A 72-year-old woman from Bangalore with atrial fibrillation (not on anticoagulation) presents with acute onset right-sided hemiparesis and aphasia at 6 hours after symptom onset. Non-contrast CT head is normal. MRI DWI shows multiple acute infarcts in the left MCA and ACA territories. Perfusion-weighted imaging (PWI) shows a larger area of hypoperfusion than the DWI lesion. What does the PWI-DWI mismatch represent, and what is its clinical significance?

Explanation

## Pathophysiology of Ischemic Penumbra **Key Point:** The ischemic penumbra is brain tissue with compromised blood flow (hypoperfused on PWI) but preserved cellular integrity (no restricted diffusion on DWI). It represents the **therapeutic target** in acute ischemic stroke. ## DWI vs. PWI: What Each Measures | Sequence | Measures | Timing | Appearance in Acute Stroke | |----------|----------|--------|----------------------------| | **DWI (Diffusion-Weighted Imaging)** | Restricted water diffusion (cytotoxic edema) | Appears within **minutes** | Hyperintense (bright) | | **ADC (Apparent Diffusion Coefficient)** | Quantifies diffusion restriction | Appears within **minutes** | Hypointense (dark) | | **PWI (Perfusion-Weighted Imaging)** | Cerebral blood flow / perfusion | Reflects hemodynamic status | Hypointense (dark) in underperfused areas | ## The Penumbra Concept ```mermaid flowchart TD A[Acute arterial occlusion]:::outcome --> B[Severe hypoperfusion]:::outcome B --> C{Tissue response depends on severity}:::decision C -->|Core: severe ischemia| D[Irreversible damage<br/>DWI positive<br/>PWI positive]:::urgent C -->|Penumbra: moderate ischemia| E[Potentially reversible<br/>DWI negative<br/>PWI positive<br/>SALVAGEABLE]:::action D --> F[Infarct core]:::outcome E --> G{Reperfusion therapy?}:::decision G -->|Yes| H[Penumbra salvaged<br/>Infarct volume minimized]:::action G -->|No| I[Penumbra progresses to infarction<br/>Larger final infarct]:::urgent ``` ## PWI-DWI Mismatch: Definition & Significance **High-Yield:** PWI-DWI mismatch = (PWI lesion volume − DWI lesion volume) / PWI lesion volume × 100%. - **Mismatch present:** PWI hypoperfusion > DWI restricted diffusion = **ischemic penumbra present** - **No mismatch:** PWI ≈ DWI = core infarction, minimal salvageable tissue **Clinical Pearl:** Presence of PWI-DWI mismatch is a strong predictor of: 1. **Benefit from reperfusion therapy** (thrombolysis, thrombectomy) 2. **Potential for infarct growth** if reperfusion is not achieved 3. **Favorable outcome** if reperfusion is successful ## Clinical Application in This Case The patient has: - **DWI:** Multiple acute infarcts (core) - **PWI:** Larger hypoperfused area (core + penumbra) - **Mismatch:** Yes → **Salvageable tissue present** **Management implication:** Even at 6 hours (beyond standard 4.5-hour IV thrombolysis window), the presence of significant PWI-DWI mismatch makes this patient a candidate for **mechanical thrombectomy** (Class IIb evidence in extended window, especially if mismatch is substantial). This is the basis for DAWN and DEFUSE 3 trial protocols. **Mnemonic:** **MISMATCH = SALVAGE** - **M**ismatch present - **I**schemic penumbra - **S**alvageable tissue - **M**echanical thrombectomy indicated - **A**cute reperfusion therapy - **T**issue rescue - **C**linical benefit - **H**ypoperfusion reversal [cite:Harrison 21e Ch 436; Neuroradiology Essentials by Osborn]