Anastomosis of the external carotid artery and the V3 segment of the vertebral artery (presumed persistent second cervical intersegmental artery) diagnosed by CT angiography.

Congenital anastomosis of the external carotid (ECA) and vertebral (VA) arteries is extremely rare. We report a case of right ECA-VA anastomosis diagnosed by computed tomography (CT) angiography. The presumed dilated ascending pharyngeal artery coursed posteriorly and entered the C1 right transverse foramen. The proximal right VA was aplastic, and the C2 right transverse foramen was absent. Based on these findings, we diagnosed congenital anastomosis of the ECA and V3 segment of the VA, which we presumed to represent persistence of the second cervical intersegmental artery. Detection of rare variations of the vertebral arteries requires careful observation of CT angiography with bone images and source images, especially to identify the level of entry into the transverse foramen. Correct diagnosis of anastomosis is important when intravascular embolization or infusion chemotherapy or surgical ligation of the ECA is planned.