A 58-year-old man is resuscitated after 18 minutes of out-of-hospital cardiac arrest. Despite targeted temperature management at 33°C, he remains comatose at 72 hours. Brain MRI with DWI sequence shows the pattern marked **A** in the diagram—bilateral symmetric restricted diffusion in the parieto-occipital regions at the border zones between major arterial territories. Which of the following best explains why the structure marked **A** is preferentially affected in this clinical scenario?

Explanation

## Why option 1 is right Watershed infarcts in hypoxic-ischemic encephalopathy (HIE) occur because these border zones between major vascular territories (anterior watershed: ACA-MCA; posterior watershed: MCA-PCA) are the most distal from arterial supply. During global hypoperfusion or hypoxia—as occurs in cardiac arrest—these zones experience the greatest drop in perfusion pressure and are therefore most vulnerable to ischemic injury. This is the hallmark pattern of mild-to-moderate HIE severity, as documented in Greenberg Neurosurgery 9e and AHA Post-Cardiac-Arrest Guidelines. ## Why each distractor is wrong - **Option 2**: While metabolically active regions (basal ganglia, thalami, hippocampi, Purkinje cells) ARE selectively vulnerable in HIE, this vulnerability is independent of perfusion pressure and occurs in severe/prolonged arrest. Watershed zones are vulnerable due to their LOCATION (distal from supply), not metabolic demand. - **Option 3**: Watershed zones are not inherently lacking collateral supply; their vulnerability in HIE is to global hypoperfusion, not to embolic occlusion of major vessels. Embolic stroke (option B in the diagram) presents with acute focal MCA territory infarction, not bilateral symmetric watershed pattern. - **Option 4**: Vasospasm is a mechanism in subarachnoid hemorrhage and acute stroke, not a primary driver of watershed infarction in global hypoxia-ischemia. The mechanism in HIE is perfusion pressure drop, not vasospasm. **High-Yield:** Watershed infarcts = distal zones between major arteries; most vulnerable to GLOBAL hypoperfusion (cardiac arrest, severe hypotension); basal ganglia/hippocampi = selectively vulnerable in SEVERE/PROLONGED arrest. [cite: Greenberg Neurosurgery 9e — Anoxic Brain Injury; AHA Post-Cardiac-Arrest Guidelines]