Internal carotid artery: correlative anatomy as a guide to surgery

The course of the internal carotid artery (ICA), particularly the C4–C6 segments, cavernous, clinoid, and ophthalmic, respectively, as described by Bouthillier et al. traverses a relatively compact region of complex anatomical relationships. The bony, dural, connective tissue, and neurovascular elements of this region frequently deter access or “block” surgical entry. The key to unlocking this region is a detailed understanding of its surgical anatomy. The treatment of aneurysms in proximity to the distal dural ring (DDR), e.g. clinoid, superior hypophyseal, or carotid ophthalmic aneurysms, presents considerable challenges to the neurosurgeon. The anterior clinoid process (ACP) and its associated anatomical variations, including a prominent middle clinoid process (MCP) and aerated optic strut, must be considered and anticipated when performing an anterior clinoidectomy. Aneurysms of the C4–C6 segments can transgress either the proximal or distal dural rings or both; this transgression (so-called transitional aneurysms) can occur in either direction making an extradural aneurysm and also intradural or vice versa. The existence of a carotid cave as originally described by Kobayashi et al. must be taken into consideration in the analysis of aneurysms of the ICA in proximity to the DDR. Again, determination of the relationship of the aneurysm to the DDR is critical to the decision making process; extradural aneurysms have a considerably different prognosis and treatment than intradural aneurysms. The correlation of cadaveric anatomy to surgical anatomy will serve as a guide to improve surgical techniques, strategies, and outcome.