What are watershed areas and why are they clinically important?

• A. Regions at border between major cerebral arterial territories; vulnerable to ischemia during systemic hypoperfusion.
• B. Areas where cerebrospinal fluid is stored.
• C. Regions where motor neurons reside.
• D. Nerve fiber tracts connecting the hemispheres.

Answer: (A)

Watershed areas are the border zones where the territories supplied by major cerebral arteries meet, such as between the anterior and middle cerebral arteries and between the middle and posterior cerebral arteries. They’re clinically important because, during systemic hypoperfusion (like shock or severe hypotension), these boundary regions have the least robust blood flow and are among the first to become ischemic. When perfusion falls, these areas can develop watershed infarcts, which may appear as wedge-shaped cortical lesions at the brain’s surface (external border zones) or in deep white matter near the ventricles (internal border zones). The resulting deficits often include proximal weakness and other cortical signs depending on which border zone is affected.