A 72-year-old woman is admitted to the medical ward with acute confusion, disorientation, and visual hallucinations. She was admitted 2 days ago for pneumonia and started on antibiotics. Her family reports she was cognitively intact until yesterday. On examination, she is drowsy, inattentive, and her consciousness fluctuates throughout the day. Which investigation is most appropriate to confirm the diagnosis and guide immediate management?

Explanation

## Clinical Context This patient presents with **acute delirium**, characterized by: - **Acute onset** (within 24 hours) - **Fluctuating consciousness** (drowsy, inattentive, varies throughout the day) - **Disorientation and hallucinations** (visual) - **Recent medical stressor** (pneumonia, antibiotics) - **Inattention** (hallmark cognitive feature of delirium) ## Investigation of Choice: Electroencephalography (EEG) **Key Point:** EEG is the investigation of choice for confirming delirium and identifying its neurophysiological signature. It provides objective evidence of diffuse brain dysfunction. ### Why EEG Is Diagnostic for Delirium 1. **Characteristic findings in delirium:** - **Diffuse slowing** of background rhythm (theta and delta waves) - **Triphasic waves** (pathognomonic but not always present) - **Loss of normal alpha rhythm** - **Generalized disorganization** of EEG pattern 2. **Confirms acute encephalopathy** — distinguishes delirium from psychiatric disorders or dementia 3. **Guides urgency of investigation** — abnormal EEG prompts immediate search for underlying cause (infection, metabolic derangement, medication toxicity) 4. **Non-invasive and rapid** — can be performed at bedside **High-Yield:** The presence of **triphasic waves** on EEG is highly specific for delirium, especially in metabolic encephalopathy (hepatic encephalopathy, uremia, hypercalcemia). **Clinical Pearl:** While EEG findings are not pathognomonic for a specific cause of delirium, they confirm the presence of acute diffuse cerebral dysfunction and help differentiate delirium from dementia or primary psychiatric illness. ## Why Other Investigations Are Not First-Line | Investigation | Role | Why Not in Acute Delirium | |---|---|---| | **MRI brain with volumetry** | Confirms chronic dementia; identifies atrophy patterns | Unnecessary in acute presentation; dementia is chronic, not acute; MRI is time-consuming and not indicated for delirium workup | | **Montreal Cognitive Assessment (MoCA)** | Screening tool for mild cognitive impairment and dementia | Not suitable for delirious patients (inattention, fluctuation prevent valid testing); does not confirm delirium diagnosis | | **Neuropsychological testing** | Comprehensive assessment of cognitive domains in stable patients | Inappropriate in acute delirium (patient cannot cooperate); requires stable consciousness and attention | ## Delirium Workup Algorithm ```mermaid flowchart TD A[Acute Confusion + Fluctuation<br/>+ Inattention]:::outcome --> B[Perform EEG]:::action B --> C{EEG shows diffuse<br/>slowing/triphasic waves?}:::decision C -->|Yes| D[Confirms DELIRIUM]:::outcome D --> E[Investigate Cause:<br/>Infection? Metabolic?<br/>Medication? Hypoxia?]:::action E --> F[Blood cultures, lactate,<br/>glucose, electrolytes,<br/>renal/liver function]:::action E --> G[Chest X-ray, urinalysis,<br/>blood cultures]:::action E --> H[Review medications<br/>for anticholinergics,<br/>sedatives, opioids]:::action C -->|No| I[Consider alternative<br/>diagnosis: Dementia?<br/>Psychiatric?]:::decision F --> J[Treat underlying cause]:::action G --> J H --> J ``` **Mnemonic: EEG in DELIRIUM** - **E** — Encephalopathy (diffuse brain dysfunction) - **E** — Electrolyte abnormalities (reflected in EEG slowing) - **G** — Generalized disorganization (loss of normal rhythm) ## Delirium vs. Dementia: EEG Distinction | Feature | Delirium EEG | Dementia EEG | |---|---|---| | **Background rhythm** | Diffuse slowing (theta/delta) | Normal or mild slowing | | **Triphasic waves** | Present (especially metabolic) | Absent | | **Reactivity** | Reduced or absent | Preserved | | **Acuity** | Acute change | Chronic, stable | | **Clinical correlation** | Fluctuating consciousness | Alert, oriented (early stages) | **Warning:** Do NOT confuse delirium with dementia based on cognitive symptoms alone. The **temporal profile** (acute vs. chronic) and **EEG findings** (abnormal vs. normal) are the key differentiators.