The transclival artery: a variant persistent carotid-basilar arterial anastomosis not previously reported.

During embryological development, primitive anastomoses exist between the carotid and vertebrobasilar arteries. These anastomoses typically regress or are incorporated into the developing vasculature. Persistence beyond fetal development, however, results in vascular anomalies that alter haemodynamic flow with a predisposition for aneurysm formation. The carotid-vertebrobasilar anastomoses mirror the primitive communications and include (from most to least common) the trigeminal, hypoglossal, proatlantal and otic arteries. The hypoglossal and proatlantal variants extend through the hypoglossal canal or foramen magnum, respectively. We present a previously undescribed variant of these persistent fetal anastomoses, the 'transclival artery', which courses through its own transclival skull base canal/foramen.