Watershed cerebral infarctions, also known as border zone infarcts, occur at the border between cerebral vascular territories with no or little anastomosis.
They account for 5-10% of all cerebral infarctions 8.
It has been proposed that both, episodes of hypoperfusion and microembolisms, play a role in pathophysiology of this entity. Clearance of the microemboli are most likely to be impaired in watershed zones due to poorer perfusion. Episodes of systemic hypotension along with severe stenosis or occlusion of the feeding arteries, in particular intra and extracranial carotid arteries can also contribute to the infarction.
Watershed zone infarct in an isolated zone is more likely to be secondary to microembolism in the absence of significant systemic hypotension. In a severe carotid stenosis lesions are usually unilateral on the side of the stenosis. Prolonged hypotension such as cardiac surgery or cardiac arrest commonly give a bilateral pattern 5.
WSI has been classified to:
cortical (external) border zones infarct
- between ACA, MCA, and PCA territories
- mostly due to microemboli
deep (internal) border zones infarct:
- between ACA, MCA, and PCA territories, and perforating medullary, lenticulostriate, recurrent artery of Heubner and anterior choroidal arteries
- mostly due to either severe hypotension or severe arterial stenosis
CT and MRI
Exact pattern depends on the bordering territories which is usually variable in different individuals. Imaging of WSI should also aim to determine presence and severity of arterial stenosis or occlusion.
Cortical (external) border zones infarct
These are usually wedge-shaped or gyriform:
- ACA/MCA: in frontal cortex extending from anterior horn to the cortex
- MCA/PCA: in parieto-occipital region extending from posterior horn to the cortex
- parallel parafalcine stripes in subcortical white matter at the vertex- this type is seen when there was profound diffuse hypoperfusion
Triple watershed zone: most vulnerable region where ACA, MCA, and PCA converge posterior to lateral ventricles in parieto-occipital region.
Deep (internal) border zones infarct
- ≥3 lesions, each ≥3 mm in diameter in a linear fashion parallel to the lateral ventricles in the centrum semiovale or corona radiata, which sometimes become more confluent and band-like 7
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- 8. Naidich TP, Castillo M, Cha S et-al. Imaging of the Brain,Expert Radiology Series,1. Saunders. (2012) ISBN:1416050094. Read it at Google Books - Find it at Amazon