Vertebrobasilar syndrome associated with subclavian origin of the right internal carotid artery.

A case of absence of the right common carotid artery with origin of the external carotid artery from the innominate artery and origin of the internal carotid artery from the right subclavian artery proximal to the right vertebral artery is presented. Atherosclerotic occlusion at the origin of the right subclavian artery and occlusion of the left internal carotid artery resulted in a vertebrobasilar syndrome. Blood flow from the right external carotid reconstituted the right vertebral artery via muscular collateral vessels, moving first retrograde to the subclavian artery and then antegrade through the right internal carotid artery. Symptoms were successfully relieved by transposition of the internal carotid to the external carotid artery. This is the second reported case in the literature and the first to be observed in a clinical setting. The anomaly can easily be explained by embryonic persistence of the right ductus caroticus associated with involution of the right third aortic arch.