This chapter describes the magnitude of the problem of mental illness, both globally and in terms of specific mental health-related visits encountered in emergency department (ED) settings. The WHO's cross-national comparisons show a globally high prevalence of major Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) mental disorders (anxiety disorders, mood disorders, impulse control disorders, substance use disorders) with 25th-75th percentiles ranging from 18.1% to 36.1%. The chapter describes the magnitude of the problem of ED presentations for specific mental disorders. The most prevalent conditions are highlighted. After anxiety disorders, mood disorders are the second most common psychiatric disorder in the general population, occurring in 10% of the U.S. adult population each year. Schizophrenia spectrum diagnoses account for approximately two thirds of all psychotic disorders. Almost 1 in 10 of the adult U.S. population is estimated to have an Axis II personality disorder in any year.

Emergency physicians are frequently required to care for unknown patients with acute undifferentiated agitation. Agitation is known to be associated with several other psychiatric and medical causes. Agitation, regardless of the etiology, is a behavioral emergency. Both typical (first-generation) and atypical (second-generation) antipsychotics are frequently used in the management of agitation. Several of the typical antipsychotics have been associated with QT prolongation and torsades de pointes. Other acute adverse effects of antipsychotic use in the treatment of acute agitation include anticholinergic effects, movement disorders, and neuroleptic malignant syndrome. Benzodiazepines are commonly used in the acute management of agitation. In addition to rapid sedation, ketamine's short duration of action, parenteral administration, and in particular the preservation of protective airway reflexes, are attractive properties in the management of patients with acute agitation. According to the Joint Commission Standards, restraints can only be used when clinically justified or when warranted by patient behavior.