Delirium is a syndrome characterized by an acute disturbance in attention, cognition, and awareness that develops within hours to days and fluctuates in severity over time. There are three subtypes of psychomotor disturbances of delirium: hyperactive, hypoactive, and mixed.¹ Hyperactive delirium is marked by agitation and restlessness and can be misdiagnosed as primary psychosis or dementia. Hypoactive delirium is characterized by lethargy and psychomotor impairment and can be mistaken for depression. Mixed delirium has hyperactive and hypoactive features.¹ Historically, the terms delirium, acute encephalopathy, acute confusional state, acute brain dysfunction, acute brain failure, and altered mental status have been used interchangeably and imprecisely. Delirium and acute encephalopathy are the most accepted terms and refer to a deviation from the patient's baseline cognition.² An accurate diagnosis of delirium is essential to prevent inappropriate treatments, prolonged hospitalization, cognitive decline, and increased mortality and morbidity.^3,4

Epidemiology

Delirium is among the most common complications in hospitalized adults 65 years and older.⁵ It is the most common surgical complication in older adults and is associated with a significant financial burden.⁶ National health care costs related to postoperative delirium are an estimated $32.9 billion per year, with most costs from the lengthening of the initial hospitalization, subsequent hospitalizations, and acute rehabilitation.⁷ Overall costs attributable to delirium are significantly higher, ranging from $143 billion to $152 billion per year, which includes factors such as prolonged hospitalizations, outpatient visits, and nursing home stays.³

The prevalence of delirium in the community is relatively low at 1% to 2% but increases to approximately 11% to 25% with hospital admission. This dramatic difference in prevalence may be due to the symptoms of delirium prompting evaluation of the patient in a more acute setting such as the emergency department.^5,8 The incidence of delirium differs significantly depending on the health care setting and the patient's associated risk factors (Table 1).^3,5,9,10 The prevalence and incidence of delirium are likely underestimated because patients with baseline cognitive impairment or dementia are often excluded from studies.⁵

Pathophysiology and Risk Factors

The pathophysiologic cause of delirium is not well understood.¹ Older age, multiple comorbidities, polypharmacy, poor baseline functional status, and recent surgery are all independent risk factors for developing delirium (Table 2).^8,11–15 Predisposing factors are the patient's background characteristics, whereas precipitating factors are acute or subacute events preceding onset of delirium.¹⁶ Patients with delirium are typically affected by more than one risk factor and often have predisposing and precipitating elements.¹⁷ It is unlikely that only one mechanism is responsible for the cause of delirium because of the variation in risk factors.^13,16 The range of potential etiologies poses a complex challenge in identifying precipitating factors but is important because delirium is reversible in 25% to 68% of patients.¹⁸

Diagnosis

Patients with suspected delirium should have a comprehensive history and physical examination for possible contributing medical conditions. Delirium can be precipitated by medications, adverse medication effects, uncontrolled pain, infections, severe organ damage, drug or alcohol withdrawal, dehydration, and constipation (Table 3^19,20). Timely identification of the etiology of symptoms is important.^14,21