Arousal and psychomotor activity changes account for the three clinical descriptive forms of delirium: ‘hyperactive’, ‘hypoactive’ and ‘mixed’ [4]. A patient with hyperactive delirium is easy to recognise, restless, paranoid and never ever seems to sleep. Pure hyperactive delirium, while memorable, is actually uncommon compared with the other two forms, occurring only in 5 % of cases [5]. Most delirious patients in critical care develop hypoactive delirium and generally appear lethargic, compliant and immobile. It is only by interacting with the patient that it can be appreciated they are inattentive and disorientated. With the mixed type of delirium, a patient’s behaviour fluctuates, often hypoactive during the day, but increasingly restless, often agitated overnight. Subsyndromal delirium describes a patient who has one or more symptoms of delirium, but does not meet the criteria for delirium diagnosis and does not progress to it [6].

At any one time, an acute hospital with 1000 beds would have around 100 patients with delirium. Delirium affects an estimated 18–35 % of hospitalised elderly, and its incidence in the Intensive Care Unit (ICU) has been documented as up to 82 % [7]. In critically ill patients, it prolongs length of hospital stay by up to 10 extra days, increases the likelihood of discharge to an institution and is a predictor of death [8]. In patients who require mechanical ventilation, those affected by delirium are three times more likely to die by 6 months, and the risk of dying is increased the longer time a patient is delirious [7].

In those patients who survive a critical illness, and developed delirium during the ICU stay, regardless of age, the risk of long-term cognitive impairment following discharge is increased nine times, and it is three times more likely to persist after discharge [9, 10]. This can be equivalent to mild Alzheimer’s disease. Patients’ quality of life and independence are consequently reduced with high detrimental impact on carers and families, financially and emotionally.

Whether or not a patient develops delirium will depend on the risk factors (Table 2.2).

Predisposing factors are those that make a patient more vulnerable to developing delirium following what may be only a relatively mild trigger. They are present on admission and are rarely modifiable: age, history of cognitive impairment, previous episodes of delirium, alcohol abuse, hypertension, age, liver impairment and other chronic medical conditions [11]. The precipitating cause of ICU delirium may be potentially treatable: often an infection or an episode of sepsis, electrolyte imbalance, renal failure, hypercarbia or the use of deliriogenic drugs. Non-modifiable causes would include stroke, traumatic brain injury and pancreatitis. Aggravating risk factors include the use of a bladder catheter, uncontrolled pain, hypnotic and narcotic drugs, visual or hearing impairment and immobility [12]. PRE-DELIRIC is a validated delirium risk prediction score for ICU patients derived from data collected during the first 24 h and is freely available for use [13].

Monitoring delirium is an essential part of routine daily assessment. Ongoing delirium and new delirium is a significant clinical sign. Several studies have demonstrated that clinicians regularly miss delirium in the ICU settings [14]. Most critically ill patients will develop hypoactive delirium, but will be able to obey direct commands, e.g. stick your tongue out, squeeze my hand and usually answer yes to most questions, e.g. do you feel better this morning? In order to detect delirium in critically ill patients, whether intubated or not, clinicians need to use a screening tool or have a meaningful exchange. Routine screening for of all acutely ill hospital patients has been recommended by the National Institute for Health and Care Excellence (NICE) and the Pain, Agitation and Delirium (PAD) guidelines from the American College of Critical Care [15, 16]

Currently, there are two non-verbal screening tools for use in intubated patients, and both are recommended in the PAD guidelines: the Confusion Assessment Method for the Intensive Care Unit (CAM-ICU) and the Intensive Care Delirium Screening Checklist (ICDSC) [17, 18]. They were developed for clinical use and have been validated against the standard DSM-4 criteria. Both tools can be used in sedated intubated patients for the assessment of delirium; however, patients must be responsive, i.e. able to open their eyes and keep them open in response to a verbal stimulus, usually their name. They require training and practice.

The CAM-ICU is a point in time assessment usually completed twice a shift or when the nurse detects a change in mental status, while the ICDSC is completed during the course of a shift on observing patient behaviour. It is worth specifying that low-arousal states of acute onset with a severely withdrawn patient who does not interact should be recognised as likely indicating delirium.

New delirium is an important early sign of clinical deterioration; it often precedes other indicators and can enable early intervention.