Introduction

In recent years there has been widespread acknowledgment that critical illness has a fundamental neurological dimension. A broad body of work has demonstrated that severe illnesses, possibly in conjunction with practices and interventions in the ICU, are responsible for neurological complications which have a major impact on short- and long-term outcome. This neurological burden is almost certainly an indirect product of intensive care itself, with increasing numbers of patients surviving to the recovery phase of critical illness. Scientific exploration of the relevance and impact of ICU-acquired neurological disorders has been led by an initially small, but rapidly expanding, group of dedicated researchers. An illustration of this process is the work on delirium which started with observational studies and now includes large, multicenter randomized trials. Delirium is a complex and fascinating syndrome as its pathophysiology, expression, and severity is heavily dependent on the underlying disorder (e.g., sepsis, hepatic failure), while understanding of its biological mechanisms draws on concepts from neurology, neuropharmacology, neuroimmunology, and the cognitive neurosciences. The association between delirium and age- or disease-associated cognitive impairment is clearly reciprocal, possibly implicating subtle shifts between chronic and acute neuroinflammatory states.