Circle of Willis variant analogous to fetal type primitive trigeminal artery.

Primitive caroticobasilar anastomoses are uncommon embryonic arterial connections between the carotid (anterior) and vertebrobasilar (posterior) circulations. With the exception of extracranial proatlantal intersegmental arteries, these embryonic arterial anastomoses are named according to the cranial nerves which they parallel. In this report, a previously undescribed variant circle of Willis with supraclinoid caroticobasilar anastomosis, functionally equivalent to a fetal type primitive trigeminal artery, is presented.