Rare carotid-vertebrobasilar anastomoses with notes on the differentiation between proatlantal and hypoglossal arteries.

Two cases of persistent anastomoses between the carotid and vertebrobasilar systems which have only once been previously reported are described. These are a proatlantal intersegmental artery which arises from the external carotid artery and a primitive trigeminal artery with direct anastomosis to the superior cerebellar artery. Angiographic criteria for differentiation of the proatlantal and more common hypoglossal arteries, as well as the rare persistent cervical intersegmental artery are also presented.