65-year-old female with history of right nasopharyngeal squamous cell carcinoma, with an incidental finding of a persistent left proatlantal intersegmental artery.

Two types of proatlantal intersegmental arteries have been described, both arising from the carotid artery and entering the foramen magnum. Type 1 proatlantal artery arises from the internal carotid artery and ascends to the level of the occipitoatlantal space without passing through the transverse foramen of any cervical vertebra. The artery takes a dorsal course cephalad to the transverse process of C1 and then travels rostrally to enter the foramen magnum. Type 2 proatlantal artery arises from the external carotid (as in this case).

The proatlantal intersegmental artery should be differentiated from a more frequently seen persistent hypoglossal artery, another type of  persistent carotid-vertebrobasilar anastomoses.