Delirium Definitions and Subtypes

Delirium has long been recognized as a clinical syndrome but has been difficult to study due to multiple definitions used in the literature over time. Our current understanding of delirium is based in the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-V), definition. Delirium diagnosis in the critically ill presents a particular challenge due to the use of sedatives and analgesics and the often nonverbal status of the patients. In this population, delirium is best diagnosed with a well-validated screening tool such as the CAM-ICU or ICDSC. These and additional tools can also help assess the severity of the acute brain dysfunction. Once delirium is diagnosed, it can be separated into motoric subtypes—hypoactive and hyperactive—which is important for prognostication, with hypoactive subtypes generally having worse outcomes. Additionally, delirium can be separated into clinical phenotypes: metabolic, hypoxic, septic, sedative-associated, and unclassified. These clinical phenotypes have also shown outcome differences. Using clear definitions of delirium in the literature will allow further study of this highly prevalent and clinically significant syndrome.