When the brain as an organ fails acutely from a wide variety of insults, a fairly stereotyped clinical syndrome, delirium, is the result. The onset of delirium is rather sudden, although an unspecific prodrome with anxiety and restlessness can predate the full-blown picture. One diagnostically useful hallmark of delirium is its fluctuation in severity over the course of the day. Delirium is fundamentally a disorder of attention: Patients are unable to pay attention, to shift attention, or to sustain attention. As attention is one of the basic brain functions that supports higher functions, other cognitive deficits are usually present. Patients are often unable to learn new information and appear puzzled, perplexed, and confused (hence the synonymous term “acute confusional state” for delirium). Patients are usually, but not obligatory, disoriented to time (often), place (sometimes), and person (only in severe cases); the key is the inability to attend. The level of consciousness can be altered in both directions, from hypervigilant to lethargic, stuporous, or comatose. Some patients are anxious, labile, and agitated (hyperactive delirium), others
withdrawn (hypoactive delirium); most show a mixed pattern. Patients might be rambling or grossly incoherent. You should not expect to get a good history from a delirious patient. In addition, the sleep-wake cycle is disturbed and patients are awake at night and sleepy during the day.

Psychosis (delusions and hallucinations) is seen in 40% of deliria (Webster and Holroyd, 2000). The psychosis of delirium is characterized by fleeting, poorly formed delusions, often more a misinterpretation of the situation. Hallucinations are often visual; you may see patients picking at things. It is not always clear if you are dealing with misperceptions (illusions) and misinterpretations, or hallucinations and delusions. Table 3.1 presents a differential diagnosis of delirium.

Have a low threshold for suspecting a delirium in the right clinical setting. An elderly, hospitalized patient with new-onset psychosis has a delirium until proven otherwise, not late-onset schizophrenia. Any sudden change in clinical status is a red flag for the presence of a delirium. However, patients with known psychiatric disorders can have a superimposed delirium: A delirium can be superimposed on a manic patient who has not eaten or had anything to drink for days on his quest for the Holy Grail; a psychotic patient who is inadvertently overdosing on his benztropine because of confusion can be delirious. “Bad behavior” can stem from a subtly confused patient.

Once you have suspected the presence of a delirium, identify its four cardinal features to make the diagnosis (these are taken from a widely used screening instrument, the Confusion Assessment Method, or CAM):