Watershed areas in the brain are regions that receive blood supply from the most distant, or terminal, branches of two major arterial systems. These zones are the border territories where the fields of supply from two different arteries overlap. Although they benefit from a dual blood supply, their location at the periphery of both territories makes them uniquely susceptible to certain types of injury. This concept explains why specific parts of the brain are affected during systemic circulatory crises.
The Anatomical Basis of Watershed Areas
Watershed zones represent the meeting points of the three main arterial territories that irrigate the cerebrum: the Anterior Cerebral Artery (ACA), the Middle Cerebral Artery (MCA), and the Posterior Cerebral Artery (PCA). These areas form linear or wedge-shaped regions between these zones of supply, creating two primary categories.
The first category is the cortical or external watershed, located on the brain’s outer surface. One major cortical watershed runs between the ACA and MCA territories in the frontal and parietal lobes. A second cortical watershed is situated in the posterior parietal and occipital lobes, marking the boundary between the MCA and the PCA territories.
The second category is the internal or terminal watershed, located deeper within the brain’s white matter. These zones are found where the terminal, penetrating branches of the superficial cortical arteries meet the deep-penetrating arteries. This places the internal watershed between the blood supply of the surface arteries and the shorter, inner arteries.
Why These Regions Are Vulnerable to Injury
The location of watershed areas at the farthest reach of two different arterial systems is the fundamental reason for their vulnerability. These regions are perfused by the most distal branches, meaning the blood pressure driving the flow is lowest here compared to the major arteries. This results in a state of hemodynamic vulnerability, where the tissue is operating with the least amount of circulatory reserve.
Damage to these areas is typically caused by global or systemic low-flow states, which reduce the overall pressure of blood entering the brain. Conditions such as severe hemorrhage, cardiac arrest, or profound hypotension due to shock can drastically lower the systemic blood pressure. When this happens, the reduced pressure is insufficient to push blood all the way to the most distant branches supplying the watershed zones.
This mechanism differs from a typical ischemic stroke, which involves a localized blood clot blocking a major artery. In a watershed event, the failure is a systemic one, affecting the entire brain’s blood flow simultaneously. Since these zones are the furthest points from the main arterial trunks, they are the first to experience critically low oxygen and nutrient supply when blood pressure drops significantly. The resulting tissue death, or infarction, appears as a characteristic pattern corresponding to these border zones.
Clinical Outcomes of Watershed Infarction
When the tissue in a watershed area dies due to insufficient blood flow, the resulting neurological symptoms are specific to the functions controlled by that particular border zone. The clinical presentation of a watershed stroke is often bilateral and symmetrical, reflecting the global nature of the underlying hypoperfusion event. This contrasts with a typical stroke caused by a single blocked vessel, which usually results in unilateral symptoms.
Man-in-the-Barrel Syndrome
One of the most recognized outcomes is the “man-in-the-barrel” syndrome, resulting from bilateral injury to the ACA/MCA cortical watershed area. This specific damage affects the motor fibers controlling the proximal upper limbs, leading to weakness in both shoulders and arms while sparing the face and lower extremities. Patients appear unable to lift their arms, as if they are stuck inside a barrel.
Aphasia and Visual Deficits
Infarction in the ACA/MCA border zone, particularly in the dominant hemisphere, can also cause transcortical aphasia. This condition manifests as difficulty initiating speech or comprehending language, depending on the precise area affected. Damage to the posterior watershed area, located between the MCA and PCA territories, can lead to complex cortical visual abnormalities. These deficits can include Balint’s syndrome, which involves difficulty directing the gaze, inability to perceive the visual field as a whole, and impaired hand-eye coordination.