Watershed strokes are named that way because they affect the watershed areas of the brain. These areas are thin strips of brain which are sandwiched in between the farthest end branches of two adjacent vascular territories. Because these are the farthest tissues supplied by an artery, adequate blood pressure must be maintained to ensure that enough blood is pumped into these areas.
Watershed areas are at high risk of developing ischemia, or lack of blood flow, during extreme drops of blood pressure. Common triggers for watershed strokes include periods of extreme dehydration, heart attacks, and sepsis (widespread infections). If ischemia to watershed areas is maintained longer than a few minutes the tissues in watershed areas begin to die, causing a stroke.
Watershed areas are particularly vulnerable to low blood pressure in people who have advanced carotid stenosis.