Functional Arterial Anatomy of the Cranial Base

Two main vascular routes supply the brain, internal carotid arteries (ICAs) and vertebral arteries. The external carotid artery (ECA) mainly supplies the skin, muscle, and skull; thus, neurointerventionalists should be aware of ECA-ICA, ECA-vertebral artery, and extracranial-intracranial anastomoses. These anastomoses are not newly formed but are arteries that are remnants of developmental processes. During development, the ICA has anastomoses with the original ECA at the first and second pharyngeal arches, and carotid-basilar anastomoses exist between the ICA and vertebral artery/basilar artery. Advances in neurointerventional technology are increasingly expanding the indications for transarterial embolization in the ECA territory. To avoid embolic complications to the brain and cranial nerve palsy, knowledge and understanding of such extracranial-intracranial anastomosis are crucial for neurointerventionalists interested in neurovascular diseases. The arterial contributions to these anastomoses are mainly the branches of the maxillary artery, ascending pharyngeal artery (extracranial), ophthalmic artery, and branches of the ICA (intracranial). Knowledge of the embryological processes is essential for understanding the routes and patterns of these anastomoses. This review discusses ECA development, the ICA segmental concept and functional anatomy of the extracranial-intracranial anastomoses in the orbit, cavernous sinus, petrosal bone, and posterior fossa from an embryological point of view.