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A watershed stroke is a stroke affecting areas of the brain furthest from direct perfusion with blood supply by the major cerebral arteries.
Watershed strokes result from hypoperfusion (a lack of blood flow), which can be caused by congestive heart failure, severe atherosclerosis of the carotid arteries, or systemic hypotension (shock). Because the brain's circulation is formed by small end-arteries branching from larger central blood vessels, poor perfusion of the brain most strongly affects tissues supplied by the most distal branches of these arteries, producing ischemia and infarction at the border of these "watershed" areas.
A watershed stroke of the anterior cerebral arteries classically presents with weakness of proximal arm and leg muscles and preservation of distal strength: colloquially, the "man in a barrel" presentation.