Delirium, also termed as 'acute confusional state', 'toxic or metabolic encephalopathy', 'acute brain failure', is essentially defined by the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) criteria as an acute change in attention and awareness that develops over a relatively short time interval and associated with additional cognitive deficits such as memory deficit, disorientation, or perceptual disturbances. It is a common phenomenon, occurring in 20% to 70% of hospitalized patients. The term 'ICU psychosis' is an unfortunate and outdated misnomer for delirium. This term, indeed, was previously used to refer to hyperactive delirium within the intensive care unit (ICU) setting and came into use when the high prevalence of delirium was recognized in this population. Several investigations proved that the higher incidence of delirium manifests in ICU patients on mechanical ventilation (MV). In this setting, delirium occurs in up to 80% of patients.
Because delirium represents the most common clinical manifestation of acute brain dysfunction in ICU, affecting up to 83% of ICU patients on mechanical ventilation (MV), new-onset confusion in the adult patient always warrants further evaluation. However, the clinical evaluation must be accurate as it can often be difficult to distinguish this phenomenon from other clinical conditions. DSM-5 criteria explicitly state that these new changes in mentation must be in the absence of a neurocognitive disorder that could explain the confusion, and do not occur in the setting of a reduced level of arousal (e.g., coma). Thus, although an identifiable cause of the delirium is often not found, a thorough evaluation for reversible causes of delirium is warranted, and multiple causes may be present in combination. In this regard, there is a large array of possible causes of delirium that range from intoxication and withdrawal states to other serious neurological insults like meningitis and stroke. The prevention, identification, and management of delirium has important consequences for patient outcomes, both during admission and after discharge.
Regardless of the classification, there are three subtypes of delirium categorized according to the psychomotor behavior:
While hyperactive delirium is the more commonly identified form of delirium outside the ICU, the hypoactive (24.5% to 43.5%) and mixed (52.5%) types are more often observed in the ICU setting. Hyperactive ICU delirium accounts for approximately 23% of cases. It is characterized by agitation, restlessness, emotional lability, and positive psychotic features such as hallucinations, illusions that often interfere with the delivery of care. It should be remembered that new-onset psychotic symptoms in older adult patients are unlikely to be a primary mental illness, and search for a pharmacological or physiological cause should be carried out. Hypoactive delirium is commonly characterized by confusion, sedation, apathy, decreased responsiveness, slowed motor function, withdrawn attitude, lethargy, and drowsiness. This type of delirium is often underrated and is associated with a worse prognosis as patients who suffered from hypoactive delirium showed increased 6-month mortality compared to the patients previously affected by other subtypes of delirium. Mixed delirium is the most frequent type, accounting for about half of the total cases. It is a combination of the two forms previously described, and patients manifest a fluctuation of hypoactive and hyperactive features.
This chapter is aimed at presenting clinical features, evaluation, prophylactic strategies, and treatment of delirium in the setting of ICU. The role of the interprofessional team in evaluating and treating critically ill patients with this condition is also addressed.
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