Anastomosis between an extracranial C1/2-origin left posterior inferior cerebellar artery and the V4 segment of the vertebral artery diagnosed by magnetic resonance angiography

Purpose To describe a case of a congenital anastomosis between the posterior inferior cerebellar artery (PICA) arising from the vertebral artery (VA) at the extracranial C1/2 level and the V4 segment of the VA, forming a large arterial ring involving the V3–V4 segments. Methods Magnetic resonance imaging (MRI), including magnetic resonance angiography (MRA), was performed in a 64-year-old man presenting with right lower-extremity weakness for evaluation of suspected cerebrovascular disease. MRI was performed using a 3-T scanner, and MRA was performed using the standard three-dimensional time-of-flight technique. Results MRI revealed no evidence of acute cerebral infarction. MRA demonstrated that the left PICA originated from the extracranial VA at the C1/2 level. A short but large anastomotic artery was identified between the left PICA and the V4 segment of the left VA, forming a large arterial ring involving the V3–V4 segments. The right VA and right PICA were normal in appearance. Other findings included a small aneurysm of the right internal carotid artery (ICA) and stenosis of the left ICA at its origin. Conclusion To our knowledge, this is the first reported case of an anastomosis between an extracranial C1/2-origin PICA and the V4 segment of the VA diagnosed by MRA. Although a tiny anastomosis between these arteries may be detected by catheter angiography or cadaveric dissection, it may not be visualized on MRA because of its limited spatial resolution. In the event of occlusion of one channel of the arterial ring, the other channel and the anastomotic artery may serve as important collateral pathways.