Delirium

• Cholinergic deficiency hypothesis: Reduced activity of cholinergic neurons projecting from the basal forebrain and pedunculopontine tegmentum to the cortex and thalamus is the most consistent neurochemical finding in delirium. Anticholinergic medications precipitate delirium by blocking acetylcholine at nicotinic and muscarinic receptors, impairing attention and executive function. Acetylcholine is critical for attention, memory consolidation, and arousal; its reduction leads to the core cognitive dysfunction. This explains why anticholinergics worsen delirium and why cholinesterase inhibitors (physostigmine) can provide rapid reversal in anticholinergic toxidrome.

• Dopaminergic hyperactivity and disruption of prefrontal function: Elevated dopamine in mesolimbic and mesocortical pathways, coupled with reduced dopamine in prefrontal circuits, disrupts executive function and attention networks. Hyperactive delirium phenotypes are associated with relative dopaminergic excess and hyperactivation of the anterior cingulate and anterior insula. This imbalance explains psychomotor hyperactivity, agitation, hallucinations, and delusional thinking in hyperactive delirium. Dopamine excess also contributes to disruption of the salience network, causing inability to filter irrelevant stimuli.

• Glutamatergic excitotoxicity and GABAergic dysfunction: Acute hypoxia, hypoglycemia, seizures, or metabolic derangement causes excessive glutamate release and activation of NMDA receptors, leading to neuronal calcium influx and excitotoxic damage. Concurrently, gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter, is depleted or its receptor sensitivity is reduced through phosphorylation changes in response to stress. This imbalance between excitatory and inhibitory tone disrupts normal rhythmic cortical activity and destabilizes neural networks. Alcohol and benzodiazepine withdrawal represent the extreme of this mechanism—sudden loss of GABA augmentation causes unopposed glutamatergic activity.

• Inflammatory cascade and cytokine hypothesis: Systemic infections, trauma, surgery, and inflammatory conditions trigger release of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α, IL-8) that cross the blood-brain barrier or trigger central microglia activation. Activated microglia produce additional cytokines and reactive oxygen species (ROS) that cause neuroinflammation, astrocyte activation, and altered blood-brain barrier permeability. The resulting neuroinflammatory state disrupts synaptic plasticity and neural oscillations, particularly in attention networks. This explains why delirium frequently accompanies sepsis, pneumonia, and other infections despite absence of direct CNS infection.

• Thalamic gating dysfunction: The thalamus serves as the central relay for sensory information to the cortex and maintains arousal and attention through reticular activating system control. In delirium, thalamic relay function is disrupted, leading to inability to filter and prioritize sensory information. Increased thalamic gamma-band oscillations and disrupted thalamo-cortical connectivity on electroencephalography (EEG) reflect this dysfunction. This mechanism explains the inability to maintain focused attention and the intrusion of irrelevant stimuli into consciousness.

• Circadian rhythm disruption: Acute illness causes loss of circadian organization through suppression of melatonin production, disruption of the suprachiasmatic nucleus function, and loss of environmental zeitgebers (light-dark cycles). Delta and theta power increase globally on EEG, reflecting loss of the normal wake-promoting activity. Sleep deprivation from ICU environment perpetuates delirium through reduction of cholinergic tone and accumulation of sleep pressure.

• Network disconnection and default mode network dysfunction: Functional MRI and EEG studies demonstrate that delirium is associated with reduced connectivity within the default mode network (posterior cingulate, medial prefrontal cortex, temporoparietal junction) and disrupted communication between the default mode network and attention networks. This network desynchronization prevents normal integration of information and self-referential processing, manifesting as disorganized thinking and temporal disorientation.

Precipitating Factors (Acute Insults)

• Infections: Urinary tract infection (most common infectious cause in elderly), pneumonia, sepsis, meningitis, encephalitis, abdominal infections. Any infection can trigger delirium through inflammatory cascade even without bacteremia; elderly patients may develop delirium before fever or leukocytosis appears.

• Medications: Anticholinergics (diphenhydramine, oxybutynin, tricyclic antidepressants, atropine), benzodiazepines (paradoxically both use and withdrawal), opioids (especially in elderly or renal failure), sedating antipsychotics, corticosteroids, H2-blockers (cimetidine), fluoroquinolones, NSAIDs, and polypharmacy causing drug-drug interactions. Anticholinergic burden is a validated predictor of delirium risk.

• Metabolic derangements: Hypoxemia, hypercapnia, hypoglycemia, hyperglycemia, hyponatremia, hypernatremia, hypokalemia, hypercalcemia, uremia, hepatic encephalopathy, hypomagnesemia, hypophosphatemia. Any acute electrolyte disturbance can precipitate delirium independent of severity.

• Hypoxia and respiratory failure: Pneumonia, pulmonary embolism, acute coronary syndrome, heart failure, severe anemia, carbon monoxide poisoning, severe hypotension. Delirium may be the first sign of hypoxia in the elderly before oxygen saturation drops significantly.

• Cardiovascular events: Myocardial infarction, arrhythmias (especially atrial fibrillation), stroke, transient ischemic attack, severe hypertension or hypotension, heart failure exacerbation causing cerebral hypoperfusion.

• Neurological conditions: Seizures (including non-convulsive status epilepticus, which presents as behavioral change), stroke, intracranial hemorrhage, subdural hematoma, meningitis, encephalitis, posterior reversible encephalopathy syndrome (PRES).

• Toxins and intoxication: Alcohol intoxication or withdrawal (most common toxicological cause), opioid intoxication, sympathomimetic intoxication (cocaine, amphetamines, pseudoephedrine), anticholinergic toxidrome, organophosphate poisoning, heavy metal exposure.

• Withdrawal syndromes: Alcohol withdrawal (develops 6-24 hours after last drink), benzodiazepine withdrawal, opioid withdrawal (less commonly causes delirium but possible), nicotine withdrawal in hospitalized patients.

• Hyperthermia and hypothermia: Sepsis-related fever, neuroleptic malignant syndrome, malignant hyperthermia, heat stroke causing direct neurotoxicity; hypothermia causing CNS depression and arrhythmias.

• Nutritional deficiencies: Thiamine deficiency (Wernicke encephalopathy presenting with delirium, ophthalmoplegia, ataxia), niacin deficiency (pellagra), B12 deficiency, folate deficiency, vitamin E deficiency.

• Endocrine emergencies: Diabetic ketoacidosis, hyperosmolar hyperglycemic state, hypoglycemia, thyroid storm, myxedema coma, adrenal crisis, severe hypoparathyroidism.

• Organ failure: Acute kidney injury (uremia), acute liver failure or decompensated cirrhosis (hepatic encephalopathy from hyperammonemia and altered neurotransmitter metabolism), acute respiratory failure.

• Blood loss and anemia: Hemorrhagic shock, severe chronic anemia causing cerebral hypoxia.

• Surgery: Postoperative delirium very common, multifactorial (anesthesia, medications, pain, sleep deprivation, infection, metabolic derangement).

• Neoplastic conditions: Brain metastases, paraneoplastic encephalitis (anti-NMDA, anti-GABA-B, anti-LGI1 antibodies), tumor lysis syndrome causing metabolic derangement.

Predisposing Risk Factors (Baseline Vulnerability)

• Advanced age (≥65 years) is the strongest predisposing factor; age >80 associated with exponentially higher risk. Older adults have reduced cholinergic reserve, increased medication sensitivity, and greater vulnerability to physiological insults.

• Dementia or cognitive impairment: Preexisting cognitive decline markedly increases delirium risk; presence of dementia indicates baseline cholinergic and dopaminergic system dysfunction.

• Severe comorbid illness: Chronic kidney disease, chronic liver disease, chronic obstructive pulmonary disease, coronary artery disease, congestive heart failure, diabetes mellitus.

• Multiple medications (polypharmacy, typically ≥5 medications): Each medication increases anticholinergic burden and risk of drug-drug interactions.

• Visual and hearing impairment: Sensory deprivation reduces ability to maintain orientation and increases misinterpretation of environmental stimuli.

• Malnutrition and frailty: Reduced physiological reserve and increased sensitivity to stressors.

• Alcohol use disorder: Baseline disruption of neurotransmitter systems and increased sensitivity to additional insults.

• Depression and anxiety: Psychological stressors and neurochemical predisposition.

• Prior episode of delirium: Indicates baseline vulnerability and risk of recurrence.

• Immobility and functional dependence: Reduced ability to adapt to acute stressors.

Core Features (Required for Diagnosis)

• Acute onset and fluctuating course (hours to days, not gradual over weeks): Change in mental status represents departure from baseline. Fluctuation is characteristic—symptoms wax and wane throughout the day, typically worse in evening ("sundowning"), with lucid intervals alternating with confusion. Morning may show improvement only to deteriorate by afternoon. This fluctuation distinguishes delirium from dementia (gradual, stable baseline cognitive decline) and other psychiatric conditions.

• Inattention (lack of focused, sustained attention): The core cognitive feature. Patient cannot maintain attention on examiner, task, or conversation; easily distracted by irrelevant stimuli; unable to shift attention purposefully. Tests with months backward, digit span, serial 7s, or Months backward all demonstrate this. Inattention makes assessment of other cognitive domains difficult because patient cannot focus long enough to process questions.

• Disorganized thinking: Manifests as incoherent speech, illogical thought processes, rambling, tangentiality, circumstantiality, or inability to follow commands. Speech may be pressured (hyperactive delirium) or sparse (hypoactive delirium). Flight of ideas, perseveration, or blocking may occur.

• Altered level of consciousness: Either hyperarousal (hyperalert, vigilant) as in hyperactive delirium, or hypoarousal (lethargic, decreased responsiveness) as in hypoactive or mixed delirium. This distinguishes delirium from primary psychiatric conditions which typically maintain normal consciousness level.

Psychomotor Phenotypes

• Hyperactive delirium (20-30% of cases): Patient is agitated, restless, loud, aggressive, with increased goal-directed but inappropriate behavior. May attempt to remove IV lines, climb out of bed, strike staff. Speech is rapid and pressured. Increased sympathetic tone with tachycardia, hypertension, hyperthermia, diaphoresis. More commonly recognized by clinicians and family as "delirium" but carries better prognosis than hypoactive form. Often associated with substance withdrawal (alcohol, benzodiazepines), anticholinergic toxidrome, or sympathomimetic intoxication.

• Hypoactive delirium (50-60% of cases, often underrecognized): Patient is lethargic, withdrawn, apathetic, with psychomotor retardation. Speech is sparse, responses delayed or absent. Patient may be quiet enough that behavioral disturbance goes unnoticed by staff ("quiet confusion"), but cognition is severely impaired. Often misattributed to depression or normal post-illness recovery. Associated with worse outcomes, higher mortality, longer ICU stays, and greater long-term cognitive impairment because diagnosis is delayed and underlying causes not rapidly addressed. Common in sepsis, hypoxia, hepatic encephalopathy, and advanced cancer.

• Mixed delirium: Features of both hyperactivity and hypoactivity, often with fluctuation between states within same day.

Cognitive Symptoms Beyond Inattention

• Disorientation: Typically to time (not knowing date, day of week, season, part of day) followed by place (hospital vs home, city), then person (names of family members, own name in severe cases). Orientation to person preserved longer than time and place. Ask "What year is it? What hospital are you in? Who is the president?"

• Memory impairment: Especially difficulty with recent (short-term) memory; cannot recall why admitted, recent conversations, or recent events. Registration may be intact but encoding and retrieval impaired. Long-term memory relatively preserved early.

• Visuospatial dysfunction: Inability to copy simple figures, draw clock, or navigate familiar environments. May become lost going to bathroom in hospital.

• Language disturbance (delirium-specific): Speech output may be incoherent or pressured (hyperactive) or sparse (hypoactive), but traditional aphasia with anomia or repetition deficits unusual; distinguishes from primary language disorders.

Perceptual Disturbances

• Hallucinations (visual > auditory > tactile): Often terrifying; patient sees insects, animals, threatening figures, or family members. Tactile hallucinations may involve sensation of bugs crawling on skin (formication) typical of withdrawal syndromes. Hallucinations are NOT pathognomonic for delirium—can occur in psychosis or other conditions—but in acute setting with inattention strongly suggest delirium.

• Delusions: Often persecutory, mood-congruent, or based on misinterpretation of hallucinations (seeing visions of dead relatives). Delusions are typically simpler and less systematized than in primary psychiatric disease.

• Misidentification: May not recognize family members or hospital staff; may mistake IV poles for people or believe healthcare workers are threatening.

Emotional and Behavioral Manifestations

• Affect instability: Rapid shifts in mood from fearful to angry to euphoric, often without apparent cause. Mood may be incongruent with thought content.

• Paranoia and fear: Often terrified of caregivers or environment, may resist medication or care believing staff are trying to harm.

• Sleep-wake cycle disruption: Severe insomnia at night with daytime somnolence is characteristic. Patient may be unable to sleep despite fatigue and may sleep fitfully with nightmares.

• Aggression: In hyperactive delirium, may strike out or kick; requires careful de-escalation and may necessitate physical or chemical restraint.

Vegetative Symptoms

• Autonomic dysregulation: Tachycardia, hypertension, tachypnea, hyperthermia (or hypothermia in severe delirium), diaphoresis. Vital sign abnormalities may be out of proportion to clinical illness.

• Tremor: Fine tremor, especially of hands; coarse tremor suggests alcohol or benzodiazepine withdrawal or severe metabolic derangement.

Physical Examination Findings

• Inattention on formal testing: Months backward test, digit span, serial 7s, or vigilance A test (tap whenever letter A appears in random letter sequence) all demonstrate inability to maintain attention.

• **Disorganized speech

Delirium is a clinical, bedside diagnosis — no laboratory test confirms it.

• DSM-5-TR criteria (APA): disturbance in attention/awareness developing acutely over hours to days, fluctuating in severity, with an additional cognitive disturbance, not better explained by a pre-existing neurocognitive disorder, and evidence from history, examination, or labs of a physiologic cause.
• **Confusion Assessment Method (CAM): the most widely used bedside instrument. Requires (1) acute onset/fluctuating course and (2) inattention, plus either (3) disorganized thinking or** (4) altered level of consciousness. CAM-ICU is the validated nonverbal version for intubated patients; ICDSC and 4AT are alternatives. Level of arousal is graded separately with the RASS.

Immediate bedside workup (do these before anything else)

• Point-of-care glucose and pulse oximetry: hypoglycemia and hypoxemia are instantly reversible and instantly lethal.
• Vital signs and medication reconciliation: fever, hypotension, or a newly started anticholinergic, opioid, or benzodiazepine is often the entire answer.

Etiologic laboratory search

• Core panel: CBC, complete metabolic panel with calcium and magnesium, urinalysis with culture, blood cultures if febrile, ECG, chest radiograph, urine toxicology. Add ammonia when cirrhosis is present, TSH, B12, and arterial blood gas when hypercapnia is plausible.
• Neuroimaging is not routine: noncontrast head CT is indicated for focal neurologic deficit, head trauma, anticoagulation, new seizure, or delirium that remains unexplained after the metabolic/infectious workup.
• Lumbar puncture: for fever with meningismus, immunosuppression, or unexplained delirium with headache.
• EEG: reserved for suspected nonconvulsive status epilepticus. Delirium classically shows diffuse background slowing (loss of the posterior alpha rhythm with generalized theta/delta); triphasic waves suggest hepatic or uremic encephalopathy. A normal-frequency background argues for a primary psychiatric cause.

Immediate stabilization

• Airway, oxygenation, glucose: correct hypoxemia and hypoglycemia at once. Give thiamine before or with dextrose in malnourished or alcohol-using patients to avoid precipitating Wernicke encephalopathy.
• Treat the precipitant: antibiotics for infection, correction of sodium/calcium, relief of urinary retention or fecal impaction, pain control, and withdrawal of the offending drug. Delirium resolves when its cause does.

First-line management is nonpharmacologic (SCCM PADIS and American Geriatrics Society)

• Multicomponent programs (Hospital Elder Life Program): repeated reorientation, glasses and hearing aids restored, early mobilization, daytime light with nighttime quiet, sleep-wake normalization, hydration, and removal of tethers (catheters, restraints, telemetry).
• Deprescribing: stop anticholinergics, benzodiazepines, sedating antihistamines, and H2 blockers per the AGS Beers Criteria.

Pharmacotherapy — symptomatic only

• Antipsychotics: SCCM PADIS does not recommend routine haloperidol or atypical agents to prevent or shorten delirium; they have not been shown to reduce duration or mortality. Reserve a butyrophenone (haloperidol) or second-generation agent (quetiapine, olanzapine) for agitation that threatens patient or staff safety, at the lowest effective dose, with ECG monitoring for QT prolongation.
• Dexmedetomidine: an alpha-2 agonist preferred over benzodiazepine sedation in mechanically ventilated patients with agitated delirium; watch for bradycardia and hypotension.

Contraindicated / exception rules

• Benzodiazepines worsen delirium and are avoided — except in alcohol or sedative-hypnotic withdrawal, where a benzodiazepine (lorazepam) is definitive therapy, and in stimulant toxicity or serotonin syndrome/NMS.
• Avoid dopamine blockers in Parkinson disease and Lewy body dementia: severe neuroleptic sensitivity; use quetiapine if a drug is unavoidable.
• Physical restraints increase injury, immobility, and delirium duration; use only as a brief last resort.
• Physostigmine is the antidote for pure anticholinergic toxidrome, not a general delirium treatment.

Complications of the delirium itself

• Missed lethal precipitant (emergency): hypoglycemia, hypoxemia, meningitis, Wernicke encephalopathy, nonconvulsive status epilepticus, and sepsis all masquerade as "confusion." The signal is delirium that persists or deepens after supportive care — reopen the workup rather than sedating.
• Delirium tremens (emergency): unopposed glutamatergic activity after loss of GABAergic tone. Signaled by delirium 48–96 hours after the last drink with fever, tachycardia, hypertension, and diaphoresis; untreated it carries substantial mortality. Requires benzodiazepines and ICU-level monitoring.
• Falls, fractures, and self-extubation or line removal: impaired attention plus psychomotor agitation. A hyperactive patient pulling at an endotracheal tube is an airway emergency.
• Aspiration pneumonia and pressure injury: depressed arousal and immobility, particularly in hypoactive delirium; signaled by new hypoxemia or infiltrate.
• Immobility cascade: deconditioning, venous thromboembolism, functional decline, and loss of independent living.
• Long-term cognitive impairment and incident dementia: neuroinflammation and network disruption outlast the acute illness; recovery may be incomplete, and duration of delirium tracks with severity of later deficits. Post-ICU PTSD from delusional memories is also described.
• Increased in-hospital and post-discharge mortality, prolonged length of stay.

Complications of treatment

• **Antipsychotic QT prolongation → *torsades de pointes*** (emergency): potassium channel blockade; watch the corrected QT interval, especially with IV haloperidol, hypokalemia, or hypomagnesemia.
• **Extrapyramidal symptoms, akathisia, and *neuroleptic malignant syndrome***: D2 blockade; NMS presents with rigidity, hyperthermia, autonomic instability, and elevated creatine kinase — an emergency. Antipsychotics also carry an FDA boxed warning for increased mortality in elderly patients with dementia.
• Benzodiazepine paradoxical agitation and respiratory depression: deepens delirium outside withdrawal states.
• Dexmedetomidine bradycardia and hypotension; restraint-associated injury, rhabdomyolysis, and worsening agitation.

• Inattention is the cardinal feature: a stem describing a patient who cannot recite the months backward, loses track mid-sentence, or must be re-asked every question is describing delirium, not dementia or psychosis.
• Single best next step in an acutely confused patient: check a fingerstick glucose and oxygen saturation, then review the medication list. The most commonly tested reversible causes are drugs (anticholinergics, benzodiazepines, opioids) and infection — urinary tract infection in the elderly, who may be afebrile with a normal white count.
• Hypoactive delirium is the most common subtype and the most dangerous because it is mistaken for depression, fatigue, or "just being old." The distractor to avoid is starting an antidepressant in a quietly withdrawn, inattentive inpatient.
• Acute + fluctuating + altered consciousness = delirium; insidious + stable + clear sensorium = dementia. Delirium can be superimposed on dementia — a sudden change from a demented baseline is still delirium until proven otherwise.
• EEG shows diffuse background slowing in delirium and is normal in primary psychiatric illness; triphasic waves point to hepatic encephalopathy. Order an EEG when nonconvulsive status epilepticus is possible.
• Benzodiazepines cause delirium — except when they cure it. Alcohol or sedative-hypnotic withdrawal is the one setting where a benzodiazepine is first-line; everywhere else, per SCCM PADIS, nonpharmacologic measures come first and antipsychotics are reserved for dangerous agitation.
• The association examiners love: anticholinergic toxidrome — hot as a hare, dry as a bone, red as a beet, mad as a hatter, blind as a bat — reversed by physostigmine, reflecting the cholinergic-deficiency mechanism of delirium.
• Never give haloperidol to a patient with Parkinson disease or Lewy body dementia; neuroleptic sensitivity can be catastrophic. Also remember the FDA boxed warning on antipsychotic mortality in elderly patients with dementia.