A 68-year-old woman with no prior psychiatric history is admitted to the hospital for pneumonia. On hospital day 2, the nursing staff reports that she is confused, calling out at night, and pulling at her IV lines. Her daughter says, 'She was fine yesterday morning.' On examination, she is disoriented to time and place, her speech is incoherent, and she appears agitated. Vital signs: BP 145/90, HR 110, RR 22, Temp 38.8°C. Chest X-ray confirms left lower lobe consolidation. Blood cultures are pending. Serum electrolytes: Na^+^ 128 mEq/L (normal 135–145), K^+^ 3.2 mEq/L (normal 3.5–5.0). Arterial blood gas shows PaO~2~ 65 mmHg on room air. What is the most likely diagnosis?

Explanation

## Diagnosis: Delirium Secondary to Multiple Precipitants ### Acute Onset with Multiple Metabolic/Infectious Triggers **Key Point:** The acute onset (day 2 of hospitalization) in a previously cognitively intact woman, combined with fever, hypoxia, hyponatremia, hypokalemia, and tachycardia, is pathognomonic for delirium, not dementia. ### Diagnostic Criteria for Delirium 1. **Acute onset** and **fluctuating course** — yes (fine yesterday, confused today) 2. **Inattention** — yes (disoriented, incoherent speech, pulling at lines) 3. **Disorganized thinking** — yes (incoherent speech) 4. **Altered level of consciousness** — yes (agitation, confusion) 5. **Identifiable medical cause(s)** — yes (multiple) **High-Yield:** Delirium is **acute** (hours to days), **fluctuating**, and **reversible** if the underlying cause is treated. This patient has all three. ### Precipitating Factors Identified | Factor | Value | Significance | |--------|-------|---------------| | **Fever** | 38.8°C | Infection (pneumonia) | | **Hypoxia** | PaO~2~ 65 mmHg | Respiratory compromise | | **Hyponatremia** | 128 mEq/L | SIADH or dilutional (common in pneumonia) | | **Hypokalemia** | 3.2 mEq/L | Electrolyte disturbance | | **Tachycardia** | HR 110 | Systemic response to infection | | **Tachypnea** | RR 22 | Hypoxia and infection | | **Hypertension** | 145/90 | Stress response | **Clinical Pearl:** Delirium is often **multifactorial**. This patient has at least 4 reversible causes: infection, hypoxia, hyponatremia, and hypokalemia. Treating all of them is essential. **Mnemonic: I WATCH DEATH** (common delirium causes) — **I**nfection (pneumonia), **W**ithdrawal, **A**cute metabolic (hyponatremia, hypokalemia), **T**oxins, **C**NS pathology, **H**ypoxia, **D**eficiency, **E**ndocrine, **A**rrhythmia, **T**emperature, **H**eavy metals. ### Why Not Dementia? - **Acute onset** (day 2) — dementia is insidious (months to years) - **Previously normal cognition** — no prior history of memory loss - **Fluctuating course** — dementia is steady and progressive - **Reversible causes present** — dementia is irreversible ### Why Not Alcohol Withdrawal? - No history of alcohol use or withdrawal mentioned - Fever and hypoxia are not typical of withdrawal alone - Hyponatremia and hypokalemia are not primary features of withdrawal - Pneumonia and abnormal CXR are the precipitant, not alcohol ### Why Not Acute Psychotic Disorder? - Acute psychosis does not explain fever, hypoxia, or electrolyte abnormalities - No prior psychiatric history to suggest primary psychosis - Disorganized speech and inattention in delirium mimic psychosis but are caused by metabolic derangement - Treatment of delirium (oxygen, antibiotics, electrolyte correction) will resolve the confusion; antipsychotics alone will not **Tip:** Always check vital signs and labs first in acute mental status change in hospitalized patients. Delirium is medical until proven otherwise. [cite:Harrison 21e Ch 383; DSM-5 Neurocognitive Disorders]