Introduction, definition of terms, and frequency of the problem

The fundamental feature of delirium is disordered attention. Delirious patients cannot focus and sustain attention on one stimulus among multiple stimuli, and cannot shift attention at will. Table 16.1 lists diagnostic criteria for delirium from the American Psychiatric Association. If delirious patients are attentive enough to allow testing, abnormalities of thinking, perception, and memory will usually be found. The abnormalities may include disorientation to time and place, impaired immediate, recent, and remote memory, dysnomia, agraphia, and visual-spatial dysfunction. Much of the difficulty with perceptual function and speech output relates to the inability to sustain attention to the task at hand. Illusions, hallucinations, and delusions may also be prominent. Sleep–wake patterns are usually abnormal with reduced wakefulness during the day and reduced, often fragmented sleep at night. Autonomic hyperactivity produces flushing, mydriasis, sweating, tachycardia, and labile blood pressures. Delirious patients may quickly shift from hyperactivity to reduced activity. During the day lethargy and even catatonic behaviour may predominate, shifting at night to agitation, shouting, and aggressive behaviour.

The terms delirium, confused, agitation, and confusional state are used variously by different neurologists and psychiatrists. The criteria cited in Table 16.1 incorporate several very different elements including acuteness of onset, altered thought and concentration, level of consciousness, and cause. Confusion is defined by Adams et al. (1997) as: