Vertebrobasilar Insufficiency

The vertebrobasilar system, also termed the "posterior circulation," constitutes a critical vascular network supplying the brainstem, cerebellum, thalamus, occipital lobes, and medial temporal lobes. This vascular territory accounts for approximately 20% of total cerebral arterial perfusion (see . Vertebral Artery Vascular Territory). The posterior circulation originates from the vertebral arteries, which typically arise from the subclavian arteries located just inferior to the clavicle. These arteries ascend through the transverse foramina of the cervical vertebrae and converge at the pontomedullary junction to form the basilar artery. The common carotid (CCAs) and vertebral arteries collectively deliver roughly 15% of cardiac output to the brain, with an approximate 4:1 contribution ratio, respectively. In most individuals, vertebral arteries arise bilaterally from the subclavian arteries. The right CCA and right subclavian artery branch from the innominate (brachiocephalic) artery, which measures less than 5 cm in length and 1.2 cm in diameter, whereas the left CCA and left subclavian artery emerge from the distal aortic arch. Blood flow and vessel diameter decrease progressively from the ascending to the descending aorta (see . Hemodynamic Features of the Aortic Arch and Its Branches). The transverse foramina, present exclusively in the cervical vertebrae, function as protective tunnels for the vertebral arteries as they ascend toward the cranium. Conventionally, the course of each vertebral artery from its origin at the subclavian artery is divided into 4 anatomic segments, as follows: V1: Extends from the subclavian artery to the transverse foramen of C6. V2: Courses through the foramina from C6 to C1. V3: Passes between the sulcus and the foramen magnum. V4: Represents the intradural segment. The vertebral arteries converge at the pontomedullary junction to form the basilar artery, which courses along the ventral surface of the pons and bifurcates into the posterior cerebral arteries (PCAs) at the pontomesencephalic junction. The basilar artery gives rise to several major branches, including the posterior inferior cerebellar artery (PICA), which supplies the inferior cerebellum and lateral medulla; the anterior inferior and superior cerebellar arteries, which supply the cerebellum and brainstem; and the PCAs, which supply the occipital cortex and inferomedial temporal lobes. Anatomic variations occur in up to 1/3 of the population. Variants include the brachiocephalico-carotid trunk, aberrant right subclavian artery, aortic origin of the left vertebral artery, duplication of the vertebral arteries, and distinct origins of the right and left CCAs (see . Anatomical Variations of Aortic Arch Arteries). Transient ischemic attack (TIA) is defined as a temporary reduction in cerebral blood flow producing stroke-like symptoms that resolve spontaneously without resulting in infarction or diffusion-weighted imaging hyperintensity within a specific vascular territory. Vertebrobasilar insufficiency (VBI) refers specifically to a transient interruption of flow within the posterior circulation and is not confined to a single vascular territory.