General introduction Chapter 1 Delirium Delirium is derived from the Latin word 'delirare', which literally means 'a disturbed state of mind' and has been known since ancient times. Hippocrates (460-370 BC) may have been the first to describe a syndrome characterized by confusion and restlessness, which fluctuated over time and which was associated with physical illness. He called this condition 'phrenitis' 1,2. In the following eras, many other names have been used to describe the syndrome, including brain failure, acute change of mental status, delirium, acute brain dysfunction, intensive care psychosis and encephalopathy. Of these, the term delirium became the official name. In the 90's, Diagnostic and Statistical Manual of Mental Disorders (DSM) criteria were developed for the syndrome and delirium became an official diagnosis. Currently, delirium is defined as a neuropsychiatric syndrome, characterized by an acute change in attention and awareness. It is always a direct consequence of an underlying medical condition 3. It is a common syndrome, affecting 10-50% of the hospitalized elderly, and is a burden for patients and their relatives 4. Delirium has several clinical manifestations, which are commonly divided into hypoactive, hyperactive and mixed subtypes. Hypoactive delirium is characterized by lethargy and lack of psychomotor behavior and speech. In contrast, the hyperactive subtype is defined by restlessness, hyper vigilance and agitation 3. The mixed type manifest both hypoactive and hyperactive elements 5. All types are associated with negative outcomes, such as prolonged hospital stay, long-term cognitive impairment and dementia 4 .