90% of cases and when the underlying cause has been treated then full recovery of mental function is the rule. Failure to recognise delirium and instigate the appropriate diagnostic routine is thus a serious clinical error.

The word delirium is derived from de (from) and lira (a ridge), thus meaning to go astray, to err from reason.

An aide-memoire to help remember the common causes of delirium is HIDEMAP:

• H - hypoxia
• I - infection
• D - drugs
• E - endocrine, e.g. diabetes
• M - metabolic, e.g. hyper/hypocalcaemia
• A - alcohol
• P - psychosis