Stepwise Dissection of the Right Cavernous Sinus Using the Extradural Approach. A, View showing completed pretemporal fronto-orbitozygomatic craniotomy and pretemporal approach, with the dura at the lateral edge of the superior orbital fissure exposed. B, View showing outer layer of the cavernous sinus separated from its inner layer after cutting the dural band at the lateral edge of the superior orbital fissure with a sharp blade. This procedure is referred to as “peeling” of the middle fossa and cavernous sinus. The nerves invested in the inner layer of the lateral wall come into view as the meningeal (outer) layer is peeled away. The anterior clinoid process has been exposed. The optic nerve and the roof of the optic canal are located medial to the anterior clinoid. The middle meningeal artery was divided when the peeling of the middle fossa reached the posterior edge of V3. After dividing the middle meningeal artery, the peeling continues posteriorly and medially and the greater petrosal nerve is exposed at the lateral edge of V3. The greater petrosal nerve usually courses above and serves as a good landmark for identifying the petrous carotid. The carotid artery may be exposed under the dura and the greater petrosal nerve at the lateral edge of the trigeminal nerve. The medial edge of the peeling of the middle fossa is at the anterior petroclinoid dural fold, and the posterior edge is at the petrous ridge. C, View showing anterior clinoid process removed extradurally using a high-speed drill with a diamond burr. Continuous irrigation is necessary to avoid heat spreading to the optic nerve and the clinoidal segment of the carotid. The drilling leaves a thin layer of bone over the optic nerve and the clinoidal segment that is removed with a microdissector. The anterior clinoid has attachments at its base to the sphenoid ridge, roof of the optic canal, and optic strut. The optic strut forms the floor of the optic canal and separates the superior orbital fissure from the optic canal. D, Enlarged view showing removal the anterior clinoid exposes the carotidoculomotor membrane, lower ring, carotid collar, and clinoidal space or clinoidal triangle. The carotidoculomotor membrane lines the lower surface of the anterior clinoid and extends medially to form the lower dural ring and upward to form the carotid collar around the clinoidal segment. The “upper floor” of the anterior portion of the roof of the cavernous sinus is formed by the anterior clinoid process. The “lower floor” of the anterior portion of the roof of the cavernous sinus is the clinoidal space, which contains, from an anterior to posterior direction, the optic strut, clinoidal segment, and roof of the anterior part of the cavernous sinus. The venous spaces in the anterior part of the roof of the cavernous sinus are opened by incising the carotidoculomotor membrane. E, View showing the inner dural layer of the lateral sinus wall removed to expose the structures in this region. In surgery, the lesion is approached in the area at which it presents in the lateral wall. The middle fossa triangles exposed are the anteromedial triangle (between V1 and V2); the anterolateral triangle (between V2 and V3); the posterolateral triangle, also called Glasscock’s triangle (between V3 and the greater petrosal nerve); and the posteromedial triangle, also called Kawase’s triangle (lateral to the trigeminal nerve and posterior to the greater petrosal nerve). The petrous carotid is exposed under the greater petrosal nerve. F, View showing some portions of the middle fossa floor and roof of the internal acoustic canal removed to expose the intraosseous segment of the greater petrosal nerve, the geniculate ganglion, and the contents of the fundus of the internal acoustic canal, including the facial and vestibulocochlear nerves. The tensor tympani muscle crosses below the middle fossa floor between the middle meningeal artery and the greater petrosal nerve. The cochlea is located at the angle formed by the greater petrosal and the facial nerves. (Images courtesy of AL Rhoton, Jr.)