A 58-year-old woman with atrial fibrillation presents with acute onset speech difficulty and right-sided weakness. CT head is unremarkable. MRI DWI shows multiple acute infarcts in different vascular territories bilaterally. What is the most common cause of acute ischemic stroke in patients with atrial fibrillation?

## Most Common Cause of Acute Ischemic Stroke in Atrial Fibrillation **Key Point:** Cardioembolism from atrial thrombus is the most common cause of acute ischemic stroke in patients with atrial fibrillation (AF), accounting for approximately 45–50% of all cardioembolic strokes. ### Pathophysiology of Cardioembolism in Atrial Fibrillation Atrial fibrillation creates a prothrombotic state through multiple mechanisms: 1. **Loss of atrial contraction** → blood stasis in the left atrial appendage (LAA) 2. **Endothelial dysfunction** → increased tissue factor expression 3. **Hypercoagulability** → elevated inflammatory markers and thrombotic factors 4. **Formation of left atrial appendage thrombus** → embolization to systemic circulation Thrombi typically lodge in the MCA territory (most common), followed by ACA and PCA territories. ### Causes of Acute Ischemic Stroke: Comparative Frequency | Cause | Frequency | Risk Factors | Imaging Features | |-------|-----------|--------------|------------------| | **Cardioembolism** | **45–50%** | **AF, MI, dilated cardiomyopathy, prosthetic valve** | **Multiple infarcts in different vascular territories (wedge-shaped, cortical)** | | Atherosclerotic thrombosis | 25–30% | Hypertension, diabetes, smoking, hyperlipidemia | Single infarct in large vessel territory; stenosis on CTA/MRA | | Small vessel disease (lacunar) | 15–20% | Hypertension, diabetes | Small (< 1.5 cm) infarcts in deep white matter, basal ganglia, pons | | Arterial dissection | 5–10% | Trauma, connective tissue disorder, recent neck manipulation | Crescent-shaped wall hematoma on MRI; tapered arterial narrowing | | Other (paradoxical embolism, hypercoagulable states) | 5–10% | PFO, thrombophilia, malignancy, antiphospholipid syndrome | Variable; depends on underlying etiology | **High-Yield:** In AF, the **CHA₂DS₂-VASc score** guides anticoagulation decisions. Patients with a score ≥ 1 (males) or ≥ 2 (females) require anticoagulation to prevent cardioembolic stroke. ### Imaging Clues to Cardioembolic Stroke **DWI-MRI findings suggestive of cardioembolism:** - Multiple acute infarcts in different vascular territories (as in this case) - Cortical (wedge-shaped) distribution - Bilateral hemispheric involvement - Infarcts in non-contiguous vascular territories **Clinical Pearl:** The presence of multiple acute infarcts in different vascular territories on DWI strongly suggests a cardioembolic source. This patient's bilateral infarcts in different territories are classic for AF-related cardioembolism. **Mnemonic — Cardioembolic Sources: CHAMP** - **C**ardiomyopathy (dilated) - **H**eart valve disease (prosthetic, rheumatic) - **A**trial fibrillation - **M**yocardial infarction (acute, with mural thrombus) - **P**atent foramen ovale (PFO) with paradoxical embolism **Warning:** Do not confuse multiple infarcts with lacunar stroke. Lacunar infarcts are small (< 1.5 cm), deep (basal ganglia, thalamus, pons, internal capsule), and typically solitary or few in number—not multiple across different vascular territories. [cite:Harrison 21e Ch 379]