Critical Procedural Anatomy of High-Risk External Carotid Artery–Internal Carotid Artery Anastomoses

Transarterial embolization of external carotid artery (ECA) branches is increasingly performed for the management of epistaxis, preoperative embolization of head and neck tumors, treatment of dural arteriovenous fistulas, and, more recently, for the treatment of chronic subdural hematoma. As new indications for ECA embolization in the management of conditions like chronic subdural hematoma continue to be identified, it is imperative that interventionalists understand the presence of intricate anastomoses between the extracranial and intracranial arterial systems, which confer significant procedural risks. Failure to account for these connections can result in devastating complications such as stroke, blindness, or cranial nerve injury due to nontarget embolization. This review examines the key anatomical territories of ECA-internal carotid artery anastomoses: the orbital region, petrous-cavernous region, and upper cervical region. These areas, often involving embryological remnants or collateral channels that enlarge in response to pathology, represent potential conduits for inadvertent embolization. We discuss the importance of preprocedural angiography to document anastomosis locations, embolic material selection, special considerations in the context of pathology, and techniques to mitigate risks. Specific considerations for each anatomical region are discussed, with a focus on critical anastomoses, embolization risks, and prevention strategies.