Delirium is defined in the Diagnostic and Statistical Manual of Mental Disorders 5 (DSM-5) as acute disturbance in attention and awareness, with additional disturbances in cognition, not explained by a pre-existing neurocognitive disorder, and caused by another medical condition (American Psychiatric Association, 2013). A significant proportion of ICU patients are affected by delirium. A meta-analysis of 16,595 critically ill patients saw an occurrence rate of 31.8% although this does differ between patient groups and studies (Slooter et al., 2017). In a study of 126, mostly older survivors of critical illness, 84% experienced delirium (Brummel et al., 2015). However, in a study of 465 nonventilated surgical ICU patients, delirium was observed in only 9.2% of patients (Serafim et al., 2012).

In the ICU, delirium has a median duration of three days and a median time to onset of two days, although this can vary widely among patients (Slooter et al., 2017). Addressing delirium is an important component of ICU care as increasing duration of delirium is an independent predictor of worse cognitive performance, worse motor-sensory function, and increased disability in carrying out activities of daily living among ICU survivors (Girard et al., 2010; Brummel et al., 2014a).