Intradural Approach to the Left Cavernous Sinus. A, View of completed fronto-orbitozygomatic craniotomy and pretemporal approach to the cavernous sinus showing wide dissection of the sylvian fissure. The carotid artery is located medial to the anterior clinoid, and the optic nerve is located superomedial to the internal carotid artery. The approach to the cavernous sinus starts with removing the anterior clinoid process to expose the anterior portion of the roof. The dura above the anterior clinoid has been opened. The oculomotor nerve penetrates the roof of the cavernous sinus by passing through the oculomotor triangle, which forms the posterior part of the roof. B, View showing the anterior clinoid removed. Continuous irrigation is necessary to avoid heat damage to the optic nerve and the clinoidal segment when a drill is used to remove the clinoid. Removing the anterior clinoid process exposes the clinoidal space. The dura extending medially from the upper surface of the anterior clinoid forms the upper ring. The carotidoculomotor membrane lines the lower surface of the anterior clinoid and extends medially to form the lower dural ring and carotid collar. C, View showing the carotid artery elevated to expose the posterior communicating and anterior choroidal arteries. The oculomotor nerve passes lateral to the posterior clinoid process and penetrates the roof of the cavernous sinus by passing through the oculomotor triangle. D, View showing the opening of the posterior portion of the roof, which begins by opening the oculomotor cistern. The incision follows the third nerve forward to the posterior edge of the clinoidal space. The posterior clinoid is exposed medial to the oculomotor nerve. E, View showing the roof of the cavernous sinus opened on the medial side of the oculomotor cistern. Gentle packing with Surgicel controls the bleeding. The posterior clinoid and adjacent part of the dorsum and upper clivus have been removed. The basilar trunk has been exposed behind the dorsum sellae. The supraclinoid carotid artery bifurcates below the anterior perforate substance in the A1 segment of the anterior cerebral artery and M1 segment of the middle cerebral artery. F, View showing a small segment of the supraclinoid carotid removed to expose the pituitary stalk. The pituitary gland can be reached between the initial supraclinoid segment of the carotid and the horizontal segment of the intracavernous carotid. (Images courtesy of AL Rhoton, Jr.)