**1.1 Definition**

Delirium is defined as a condition highlighted by an acute disturbance in attention, awareness and cognition that is not explained by a preexisting neurocognitive disorder. Delirium is characterized by reduced capacity to direct, focus, sustain, or shift attention, as well as reduced orientation to the environment [1, 3]. These symptoms must present acutely and fluctuate throughout the day. Importantly, the diagnosis of delirium identifies the constellation of symptoms representing altered brain function, but does not identify the etiology (**Figure 1**).

Delirium can be classified into three subtypes based on psychomotor behavior: hyperactive, hypoactive and mixed type delirium. Delirium is under diagnosed due to inconsistent screening, but also because delirium has varying and inconsistent presentations especially in patients suffering from hypoactive delirium. Hypoactive delirium is characterized by slowed mentation, lethargy, and decreased movement, whereas hyperactive delirium is marked by agitated behavior, confusion and difficulty with re-orientation. Without consistent, evidence-based screening methods, hypoactive delirium is more likely to be overlooked compared to hyperactive delirium. In addition, the different forms of delirium carry different prognosis. In a study of patients admitted to the intensive care unit after elective operations, patients that suffered from hypoactive delirium had an increased six-month mortality compared to patients with other subtypes of delirium (32 vs. 8.7%, P = 0.04) [4]. Therefore, it is important understand the various forms of delirium and the clinical scenarios in which it can present to allow timely diagnosis and management.
