Delirium, or acute confusional state, is a syndrome that presents in two basic forms. In its hyperactive form, it is manifested as severe confusion and disorientation, developing with relatively rapid onset and fluctuating in intensity. In its hypoactive form, it is manifested by an equally sudden withdrawal from interaction with the outside world. Delirium as a syndrome is one which occurs more frequently in people in their later years. However, when it occurs in the course of a critical illness, delirium has been found to occur in young and old patients at relatively even rates.