Delirium has been defined in many ways. Some use the term to refer to an acute, hyperactive, confusional state. Psychiatrists define it more broadly to describe clinical states characterized by a reduced level of consciousness, an inability by the affected individual to sustain or shift attention appropriately, disorganized thinking, dis-orientation to time, place, or person, and memory impairment. In addressing the affected individual, questions need to be repeated, the individual may perseverate in responses, and speech may be rambling or incoherent. Additional features include an altered sleep-wake cycle, sensory misperceptions, disturbances in the pace of psychological and motor activity, and varying mood states (e.g., apathy, euphoria). Sensory misperceptions—usually visual ones—may include illusions (e.g., specks on the floor are thought to be insects) or hallucinations (one "sees" a relative in the room when there is actually no one there). Delusions may be present (e.g...