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Aneurysms of the Ophthalmic (C6) Segment of the Internal Carotid Artery: Embryology, Surgical Anatomy, and Radiology (Part 1)

Aneurysms arising from the intradural ophthalmic (C6) segment of the internal carotid artery (ICA) constitute 5% to 11% of all intracranial aneurysms; 25% of these aneurysms are giant, and 50% are multiple. Critical to the successful treatment of these aneurysms is an understanding of the embryologic development, regional anatomy, and radiographic techniques. In part 1 of this review, we discuss theories of Dorcas Hager Padget (neuroembryologist and neurosurgical illustrator) and Pierre Lasjaunias to clarify the complex embryology of this cerebral vasculature and to identify the vascular anomalies of the C6 segment. Next, we provide comprehensive detail of the bone, connective tissue, and vascular structures that are intimately related in the confining space surrounding the distal dural ring (DDR) and anterior clinoid process. The anatomic complexity of the region surrounding the C6 segment of the ICA partly explains the surgical challenge in treating aneurysms of this segment and the relatively high incidence of postoperative complications. We review the radiologic techniques to assess these aneurysms when planning treatment, such as the role of cerebral angiography in providing details about the hemodynamic function of the vessels (eg, collateral circulation pattern, potential recipient vessels for bypass) and the role of the DDR in distinguishing aneurysms of the extradural or intradural segment of the ICA. Although these aneurysms remain challenging, the information reviewed in part 1 helps to understand these aneurysms more clearly in terms of embryology, anatomy, and radiology for successful treatment strategies. Applications of this information are discussed in part 2, in which we review clinical experience and treatment strategies.