Clinical meaning
Delirium is an acute, fluctuating disturbance in attention, awareness, and cognition that develops over hours to days and represents a medical emergency indicating underlying physiological derangement. It is NOT a psychiatric disorder but rather a manifestation of acute brain dysfunction caused by medical illness, medication effects, metabolic derangement, or physiological stress. Delirium affects 20-30% of general medical inpatients, 50-80% of ICU patients on mechanical ventilation, 15-53% of post-surgical patients (highest after hip fracture repair and cardiac surgery), and up to 87% of patients receiving palliative care at the end of life. Despite its high prevalence, delirium is underdiagnosed in 50-70% of cases, particularly the hypoactive subtype, because it is often mistaken for depression, fatigue, or dementia. The pathophysiology of delirium is multifactorial and incompletely understood, but several interconnected mechanisms have been identified. The neuroinflammatory hypothesis proposes that systemic inflammation (from infection, surgery, trauma, or critical illness) activates peripheral immune cells that release pro-inflammatory cytokines (IL-1beta, IL-6, TNF-alpha) into the systemic circulation. These cytokines cross the blood-brain barrier (BBB) -- either through circumventricular organs where the BBB is naturally fenestrated, through active cytokine transport mechanisms, or through a BBB that has been disrupted by illness -- and activate microglial cells (the brain's resident immune cells). Activated microglia release additional pro-inflammatory mediators within the brain parenchyma, causing neuroinflammation that disrupts synaptic transmission, damages neurons, and impairs the integrated cortical and subcortical network function required for normal attention, awareness, and cognition. The neurotransmitter imbalance hypothesis identifies specific neurotransmitter disturbances that produce the clinical features of delirium. The most consistently identified abnormality is CHOLINERGIC DEFICIENCY -- reduced acetylcholine (ACh) activity in the cerebral cortex. Acetylcholine is the primary neurotransmitter mediating sustained attention and cognitive processing in the cortex, and its deficiency produces the hallmark attention deficit of delirium. This cholinergic deficiency explains why anticholinergic medications (diphenhydramine, oxybutynin, atropine, promethazine, scopolamine, and many others) are among the most common precipitants of delirium, particularly in elderly patients with already-reduced cholinergic reserve. Concurrently, DOPAMINERGIC EXCESS (increased dopamine activity) contributes to the hallucinations, psychomotor agitation, and perceptual disturbances seen in hyperactive delirium. This is why dopamine-blocking agents (haloperidol, other antipsychotics) can help manage the agitation and psychosis of delirium. Other neurotransmitter systems involved include: excess glutamate release (excitotoxicity from metabolic stress, contributing to neuronal injury), excess norepinephrine (from sympathetic hyperactivation during illness, contributing to hyperarousal and agitation), excess cortisol (from HPA axis activation during stress, which is neurotoxic and impairs hippocampal function), and altered GABA-ergic transmission (both excess and deficiency depending on the clinical context -- excess GABA causes the somnolence of hepatic encephalopathy, while GABA deficiency causes the agitation of alcohol withdrawal delirium). The oxidative stress hypothesis proposes that impaired cerebral oxidative metabolism (from hypoxia, hypoperfusion, metabolic derangement, or mitochondrial dysfunction) reduces ATP production in neurons, leading to failure of ATP-dependent ion pumps, loss of membrane potential, calcium influx, free radical generation, and neuronal dysfunction or death. This mechanism explains why conditions that impair cerebral oxygen delivery (hypoxia, anemia, heart failure, shock) or cerebral metabolism (hepatic/renal failure, hypoglycemia, thiamine deficiency) are potent delirium triggers. Delirium is clinically classified into three psychomotor subtypes. Hyperactive delirium (25% of cases) presents with agitation, restlessness, pulling at lines and tubes, combativeness, hallucinations, and autonomic hyperactivity. Hypoactive delirium (50% of cases -- the most common and most frequently MISSED subtype) presents with lethargy, decreased responsiveness, reduced psychomotor activity, flat affect, and withdrawal -- it is often mistaken for depression, fatigue, medication effect, or simply 'being a good patient.' Mixed delirium (25% of cases) alternates between hyperactive and hypoactive features. The Confusion Assessment Method (CAM) is the most widely validated and commonly used delirium screening tool. It requires the presence of: (1) ACUTE onset and FLUCTUATING course, AND (2) INATTENTION, AND either (3) DISORGANIZED THINKING or (4) ALTERED LEVEL OF CONSCIOUSNESS. The sensitivity of the CAM is 94-100% and specificity is 90-95% when administered by trained assessors. The CAM-ICU is the adapted version for mechanically ventilated patients who cannot speak. Delirium prevention (multicomponent non-pharmacological strategies) is more effective than treatment of established delirium. The Hospital Elder Life Program (HELP) is the most evidence-based delirium prevention program and reduces delirium incidence by 33-40% through systematic implementation of: reorientation protocols (clocks, calendars, family photos, consistent caregivers), sleep promotion (minimize nighttime interruptions, dim lights, reduce noise, warm milk/relaxation techniques), early mobilization (get patients out of bed, ambulate, reduce restraint use), cognitive stimulation (conversation, reminiscence, newspapers, puzzles), visual/hearing optimization (ensure glasses and hearing aids are in place), hydration and nutrition assistance, and medication review (discontinue or minimize deliriogenic medications, particularly anticholinergics, benzodiazepines, and opioids). Treatment of established delirium focuses on identifying and treating the underlying cause (infection, metabolic abnormality, medication effect, pain, urinary retention, constipation) rather than simply medicating the symptoms. Pharmacological management with low-dose haloperidol or atypical antipsychotics is reserved for patients with severe agitation that poses a safety risk -- antipsychotics do NOT shorten the duration of delirium but can manage dangerous agitation. Benzodiazepines should be AVOIDED in delirium management (they worsen delirium in most cases) EXCEPT for alcohol or benzodiazepine withdrawal delirium, where benzodiazepines are the specific treatment.