The anatomy and pathophysiology of extracranial atherosclerotic cerebrovascular disease

The brain is supplied by two pairs of arteries, the large carotid arteries anteriorly and the vertebral arteries (so called because they are so close to the vertebral column) posteriorly. All four arteries enter the skull at the base of the brain and are connected in the arterial circle of Willis, a unique safety device that permits arterial blood to cross from one side to the other, in case of need, or from front to back or back to front. When one inflow artery is narrowed or occluded, flow increases via other inflow arteries to maintain pressure and flow within the circle. This collateral circulation is also assisted by inflow to the circle through the orbit. When needed, arterial blood reaches the circle of Willis from the face by traversing the orbit in reverse direction. In unusual circumstances, blood in the circle of Willis can leave the brain and flow down the vertebral artery-the so-called ''vertebral steal.'' The authors provide a detailed description of the anatomy, particularly as it affects the signs, symptoms, and noninvasive diagnosis of cerebrovascular disease