Internal carotid artery agenesia is a rare anomaly of the embryonic development. We present a case with this anomaly and a vascular feeding of this territory out of the contralateral carotid system and the posterior vascular system. There were multiple aneurysms in the right posterior communicating artery, never described before in association with carotid agenesia, that are similar to the angiographic image of the fibromuscular dysplasia ("atypical" pattern). We review and discuss the embryonic development of this anomaly and its relationship with the hemodynamic stress and aneurysm formation.
