A, The clinoid segment of the carotid artery is the segment situated medial to the anterior clinoid process. The upper dural ring, which surrounds the upper edge of the clinoid segment, is formed by the dura that extends medially from the upper margin of the anterior clinoid process. The lower dural ring extends medially from the lower margin of the anterior clinoid process. The ophthalmic artery arises from the superior surface of the initial supra-clinoid segment of the carotid artery and passes forward under the optic nerve to enter the optic foramen. The upper ring often seems to be adherent to and forms a collar around the carotid artery. However, in many cases there is a space between this ring and the anteromedial aspect of the artery that extends downward to form a cave around the artery, referred to as the carotid cave. The cave is most prominent on the anteromedial side of the carotid artery at the roof of the cavernous sinus. If the ophthalmic artery arises within the carotid cave, the neck of the aneurysm will also be located in the cave, and the aneurysm will extend upward out of the cave into the subarachnoid space. The superior hypophyseal artery arises from the medial wall of the internal carotid artery and courses toward the pituitary stalk. The optic strut is the bridge of bone that separates the optic foramen from the superior orbital fissure. This strut extends from the lower surface of the anterior clinoid process to the body of the sphenoid bone. The strut forms the inferolateral margin of the optic foramen. The anterior and middle cerebral arteries are also in the exposure. B, Sagittal cross section through the clinoid segment and carotid cave. The cave extends downward between the upper dural ring and the wall of the carotid artery. The ophthalmic artery usually arises from the carotid artery immediately above the carotid cave and upper dural ring. A probe is inserted in the carotid cave, the space between the upper dural ring and the wall of the carotid artery. This clinoid segment of the carotid artery is situated medial to the anterior clinoid process. C, Various patterns (1–5 in C) of the origin and passage of the ophthalmic artery that determine the degree of involvement by an aneurysm of the clinoid segment and carotid cave. 1, the ophthalmic artery arises from the superomedial wall of the artery well above the carotid cave. An aneurysm arising at the origin of this ophthalmic artery will mimic a superior hypophyseal aneurysm. 2, the ophthalmic artery arises in the carotid cave. 3, the artery arises just above the carotid cave. 4, the artery arises in the carotid cave and passes through the optic strut to enter the optic canal. 5, the artery arises in the cavernous sinus and passes through the superior orbital fissure. D and E, Superior and anterior views of the most common ophthalmic aneurysm. This aneurysm arises above the clinoid segment and the carotid cave from the medial part of the superior wall of the carotid artery and projects upward into the optic nerve. The cavernous sinus is located below the anterior clinoid process in the anterior view. F and G, superior and anterior view of an ophthalmic aneurysm that mimics a superior hypophyseal aneurysm. The ophthalmic artery has a relatively long course to the optic foramen. This aneurysm projects medially below the optic chiasm and mimics the superior hypophyseal aneurysm, although it arises at the origin of the ophthalmic artery. The neck of the aneurysm is proximal to the origin of the superior hypophyseal artery. This aneurysm, on lateral angiography, may be seen medial to and below the upper margin of the anterior clinoid process. (Images courtesy of AL Rhoton, Jr.)