Carotid-vertebrobasilar anastomoses are transient connections which appear during embryonic development between the anterior and posterior circulations. These embryonic anastomotic vessels usually regress and disappear in post-natal period.  In rare cases these vessels persist in adults. They are the persistent trigeminal artery, otic (acoustic) artery, hypoglossal artery, and proatlantal intersegmental artery. The most common type is persistent trigeminal artery.