Intracranial Collateral Anastomoses: Relevance to Endovascular Procedures Adnan H. Siddiqui, MD, PhD a, *, Peng R. Chen, MD b Endovascular strategies for addressing intracra- nial and extracranial disease continue to gain momentum. These techniques are limited princi- pally by technology and imagination. As newer devices and implements are introduced to the endovascular surgeon, more diseases previously construed to be the realm of open surgery or un- treatable are becoming amenable to endovascular interventions. Because of the nature of endovas- cular procedures, with liquid agents, flow-directed therapies, and embolic materials, it is critical for the endovascular surgeon to be aware of the collaterals that exist between the vessels being embolized and other critical collaterally connected vessels, occlusion of which may result in undesir- able outcomes. Similarly, for other occasions, such collaterals may provide unique conduits that may afford access in novel ways to the intra- cranial or extracranial circulation. The under- standing of these collaterals is best undertaken with an initial understanding of the development of the cranial vasculature. The rich anastomotic connections and interlinked development shed great light and provide a firm basis for under- standing the cranial collaterals. The collateral circulation may be divided by collaterals between extracranial and intracranial systems and collat- erals between the internal carotid and vertebrobasilar (VB) systems. This article provides a brief overview of cranial vascular development, followed by specific clinically relevant examples of extracranial and intracranial anastomoses and the internal carotid artery (ICA) and VB anastomoses. CRANIAL VASCULAR EMBRYOLOGY The cranial vasculature begins with the develop- ment of a vascular supply to the paired pharyngeal arches. This supply develops as vascular arches that emanate from the ventral aortic sac connect with the paired dorsal aortae. Each pharyngeal arch gets its own vascular arch. These vascular arches then develop and regress in rostrocaudal fashion. The pharyngeal arches become apparent at approximately 3 to 4 weeks’ gestation. The pharyngeal arches develop plexiform vascular channels that ultimately connect the ventral aortic sac with the paired dorsal aortae, forming the vascular arch. The first arch gives rise to the prim- itive stapedial artery, whereas the second gives rise to the hyoid artery. These arches then regress and coalesce to form the primitive hyoidostapedial artery. These vessels are critical to the vascular development of the skull base. This primitive branch follows the three divisions of the trigeminal a Department of Neurosurgery, School of Medicine and Biomedical Sciences, Millard Fillmore Gates Hospital, Kaleida Health, State University of New York, University at Buffalo, 3 Gates Circle, Buffalo, NY 14209, USA b Department of Neurosurgery, University of Texas at Houston, Houston, TX, USA * Corresponding author. E-mail address: adnan.h.siddiqui@gmail.com (A.H. Siddiqui). KEYWORDS  Complication avoidance  Extracranial-to-intracranial anastomoses  Intracranial anastomoses  Intracranial collateralization  Neurointerventional procedures  Vascular anatomy Neurosurg Clin N Am 20 (2009) 279–296 doi:10.1016/j.nec.2009.04.013 1042-3680/09/$ – see front matter ª 2009 Published by Elsevier Inc. neurosurgery.theclinics.com