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Background
The carotid-vertebrobasilar anastomoses are primitive embryonic vessels that temporarily provide fetal arterial blood flow from the internal carotid artery to the future vertebrobasilar artery in the hindbrain. These arterial communications include the trigeminal, otic, hypoglossal and proatlantal intersegmental arteries, which remain functional during early stage of fetal development and obliterate at the rate of posterior communicating artery development. Failure to regress results in persistence of the fetal artery. As such, these persistent communications are occasionally encountered in practice, discovered as incidental anatomic variants. This exhibit aims to provide an imaging review of the different types of persistent carotid-vertebrobasilar anastomoses.

Educational Goals / Teaching Points
The purpose of this educational exhibit is to review the background and development of the carotid-vertebrobasilar anastomoses, discuss their importance and clinical relevance, and provide examples of imaging findings of the different types.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The various persistent carotid-vertebrobasilar artery types are most readily identified on CTA, MRA or conventional catheter angiogram. The trigeminal artery is the most common persistent fetal artery and is separated into 2 subtypes, Saltzman type 1 and Saltzman type 2, based on its communication with the basilar artery and appearance of the posterior communicating arteries. The hypoglossal artery is identified as an arterial vessel originating from the cervical internal carotid artery at the C1-C2 level which transits through the hypoglossal canal to the basilar artery. The proatlantal intersegmental artery is the most caudally located carotid-basilar artery and communicates the cervical internal carotid artery at the C2-C3 level, or rarely the external carotid artery, to the vertebral artery through the foramen magnum. The otic artery is rare but when seen, courses from the petrous internal carotid artery to the basilar artery through the internal auditory canal. When present, the vertebrobasilar system proximal to the persistent artery is typically hypoplastic. Although typically asymptomatic, the persistent embryonic vessels can succumb to disease such as atherosclerosis and aneurysm. Additionally, posterior circulation embolic infarcts from anterior circulation atherosclerosis are possible with this communication. Knowledge of these persistent communications is important when interpreting diagnostic imaging and when treating diseases that occur with them.

Conclusion
Carotid-vertebrobasilar anastomoses are rare and typically found incidentally. Four persistent carotid-vertebrobasilar anastomoses have been described which include the trigeminal, hypoglossal, proatlantal and otic arteries. Although these primitive fetal artery connections are typically asymptomatic, identifying their presence becomes important when describing intracranial arterial pathology and variance.