**3. Etiology**

Delirium is divided into subtypes according to the etiological factors. When there is evidence from the history, physical examination, or laboratory findings that the disturbance is caused by the direct physiological consequences of a general medical condition, it is called as *Delirium due to a general medical condition.* When the symptoms of delirium are due to substance intoxication, it is called as *Substance intoxication delirium*. When the delirium is due to substance withdrawal, it is called as *Substance withdrawal delirium.* When there is evidence from the history, physical examination, or laboratory findings that the delirium has more than one etiology, it is called as *Delirium due to multiple etiologies.* Delirium that is of unclear etiology is called as *Delirium not otherwise specified* (American Psychiatric Association, 2000).

Delirium usually has a multifactorial etiology. It has been reported that 90% of patients with delirium had three to four identifiable etiologic factors, 27% had two factors, and only 16% had one identifiable etiologic factor (Camus et al., 2000). The etiology of delirium is complex and multifactorial, with the interaction of precipitating factors (acute insults) on a vulnerable patient with predisposing conditions (Inouye, 1999).
