9/18/2023 0 Comments Warning signs and symptoms of picaIncomplete circle of Willis and the fetal PCAĪn incomplete COW is present in 48–58% of the population ( 6) (Figure (Figure2B). Pertinent anatomical variants and their clinical significance are highlighted. The labyrinthine artery usually arises from the anterior inferior cerebellar artery (AICA) however this vessel may arise directly from the BA. Other intracranial anomalies include fenestration of the vertebrobasilar junction (0.3–0.6%) which may predispose to PC aneurysm formation ( 8) (Figure (Figure2C), 2C), persistent hypoglossal artery, the artery of Percheron, fetal origin of one or both PCAs (fPCA) (Figure (Figure2D), 2D), and absence of one or both posterior communicating arteries (PCOMs) (Figure (Figure2B). A persistent trigeminal artery is the most common carotid–vertebrobasilar anastomosis ( 7). Although typically discovered incidentally, they are infrequently related to intracranial vascular pathology.Īsymmetric VAs occur in over two-thirds of individuals ( 4, 5) (Figure (Figure2A) 2A) and an incomplete circle of Willis (COW) may be seen in over half ( 6) (Figure (Figure2B). Familiarity with these variants is important as not to mistake them as pathological findings. The topographic classification described by Caplan in the New England Medical Center-posterior circulation registry (NEMC-PCR) ( 3) divides the intracranial vertebrobasilar system into proximal, middle, and distal territories.Īnatomical Variants of the Posterior Circulation and Clinical SignificanceĪnatomical variations of the PC are frequent but commonly asymptomatic. Segments of the vertebral artery and branches of the vertebrobasilar system are depicted in Figure Figure1. The BA travels rostral along the ventral medulla and basis pontis until it bifurcates into the right and left posterior cerebral arteries (PCAs) at the pontomesencephalic junction. The most distal segment representing the intracranial portion of the VA after entering the dura mater is the V4 segment. The V3 segment begins after the artery exits the foramen transversarium of the axis (C2) and arches behind the atlas (C1) before entering the cranium. The segment occupying the course of the VA from the initial to last transverse foramen is known as V2. V1 is the most proximal segment from the VA origin to point of entry into the initial foramen transversarium, usually at the sixth or seventh cervical vertebral body. Both VAs join at the pontomedullary junction forming the basilar artery (BA). When reaching the foramen magnum, they pierce the dura mater to start their intracranial course. The vertebral arteries (VAs) arise from the right and left subclavian arteries and travel cranially through the transverse foramina of the cervical vertebrae. A posterior circulation (PC) stroke is classically defined by infarction occurring within the vascular territory supplied by the vertebrobasilar (VB) arterial system.
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