Watershed cerebral infarctions, also known as border zone infarcts, occur at the border between cerebral vascular territories where the tissue is furthest from arterial supply and thus most vulnerable to reductions in perfusion.
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Epidemiology
Watershed cerebral infarction accounts for 5-10% of all cerebral infarctions 8. They tend to occur in the elderly, who have a higher incidence of arterial stenosis and hypotensive episodes, as well as sources of microemboli.
Pathology
Although traditionally watershed infarction has been thought of as being due primarily to hypoperfusion, there is mounting evidence that both episodes of hypoperfusion and/or microemboli from inflamed atherosclerotic plaques or other sources can be causative 3,5.
Episodes of systemic hypotension, particularly combined with severe inflow stenosis or occlusion (e.g. carotid stenosis, intracranial atherosclerotic disease), is the typical scenario in which a watershed infarction is encountered and classically affects the deep border zone (string of pearls sign) but can also involve the external border zone 3. The location of stenoses and anatomy of the circle of Willis will contribute to the location of infarcts.
In contrast, clearance of the microemboli, which may form on the surface of inflamed plaques or are the result of an embolic shower (including fat embolism and air embolism 11,12), usually only affect the external (cortical) border zone presumably as the absence of abundant collateral supply makes these areas more likely to infarct from small occlusions 3,10.
Thus, in the absence of significant inflow impairment, a border zone infarct in an isolated area is more likely to be secondary to microembolism.
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cortical (external) border zones infarct
due to either hypoperfusion or microemboli
histologically, these can be wedges of cortical and subcortical infarction or cortical laminar necrosis
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deep (internal) border zones infarct:
due to hypoperfusion
between ACA, MCA, and PCA territories and perforating medullary, lenticulostriate, recurrent artery of Heubner and anterior choroidal arteries
Radiographic features
CT and MRI
The exact pattern depends on the bordering territories, which are usually variable in different individuals. Imaging of watershed infarction should also aim to determine the presence and severity of arterial stenosis and occlusion.
Cortical (external) border zones infarct
These are usually wedge-shaped or gyriform:
ACA/MCA: in the frontal cortex, extending from the anterior horn to the cortex
MCA/PCA: in the parieto-occipital region, extending from the posterior horn to the cortex
parallel parafalcine stripes in the subcortical white matter at the vertex - this type is seen with profound diffuse hypoperfusion
Triple watershed zone: most vulnerable region where ACA, MCA, and PCA converge in the parieto-occipital region posterior to the lateral ventricles.
Deep (internal) border zones infarct
≥3 lesions, each ≥3 mm in diameter, in a linear fashion (string of pearls) 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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