1. Epidemiology and Impact

Delirium, agitation, and anxiety are profoundly common in the intensive care unit, affecting up to 80% of mechanically ventilated patients. These conditions are not mere side effects of critical illness but are independent drivers of worse outcomes, including increased mortality and long-term cognitive decline.

Prevalence and Outcomes

• Prevalence: Delirium is identified in 65–80% of mechanically ventilated patients, compared to 20–50% in non-ventilated ICU patients.
• Mortality: In older adults, each additional day of delirium is associated with an approximate 10% increase in the hazard of death at one year.
• Resource Utilization: Delirium contributes to increased ventilator days, longer ICU and hospital stays, and a higher risk of self-extubation and removal of critical lines.
• Long-Term Sequelae: Between 30–40% of ICU delirium survivors experience persistent, disabling cognitive deficits, a key component of Post-Intensive Care Syndrome (PICS).

2. Pathophysiological Mechanisms

ICU-related brain dysfunction is not caused by a single insult but rather a convergence of disturbances in neurotransmission, neuroinflammation, and cellular metabolism that disrupt normal brain function.

Core Mechanisms

• Neurotransmitter Imbalance: A central theory involves a functional decrease in acetylcholine (leading to inattention and memory loss) and an excess of dopamine (contributing to psychosis and agitation). Dysregulation of GABA, glutamate, and serotonin also plays a significant role in anxiety and excitotoxicity.
• Neuroinflammation & Blood-Brain Barrier (BBB) Disruption: Systemic inflammation from conditions like sepsis allows inflammatory cytokines (e.g., IL-6) to cross a compromised BBB. This activates microglia, the brain’s resident immune cells, promoting a state of neuroinflammation.
• Oxidative Stress: Hypoxia, systemic inflammation, and metabolic derangements lead to the production of reactive oxygen species, which cause mitochondrial dysfunction and promote neuronal injury and apoptosis.

3. Predisposing Risk Factors

Patient-specific factors determine an individual’s baseline susceptibility to developing delirium. These can be categorized as nonmodifiable and modifiable.

Nonmodifiable Factors

• Advanced Age: Age ≥65 years is one of the strongest predictors.
• Baseline Cognitive Status: Preexisting dementia, mild cognitive impairment, or history of delirium significantly lowers the threshold for future episodes.
• Frailty and Illness Severity: High admission severity scores (e.g., APACHE II) and underlying frailty indicate reduced physiologic reserve.

Modifiable or Chronic Health Factors

• Chronic Organ Dysfunction: Hepatic or renal failure impairs the clearance of toxins and drugs, increasing the risk of metabolic encephalopathy. Cardiovascular disease can alter cerebral perfusion.
• Medication Burden: Polypharmacy, particularly a high anticholinergic drug load from medications like tricyclic antidepressants, first-generation antihistamines, or certain muscle relaxants, is a major risk.
• Social Determinants: Factors like poor health literacy, limited access to medications, and inadequate caregiver support can contribute to a patient’s vulnerability.

4. Precipitating Factors

In a patient with predisposing risks, acute ICU exposures and environmental stressors can act as triggers that precipitate an episode of delirium.

Common ICU Triggers

• Sedative-Analgesics: Benzodiazepines, particularly via continuous infusion, are independent risk factors for increased delirium risk and duration. While data on opioids is mixed, high doses and accumulation in renal failure may contribute.
• Environmental Stressors: Sensory deprivation (e.g., hearing or vision impairment without aids), excessive noise and light, and severe sleep fragmentation from frequent interruptions disrupt the normal sleep-wake cycle.
• Physiologic Insults: Any acute infection (especially sepsis), metabolic derangement (e.g., electrolyte abnormalities, hypoglycemia), or episode of hypoxia can directly trigger delirium.

Clinical Case Snapshot: A 78-year-old with chronic kidney disease, admitted for pneumonia, develops sudden inattention and lethargy on day 3 of his ICU stay. He is receiving a continuous lorazepam infusion for agitation. This presentation is highly suspicious for hypoactive delirium, precipitated by the benzodiazepine in a vulnerable patient. The immediate next steps should include reviewing the sedative choice and implementing nonpharmacologic sleep promotion strategies.

5. Clinical Presentation and Subtypes

Delirium is primarily a disorder of inattention and acute cognitive change, but its presentation varies. It is crucial to differentiate delirium from pure anxiety, which can have overlapping signs.

Motoric Subtypes of Delirium

• Hyperactive: The most recognized subtype, characterized by agitation, restlessness, emotional lability, and sometimes hallucinations or delusions. These patients are often a safety risk to themselves and staff.
• Hypoactive: The most common subtype, presenting with lethargy, apathy, reduced speech, and inattention. It is often mistaken for depression or fatigue and is associated with a worse prognosis.
• Mixed: Patients fluctuate, often within hours, between hyperactive and hypoactive features.

Anxiety Overlap

Anxiety may present with tachycardia, diaphoresis, and restlessness. However, if the patient remains attentive and can follow commands, it is less likely to be delirium. Always consider underlying causes like pain, hypoxemia, or substance withdrawal.

6. Clinical Implications and Early Recognition

Given the profound impact of delirium, a proactive approach focused on risk stratification, routine screening, and prevention is the standard of care.

Strategies for Early Recognition

• Risk Stratification Models: Tools like the PRE-DELIRIC model can be used on admission to predict a patient’s risk of developing delirium. This model incorporates factors like age, admission diagnosis, coma, and illness severity with good predictive accuracy (AUC ~0.87).
• Routine Screening: Performing the CAM-ICU or ICDSC at least once per nursing shift, and more often if a patient’s mental status changes, dramatically improves detection rates over unstructured observation.
• Interdisciplinary Collaboration: An effective delirium program requires a team approach. Pharmacists can perform medication reviews, nurses are central to screening and implementing non-pharmacologic interventions, and physicians lead the diagnostic process and overall management strategy. Integrating alerts for high-risk patients into the electronic health record (EHR) can further support this process.