A 72-year-old woman from Bangalore with a history of atrial fibrillation (not on anticoagulation) presents with acute right-sided weakness and facial droop of 6 hours duration. Non-contrast CT head is normal. MRI is performed: DWI shows multiple acute infarcts in the left middle cerebral artery (MCA) territory with restricted diffusion. ADC map shows low signal in the same regions. What is the most likely mechanism of stroke in this patient?

Explanation

## Clinical Context A 72-year-old woman with untreated atrial fibrillation (major cardioembolic risk factor) presents with acute neurological deficit. MRI shows **multiple acute infarcts in a single vascular territory (left MCA)**, which is the key diagnostic clue. ## Why Cardioembolic Stroke? **Key Point:** Multiple acute infarcts in different vascular territories or scattered throughout a single large territory ("showering" pattern) is the hallmark of cardioembolic stroke. This occurs because emboli from the heart can lodge in multiple distal branches. **High-Yield:** Cardioembolic strokes have these imaging characteristics: 1. **Multiple infarcts** in different vascular territories (or scattered in one territory) 2. **Cortical distribution** — typically at the gray-white matter junction 3. **Acute presentation** — sudden onset due to embolic occlusion 4. **DWI positivity** — confirms acute ischemia within minutes ## Imaging Patterns in Stroke Subtypes | Stroke Type | DWI Pattern | Distribution | Vascular Territory | Clinical Clue | | --- | --- | --- | --- | --- | | **Cardioembolic** | Multiple acute infarcts | Scattered or multi-territorial | Often cortical, MCA territory | Atrial fibrillation, cardiac source | | **Large artery atherosclerotic** | Single large infarct | Territorial (wedge-shaped) | MCA, ACA, PCA territory | Carotid stenosis, hypertension | | **Lacunar (small vessel)** | Small deep infarcts | Basal ganglia, thalamus, pons | Penetrating arteries | Hypertension, diabetes | | **Arterial dissection** | Infarct in ICA/VA territory | Cortical or brainstem | ICA or VA distribution | Neck pain, Horner's syndrome | ## Pathophysiology of Cardioembolic Stroke ```mermaid flowchart TD A[Atrial fibrillation]:::outcome --> B[Irregular atrial contractions]:::outcome B --> C[Blood stasis in left atrial appendage]:::outcome C --> D[Thrombus formation]:::outcome D --> E[Embolus enters systemic circulation]:::outcome E --> F{Lodges in cerebral artery?}:::decision F -->|Yes| G[Acute arterial occlusion]:::urgent G --> H[Ischemia in distal territory]:::urgent H --> I[Multiple infarcts if emboli fragment]:::outcome F -->|No| J[Embolus to other organs]:::outcome ``` **Clinical Pearl:** Cardioembolic strokes are often **multiple** and **simultaneous** because the embolus may fragment as it travels, causing infarction in multiple distal branches. This is why the patient has multiple acute infarcts in the MCA territory rather than a single large territorial infarct. ## Risk Factors for Cardioembolic Stroke **Mnemonic: CHADS~2~-VASc** (for atrial fibrillation): - **C**ongestive heart failure - **H**ypertension - **A**ge ≥75 years - **D**iabetes - **S**troke/TIA/thromboembolism - **V**ascular disease - **A**ge 65–74 years - **Sc**ex (female) Other cardioembolic sources: prosthetic valve, endocarditis, myocardial infarction with thrombus, dilated cardiomyopathy, atrial myxoma. **Reasoning:** The combination of untreated atrial fibrillation (major cardioembolic source) and multiple acute infarcts in a single vascular territory on DWI-MRI is pathognomonic for cardioembolic stroke. The "showering" of emboli into distal MCA branches causes the scattered infarct pattern.