Segmental Agenesis of External Carotid Artery from Common Carotid Artery with Anomalous Reformation through Occipital Artery Anastomoses

Anomalous origin of extracranial and intracranial carotid system is subject to permutations of embryonal regression of the developing primitive embryonal vasculature. We present a case of segmental agenesis of left external carotid artery from the cervical carotid circulation. The entire left external carotid artery was supplied by the vertebral artery through muscular branches via the occipital artery. This anomaly of proximal external carotid agenesis with the entire vascular tree dependent on the posterior circulation via occipital artery muscular collaterals is unique and has not been described before. We discuss this anomaly with its embryology, clinical implications, and its relationship to a Proatlantal artery.