Microsurgical anatomic features and nomenclature of the paraclinoid region

OBJECTIVE: We describe the detailed microsurgical anatomic features of the clinoid (C5) segment of the internal carotid artery (ICA) and surrounding s tructures, clarify the anatomic relationships of structures in this region, and emphasize the clinical relevance of these observations. Furthermore, b ecause the nomenclature of the paraclinoid region is confusing and lacks st andardization, this report provides a glossary of terms that are commonly u sed to describe the anatomic features of the paraclinoid region. METHODS: The region surrounding the anterior clinoid process was observed i n 70 specimens from 35 formalin-fixed cadaveric heads. Detailed microanatom ic dissections were performed in 10 specimens. Histological sections of thi s region were obtained from the formalin-fixed cadaveric specimens. RESULTS: The clinoid segment of the ICA is the portion that abuts the clino id process. This portion of the ICA can be directly observed only after rem oval of the clinoid process. The dura of the cavernous sinus roof separates to enclose the clinoid process. The clinoid segment of the ICA exists only where this separation of dural layers is present. Because the clinoid proc ess does not completely enclose the ICA in most cases, the clinoid segment is shaped more like a wedge than a cylinder. The outer layer of the dura (d ura propria) is a thick membrane that fuses with the adventitia of the ICA to form a competent ring that separates the intradural ICA from the extradu ral ICA. The thin inner membranous layer of the dura loosely surrounds the ICA throughout the entire length of its clinoid segment. The most proximal aspect of this membrane defines the proximal dural ring. The proximal ring is incompetent and admits a variable number of veins from the cavernous ple xus that accompany the ICA throughout its clinoid segment. CONCLUSION: The narrow space between the inner dural layer and the clinoid ICA is continuous with the cavernous sinus via an incompetent proximal dura l ring. This space between the clinoid ICA and the inner dural layer contai ns a variable number of veins that directly communicate with the cavernous plexus. Given the inconstancy of the venous plexus surrounding the clinoid ICA, we think that categorical labeling of the clinoid ICA as intracavernou s or extracavernous cannot be justified.