10-Minute Diagnosis Manual

The neurobiologic mechanism of delirium is poorly understood, because it is not a disease but a syndrome with multiple disparate causes. The causes delirium can be categorized according to whether they are predisposing or precipitating factors. The most common predisposing factors are advanced age and dementia. Other risk factors include immobility and functional dependence, sensory impairment, dehydration, malnutrition, and alcoholism. Nearly any acute systemic illness or medical condition may precipitate delirium in a susceptible patient. The common causes are listed in Table 4.3.1.

Variation in case identification and sample bias make estimates of the prevalence and incidence of delirium difficult to determine. Clinicians often fail to recognize delirium, although it is one of the most common mental disorders encountered in patients with medical illnesses, especially those who are critically ill or elderly. Most studies of mixed hospital inpatient populations report prevalences of 10% to 20%.

Determining that a cognitive impairment or perceptual disturbance is not due to a preexisting or progressing dementia or other mental disorder requires knowledge of the patient's baseline mental status, and level of functioning. If this is not known, information should be sought from family, friends, and other care providers. Because it is difficult or impossible to obtain a history from a confused or uncooperative patient, important clues as to the cause of delirium may also be elicited from these historical sources (recent febrile illness, recent trauma, history of drug abuse or alcoholism) and a careful review of the medical history. Because drug toxicity accounts for a significant percentage of all cases of delirium, clinicians should not neglect considering over-the-counter drugs, drugs belonging to other family members, drugs prescribed by other physicians, or illicit drugs. Because the features of delirium fluctuate during the course of the day, a review of nursing notes, especially from the evening and night shifts, can be very helpful for discovering or documenting changes in consciousness and cognition.

The examination must focus on two issues: (i) confirming that delirium is present, and (ii) uncovering the medical illness that has likely caused the delirium. A comprehensive examination is often difficult in a confused and uncooperative patient. Clinicians should perform a focused examination guided by the history and context, keeping in mind the multifactorial nature of delirium.

The history and physical examination should guide most of the diagnostic investigation. First-line investigations should include electrolytes, complete blood count, urinalysis, liver and thyroid function tests, glucose, creatinine, calcium, chest x-ray, and electrocardiogram. Blood gas determinations are often helpful. Drug levels can be obtained when appropriate, but the clinician should be aware that delirium can occur even with therapeutic levels. The following diagnostic tests may be indicated when a cause of delirium is not apparent after the initial evaluation: urine and blood toxicology screen, syphilis serology, human immunodeficiency virus antibody, autoantibody screen, vitamin B₁₂ level, head computed tomography or magnetic resonance imaging, a lumbar puncture with cerebrospinal analysis, and electroencephalogram testing.

The most common issue in the differential diagnosis is whether the patient has dementia rather than delirium, has delirium only, or a delirium superimposed on a preexisting dementia. Careful attention to the key features (disturbed consciousness, change in cognition or perceptual disturbance, acute onset, and fluctuating course) should readily distinguish delirium from dementia and other primary psychiatric disorders such as depression, psychosis, or mania. Nonconvulsive status epilepticus and several lobar or focal neurologic syndromes (Wernicke's aphasia, transient global amnesia, Anton's syndrome, frontal lobe tumors) can result in features that may overlap with those of delirium.

Engaging in conversation with a patient in delirium can be difficult because he or she may become easily distracted, unpredictably switch from subject to subject, or persevere with answers to a previous question. In more advanced cases of delirium, the patient may be drowsy or lethargic. Cognitive changes may include memory impairment (most commonly short-term disorientation (usually to time and place), difficulty with language or speech (dysarthria, dysnomia, dysgraphia, or aphasia), and perceptual disturbances (illusions, hallucinations, or misperceptions). The patient may be so inattentive and incoherent that it may be difficult or impossible to assess cognitive function. Other associated features of delirium may include sleep disturbance or a reversal of the night-day sleep-wake cycle, hypersensitivity to light and sound, anxiety, anger, depressed affect, and emotional lability. Because of confusion, disorientation, and agitation, patients with delirium may harm themselves by climbing over bedrails or pulling out their intravenous line or Foley catheter.