An 82-year-old woman admitted to the medical ward for pneumonia develops acute confusion, agitation, and visual hallucinations over 12 hours. Her daughter reports the patient was cognitively intact 2 days ago. On examination, her consciousness level fluctuates, and she is inattentive. Blood cultures and chest X-ray confirm bacterial pneumonia. Which feature most reliably distinguishes this acute presentation from a chronic neurodegenerative dementia?

Explanation

## Distinguishing Feature: Acute Onset + Identifiable Cause + Fluctuation **Key Point:** Delirium is distinguished from dementia by three cardinal features: **acute onset (hours to days), an identifiable medical/toxic precipitant, and fluctuating consciousness and attention.** ### Core Diagnostic Criteria Comparison | Criterion | Delirium | Dementia | |-----------|----------|----------| | **Onset** | Acute (hours–days) | Insidious (months–years) | | **Precipitant** | Medical, toxic, or metabolic (identifiable) | Neurodegenerative; no acute trigger | | **Attention** | Severely impaired, fluctuating | Relatively preserved early; steady decline | | **Reversibility** | Often reversible with treatment | Irreversible | | **Consciousness** | Altered (hypoactive/hyperactive/mixed) | Alert until late stages | **High-Yield:** In this case: - **Acute onset** over 12 hours (not months) - **Identifiable cause**: bacterial pneumonia with positive blood cultures - **Fluctuating attention** and consciousness (noted on examination) These three features together are pathognomonic for **delirium superimposed on normal baseline cognition**. ### Pathophysiology: Why Infection Causes Delirium 1. Bacterial infection → systemic inflammatory response (cytokine release) 2. Blood-brain barrier disruption → neuroinflammation 3. Cholinergic dysfunction and dopaminergic imbalance 4. Acute encephalopathy with fluctuating attention and consciousness **Clinical Pearl:** Delirium in the elderly is a medical emergency. Pneumonia, UTI, sepsis, and medication toxicity are the most common precipitants. Identifying and treating the underlying cause often reverses the delirium. **Mnemonic:** **I WATCH DEATH** (causes of delirium) - **I**nfection (pneumonia, UTI, sepsis) - **W**ithdrawal (alcohol, benzodiazepines) - **A**cute metabolic (hyponatremia, hypoglycemia) - **T**oxins (medications, drugs) - **C**NS pathology (stroke, seizure, bleed) - **H**ypoxia, **H**ypertension - **D**eficits (sensory, sleep deprivation) - **E**ndocrinopathy (thyroid, adrenal) - **A**rrhythmia (cardiac) - **T**emperature (infection) - **H**eavy metals [cite:Harrison 21e Ch 25]