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Cavernous Sinus and Carotid Collar

Last Updated: August 23, 2020

INTRODUCTION

The paired cavernous sinuses are located near the center of the head on each side of the sella, pituitary gland, and sphenoid sinus. Each sinus has dural walls that surround a venous space through which a segment of the internal carotid artery courses. The sinus extends from the superior orbital fissure in front to the area lateral to the dorsum sellae behind (Fig. 9.1). Its anterior edge is attached to the margins of the superior orbital fissure and its posterior wall is located between the dorsum sellae medially and the ostium of Meckel’s cave laterally. The oculomotor, trochlear, and ophthalmic nerves course in the lateral wall. The abducens nerve courses on the medial side of the ophthalmic nerve between it and the internal carotid artery. The lateral wall faces the temporal lobe, the roof faces the basal cisterns, the medial wall faces the sella, pituitary gland, and body of the sphenoid bone, and the lower edge is located below the horizontal portion of the intracavernous segment of the internal carotid artery. The cavernous sinus has venous connections with the cerebrum, cerebellum, brainstem, face, eye, orbit, nasopharynx, mastoid, and middle ear (6, 7). These connections and the relationships of the cavernous sinus to the carotid artery, extraocular nerves, and pituitary gland make the sinus of special interest to neurologists, ophthalmologists, otolaryngologists, and endocrinologists, in addition to neurosurgeons (1).

Figure 9.1 (A–F). Stepwise dissection of the right cavernous sinus. A, the lateral wall of the cavernous sinus extends downward from the tentorial edge and blends into the dura covering Meckel’s cave and the middle fossa. The oculomotor and trochlear nerves enter the roof of the cavernous sinus. The carotid artery exits the cavernous sinus on the medial side of the anterior clinoid process. B, the outer layer of dura has been peeled away from the lateral wall of the cavernous sinus and Meckel’s cave. This exposes the oculomotor and trochlear nerves entering the roof of the cavernous sinus and passing forward through the superior orbital fissure. The thin layer covering Meckel’s cave consists in part of the arachnoid membrane extending forward from the posterior fossa and surrounding trigeminal nerve to the level of the trigeminal ganglion. The superior petrosal sinus passes above the ostium of Meckel’s cave and joins the posterior part of the cavernous sinus. C, the oculomotor nerve enters a short cistern in the sinus roof (red arrow) and does not become incorporated into the lateral wall until it reaches the lower margin of the anterior clinoid process (yellow arrow). The arachnoid covering of Meckel’s cave, which extends forward around the posterior trigeminal root to the level of the midportion of the ganglion, has been removed. The cavernous sinus extends from the superior orbital fissure to the petrous apex. It is located medial to the upper third of the gasserian ganglion. The pericavernous venous plexus surrounds the maxillary and mandibular nerves in the region of the foramen rotundum and foramen ovale. D, the remaining dura covering the lateral wall has been removed. The oculomotor, trochlear, and ophthalmic nerves pass forward to converge on the superior orbital fissure. The segment of the superior petrosal sinus above the posterior trigeminal root has been removed. E, the posterior trigeminal root has been reflected forward to expose the posterior part of the lower margin of the cavernous sinus (yellow arrow) in the area medial to the trigeminal impression on the petrous apex, in which Meckel’s cave sits. The superior ophthalmic vein exits the orbit through the superior orbital fissure and passes posteriorly below the ophthalmic nerve to enter the cavernous sinus. F, the trigeminal nerve and its three divisions have been reflected forward to expose the venous spaces of the cavernous sinus. The lower margin of the cavernous sinus (broken line) is located at the site where the internal carotid artery exits the carotid canal by passing below the petrolingual ligament. The venous spaces in the cavernous sinus communicate posteriorly with the inferior and superior petrosal and basilar sinuses. In addition, the cavernous sinus communicates with the superior ophthalmic veins and the venous plexus around the maxillary and mandibular nerves and pituitary gland. A., artery; Alv., alveolar; Ant., anterior; Bas., basilar; Brs., branches; Car., carotid; Cav., cavernous; Cist., cistern; Clin., clinoid; CN, cranial nerve; Cond., condyle; Div., division; Fiss., fissure; For., foramen; Gang., ganglion; Gen., geniculate; Gr., greater; Impress., impression; Inf., inferior; Inf. Lat., inferolateral; Infraorb., infraorbital; Infratemp., infratemporal; Lat., lateral; Less., lesser; Lig., ligament; M., muscle; Mandib., mandibular; Max., maxillary; Men., meningeal; Mid., middle; N., nerve; Oculom., oculomotor; Ophth., ophthalmic; Orb., orbital; Pericav., pericavernous; Pet., petrosal, petrous; Petroling., petrolingual; Petrosphen., petrosphenoid; Plex., plexus; Post., posterior; Proc., process; Pteryg., pterygoid; Pterygopal., pterygopalatine; Rec., recurrent; Seg., segment; Sphen., sphenoid; Sup., superior; Tent., tentorial; Tr., trunk; Trig., trigeminal; Troch., trochlear; V., vein; Ven., venous; Zygo., zygomatic. (Images courtesy of AL Rhoton, Jr.)

Figure 9.1 (G–L). Stepwise dissection of the right cavernous sinus. G, the venous plexus surrounding the nerves has been removed to expose the trigeminal divisions and the nerves coursing in the wall of the cavernous sinus. H, the ophthalmic nerve has been depressed to expose the abducens nerve, which passes under the petrosphenoid ligament roofing Dorello’s canal, and courses medial to the ophthalmic nerve. The abducens nerve crosses laterally below the ophthalmic nerve as it passes through the superior orbital fissure. I, the anterior clinoid process has been removed. The optic strut separates the optic canal and superior orbital fissure. The dura extending medially off the upper surface of the anterior clinoid forms the upper dural ring around the internal carotid artery, and the dura lining the lower margin of the clinoid extends medially to form the lower dural ring. The clinoid segment of the carotid artery, located between the upper and lower ring, is enclosed in the dura sheath, referred to as the carotid collar. J, the trigeminal nerve has been folded downward to expose the petrolingual ligament, which extends above the internal carotid artery, just proximal to where the artery enters the cavernous sinus. The abducens nerve passes around the internal carotid artery and courses medial to the ophthalmic nerve in the lower part of the cavernous sinus. The margins of the cavernous sinus are shown with an broken line. The cavernous sinus does not extend laterally into the area of the trigeminal impression where Meckel’s cave sets. K, enlarged view. The optic nerve has been elevated to expose the ophthalmic artery coursing within the optic sheath. At the orbital apex, the artery penetrates the optic sheath and enters the orbital apex on the lateral side of the optic nerve. Removal of additional optic strut exposes the mucosa lining the sphenoid sinus on the medial side of the optic strut. L, the bone between the first and second and the second and third trigeminal divisions has been drilled to expose the lateral wing of the sphenoid sinus. The vidian nerve, which passes forward to enter the sphenopalatine ganglion in the pterygopalatine fossa, is exposed between the second and third trigeminal divisions. A., artery; Alv., alveolar; Ant., anterior; Bas., basilar; Brs., branches; Car., carotid; Cav., cavernous; Cist., cistern; Clin., clinoid; CN, cranial nerve; Cond., condyle; Div., division; Fiss., fissure; For., foramen; Gang., ganglion; Gen., geniculate; Gr., greater; Impress., impression; Inf., inferior; Inf. Lat., inferolateral; Infraorb., infraorbital; Infratemp., infratemporal; Lat., lateral; Less., lesser; Lig., ligament; M., muscle; Mandib., mandibular; Max., maxillary; Men., meningeal; Mid., middle; N., nerve; Oculom., oculomotor; Ophth., ophthalmic; Orb., orbital; Pericav., pericavernous; Pet., petrosal, petrous; Petroling., petrolingual; Petrosphen., petrosphenoid; Plex., plexus; Post., posterior; Proc., process; Pteryg., pterygoid; Pterygopal., pterygopalatine; Rec., recurrent; Seg., segment; Sphen., sphenoid; Sup., superior; Tent., tentorial; Tr., trunk; Trig., trigeminal; Troch., trochlear; V., vein; Ven., venous; Zygo., zygomatic. (Images courtesy of AL Rhoton, Jr.)

Figure 9.1 (M–P). Stepwise dissection of the right cavernous sinus. M, the trigeminal nerve has been reflected forward to expose the opening into the lateral wing of the sphenoid sinus. The vidian nerve, formed by the union of the greater and deep petrosal nerves, courses forward in the vidian canal to reach the pterygopalatine fossa. N, enlarged view of the petrolingual and petrosphenoid ligaments. The petrosphenoid ligament extends from the lower part of the lateral margin of the dorsum sellae above the abducens nerve to the petrous apex. The lower margin of the posterior wall of the cavernous sinus is located at the lower margin of Dorello’s canal. Anteriorly, the lower margin of the cavernous sinus is located at the level that the internal carotid artery exits the area below the petrolingual ligament and enters the posterior part of the cavernous sinus. O, the exposure has been extended down to the infratemporal and pterygopalatine fossae. The infratemporal fossa contains branches of the mandibular nerve and maxillary artery, the pterygoid muscles, and the pterygoid venous plexus. The maxillary nerve passes through the foramen rotundum to enter the pterygopalatine fossa. P, enlarged view. The pterygopalatine fossa is located between the posterior maxillary wall and the pterygoid process. The vidian nerve penetrates the upper part of the pterygoid process and the area below the foramen rotundum to enter pterygopalatine fossa. The maxillary nerve gives rise to zygomatic, infraorbital, and posterosuperior alveolar nerves, and branches and rami to the pterygopalatine ganglion. A., artery; Alv., alveolar; Ant., anterior; Bas., basilar; Brs., branches; Car., carotid; Cav., cavernous; Cist., cistern; Clin., clinoid; CN, cranial nerve; Cond., condyle; Div., division; Fiss., fissure; For., foramen; Gang., ganglion; Gen., geniculate; Gr., greater; Impress., impression; Inf., inferior; Inf. Lat., inferolateral; Infraorb., infraorbital; Infratemp., infratemporal; Lat., lateral; Less., lesser; Lig., ligament; M., muscle; Mandib., mandibular; Max., maxillary; Men., meningeal; Mid., middle; N., nerve; Oculom., oculomotor; Ophth., ophthalmic; Orb., orbital; Pericav., pericavernous; Pet., petrosal, petrous; Petroling., petrolingual; Petrosphen., petrosphenoid; Plex., plexus; Post., posterior; Proc., process; Pteryg., pterygoid; Pterygopal., pterygopalatine; Rec., recurrent; Seg., segment; Sphen., sphenoid; Sup., superior; Tent., tentorial; Tr., trunk; Trig., trigeminal; Troch., trochlear; V., vein; Ven., venous; Zygo., zygomatic. (Images courtesy of AL Rhoton, Jr.)

Figure 9.1 (Q–S). Stepwise dissection of the right cavernous sinus. Q, overview. The posterior wall of the cavernous sinus extends laterally from the dorsum sellae to the medial edge of the ostium of Meckel’s cave. The floor of the middle fossa has been removed to expose the infratemporal fossa, which contains the branches of the maxillary artery and the mandibular nerve, the pterygoid venous plexus, and the pterygoid muscles. The infratemporal fossa opens medially into the pterygopalatine fossa. The maxillary nerve passes through the foramen rotundum to enter the pterygopalatine fossa and send branches along the orbital floor. The ophthalmic nerve passes through the superior orbital fissure and sends branches along the orbital roof. R, lateral view of the left cavernous sinus. The oculomotor, trochlear, and trigeminal nerves are all enclosed in an arachnoid sac that surrounds the nerve for a short distance after they enter into the dura. The lateral wall of all three cisterns has been removed. Meckel’s cave, the cistern around the posterior trigeminal root, extends forward to the middle portion of the gasserian ganglion. The oculomotor cistern extends forward in the roof of the cavernous sinus to where the nerve passes under the anterior clinoid process. The thin cistern around the trochlear nerve extends forward below the oculomotor cistern. S, the posterior trigeminal root has been removed to expose the trigeminal impression and the lower margin of the cavernous sinus, which is located at the level of the upper and middle thirds of the trigeminal ganglion (arrows). A., artery; Alv., alveolar; Ant., anterior; Bas., basilar; Brs., branches; Car., carotid; Cav., cavernous; Cist., cistern; Clin., clinoid; CN, cranial nerve; Cond., condyle; Div., division; Fiss., fissure; For., foramen; Gang., ganglion; Gen., geniculate; Gr., greater; Impress., impression; Inf., inferior; Inf. Lat., inferolateral; Infraorb., infraorbital; Infratemp., infratemporal; Lat., lateral; Less., lesser; Lig., ligament; M., muscle; Mandib., mandibular; Max., maxillary; Men., meningeal; Mid., middle; N., nerve; Oculom., oculomotor; Ophth., ophthalmic; Orb., orbital; Pericav., pericavernous; Pet., petrosal, petrous; Petroling., petrolingual; Petrosphen., petrosphenoid; Plex., plexus; Post., posterior; Proc., process; Pteryg., pterygoid; Pterygopal., pterygopalatine; Rec., recurrent; Seg., segment; Sphen., sphenoid; Sup., superior; Tent., tentorial; Tr., trunk; Trig., trigeminal; Troch., trochlear; V., vein; Ven., venous; Zygo., zygomatic. (Images courtesy of AL Rhoton, Jr.)

THE SINUS

The cavernous sinus is defined as the dural envelope in which the cavernous segment of the internal carotid artery courses (Fig. 9.2). The dural envelope contains not only the cavernous carotid artery, but is also the site of a venous confluence that receives the terminal end of multiple veins draining the orbit, sylvian fissure, and middle and anterior fossae and has free communication with the basilar, superior and inferior petrosal, and intercavernous sinuses. Overall, the sinus is shaped like a boat with its narrow keel located at the superior orbital fissure and its broader bow (posterior wall) located lateral to the dorsum sellae above the petrous apex. It has four walls: a roof and lateral, medial, and posterior walls. The deck or roof of the sinus faces upward and the lower edge, formed at the junction of the medial and lateral walls below the intracavernous segment of the internal carotid artery, gives the sinus a triangular shape in cross section (Fig. 9.3). The roof is formed by the dura lining the lower margin of the anterior clinoid process anteriorly and the patch of dura, called the oculomotor triangle, between the anterior and posterior clinoid processes and the petrous apex through which the oculomotor nerve penetrates the sinus roof. The medial edge of the oculomotor triangle is formed by the interclinoid dural fold, which extends from the anterior to the posterior clinoid process; the lateral margin is formed by the anterior petroclinoid fold, which extends from the anterior clinoid process to the petrous apex; and the posterior margin is formed by the posterior petroclinoid fold, which extends from the posterior clinoid process to the petrous apex.

Figure 9.2 (A–C). A, superior view of the cranial base in the region of the cavernous sinus. The cavernous sinus extends from the superior orbital fissure anteriorly, to the petrous apex posteriorly, and it is bordered by the sella medially and the middle fossa laterally. It fills the posterior margin of the superior orbital fissure, which is located below the anterior clinoid process and its posterior wall, extends from the lateral edge of the dorsum sellae to the medial margin of the trigeminal impression and Meckel’s cave. Numerous venous channels open into the cavernous sinus. These include the basilar, anterior and posterior intercavernous, and the superior and inferior petrosal sinuses; the sylvian and ophthalmic veins, and the veins exiting the foramen ovale, rotundum, and spinosum; and the carotid canal and the sphenoidal emissary foramen. Each structure is shown by colored arrows. The basilar sinus is the largest communicating channel between the cavernous sinuses. B, superior view of the roof of the cavernous sinus. The oculomotor triangle, through which the oculomotor enters the roof of the cavernous sinus, is located between the anterior and posterior petroclinoid and the interclinoid dural folds. The interclinoid fold extends from the anterior to the posterior clinoid process. The anterior petroclinoid fold extends from the petrous apex to the anterior clinoid process and the posterior petroclinoid fold extends from the petrous apex to the posterior clinoid process. The internal carotid artery has been divided at the point it exits the roof of the cavernous sinus. A probe has been inserted under the falciform dura fold, which extends medially from the anterior clinoid process and above the optic canal to the chiasmatic sulcus. C, the dura has been removed from the roof and lateral wall of another cavernous sinus. The anterior clinoid process has been preserved. The abducens nerve passes around the lateral surface of the internal carotid artery. The oculomotor and ophthalmic nerves have been divided to expose the floor of the cavernous sinus, which is located at the level of the lower edge of the carotid sulcus on the sphenoid body below the intracavernous carotid. A., artery; Ant., anterior; Bas., basal; Car., carotid; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Comm., communicating; Diaph., diaphragm; Em., emissary; Falc., falciform; For., foramen; Hyp., hypophyseal; Impress., impression; Inf., inferior; Intercav., intercavernous; Interclin., interclinoid; Lat., lateral; Lig., ligament; Med., medial; Men., meningeal; Men. Hyp., meningohypophyseal; Olf., olfactory; Ophth., ophthalmic; P.C.A., posterior cerebral artery; Pet., petrosal; Petroclin., petroclinoid; Post., posterior; Seg., segment; Sphen., sphenoid; Sup., superior; Trig., trigeminal; V., vein; Ven., venous. (Images courtesy of AL Rhoton, Jr.)

Figure 9.2 (D–G). D, superior view of the cavernous sinus with the anterior clinoid process and roof removed. The roof of the sinus extends backward from the area below the anterior clinoid process, which has been removed, into the area between the middle fossa and sella and posteriorly, to the area lateral to the dorsum sellae. The lateral wall of the cavernous sinus, in which the oculomotor, ophthalmic, and trochlear nerves course, has been retracted laterally to show the lower margin of the sinus from inside. E, enlarged view of the sinus opened from above. Numerous ostia of veins drain the surrounding areas, which open into the cavernous sinus (arrows). The ophthalmic artery enters the optic canal. F, the cavernous sinus shown in C and D is viewed from laterally. The pins have been inserted along the lower edge of the interior of the sinus shown in E. The lower edge of the sinus is located medial to the upper third of Meckel’s cave and trigeminal ganglion and extends forward below the ophthalmic nerve, but does not include the area of the maxillary nerve. G, lateral view of the specimen shown in C. A segment of the oculomotor, trigeminal, trochlear, and abducens nerves has been removed. The arrows have been placed along the lower margin of the dural envelope forming the lower margin of the cavernous sinus. The cavernous sinus does not extend laterally to the mandibular and maxillary nerves, but extends down to just below the carotid sulcus on the body of the sphenoid bone. A., artery; Ant., anterior; Bas., basal; Car., carotid; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Comm., communicating; Diaph., diaphragm; Em., emissary; Falc., falciform; For., foramen; Hyp., hypophyseal; Impress., impression; Inf., inferior; Intercav., intercavernous; Interclin., interclinoid; Lat., lateral; Lig., ligament; Med., medial; Men., meningeal; Men. Hyp., meningohypophyseal; Olf., olfactory; Ophth., ophthalmic; P.C.A., posterior cerebral artery; Pet., petrosal; Petroclin., petroclinoid; Post., posterior; Seg., segment; Sphen., sphenoid; Sup., superior; Trig., trigeminal; V., vein; Ven., venous. (Images courtesy of AL Rhoton, Jr.)

Figure 9.3. Stepwise dissection of the roof of the cavernous sinus. A, superior view. The dura covering the upper surface of the right anterior clinoid process, optic canal, and adjacent part of the planum has been removed. The roof of the cavernous sinus is formed anteriorly by the dura lining the lower margin of the anterior clinoid and posteriorly by the dura covering the oculomotor triangle located between the anterior and posterior petroclinoidal and intraclinoidal dural folds. The oculomotor nerve enters the roof of the cavernous sinus through the oculomotor triangle. B, the anterior clinoid and roof of the optic canal has been removed. The optic nerve has been elevated to expose the ophthalmic artery entering the optic foramen. Removing the anterior clinoid exposes the floor of the clinoidal triangle located between the optic and oculomotor nerves. The dura separating the lower surface of the clinoid from the oculomotor nerve and extending medially around the carotid artery, referred to as the carotidoculomotor membrane, forms the floor of the clinoidal triangle and the anterior part of the roof of the cavernous sinus. The dura extending medially off the upper surface of the clinoid forms the upper dural ring, and the carotidoculomotor membrane extending medially from the lower surface of the clinoid forms the lower dural ring. C, the dura in the floor of the clinoidal triangle and roof of the oculomotor triangle, which together form the roof of the cavernous sinus, have been removed to expose the upper part of the cavernous sinus. The dura has been elevated from the lateral wall of the cavernous sinus and middle fossa floor to expose the nerves coursing in the lateral wall of the cavernous sinus. D, the sinus has been cleared to expose the clinoid segment of the internal carotid artery in the clinoidal triangle and the posterior bend of the intercavernous carotid below the oculomotor triangle. The anterior part of the roof is formed by the dura that separates the anterior clinoid and oculomotor nerve and that extends medially to form the lower dural ring. The posterior part of the roof is formed by the dura forming the oculomotor triangle. A., artery; Ant., anterior; Car., carotid; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Hyp., hypophyseal; Interclin., interclinoid; Oculom., oculomotor; Ophth., ophthalmic; Petroclin., petroclinoid; Post., posterior; S.C.A., superior cerebellar artery; Seg., segment; Sphen., sphenoid; Sup., superior; Tent., tentorial; Triang., triangle. (Images courtesy of AL Rhoton, Jr.)

The cavernous sinus has a wide posterior dural wall that it shares with the lateral part of the posterior wall of the basilar sinus, the venous connection extending across the back of the upper clivus and dorsum sellae. The basilar sinus is the largest venous connection between the paired cavernous sinuses. The cavernous sinus opens into and communicates widely at its posterior end with the basilar sinus. The superior and inferior petrosal sinuses also open into the lateral part of the basilar sinus, thus creating a large venous confluence along the posterior wall of the cavernous sinus at the area where the cavernous, basilar, and superior and inferior petrosal sinuses converge. This part of the posterior wall of the cavernous sinus that it shares with the basilar sinus is located lateral to the dorsum sellae, where the basilar sinus opens into the cavernous sinus and communicates with the superior and inferior petrosal sinuses. The lower margin of the posterior wall of the cavernous sinus is located at the upper margin of the petroclival fissure at the junction of the temporal and sphenoid bones. The abducens nerve passes through the lower margin of the posterior wall and under the petrosphenoid ligament to enter the sinus. The upper edge of the posterior wall is located at the level of the posterior petroclinoid dural fold, which extends from the petrous apex to the posterior clinoid process. The lateral edge of the posterior wall is located just medial to the ostium of Meckel’s cave, and the medial edge is located at the lateral margin of the dorsum sellae.

The lateral wall extends from the medial edge of Meckel’s cave posteriorly to the lateral margin of the superior orbital fissure anteriorly, and from the anterior petroclinoid dural fold above to the lower margin of the carotid sulcus below. The carotid sulcus is the groove on the lateral aspect of the body of the sphenoid along which the internal carotid artery courses (Fig. 9.4). The sheet of dura forming the posterior part of the lateral wall of the sinus also forms the upper third of the medial wall of Meckel’s cave, which is located lateral to and is separated from the posterior part of the cavernous sinus by their shared dural wall. The medial wall is formed by the dura that constitutes the lateral wall of the sella turcica and covers the lateral surface of the body of the sphenoid bone. The medial wall extends from the lateral edge of the dorsum sellae posteriorly to the medial edge of the superior orbital fissure anteriorly, and from the interclinoid dural fold above to the lower margin of the carotid sulcus below. Anteriorly, the lower edge of the sinus, where the medial and lateral walls meet, is located just below where the ophthalmic nerve courses in the lateral sinus wall; proceeding posteriorly, it is located medial to the junction of the upper and middle third of the gasserian ganglion. Finally, at the posterior part, it slopes upward medial to the upper part of Meckel’s cave (Fig. 9.1E). Behind the site where the ophthalmic nerve arises from the trigeminal ganglion, the lower edge of the medial and lateral walls of the sinus come together at the lateral edge of the carotid sulcus on the medial side of the upper part of Meckel’s cave. Only the upper part of the medial wall of Meckel’s cave and the upper part of the gasserian ganglion are located directly lateral to the cavernous sinus; thus almost all of Meckel’s cave is located below and lateral to the posterior part of the cavernous sinus (Figs. 9.1 and 9.2). Meckel’s cave extends forward from the posterior fossa, where its ostium is located between the medial part of the petrous ridge below, the superior petrosal sinus above, and the lateral edge of the cavernous sinus medially. The subarachnoid space extends forward within Meckel’s cave to approximately the level of the midportion of the gasserian ganglion. The terminal part of the petrous carotid exits the carotid canal and passes under the trigeminal nerve and the petrolingual ligament, where it turns upward to enter the posterior part of the cavernous sinus. The artery becomes enclosed in the dural envelope of the cavernous sinus only when it exits the region of the foramen lacerum and turns upward, after traveling below the petrolingual ligament to reach the carotid sulcus on the lateral surface of the sphenoid body (Fig. 9.1).

Figure 9.4. Osseous relationships of the cavernous sinus and carotid collar. A, superior view. The osseous structures, which nearly encircle the clinoid segment of the internal carotid artery, include the anterior clinoid laterally, the optic strut anteriorly, and the carotid sulcus medially. The carotid sulcus begins lateral to the dorsum sellae at the intracranial end of the carotid canal, extends forward just below the sellar floor, and turns upward along the posterior surface of the optic strut. The anterior clinoid process projects backward from the lesser wing of the sphenoid bone, often overlapping the lateral edge of the carotid sulcus. The anterior root of the lesser sphenoid wing extends medially to form the roof of the optic canal. The posterior root of the lesser wing, referred to as the optic strut, extends from the inferomedial aspect of the anterior clinoid to the sphenoid body. The bony collar around the carotid artery formed by the anterior clinoid, optic strut, and carotid sulcus is inclined downward as it slopes medially from the upper surface of the anterior clinoid to the carotid sulcus. Another small prominence, the middle clinoid process, situated on the medial side of the carotid sulcus at the level of the tip of the anterior clinoid process, projects upward and laterally. In some cases, there is an osseous bridge extending from the tip of the middle clinoid to the tip of the anterior clinoid. In well pneumatized sphenoid bones, the carotid sulcus is seen as a prominence in the lateral wall of the sphenoid sinus just below the floor of the sella. B, posterior view of the optic strut, optic canal, and the superior orbital fissure. The optic strut separates the optic canal and superior orbital fissure and forms the floor of the optic canal and the superomedial part of the roof of the superior orbital fissure. The posterior surface of the strut is shaped to accommodate the anterior wall of the clinoid segment. The artery courses along and may groove the medial half of the lower aspect of the anterior clinoid before turning upward along the medial edge of the clinoid. The air cells in the sphenoid sinus may extend into the optic strut and anterior clinoid. In this case, the sphenoid sinus has pneumatized to a degree that bone is absent over the anterior part of the carotid sulcus, just medial to where the optic strut attaches to the body of the sphenoid bone. The maxillary strut is the bridge of bone separating the superior orbital fissure from the foramen rotundum. C, oblique posterior view of the right optic strut. The lateral part of the bony collar around the clinoid segment is formed by the anterior clinoid, the anterior part is formed by the posterior surface of the optic strut and the part of the carotid sulcus located medial to the anterior clinoid process. The posterior surface of the optic strut is wider medially adjacent to the carotid sulcus than it is laterally at the site of attachment to the anterior clinoid process. The optic strut slopes downward from its lateral end so that the medial part of the bony collar is located below the level of the part of the collar joining the anterior clinoid. The inferomedial aspect of the right anterior clinoid is grooved by the artery. D, superior view of specimen with bilateral caroticoclinoidal foramen and interclinoidal osseous bridges. An osseous bridge connects the tips of the anterior and middle clinoid processes bilaterally, thus creating a bony ring around the artery, called a caroticoclinoidal foramen, on each side. There is also an interclinoidal osseous bridge connecting the anterior and posterior clinoid processes on both sides. E, superior view of another specimen, in which the lesser sphenoid wings and the base of the anterior clinoids and roof of the optic canals have been removed. The remaining part of the anterior clinoid is held in place by its attachment to the optic strut. The medial side of the anterior clinoid is grooved to accommodate the clinoid segment. F, enlarged view of the left half of E. The posterior face of the optic strut is shaped to accommodate the anterior surface, and the medial aspect of the anterior clinoid is grooved to accommodate the lateral surface of the clinoid segment. The tip of the anterior clinoid process is the site of a small bony projection directed toward the middle clinoid process, with the anterior and middle clinoids nearly completing a ring around the clinoid segment at the level of the cavernous sinus roof. A., artery; Ant., anterior; Car., carotid; Caroticoclin., caroticoclinoid; Clin., clinoid; Em., emissary; Fiss., fissure; For., foramen; Gr., greater; Interclin., interclinoid; Lac., lacrimal; Less., lesser; Mid., middle; Orb., orbital; Pit., pituitary; Post., posterior; Sphen., sphenoid; Sulc., sulcus; Sup., superior; Tuberc., tuberculum; V., vein. (Images courtesy of AL Rhoton, Jr.)

The maxillary nerve does not course in the lateral wall of the dural envelop of the sinus as does the ophthalmic nerve. It courses beneath the dura of the middle fossa, below the level where the medial and lateral walls of the cavernous sinus join at the lower edge of the ophthalmic nerve. The superior ophthalmic vein usually courses posteriorly along the sphenoid body in the interval between the ophthalmic and maxillary nerves to reach the anteroinferior part of the sinus. As the dura is elevated from the floor of the middle fossa, it can be stripped upward off the lateral aspects of both the maxillary and ophthalmic nerves, but only the ophthalmic nerve has the venous space of the cavernous sinus on its medial side. The medial side of the maxillary nerve sits against the bone and is located below the lower edge of the anterior part of the sinus.

Numerous venous channels course along the lateral margin of the sella, the medial part of the middle fossa, the superior and inferior orbital fissures, the foramina ovale, rotundum, and spinosum and surrounding the pituitary gland. These channels converge on the cavernous sinus and may be, mistakenly, considered to be part of the cavernous sinus (Fig. 9.2). However, they course outside the dural envelope containing the internal carotid artery and open into the sinus through discrete ostia. They are part of the pericavernous venous plexus, but are not within the sinus. There is a tendency in classifying tumors in the region to include those tumors along the anterior clinoid process and in the region of Meckel’s cave and petrous apex as cavernous sinus tumors; however, the author would not define those as cavernous sinus tumors. Those tumors that involve this region, but do not involve or extend into the dural envelope around the carotid artery, are much more amenable to resection than those involving and extending inside the dural envelope. The important osseous relationships in the area are reviewed before the dural relationships in the area are considered.

OSSEOUS RELATIONSHIPS

The cavernous sinus sits on the lateral aspect of the body of the sphenoid bone (Figs. 9.2 and 9.4) (19). The posterior part of the lower edge of the sinus is located above the junction of the petrous apex and body of the sphenoid bone at the upper end of the petroclival fissure, and the posterior edge of the medial wall rests against the lateral edge of the dorsum sellae. The cavernous sinus extends downward and laterally from the lateral margin of the sella, across the sphenoid body to the junction of the body and greater sphenoid wing of the sphenoid, but does not extend laterally to include the margins of the foramina ovale, rotundum, or spinosum, although venous channels coursing through and around these foramina empty into the sinus and are part of the pericavernous venous plexus. The inconsistently occurring sphenoid emissary foramen, situated medial to the foramen ovale, transmits an emissary vein from the cavernous sinus.

The carotid sulcus is the shallow groove on the lateral aspect of the body of the sphenoid bone along which the internal carotid courses in the cavernous sinus. The intracavernous carotid sits against and is separated from the carotid sulcus by the dura of the medial sinus wall (Fig. 9.4). The carotid sulcus begins below and lateral to the dorsum sellae at the intracranial end of the carotid canal, turns forward to groove the body of the sphenoid immediately below the lateral edge of the floor of the sella, and turns upward to end medial to the anterior clinoid process. The segment of the internal carotid artery that courses along the medial side of the clinoid is referred to as the clinoid segment. The carotid sulcus, in well-pneumatized sphenoid bones, forms a serpiginous prominence that can be seen in the lateral wall of the sphenoid sinus below the pituitary fossa. The bone in the lateral wall of the sphenoid sinus may be thin or even absent in some areas, thus allowing the artery to be observed through the sinus wall.

Anterior and Middle Clinoid Processes

The anterior clinoid process projects posteriorly from the lesser wing of the sphenoid bone above the anterior part of the roof of the sinus (Fig. 9.4). The base of the clinoid has three sites of continuity with the adjacent part of the sphenoid bone. The base is attached anteriorly at the medial edge of the sphenoid ridge, formed by the lesser sphenoid wing, and is attached medially to the anterior and posterior roots of the lesser wing. The anterior root of the lesser wing extends medially from the base of the anterior clinoid to the body of the sphenoid bone and forms the roof of the optic canal. The posterior root of the lesser wing, called the optic strut, extends medially below the optic nerve to the sphenoid body and forms the floor of the optic canal. The base of the anterior clinoid forms the lateral margin of the optic canal. The segment of the internal carotid artery that courses along the medial aspect of and is exposed by removing the anterior clinoid is referred to as the clinoid segment. The clinoid segment courses below the medial half of the lower margin of the clinoid, where it grooves the bone before coursing upward along the medial edge of the clinoid (Fig. 9.4F). The medial edge of the clinoid, just behind the base, is frequently the site of a shallow rounded indention that accommodates the lateral surface of the clinoid segment. The posterior tip of the clinoid often projects medially behind the lateral part of the clinoid segment. The anterior clinoid is the site of attachment of the anteromedial part of the tentorium and the anterior petroclinoid and interclinoid dural folds. Another dural fold, the falciform ligament, extends from the base of the clinoid across the roof of the optic canal to the planum sphenoidale. The chiasmatic sulcus is a shallow trough on the upper surface of the sphenoid bone between the intracranial end of the optic canals. The tuberculum sellae is located in the midline along the ridge forming the posterior margin of the chiasmatic sulcus. The anterior clinoid has a dense surface of cortical bone and a weak diploe of cancellous bone that is sometimes crossed by small venous channels that communicate with the cavernous sinus and the diploic veins of the orbital roof. The air cells in the sphenoid sinus may also extend through the optic strut into the anterior clinoid.

There is another small prominence, the middle clinoid process, that projects upward on the medial side of the terminal part of the carotid sulcus and medial to the tip of the anterior clinoid process (Fig. 9.3). An osseous bridge may extend from the tip of the middle clinoid to the tip of the anterior clinoid, thus converting the roof of the anterior part of the cavernous sinus into a bony ring or foramen, referred to as a caroticoclinoidal foramen, through which the internal carotid artery passes (Fig. 9.4D). This type of variant may infrequently occur bilaterally (15). There may also be interclinoidal osseous bridges that extend from the anterior to the posterior clinoid unilaterally or bilaterally (Fig. 9.4D). Such bridges make it difficult to remove the anterior clinoid process.

Optic Strut

The optic strut (posterior root of the lesser wing) is a small bridge of bone that extends from the inferomedial aspect of the base of the anterior clinoid process to the body of the sphenoid just in front of the carotid sulcus (Figs. 9.1 and 9.4) (18). The strut, from its junction with the clinoid, slopes gently downward as it approaches the body of the sphenoid. The strut separates the optic canal and superior orbital fissure. The superior surface of the strut, which slopes downward and forward from its intracranial edge, forms the floor of the optic canal. The inferior surface of the optic strut forms the medial part of the roof of the superior orbital fissure and the anterior part of the roof of the cavernous sinus. The strut sits at the junction of the orbital apex anteriorly, with the superior orbital fissure and optic canal posteriorly. The anterior edge of the strut is a narrow ridge located at the junction of its superior and inferior surfaces. The posterior face of the optic strut, which faces slightly downward, is shaped to accommodate the anterior surface of the anterior bend of the intracavernous carotid, which rests against the posterior surface of the optic strut as it ascends on the medial side of the anterior clinoid process. The posterior face of the strut also widens as it slopes medially. The site at which the strut blends into the sphenoid body is marked on the surface of the sphenoid bone facing the sphenoid sinus by a recess, the opticocarotid recess, which extends laterally from the superolateral part of the sphenoid sinus between the prominences in the sinus wall overlying the carotid sulcus and optic canal. This recess may extend deeply into the strut, so that the strut is partially or completely aerated by a lateral extension of the sphenoid sinus. The aeration may extend through the strut into the anterior clinoid process. Venous channels connecting the cavernous sinus with diploic veins of the orbital roof and anterior clinoid process may extend into or through the optic strut.

DURAL RELATIONSHIPS

The dural relationships of the anterior clinoid process are especially important in planning surgical approaches to the area (Figs. 9.1–9.5). The dura lining and extending medially from the upper surface of the anterior clinoid forms the lateral part of a dural ring, referred to as the upper or distal ring, which defines the upper margin of the carotid’s clinoid segment (22). The dura forming the lateral part of the upper ring extends forward and medially below the optic nerve to line the upper surface of the optic strut and form the anterior part of the upper ring. The dura lining the upper surface of the optic strut extends medially and posteriorly at the level of the upper part of the carotid sulcus to form the medial part of the upper ring. Further medially, the dura forming the upper ring blends into the diaphragma sellae. The dura, extending medially above the optic nerve from the clinoid process to line the anterior root of the lesser wing and attaching to the posterior edge of the planum sphenoidale, is located at the horizontal level of the upper surface of the clinoid. However, the dura that extends medially off the upper surface of the clinoid to line the upper surface of the optic strut and form the upper dural ring slopes downward as it proceeds medially, so that the medial part of the upper dural ring actually lies at the level of the lower rather than the upper surface of the anterior clinoid and optic canal.

Figure 9.5 (A–B). Triangles in the region of the cavernous sinus and middle fossa formed by the convergence and divergence of the cranial nerves. A–B, lateral aspect of brainstem and posterior fossa showing the brainstem origin of the cranial nerves, which form the margins of the cavernous sinus and middle fossa triangles. The tentorial edge was preserved in A and removed in B. There are four cavernous sinus triangles, four middle fossa triangles, and two paraclival triangles. The cavernous sinus triangles are the clinoidal, oculomotor, supratrochlear, and infratrochlear triangle. The clinoidal triangle, exposed by removing the anterior clinoid process, is situated in the interval between the optic and oculomotor nerves. The optic strut is in the anterior part, the clinoid segment is in the midportion and the thin roof of the cavernous sinus is in the posterior part of this triangle. The oculomotor triangle is the triangular patch of dura through which the oculomotor nerve enters the roof of the cavernous sinus. The posterior margin of this triangle is formed by the posterior petroclinoid dural fold, which extends from the petrous apex to the posterior clinoid process. The lateral margin is formed by the anterior petroclinoid dural fold, which extends from the petrous apex to the anterior clinoid process. The medial margin is formed by the intraclinoid dural fold, which extends from the anterior to the posterior clinoid. The supratrochlear triangle is situated between the lower surface of the oculomotor nerve and the upper surface of the trochlear nerve, and has a line joining the points of entrance of these nerves into the dura as its third margin. This triangle is very narrow. The infratrochlear triangle (Parkinson’s triangle) is located between the lower margin of the trochlear nerve and the upper margin of the ophthalmic nerve, and has a third margin formed by a line connecting the point of entry of the trochlear nerve into the dura to the site where the trigeminal nerve enters Meckel’s cave. The posterior bend of the carotid artery and the origin of the meningohypophyseal trunk are located in this triangle. The middle fossa triangles are the anteromedial, anterolateral, posterolateral, and the posteromedial triangles. The anteromedial triangle is situated between the lower margin of the ophthalmic and the upper margin of the maxillary nerves, and has a third edge formed by a line connecting the point where the ophthalmic nerve passes through the superior orbital fissure and the maxillary nerve passes through the foramen rotundum. Removing bone in the medial wall of this triangle will create an opening into the sphenoid sinus. The anterolateral triangle is located between the lower surface of the maxillary nerve, the upper surface of the mandibular nerve, and a line connecting the foramen ovale and rotundum. Opening the bone in the medial wall of this triangle exposes the sphenoid sinus. The posterolateral triangle (Glasscock’s triangle) is formed on the anterolateral side by the lateral surface of the mandibular nerve distal to the point at which the greater petrosal nerve crosses below the lateral surface of the trigeminal nerve, and on the posterolateral side is formed by the anterior margin of the greater petrosal nerve. This triangle encompasses the floor of the middle cranial fossa between these two structures. The middle meningeal artery passes through the foramen spinosum in this triangle. Opening the floor of the middle fossa in this triangle exposes the infratemporal fossa. The posteromedial triangle (Kawase’s triangle) is located between the greater petrosal nerve and the lateral edge of the trigeminal nerve behind the point where the greater petrosal nerve passes below the lateral edge of the trigeminal nerve, and a line along the connecting hiatus fallopii to the dural ostium of Meckel’s cave. The petrous carotid crosses the anterior margin of this triangle. The cochlea is located below the floor of the middle fossa in the lateral apex of the triangle. Drilling the bony floor of the triangle in the area behind the internal carotid artery and medial to the cochlea exposes the lateral edge of the clivus. The paraclival triangles are the inferomedial and inferolateral triangles. The inferolateral paraclival triangle is located on the posterior surface of the clivus and temporal bone. The medial margin is formed by a line connecting the dural entry sites of the trochlear and abducens nerves; the upper margin extends from the dural entrance of the trochlear nerve to the point at which the first petrosal vein lateral to Meckel’s cave joins the superior petrosal sinus (removed); and the lower margin is formed by a line connecting the point at which the abducens nerve enters the dura to the site at which the first petrosal vein, lateral to the trigeminal nerve, joins the superior petrosal sinus. The porus, through which the posterior trigeminal root enters Meckel’s cave, is situated in the center of the inferolateral paraclival triangle. The inferomedial paraclival triangle is formed above by a line extending from the posterior clinoid process to the dural entrance of the trochlear nerve, laterally by a line connecting the dural entrances of the trochlear and abducens nerves, and medially by a line extending from the dural entrance of the abducens nerve to the posterior clinoid process. The dura in this triangle forms the posterior wall of cavernous sinus. A., artery; Anterolat., anterolateral; Anteromed., anteromedial; Bas., basilar; Car., carotid; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Dors., dorsal; Fiss., fissure; Gr., greater; Inf., inferior; Infratroch., infratrochlear; Men., meningeal; Men. Hyp., meningohypophyseal; N., nerve; Oculom., oculomotor; Opticocar., opticocarotid; Orb., orbital; P.C.A., posterior cerebral artery; Pet., petrosal; Post., posterior; Posterolat., posterolateral; Posteromed., posteromedial; Prom., prominence; Pterygopal., pterygopalatine; Rec., recess; S.C.A., superior cerebellar artery; Seg., segment; Sup., superior; Supratroch., supratrochlear; Tent., tentorial; Tr., tract, trunk; Triang., triangle. (Images courtesy of AL Rhoton, Jr.)

Figure 9.5 (C–H). Triangles in the region of the cavernous sinus and middle fossa formed by the convergence and divergence of the cranial nerves. C, lateral view of the parasellar area and the oculomotor triangle. The temporal lobe has been elevated to expose the oculomotor and trochlear nerves as they enter the roof of the cavernous sinus. The oculomotor triangle is the triangular patch of dura through which the oculomotor nerve enters the roof of the cavernous sinus. The optic tract passes backward on the medial side of the uncus. D, enlarged view of the clinoidal, oculomotor, supratrochlear, and infratrochlear cavernous sinus triangles. The optic strut is exposed in the anterior part of the clinoidal triangle, the clinoid segment is exposed in the midportion and the roof of the cavernous sinus is exposed in the posterior part. The upper margin of the clinoid segment is surrounded by the upper dural ring, which is formed by the dura extending medially from the upper surface of the anterior clinoid. The lower margin of the clinoid segment is defined by the lower dural ring, which is formed by the dura extending medially from the lower margin of the anterior clinoid. The dura on the lower margin of the anterior clinoid, referred to as the carotidoculomotor membrane, separates the lower surface of the anterior clinoid from the upper surface of the oculomotor nerve and extends medially to form the lower dural ring. The posterior bend of the internal carotid artery and the origin of the meningohypophyseal trunk, which gives rise to the tentorial and dorsal meningeal arteries, are exposed in the infratrochlear triangle. The abducens nerve passes through Dorello’s canal and between the lateral surface of the intracavernous carotid and the medial side of the ophthalmic nerve. The inferolateral trunk arises from the horizontal segment of the intracavernous carotid and passes above the abducens nerve. E and F, side by side comparison of medial and lateral aspects of the cavernous sinus. E, lateral view of cavernous sinus. F, view, through the sphenoid sinus, of the medial side of the cavernous sinus. The optic nerve is exposed at the upper margin of the clinoidal triangle and above the optic strut. On the sphenoid sinus side of the specimen, the optic canal is seen above the opticocarotid recess, which leads into the optic strut. The clinoid segment rests against the posterior aspect of the optic strut in both views. In the lateral view, the superior orbital fissure through which the ophthalmic, trochlear, abducens nerves pass is seen below the optic strut. In the view through the sphenoid sinus, the medial edge of the superior orbital fissure produces a wide rounded prominence below the optic strut, and the maxillary nerve produces a prominence in the lower part of the sphenoid sinus just distal to the foramen rotundum. The lateral wing of the sphenoid sinus extends laterally under the maxillary nerve into the medial part of the floor of the middle fossa. Opening the middle fossa floor in the anteromedial and anterolateral triangles exposes the sphenoid sinus. G, posterior view of the inferolateral triangle. The medial edge of the inferolateral triangle extends between the dural entrances of the IVth and VIth nerves. The inferior limb extends from the VIth nerve to where the first vein lateral to Meckel’s cave joins the superior petrosal sinus and the superior limb extends from that vein to the dural entrance of the IVth nerve. The ostium of Meckel’s cave is located within the inferolateral triangle. H, posterior view of the inferomedial triangles. The medial limb of the inferomedial triangle extends from the posterior clinoid to the dural entrance to the abducens nerve. The lateral limb extends between the dural entrances of the IVth and VIth nerves and the superior limb extends from the IVth nerve to the posterior clinoid. On the right side, there is an abnormal projection of the posterior clinoid process, which extends below the oculomotor nerve toward the petrous apex. A., artery; Anterolat., anterolateral; Anteromed., anteromedial; Bas., basilar; Car., carotid; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Dors., dorsal; Fiss., fissure; Gr., greater; Inf., inferior; Infratroch., infratrochlear; Men., meningeal; Men. Hyp., meningohypophyseal; N., nerve; Oculom., oculomotor; Opticocar., opticocarotid; Orb., orbital; P.C.A., posterior cerebral artery; Pet., petrosal; Post., posterior; Posterolat., posterolateral; Posteromed., posteromedial; Prom., prominence; Pterygopal., pterygopalatine; Rec., recess; S.C.A., superior cerebellar artery; Seg., segment; Sup., superior; Supratroch., supratrochlear; Tent., tentorial; Tr., tract, trunk; Triang., triangle. (Images courtesy of AL Rhoton, Jr.)

The layer of dura that lines the lower margin of the anterior clinoid and extends medially to form the lower or proximal dural ring is called the carotidoculomotor membrane because it separates the lower margin of the clinoid from the oculomotor nerve and extends medially around the carotid artery (Figs. 9.5 and 9.6). This membrane extends medially and forward to line the lower surface of the optic strut and forms the anterior part of the lower ring. The dura lining the lower margin of the optic strut blends medially and backward into the dura lining the carotid sulcus, but does not form as distinct a lower ring on the medial side of the artery as it does along the anterior and lateral margins. The medial part of the lower ring is located at the level of a line extending from the lower margin of the optic strut to the floor of the sella (Fig. 9.6H). This line approximates the lower margin of the segment of the artery, which tightly hugs the carotid sulcus. Below the level of the sellar floor, the artery, not being enclosed in the bony ring formed by the clinoid and strut, becomes more widely separated from the carotid sulcus and its dural lining to admit the large venous channels forming the medial venous space of the cavernous sinus.

Figure 9.6 (A–F). Stepwise dissection of the carotid collar. A, lateral view of the paraclinoidal area. The anterior clinoid process projects backward from its base on the lesser wing. The carotid artery passes upward, and the optic nerve enters the optic canal on the medial side of the anterior clinoid. The origin of the ophthalmic artery bulges upward, below the optic nerve. The oculomotor nerve enters the roof of the cavernous sinus and passes along the lower margin of the anterior clinoid process. B, the anterior clinoid and the bone forming the roof and lateral wall of the optic canal have been removed. The clinoid segment of the internal carotid artery is located in the medial wall of the space created by removal of the anterior clinoid. The dura that covers the medial and lower surfaces of the anterior clinoid has been preserved. The layer of dura that covers the medial side of the anterior clinoid forms the lateral part of the carotid collar. The upper surface of the optic strut forms the floor of the optic canal. The anterior wall of the clinoid segment rests against the posterior surface of the optic strut. The dura lining the upper and lower surfaces of the optic strut extends backward to form the anterior part of the upper and lower dural rings and the dura lining the posterior surface of the strut forms the anterior part of the carotid collar. The falciform ligament is a dural fold, which extends above the optic nerve at the posterior edge of the optic canal. C, enlarged view of the lateral aspect of the carotid collar and optic strut. The anterior surface of the clinoid segment rests against the posterior surface of the optic strut. This strut is triangular in cross section. The upper surface of the optic strut, which forms of the lower margin of the optic canal, slopes forward and downward from its intracranial edge. The lower surface of the optic strut forms part of the upper margin of the superior orbital fissure. The posterior surface of the optic strut is contoured to accommodate the anterior wall of the clinoid segment. The dura that lines the lower margin of the anterior clinoid process and extends medially above the oculomotor nerve to surround the internal carotid artery and form the lower dural ring is referred to as the carotidoculomotor membrane. The lower ring does not tightly adhere to the surface of the artery, as does the upper ring. The lower ring admits venous tributaries of the cavernous sinus, referred to as the clinoid venous plexus, which course between the clinoid segment and the carotid collar and can be seen through the collar. Thus, the clinoid segment of the internal carotid artery, which was once considered to lie outside the cavernous sinus, is partially surrounded by the venous tributaries of the cavernous sinus. D, the optic strut has been removed. The layer of dura that lines the posterior surface of the optic strut forms the anterior part of the carotid collar. The medial end of the strut attaches to the body of the sphenoid bone immediately in front of the carotid sulcus. In some cases, the interior of the optic strut is aerated by a lateral extension of the sphenoid sinus, which may also extend through the strut into the anterior clinoid. The dura lining the lower margin of the anterior clinoid, which forms the carotidoculomotor membrane, also forms the most anterior portion of the roof of the cavernous sinus. The venous channels of the cavernous sinus extend upward between the carotid collar and the clinoid segment. The layers of dura lining the upper and lower surfaces of the clinoid process come together at the posterior tip of the anterior clinoid. E, the outer layer of dura in the lateral wall of the cavernous sinus has been removed to expose the trigeminal, oculomotor, and trochlear nerves, which are enmeshed in the inner layer of dura. The maxillary strut, the bridge of bone located between the superior orbital fissure and the foramen rotundum, has been removed. The oculomotor and trochlear nerves course on the lower surface of the carotid-oculomotor membrane. A tentorial artery passes above the ophthalmic nerve. The openings created by removal of the optic and maxillary struts and the bone below the maxillary nerve open into the sphenoid sinus. F, the segment of the optic nerve that courses above the optic strut has been removed. The carotidoculomotor membrane has been separated from the oculomotor nerve and folded upward to expose the tributaries of the cavernous sinus, which course between the carotid collar and the clinoid segment, and extend upward to near the upper ring. A., artery; Ant., anterior; Anterolat., anterolateral; Anteromed., anteromedial; Bas., basilar; Car., carotid; Car. Oculo., carotidoculomotor; Cav., cavernous; Chor., choroid; Clin., clinoid; CN, cranial nerve; Comm., communicating; Dors., dorsal; Falc., falciform; Fiss., fissure; Front., frontal; Hyp., hypophyseal; Inf., inferior; Inf. Lat., inferolateral; Intercav., intercavernous; Lat., lateral; Lig., ligament; M., muscle; Max., maxillary; Med., medial; Memb., membrane; Men., meningeal; Ophth., ophthalmic; Orb., orbital; Pet., petrous; Pit., pituitary; Plex., plexus; Post., posterior; Rec., rectus; Seg., segment; Sphen., sphenoid; Sup., superior; Temp., temporal; Tent., tentorial; Tr., trunk; Ven., venous. (Images courtesy of AL Rhoton, Jr.)

Figure 9.6 (G–L). Stepwise dissection of the carotid collar. G, the trigeminal, oculomotor, and trochlear nerves have been removed and the abducens nerve preserved. The dura forming the carotid collar has been folded upward after dividing the collar beginning below at the level of the lower dural ring and extending upward to just below the upper ring. The inferolateral trunk, a branch of the intracavernous carotid, passes above the abducens nerve. The tentorial artery arises from the inferolateral trunk, and the inferior hypophyseal artery arises directly from the internal carotid artery rather than from the meningohypophyseal artery, which is absent in this case. H, the bone and mucosa in the lateral wall of the sphenoid sinus have been removed to expose the dura that forms the medial wall of the cavernous sinus. The optic canal and optic sheath are located in the superolateral margin of the sphenoid sinus. The optic strut forms the floor of the optic canal and separates the lower margin of the optic canal from the upper margin of the superior orbital fissure. The maxillary nerve crosses along the midportion of the lateral wall of the sphenoid sinus and is separated from the superior orbital fissure by the maxillary strut, a narrow bridge of bone between the fissure and the foramen rotundum. The medial aspect of the superior orbital fissure produces a bulge in the lateral wall of the sphenoid sinus between the optic and maxillary struts. The intracavernous carotid can be seen through the dura in the area behind the optic strut. The medial side of the carotid collar, which extends around the medial side of the artery in the area behind the optic strut, is not as well defined and is wider than on the lateral side of the artery. The lower margin of the collar on the medial side of the clinoid segment is located at approximately the level of the broken line extending from the lower margin of the optic strut to the floor of the sella. The small venous channels within the collar can be seen through the dura. The anterior intracavernous sinus courses at the posterior margin of the upper dural ring. The lateral wing of the sphenoid  sinus extends below the maxillary nerve and below the middle fossa floor. I, the optic strut has been removed. A small opening in the dura admits a probe that has been passed forward between the dura and the internal carotid artery along the lower margin of the carotid collar. The dura that lines the upper surface of the optic strut and the lower part of the optic sheath is continuous posteriorly with the upper ring. The lower margin of the collar is located at approximately the level of the broken line extending from the lower margin of the optic strut to the floor of the sella. J, the segment of the optic nerve coursing above the optic strut has been removed, but the segment passing through the annular tendon, from which the rectus muscles arise, has been preserved. The dura has been opened along the margin of the lower ring to expose the venous spaces within the cavernous sinus which extend upward inside the carotid collar. The venous spaces inside the envelope of the cavernous sinus increase in size in the area below the collar. K and L, medial aspect of another cavernous sinus on the right side. K, the bone in the lateral wall of the sphenoid sinus has been removed to expose the medial aspect of the optic canal, superior orbital fissure, the prominence over the maxillary nerve, and the middle fossa dura. The dura in the medial wall of the cavernous sinus has been removed to expose the intercavernous carotid and the abducens nerve. The lower margin of the cavernous sinus is marked with yellow arrows and is located below the intracavernous segment of the carotid and the abducens nerve. The optic canal and opticocarotid recess are located above the superior orbital fissure and anterior bend of the cavernous carotid. The prominence of the superior orbital fissure is located below the opticocarotid recess. Another recess extends laterally into the maxillary strut, the bridge of bone that separates the superior orbital fissure and the foramen rotundum. The maxillary nerve produces a prominence in the lateral wall of the sphenoid sinus below which is located the lateral wing of the sphenoid, which extends below the floor of the middle fossa in the region of the anterolateral triangle. The basilar sinus is the largest connection across the midline between the cavernous sinuses. L, the middle fossa dura, lateral to the sphenoid sinus and between the superior orbital fissure and maxillary nerve, has been opened to expose the medial aspect of the temporal lobe. Bone has been removed below the maxillary nerve to expose the floor of the middle fossa below the temporal lobe. The optic, oculomotor, and trochlear nerves and the internal carotid artery are exposed above the sella. A., artery; Ant., anterior; Anterolat., anterolateral; Anteromed., anteromedial; Bas., basilar; Car., carotid; Car. Oculo., carotidoculomotor; Cav., cavernous; Chor., choroid; Clin., clinoid; CN, cranial nerve; Comm., communicating; Dors., dorsal; Falc., falciform; Fiss., fissure; Front., frontal; Hyp., hypophyseal; Inf., inferior; Inf. Lat., inferolateral; Intercav., intercavernous; Lat., lateral; Lig., ligament; M., muscle; Max., maxillary; Med., medial; Memb., membrane; Men., meningeal; Ophth., ophthalmic; Orb., orbital; Pet., petrous; Pit., pituitary; Plex., plexus; Post., posterior; Rec., rectus; Seg., segment; Sphen., sphenoid; Sup., superior; Temp., temporal; Tent., tentorial; Tr., trunk; Ven., venous. (Images courtesy of AL Rhoton, Jr.)

The segment of the internal carotid artery located between the upper and lower dural rings, which is exposed by removing the anterior clinoid process, is referred to as the clinoid segment. It may be necessary to divide the dural rings to mobilize the carotid artery for dealing with aneurysms arising at the level of the roof of the cavernous sinus from the internal carotid artery at the origin of the ophthalmic artery.

Carotid Collar

The dura forming the lateral and anterior edges of the lower ring, as it approaches the clinoid segment, turns upward inside the bony ring to form a collar (carotid collar) around the artery, which does not adhere or fuse to the wall of the artery until it reaches the level of the upper ring (Figs. 9.4 and 9.6) (22). The site at which the dura forming the lower ring turns upward to form the collar around the clinoid segment is not attached to the arterial wall, but is separated from the artery by a narrow space through which course venous channels communicating with the anterior part of the cavernous sinus. These venous channels extend to just below the level of the upper dural ring. The dura forming the collar is so thin that the artery and the thin venous channels, referred to as the clinoid venous plexus, can be seen through the collar (Fig. 9.6). The transition between the medial side of the carotid collar and the lower ring, which is located at the level of the floor of the pituitary fossa, is not as sharply defined as it is on the anterior and lateral sides, where the dura forming the lower ring turns sharply upward around the artery at the edge of the optic strut and anterior clinoid.

The dural collar and the upper and lower rings slope downward as they extend medially from the clinoid (Fig. 9.6). The upper and lower rings also diverge as they slope medially, making the collar wider in the area facing the carotid sulcus and the medial end of the optic strut than in the areas facing the anterior clinoid. The carotid collar disappears posterior to the tip of the anterior clinoid process, where the dura lining the upper and lower surfaces of the clinoid process fuse into a single dural layer that forms the oculomotor triangle and posterior part of the roof of the cavernous sinus. The anterior part of the roof is formed by the dura lining the lower margin of the anterior clinoid.

The upper dural ring, at its junction with the collar, is adherent to the surface of the artery and serves as a barrier between the intra- and extradural spaces. In contrast, the lower dural ring and lower part of the collar are separated from the wall of the artery, creating a narrow space in which courses a thin layer of venous channels that are continuous through the lower part of the collar and lower dural ring with the venous channels within the cavernous sinus.

NEURAL RELATIONSHIPS

The nerves in the sinus wall or sinus are, from superior to inferior, the IIIrd cranial nerve followed by the trochlear, ophthalmic, and abducens nerves (Figs. 9.1, 9.5, and 9.7–9.9) (6, 7, 17). The oculomotor, trochlear, and ophthalmic nerves course in the inner part of the lateral sinus wall. The abducens courses medial to the ophthalmic nerve and is adherent to the lateral surface of the intracavernous carotid medially, but it also is adherent laterally to the medial surface of the ophthalmic nerve and the inner part of the lateral sinus wall.

Figure 9.7 (A–F). Intradural approach to the cavernous sinus. A, this is the dissection that the participants complete in our microsurgery courses to demonstrate the intradural approach to the cavernous sinus. At this stage of the course, the cerebral hemisphere has been removed and the suprasellar area and the lateral wall of the cavernous sinus have been exposed. The optic nerve has been elevated to expose the ophthalmic artery entering the optic canal. B, the dura over the upper surface of the anterior clinoid, optic canal, and planum has been removed in preparation for anterior clinoidectomy and removal of the roof of the optic canal. The falciform ligament extends across the optic nerve just proximal to the nerve’s entrance to the optic canal. C, the anterior clinoid and optic strut have been removed to expose the clinoid segment of the carotid artery enclosed in the dural carotid collar. The carotid artery, within this cavernous sinus, is quite tortuous and bulges upward medial to the oculomotor nerve to distort the roof of the sinus. Removal of the optic strut exposed the sphenoid sinus mucosa extending into the base of the strut. The carotid collar is the cuff of dura that encloses the clinoid segment between the upper and lower dural rings. The lower dural ring is loosely adherent to the artery, but the upper dural ring adheres tightly to the artery. D, a dural incision extending around the margin of the lateral wall of the cavernous sinus and the trigeminal nerve has been completed, and the outer layer of dura in the lateral sinus wall has been removed to expose the thin inner layer in which the nerves course. The pericavernous venous plexus extends around all three trigeminal divisions. The greater petrosal nerve is exposed lateral to the trigeminal ganglion. E, the outer layer of the dural roof of sinus has been removed, while the thin layer investing the nerves has been preserved. The oculomotor nerve enters the dura through the oculomotor triangle located between the anterior and posterior clinoid processes and the petrous apex, and sits in a narrow cistern in the sinus roof that extends a variable length along the course of the nerve. The dura, which lines the lower surface of the anterior clinoid and separates the clinoid and the oculomotor nerve, referred to as the carotidoculomotor membrane, extends medially around the lower edge of the clinoid segment to form the lower dural ring. F, the thin inner layer of dura remaining over the lateral wall of the oculomotor triangle has been removed. The dura covering the lower margin of the anterior clinoid process and in the oculomotor triangle forms the roof of the cavernous sinus. The level at which the oculomotor nerve enters the oculomotor cistern is marked with a green arrow and the level at which the cistern ends and the nerve become tightly invested by dura is shown with a yellow arrow. A., artery; Ant., anterior; Car., carotid; Cav., cavernous; Cist., cistern; Clin., clinoid; CN, cranial nerve; Comm., communicating; Dors., dorsal; Falc., falciform; Gr., greater; Inf. Lat., inferolateral; Lig., ligament; Men., meningeal; Men. Hyp., meningohypophyseal; Mid., middle; N., nerve; Oculom., oculomotor; Olf., olfactory; Ophth., ophthalmic; P.C.A., posterior cerebral artery; Pericav., pericavernous; Pet., petrosal, petrous; Petrosphen., petrosphenoid; Pit., pituitary; Plex., plexus; Post., posterior; S.C.A., superior cerebellar artery; Seg., segment; Sphen., sphenoid; Sup., superior; Tent., tentorial, tentorium; Tr., tract, trunk; V., vein; Ven., venous. (Images courtesy of AL Rhoton, Jr.)

Figure 9.7 (G–L). Intradural approach to the cavernous sinus. G, the dura between the upper and lower dural ring that forms the carotid collar has been separated from the internal carotid artery and folded upward to expose the thin venous plexus extending inside the carotid collar. H, the venous space surrounding the carotid artery and the trigeminal nerve has been cleared. This exposes the full course of the intracavernous carotid, which is quite tortuous and bulges upward to elevate the roof of the cavernous sinus. The upper dural ring, which is tightly adherent to the artery, extends below the optic nerve and across the upper surface of the optic strut. I, enlarged view of the intravenous carotid artery. The ophthalmic nerve has been depressed to expose the abducens nerve as it passes around the lateral margin of the carotid artery. The inferolateral trunk arises from the intracavernous carotid, passes above the VIth nerve, and gives rise to a branch that passes forward toward the foramen rotundum. The meningohypophyseal artery has been distorted by the tortuous course of the internal carotid artery. The dorsal meningeal branch of the meningohypophyseal trunk passes backward toward Dorello’s canal and the clivus. The petrosphenoid ligament (Gruber’s ligament) roofs Dorello’s canal through which the abducens nerve enters the lower margin of the sinus. J, the lower margin of the cavernous sinus is marked with a yellow line, and Meckel’s cave, the arachnoid cistern around the trigeminal nerve, is outlined with a green line. The lower edge of the cavernous sinus is located medial to the junction of the upper and middle third of Meckel’s cave. The bone in the anteromedial and anterolateral triangles has been opened to expose the lateral wing of the sphenoid sinus extending laterally below the trigeminal nerve. K, the trigeminal ganglion and the adjacent part of the posterior root and division have been removed to expose the petrous segment of the internal carotid artery and the lateral wing of the sphenoid sinus. L, bone has been removed in the floor of the sphenoid sinus to “unroof” the vidian canal in which the vidian nerve courses. The vidian nerve is formed by the union of the greater petrosal nerve and the deep petrosal nerve, the latter arising from the periarterial carotid plexus. The two roots join in the region of the foramen lacerum and enter the vidian canal to reach the pterygopalatine ganglion in the pterygopalatine fossa. A., artery; Ant., anterior; Car., carotid; Cav., cavernous; Cist., cistern; Clin., clinoid; CN, cranial nerve; Comm., communicating; Dors., dorsal; Falc., falciform; Gr., greater; Inf. Lat., inferolateral; Lig., ligament; Men., meningeal; Men. Hyp., meningohypophyseal; Mid., middle; N., nerve; Oculom., oculomotor; Olf., olfactory; Ophth., ophthalmic; P.C.A., posterior cerebral artery; Pericav., pericavernous; Pet., petrosal, petrous; Petrosphen., petrosphenoid; Pit., pituitary; Plex., plexus; Post., posterior; S.C.A., superior cerebellar artery; Seg., segment; Sphen., sphenoid; Sup., superior; Tent., tentorial, tentorium; Tr., tract, trunk; V., vein; Ven., venous. (Images courtesy of AL Rhoton, Jr.)

Figure 9.8 (A–F). Extradural approach to the cavernous sinus. This is the dissection participants complete in our microsurgery courses to demonstrate the intradural approach to the cavernous sinus. A, at this stage of the course, part of the frontal and temporal lobes has been removed and the sylvian fissure has been exposed. B, the sphenoid ridge has been removed and dura has been elevated from the superior and lateral wall of the orbit to expose the superior orbital fissure. At the lateral margin of the superior orbital fissure, the dura becomes thick and tough at the point it blends into the periorbita, making it necessary to make a sharp shallow cut in this dura at the lateral margin of the fissure to continue peeling the outer layer of dura away from the wall of the cavernous sinus. C, the outer layer of dura has been pealed back to expose the anterior clinoid process and the oculomotor and trochlear nerves and trigeminal divisions. The nerves course in the thin inner layer of the lateral wall of the cavernous sinus. D, the anterior clinoid has been removed to expose the clinoid segment of the internal carotid artery. The thin inner layer of dura within the wall of the sinus has been removed and the ophthalmic nerve has been depressed to expose the abducens nerve coming through Dorello’s canal, located below the petrosphenoid (Gruber’s) ligament. The lower margin of the posterior wall of the sinus is located at the petrous apex just below where the abducens nerve enters Dorello’s canal (red arrow). The anterior edge of the lower margin is located at the lower margin of the superior orbital fissure (yellow arrow). E, the bone in the anteromedial and anterolateral middle fossa triangles has been drilled to open into the lateral wing of the sphenoid sinus. The motor root of the trigeminal nerve courses along the medial side of the trigeminal ganglion and enters the medial part of the foramen ovale with the mandibular nerve. F, the bone behind the greater petrosal nerve and lateral to the trigeminal nerve in Kawase’s triangle has been drilled to expose the nerves and anteroinferior cerebellar artery in the internal acoustic meatus. A.I.C.A., anteroinferior cerebellar artery; Ant., anterior; Caud., caudal; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Coch., cochlear; Fiss., fissure; Front., frontal; Gang., ganglion; Genic., geniculate; Gr., greater; Inf., inferior; Intermed., intermedius; Lac., lacrimal; Lat., lateral; Less., lesser; Lev., levator; Lig., ligament; M., muscle; M.C.A., middle cerebral artery; N., nerve; Nasocil., nasociliary; Nerv., nervus; Ophth., ophthalmic; Orb., orbital; Pet., petrosal, petrous; Petrosphen., petrosphenoid; Post., posterior; Rec., rectus; Rost., rostral; Seg., segment; Sphen., sphenoid; Sup., superior; Temp., temporal, temporalis; Tent., tentorial; Tr., trunk; Trig., trigeminal; V., vein; Vert., vertebral; Vest., vestibular. (Images courtesy of AL Rhoton, Jr.)

Figure 9.8 (G–L). Extradural approach to the cavernous sinus. G, the superior vestibular and facial nerves have been retracted to expose the superior and inferior vestibular nerves posteriorly and the facial and cochlear nerves anteriorly within the meatus. The anteroinferior cerebellar artery loops into the meatus. The nervus intermedius courses with the VIIIth nerve near the brainstem and jumps to the facial nerve, either in the cerebellopontine angle or internal acoustic meatus. H, the petrous apex has been drilled downward to the level of the inferior petrosal sinus and medially to the lateral edge of Dorello’s canal. This exposure includes the anteroinferior cerebellar artery bifurcation and extends down to the level of the vertebral artery. I, the facial nerve has been retracted backward to expose the nervous intermedius and the superior and inferior vestibular and cochlear nerves. The rostral trunk of the anteroinferior cerebellar artery loops into the internal acoustic meatus, and the caudal trunk passes downward to the inferolateral part of the cerebellum. J, the roof of the cavernous sinus is located medial to the tentorial edge and includes the oculomotor triangle, through which the oculomotor nerve passes, and the dura lining the lower margin of the anterior clinoid. The optic nerve has been elevated to expose the origin of the ophthalmic artery, which enters the optic canal and penetrates the optic sheath to enter the orbit on the lateral side of the optic nerve. K, the roof and lateral wall of the orbit have been removed to expose the orbital contents. L, enlarged view of the structures in the upper part of the orbit. The levator and superior rectus muscles have been elevated and the lateral rectus muscle has been depressed. The superior ophthalmic vein passes backward and empties into the cavernous sinus after passing downward along the lower margin of the ophthalmic nerve. The nasociliary nerve, ophthalmic artery, and superior ophthalmic vein course above the optic nerve and are located on the lateral side of the optic nerve at the orbital apex. A.I.C.A., anteroinferior cerebellar artery; Ant., anterior; Caud., caudal; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Coch., cochlear; Fiss., fissure; Front., frontal; Gang., ganglion; Genic., geniculate; Gr., greater; Inf., inferior; Intermed., intermedius; Lac., lacrimal; Lat., lateral; Less., lesser; Lev., levator; Lig., ligament; M., muscle; M.C.A., middle cerebral artery; N., nerve; Nasocil., nasociliary; Nerv., nervus; Ophth., ophthalmic; Orb., orbital; Pet., petrosal, petrous; Petrosphen., petrosphenoid; Post., posterior; Rec., rectus; Rost., rostral; Seg., segment; Sphen., sphenoid; Sup., superior; Temp., temporal, temporalis; Tent., tentorial; Tr., trunk; Trig., trigeminal; V., vein; Vert., vertebral; Vest., vestibular. (Images courtesy of AL Rhoton, Jr.)

Figure 9.9. Lateral aspect of the left cavernous sinus, superior orbital fissure, and orbit. A, the lateral wall and roof of the right orbit and the intraorbital fat have been removed. The anterior clinoid process and dura in the lateral wall of the cavernous sinus have been preserved. The oculomotor, trochlear, and ophthalmic nerves have been exposed by removing the dura in the lateral wall of the cavernous sinus. The superior and inferior ophthalmic veins arise inside the muscle cone, but exit the intraconal area as they converge on the orbital apex. The superior ophthalmic vein passes downward along the origin of the lateral rectus muscle from the annular tendon, where it is joined by the inferior ophthalmic vein to form a common trunk that passes backward below and medial to the ophthalmic nerve to enter the cavernous sinus. B, another specimen with the anterior clinoid, lateral wall of the orbit, and cavernous sinus removed. The superior ophthalmic vein exits the muscle cone to pass along the lateral margin of the superior orbital fissure and below the ophthalmic nerve (blue arrow) to enter the anteroinferior part of the cavernous sinus on the medial side of the ophthalmic nerve. C, the superior ophthalmic vein has been removed. The trochlear nerve passes medially above the oculomotor and ophthalmic nerves to reach the superior oblique muscles. The frontal, lacrimal, and trochlear nerves pass outside the annular tendon, and the nasociliary, oculomotor, and abducens nerves pass through the tendon. D, the frontal and lacrimal nerves have been depressed to show the nasociliary nerve arising from the medial side of the ophthalmic nerve. The oculomotor foramen is the portion of the opening in the annular tendon lateral to the optic foramen through which the superior and inferior divisions of the oculomotor nerve and the nasociliary and abducens nerves pass. The oculomotor nerve divides into superior and inferior divisions just behind the superior orbital fissure and annular tendon. The abducens nerve courses on the medial side of the ophthalmic nerve in the cavernous sinus, but in the fissure, it turns laterally below the nerve to enter the medial side of the lateral rectus muscle. E, the annular tendon has been divided between the origin of the superior and lateral rectus muscles. The superior division of the oculomotor nerve passes upward to innervate the levator and superior rectus muscles. The inferior division innervates the inferior oblique, inferior rectus, and medial rectus muscles and gives rise to the parasympathetic pupilloconstrictor fibers to the ciliary ganglion. F, another specimen to show the oculomotor nerve splitting into the superior and inferior divisions at the anterior edge of the cavernous sinus just behind the superior orbital fissure. A., artery; Clin., clinoid; CN, cranial nerve; Div., division; For., foramen; Front., frontal; Inf., inferior; Lac., lacrimal; Lat., lateral; M., muscle; N., nerve; Nasocil., nasociliary; Oculom., oculomotor; Ophth., ophthalmic; Rec., rectus; Seg., segment; Sup., superior; V., vein. (Images courtesy of AL Rhoton, Jr.)

The oculomotor nerve pierces the roof of the cavernous sinus near the center of the oculomotor triangle, and the IVth nerve enters the dura at the posterolateral edge of the triangle. A short length of both trochlear and oculomotor nerves are surrounded by a dural and arachnoid cuff to create the oculomotor and trochlear cisterns as they pass through the roof of the cavernous sinus and below the anterior clinoid process. Both nerves are situated medial to and slightly beneath the level of the free edge of the tentorium at their point of entry.

The oculomotor nerve enters the cavernous sinus slightly lateral and anterior to the dorsum sellae, almost directly above the origin of the meningohypophyseal trunk from the intracavernous carotid, and courses along the lower margin of the anterior clinoid and the carotidoculomotor membrane. The oculomotor nerve pierces the sinus roof between 2 and 7 mm posterior to the initial supraclinoid segment of the carotid artery; the average separation is 5 mm (6).

The trochlear nerve enters the roof of the sinus posterolateral to the IIIrd nerve and courses below the oculomotor nerve in the posterior part of the lateral wall. Anteriorly, below the base of the anterior clinoid process, it passes upward along the lateral surface of the oculomotor nerve. From there, the trochlear nerve passes medially between the oculomotor nerve and dura lining the lower margin of the anterior clinoid and optic strut to reach the medial part of the orbit and the superior oblique muscle.

The ophthalmic nerve is the smallest of the three trigeminal divisions. It is inclined upward as it passes forward near the medial surface of the dura, forming the lower part of the lateral wall of the cavernous sinus, to reach the superior orbital fissure. It is flattened in the wall of the cavernous sinus, but at the superior orbital fissure, it takes on an oval configuration. The ophthalmic nerve splits into the lacrimal, frontal, and nasociliary nerves as it approaches the superior orbital fissure.

The superior petrosal sinus passes above the posterior root of the trigeminal root to form the upper margin of the ostium of Meckel’s cave, the dural and subarachnoid cavern, which communicates with the subarachnoid space in the posterior fossa (Figs. 9.1 and 9.8). The cave extends forward around the posterior trigeminal root to the midportion of the ganglion. The motor root of the trigeminal nerve courses on the medial side of the sensory fibers at the level of Meckel’s cave (Fig. 9.7).

The abducens nerve pierces the dura forming the lower part of the posterior wall of the sinus at the upper border of the petrous apex and enters a dural cave, referred to as Dorello’s canal, where it passes below the petrosphenoid ligament (Gruber’s ligament) that extends from the lower part of the lateral edge of the dorsum sellae to the petrous apex, to enter the cavernous sinus (Figs. 9.1, 9.7, and 9.8). The nerve bends laterally around the proximal portion of the intercavernous carotid and gently ascends as it passes forward inside the cavernous sinus medial to the ophthalmic nerve, on the lateral side of the internal carotid artery, and below and medial to the nasociliary nerve. It has the most medial site of entry of the nerves coursing in the sinus wall and maintains that position in its course through the sinus. The nerve usually enters the sinus as a single bundle, but may persist as two bundles in the subarachnoid space. After entering the sinus, it may split into as many as five rootlets as it courses between the internal carotid artery and ophthalmic nerve. In a study of 50 sinuses, the intracavernous segment of the nerve consisted of a single rootlet in 34 specimens, two rootlets in 13, three rootlets in 2, and five rootlets in 1 specimen (6).

Sympathetic fiber bundles large enough to be recognized without a surgical microscope travel on the surface of the carotid as it emerges from the foramen lacerum. Some of the bundles join the VIth nerve within the sinus before ultimately being distributed to the first trigeminal division, which sends sympathetic fibers that reach the pupillodilator through the long ciliary nerves and by passing through the ciliary ganglion (8, 12). Some sympathetic fibers pass directly from the carotid plexus to the ciliary ganglion and others may travel along the ophthalmic artery to the globe (23).

CAVERNOUS SINUS AND MIDDLE FOSSA TRIANGLES

Parkinson (13) described a triangle within the lateral wall of the cavernous sinus through which the intracavernous portion of the carotid artery and its branches might be exposed for the surgical treatment of carotid-cavernous fistulae (Fig. 9.5). Since his pioneering work, a number of significant triangular relationships formed by the convergence and divergence of the cranial nerves in the region of the cavernous sinus and middle fossa have been defined. There are four triangles in the cavernous sinus, four middle fossae lateral to the cavernous sinus, and two fossae in the paraclival area that are helpful in understanding and planning approaches to the cavernous sinus. The cavernous sinus triangles are formed by the optic, oculomotor, trochlear, and ophthalmic nerves converging on the optic canal and superior orbital fissure. The middle fossa triangles are formed by the trigeminal divisions diverging as they pass from the gasserian ganglion to reach their foramina (17).

Cavernous Sinus Triangles

Clinoidal Triangle

This triangle is situated in the interval between the optic and oculomotor nerves. This triangle is exposed by removing the anterior clinoid process. The optic strut is in the anterior part, the clinoid segment of the internal carotid artery is in the midportion, and the thin roof of the cavernous sinus is in the posterior part of this triangle.

Oculomotor Triangle

This triangle is formed by the triangular patch of dura through which the oculomotor nerve enters the roof of the cavernous sinus. Two margins of this triangle are formed by the anterior and posterior petroclinoidal dural folds that extend, respectively, from the anterior and posterior clinoid processes to the petrous apex. The third side is formed by the interclinoidal dural fold that extends from the anterior to the posterior clinoid process.

Supratrochlear Triangle

This triangle is situated between the lower surface of the oculomotor nerve and the upper surface of the trochlear nerve. A line joining the points of entrance of these nerves into the dura forms the third margin. This triangle is very narrow.

Infratrochlear Triangle (Parkinson’s Triangle)

This triangle is located between the lower margin of the trochlear nerve and the upper margin of the ophthalmic nerve. The third margin is formed by a line connecting the point of entry of the trochlear nerve into the dura to the site where the trigeminal nerve enters Meckel’s cave. The posterior bend of the internal carotid artery and the origin of the meningohypophyseal  trunk from the posterior bend are located in this triangle, except when the carotid artery is elongated and tortuous. In that case, the origin may be pushed upward into the oculomotor triangle. Parkinson (12–14) first described the surgical exposure of the intercavernous portion of the carotid artery through this triangle for the treatment of carotid-cavernous fistulas. In an earlier study, we found that the superior margin of the triangle formed by the lower margin of the IVth nerve averaged 13 mm (range, 8–20 mm); the inferior margin formed by the upper margin of the Vth cranial nerve averaged 14 mm (range, 5–24 mm); and the posterior margin represented by the slope of the dorsum and clivus averaged 6 mm (range, 3–14 mm). The average triangle measured 13 x 14 x 6 mm; however, it could be very small, measuring only 8 x 5 x 3 mm, and it may not be large enough to provide a good surgical exposure of all of the arterial branches within the sinus (6). Parkinson (12), through an incision starting 4 mm beneath the dural entrance of the IIIrd nerve and extending anteriorly approximately 2 cm parallel to the slope of the IIIrd and IVth nerves, exposed the meningohypophyseal trunk and the artery of the inferior cavernous sinus. The VIth nerve at the bottom edge of the exposure was seen on retracting the superior aspect of the trigeminal nerve. The most anterior aspect of the cavernous carotid was not seen through this exposure unless the carotid artery was grasped and pulled backward. Parkinson thought that the triangle would provide access to most spontaneous fistulas, assuming that they are due to ruptured aneurysms developing at the point of departure of the meningohypophyseal trunk or artery of the inferior cavernous sinus.

Middle Fossa Triangles

Anteromedial Middle Fossa Triangle

This triangle is situated between the lower margin of the ophthalmic and the upper margin of the maxillary nerves. The third edge is formed by a line connecting the point where the ophthalmic nerve passes through the superior orbital fissure and the maxillary nerve passes through the foramen rotundum (Fig. 9.5). Removing bone in the triangular space between the ophthalmic or maxillary nerve opens into the sphenoid sinus.

Anterolateral Middle Fossa Triangle

This triangle is located between the lower surface of the maxillary nerve, the upper surface of the mandibular nerve, and a line connecting the foramen ovale and rotundum. Opening the bone in the medial wall of this triangle exposes the lateral wing of the sphenoid sinus.

Posterolateral Middle Fossa Triangle (Glasscock’s Triangle)

This triangle is formed on the anteromedial side by the lateral surface of the mandibular nerve distal to the point at which the greater petrosal nerve crosses below the lateral surface of the trigeminal nerve. On the posterolateral side, it is formed by the anterior margin of the greater petrosal nerve. This triangle opens laterally to encompasses the floor of the middle cranial fossa between these two structures. The middle meningeal artery passes through the foramen spinosum in this triangle. Opening the floor of the middle fossa in this triangle exposes the infratemporal fossa.

Posteromedial Middle Fossa Triangle (Kawase’s Triangle)

            This triangle is located between the greater petrosal nerve, and the lateral edge of the trigeminal nerve behind the point where the greater petrosal nerve passes below its lateral surface, and a line along the connecting hiatus fallopii to the dural ostium of Meckel’s cave. The petrous segment of the internal carotid artery crosses the anterior margin of this triangle. The cochlea is located below the floor of the middle fossa in the lateral apex of the triangle. Removing the bone in the lateral part of the posteromedial triangle exposes the cochlea and the anterior wall of the internal auditory canal, and removing the bone in the medial part of the posteromedial triangle exposes the side of the clivus and the inferior petrosal sinus. The approach directed through the temporal bone in this triangle is referred to as an anterior petrosectomy.

Paraclinoid Triangles

Inferolateral Paraclival Triangle

This triangle is located on the posterior surface of the clivus and temporal bone (Fig. 9.5). The medial margin is formed by a line connecting the dural entry sites of the trochlear and abducens nerves; the upper margin extends from the dural entrance of the trochlear nerve to the point at which the first petrosal vein lateral to Meckel’s cave joins the superior petrosal sinus; and the lower margin is formed by a line connecting the point at which the abducens nerve enters the dura to the site at which the first petrosal vein, lateral to the trigeminal nerve, joins the superior petrosal sinus. The porus through which the posterior trigeminal root enters Meckel’s cave is situated in the center of the inferolateral paraclival triangle.

Inferomedial Paraclival Triangle

This triangle is formed above by a line extending from the posterior clinoid process to the dural entrance of the trochlear nerve; laterally by a line connecting the dural entrances of the trochlear and abducens nerves; and medially by a line extending from the dural entrance of the abducens nerve to the posterior clinoid process. The abducens nerve enters the cavernous sinus at the lower edge of this triangle. This triangle extends along the posterior sinus wall. Removing the medial part of the inferomedial triangle behind the internal carotid artery exposes the lateral edge of the dorsum sellae, the upper end of the petroclival suture, and the VIth nerve passing below Gruber’s ligament.

ARTERIAL RELATIONSHIPS

The internal carotid artery exits the foramen lacerum lateral to the posterior clinoid process where it passes under the petrolingual ligament and turns abruptly forward to course along the carotid sulcus and lateral part of the body of the sphenoid. It passes forward in a horizontal direction for approximately 2 cm and terminates by passing upward along the medial side to the anterior clinoid process and the posterior surface of the optic strut where it penetrates the roof of the cavernous sinus. The clinoid segment of the carotid artery is tightly surrounded by the anterior clinoid process laterally, the optic strut anteriorly, and the carotid sulcus medially, leaving only a narrow space between the bone and artery (Figs. 9.1, 9.5, and 9.6). The dura lining the surface of these osseous structures facing the clinoid segment forms the carotid collar around the clinoid segment. The intracavernous carotid is relatively fixed by the bony ring, but despite this, large extensions of pituitary tumor may produce lateral displacement of the artery.

Just proximal to the cavernous sinus in the foramen lacerum the artery lies beneath the trigeminal nerve (6). In surgical approaches to the trigeminal nerve directed through the middle cranial fossa, there is a tendency to assume that the carotid artery is distant from the trigeminal nerve. However, nearly 85% of carotid arteries are exposed under some portion of Meckel’s cave and the trigeminal nerve with only dura, and no bone, separating the nerve from the artery (Figs. 9.1 and 9.5) (6). In the remainder, the bone separating the nerve and artery is often paper-thin. The absence of bone over the carotid often extends to the lateral edge of the trigeminal nerve and, in more than a third, the bone covering the carotid is defective lateral to the edge of the third division. The maximum length of artery exposed lateral to the nerve was 7 mm in our study.

The branches of the intracavernous carotid are the meningohypophyseal trunk, the largest branch, present in 100% of our specimens; the artery of the inferior cavernous sinus, present in 84%; and McConnell’s capsular arteries, present in 28% (Fig. 9.1). Less frequent branches of the intracavernous carotid were the ophthalmic artery (8%) and the dorsal meningeal artery (6%) (6).

Ophthalmic Artery

The ophthalmic artery commonly arises just above the upper ring from the medial half of the anterior wall of the internal carotid artery (Figs. 9.1 and 9.6–9.8). From its origin, it runs anteriorly and laterally on the upper surface of the optic strut and below the optic nerve. It runs freely above the optic strut inside the posterior part of the optic canal, but anteriorly it pierces the dura on the upper surface of the optic strut and exits the optic canal outside the optic sheath to course on the inferolateral aspect of the optic nerve and sheath at the orbital apex. The ophthalmic artery may also arise in the cavernous sinus or from the clinoid segment, in which case it usually passes through the superior orbital fissure. It may rarely arise rom the middle meningeal artery (10).

Intracavernous Branches

The meningohypophyseal trunk, the most proximal intracavernous branch, arises lateral to the dorsum sellae at or just before the apex of the first curve of the intracavernous carotid where it turns forward after leaving the foramen lacerum (Figs. 9.1 and 9.8). It is approximately the same size as the ophthalmic artery. The IIIrd and IVth nerves enter the dural roof of the cavernous sinus just above or slightly behind the trifurcation of the meningohypophyseal trunk. The meningohypophyseal trunk divides near the roof of the cavernous sinus and typically gives rise to three branches: 1) the tentorial artery, also called the artery of Bernasconi-Cassinari, which courses lateral to the tentorium; 2) the inferior hypophyseal, which travels medially to supply the posterior pituitary capsule; and 3) the dorsal meningeal artery, which enters the dura of the posterior sinus wall and supplies the clival dura and VIth nerve. The artery of the inferior cavernous sinus, also called the inferolateral trunk, may infrequently arise from the meningohypophyseal trunk (6).

The tentorial artery, the most constant branch of the meningohypophyseal trunk, present in 100% of instances, passes forward to the roof of the cavernous sinus and then posterolaterally along the free edge of the tentorium (Figs. 9.5 and 9.6). It sends branches to the IIIrd and IVth cranial nerves, and anastomoses with the meningeal branches of the ophthalmic artery and its mate of the opposite side. Bernasconi and Cassinari (2) first reported the angiographic visualization of a tentorial artery supplying tentorial meningiomas. It has a wavy appearance and ranges in length from 5 to 35 mm in normal angiograms. If longer than 40 mm, a pathological lesion, usually a tumor, is considered probable, although it may be seen angiographically in arteriovenous malformations (26).

The dorsal meningeal artery arises from the meningohypophyseal trunk in 90% of cavernous sinuses (Figs. 9.5 and 9.8). It passes posteriorly through the cavernous sinus with the abducent nerve to reach the dura over the dorsum and clivus. It sends a branch to the VIth cranial nerve and anastomoses with its mate of the opposite side. Six percent of dorsal meningeal arteries arise directly from the intracavernous carotid, below the meningohypophyseal trunk.

The inferior hypophyseal artery, the least frequent of the three common branches of the meningohypophyseal trunk, arises from the meningohypophyseal trunk in 80% of cavernous sinuses (6, 7). It passes medially to the posterior pituitary capsule and lobe and anastomoses with its mate of the opposite side after supplying the dura of the sellar floor. It may supply pituitary adenomas and tumors of the sphenoid sinus. It may also arise directly from the intracavernous carotid.

The inferolateral trunk (artery of the inferior cavernous sinus) arises from the lateral side of the midportion of the horizontal segment of the intracavernous carotid approximately 5 to 8 mm distal to the origin of the meningohypophyseal trunk (Figs. 9.1 and 9.8). It arises directly from the carotid artery in 84% of cavernous sinuses and from the meningohypophyseal artery in another 6% (6, 7). It passes above or below the VIth nerve and downward medial to the first trigeminal division to supply the dura of the inferior lateral wall of the cavernous sinus and the area of the foramina rotundum and ovale. It may anastomose with the middle meningeal artery at the foramen spinosum. Some branches pass to the trigeminal ganglion.

McConnell’s capsular arteries arise from the medial side of the carotid artery and pass to the capsule of the gland or the dura lining the anterior wall and floor of the sella. They are frequently absent, being found in approximately a quarter of cavernous sinus (6). They arise approximately distal to the origin of the artery of the inferior cavernous sinus. Some run medially in the dura covering the sellar floor and the anterior lobe of the pituitary and anastomose with the branches of the inferior hypophyseal artery. Others originate just before the carotid artery pierces the dural roof of the cavernous sinus and run medially in the dura of the anterior sellar wall, anastomosing with its opposite mate (11).

The branches of the intracavernous carotid anastomose with carotid branches from the opposite side and provide an important collateral pathway in occlusion of the internal carotid artery below the cavernous sinus. These branches also enlarge and are of significance in the diagnosis and management of carotid-cavernous fistulas. The demonstration of these arteries does not necessarily indicate the presence of a lesion, but their presence should precipitate careful review of the base of the cranium and tentorium.

Parkinson (14) noted that spontaneous carotid-cavernous fistulas, which are presumed to be due to aneurysm rupture, tend to occur at the junction of one of the branches with the intracavernous carotid. Traumatic fistulas due to tears of the carotid and / or one or more of its intracavernous branches may have several sources, and are commonly located anteriorly in the sinus. Another artery, larger than the meningohypophyseal trunk, that may pass through the cavernous sinus, is a persistent trigeminal artery. This artery arises from the carotid artery in the cavernous sinus proximal to the origin of the meningohypophyseal trunk and joins the basilar artery between the superior cerebellar and anteroinferior cerebellar arteries.

Arteriovenous fistulae between the branches of the meningohypophyseal trunk, especially the dorsal meningeal branch and the basilar sinus, may produce all the signs and symptoms associated with a fistula between the internal carotid artery and the cavernous sinus (6). A fistula between the dorsal meningeal branch of the meningohypophyseal trunk and the basilar sinus may be of the low-flow type. The fistulas, because of a communication between a branch of the intracavernous carotid or a branch of the external carotid artery in the floor of the middle fossa on one of the venous channels, will be more amenable to direct occlusion of the fistula than one in which there has been a traumatic rupture of the carotid artery into a large venous cavern. In dealing with a carotid-cavernous fistula, it is important to remember that the proptosis may occur on the side opposite the cavernous sinus harboring the fistula. Proptosis may also occur with arteriovenous fistulas in intracranial locations other than the cavernous sinus. Proptosis may result from fistulas between the branches of the external carotid artery and the lateral sinus, caused by high-pressure flow from the fistula through the vein of Labbé to the sylvian veins and then into the cavernous sinus and ophthalmic veins or through the petrosal sinuses into the cavernous sinus and then to the orbit.

VENOUS RELATIONSHIPS

The cavernous sinus is narrowest anteriorly adjacent to the superior orbital fissure and widest posteriorly lateral to the dorsum sellae where it opens into the venous confluence formed by the junction of the basilar, cavernous, and superior and inferior petrosal sinuses (Fig. 9.2 and 9.3). The sinus is connected to the orbit by the superior and inferior ophthalmic veins, to the cerebral hemispheres through the middle and inferior cerebral veins, to the retina by the central retinal vein, to the dura by tributaries of the middle meningeal veins, to the transverse sinus via the superior petrosal sinus, to the jugular bulb by way of the inferior petrosal sinus, to the pterygoid venous plexus by the emissary veins passing through the cranial foramina, and to the facial veins through the ophthalmic veins. The basilar sinus, the largest and most constant intercavernous connection across the midline, passes posterior to the dorsum sellae and upper clivus and connects the posterior aspect of both cavernous sinuses.

The three main venous spaces within the sinus, identified by their relation to the carotid artery, are the medial, the anteroinferior, and the posterosuperior compartments (Figs. 9.1–9.3). The widest spaces are located posteriorly near the junction with the basilar sinus and anteriorly near the superior orbital fissure. The medial compartment is situated between the pituitary gland and the carotid artery. The medial space may be as wide as 7 mm, but may be obliterated by a tortuous carotid that indents the pituitary gland (6). The anteroinferior space is located in the concavity below the first curve of the intracavernous carotid where the superior and inferior ophthalmic veins commonly open into the sinus (Fig. 9.1). The VIth nerve enters the anteroinferior space after passing laterally around the intracavernous portion of the carotid (6). The superior or common trunk of the superior and inferior ophthalmic veins commonly empties into this space. The posterosuperior space is located between the carotid and the posterior half of the roof of the sinus (Fig. 9.1). The basilar sinus opens into the posterosuperior space. The meningohypophyseal artery arises in this space. A tortuous elongated intracavernous carotid may obliterate the posterosuperior space (Fig. 9.8). These three venous spaces are larger than the lateral space located between the carotid artery and the lateral sinus wall. The lateral space is usually so narrow that the VIth nerve that passes through it is adherent to the carotid on its medial side and to the sinus wall on its lateral side. Venous spaces that extend lateral to the carotid artery and VIth nerve are found in less than 10% of sinuses (1). The fact that the medial or posterior spaces are the largest make them the most suitable areas for entering the sinus. The medial space can be entered through the roof on the medial side of the oculomotor nerve. Attempting to enter the sinus through the roof on the lateral side of the oculomotor nerve carries a greater risk of damaging the abducens nerve than entry on the medial side, because the lateral space in which the abducens nerve courses is very narrow.

Intercavernous Sinus

The venous sinuses commonly found in the margins of the diaphragma and sella and connecting both cavernous sinuses are termed the intercavernous sinuses (Figs. 9.1, 9.2, 9.7, and 9.8) (16). These intercavernous connections within the sella are named on the basis of their relationship to the pituitary gland; the anterior intercavernous sinuses pass anterior to the hypophysis, and the posterior intercavernous sinuses pass behind the gland. These transsellar connections between the cavernous sinuses may exist at any point from the anterior to posterior wall of the sella, including the diaphragma, or all connections between the two sides may be absent. They can occur at any site along the anterior, inferior, or posterior surface of the gland. The anterior intercavernous sinuses are usually larger than the posterior ones and may cover the whole anterior wall of the sella, but either the anterior or posterior connections, or both, may be absent. If the anterior and posterior connections coexist and join with the cavernous sinus to form a venous ring around the gland, the whole structure constitutes the “circular sinus.” Entering an anterior intercavernous connection that extends downward in front of the gland during transsphenoidal operation may produce brisk bleeding. However, this usually stops with temporary compression of the channel or with gentle coagulation, which serves to glue the walls of the channel together. The anterior intercavernous sinus, which crosses the upper anterior margin of the sella, joins the cavernous sinus immediately behind the site at which the upper and lower rings fuse at the posterior tip of the clinoid process (Fig. 9.6) (22).

Clinoid Venous Space

The clinoid venous space, the upward extension of the cavernous sinus through the lower dural ring and inside the carotid collar, is widest at the level of the lower ring through which it communicates with the larger venous channels in the anterior part of the cavernous sinus and narrows superiorly to a thin serpiginous venous plexus, which disappears as the upper ring is approached, thus demonstrating that the clinoid segment is intracavernous (Fig. 9.6). The clinoid venous space has connections with the diploic veins of the orbital roof by way of small emissary veins that pass through small foramina in the surface of the anterior clinoid process and the optic strut facing the dural collar.

DISCUSSION

A controversy has arisen as to whether the sinus is an unbroken, trabeculated, venous cavern or a plexus of various sized veins that divide and coalesce and incompletely surround the carotid artery (1, 14, 24). Both concepts are, in part, correct. Numerous veins, such as those from the orbit, middle fossa dura, and sylvian fissure, maintain their integrity as they wind their way around the carotid arteries and nerves in the sinus wall before opening into the sinus. In other areas, numerous dural sinuses converge and form large venous spaces in the sinus, such as in the area where the basilar and the superior and inferior petrosal sinuses open into the posterior part of the cavernous sinus, or on the medial side of the carotid artery where the intercavernous sinus joins the cavernous sinus. The degree to which a cavern or a venous plexus will predominate varies from sinus to sinus and from one area to another in the same sinus. A dural venous plexus, which is referred to here as the pericavernous venous plexus and not the cavernous sinus, predominates in the middle fossa dura lateral to the cavernous sinus and anteriorly near the orbital apex and superior orbital fissure, but becomes a cavern where these numerous venous channels join with the dural envelope defining the sinus (Figs. 9.2, 9.3, and 9.5). The cavern is largest at the junction of the basilar and superior and inferior petrosal sinuses with the cavernous sinus and on the medial side of the carotid where the intercavernous sinus joins the paired cavernous sinuses. Parkinson’s observations are valuable in that they form the basis for carefully examining the anatomy of the carotid-cavernous fistulae and identifying those involving the veins in the area that can be repaired with a single clip while preserving the carotid artery.

Operative Considerations

The walls of the cavernous sinus are exposed in the operative approaches that reach the intra-, supra-, and parasellar areas (3, 4, 20, 21). A pterional (frontotemporal) or orbitozygomatic craniotomy is commonly selected to expose the roof and lateral wall of the sinus and also to provide access to the adjoining orbit and suprasellar area (Fig. 9.9). The lateral sinus wall can also be exposed by the subtemporal route, but reaching the sinus roof by this route often requires significant temporal lobe retraction, and the angle of view is not suitable for examining the roof. The orbitozygomatic approach is a variant of the frontotemporal craniotomy in which a variable amount of the upper and lateral orbital rim and zygomatic arch are elevated as a single piece in continuity with the bone flap or as a second step after elevation of a frontotemporal (pterional) bone flap (Figs. 9.10 and 9.11). The orbitozygomatic craniotomy provides an excellent exposure of the cavernous sinus and orbital contents and the structures passing through the optic canal and superior orbital fissure (Figs. 9.7 and 9.9). The exposure of the orbital contents by this approach is reviewed in Chapter 7. The frontotemporal approach without the orbitozygomatic osteotomy is suitable for lesions that are more strictly intracranial in the supra- and parasellar areas.

Figure 9.10. Pterional craniotomy and extradural approach to the cavernous sinus. A, the inset (upper left) shows the site of the scalp incision. The scalp has been reflected using subgaleal dissection to expose the frontal bone and the temporalis muscle and fascia. The facial nerve branches to the frontalis muscle course in the fat pad above the zygomatic arch. B, the superficial layer of temporalis fascia has been divided just above the fat pad so that the superficial layer of temporalis fascia and the fat pad can be folded downward with the scalp flap to protect the branches of the facial nerve. The inset shows the burr holes and craniotome cuts for the bone flap. A cuff of temporalis fascia is preserved along the superior temporal line to aid in anchoring the temporal muscle to the line at the time of closure. The keyhole burr hole is located above and behind the frontozygomatic suture. C, the sphenoid ridge has been flattened and a thin shell of bone has been left along the roof and lateral wall of the orbit. The dura has been elevated from the orbital root and lateral wall to the lateral edge of the superior orbital fissure, where the dura blends into the periorbita. It is often necessary to make a shallow cut in the dura at this lateral apex of the fissure to continue to peel the thick outer layer of dura in the lateral sinus wall from the inner layer investing the nerves. D, the outer layer of dura in the lateral sinus wall and covering Meckel’s cave has been elevated to expose the three trigeminal divisions, which course in the thin inner layer of the lateral wall. E, the dura has been elevated from the anterior clinoid process and backward along the sinus wall to expose the greater petrosal nerve and petrous ridge. F, the anterior clinoid process has been removed to expose the clinoid segment and the anterior part of the sinus roof formed by the dura lining the lower surface of the clinoid. A., artery; Ant., anterior; Clin., clinoid; CN, cranial nerve; Fiss., fissure; Frontozyg., frontozygomatic; Gang., ganglion; Gen., geniculate; Gr., greater; Lat., lateral; M., muscle; Men., meningeal; Mid., middle; N., nerve; Orb., orbital; Pet., petrosal; Seg., segment; Sup., superior; Temp., temporal, temporalis; Zygo., zygomatic. (Images courtesy of AL Rhoton, Jr.)

Figure 9.11. One-piece orbitozygomatic craniotomy and intradural approach to the cavernous sinus. A, a frontotemporal scalp flap has been reflected forward to expose the superior and lateral orbital rim and the zygomatic arch. An incision has been made in the superficial temporalis fascia along the upper margin of the temporal fat pad, which overlies the lower part of the temporalis fascia and contains the frontal branches of the facial nerve, so that the fat pad can be reflected downward with the scalp to protect the branches of the facial nerve as the flap is elevated. The temporalis muscle, under the fascial incision, has not been incised. The pericranium in the frontal region has been elevated with the scalp flap. B, the bone flap, which includes part of the upper and lateral orbital rim and the zygomatic arch, has been outlined with the craniotome. The periorbita has been separated from the orbital roof and lateral wall, and the zygomatic arch at the lower margin of the bone flap has been divided. The keyhole, located just behind the junction of the temporal line with the lateral orbital rim and just above and behind the frontozygomatic suture, is the site of a burr hole that will have the frontal dura and periorbita exposed in the depths of its upper and lower margin, respectively. The bone flap is shown in the inset. In closing the incision, the temporalis muscle and fascia are anchored to the cuff of temporal fascia remaining along the temporal line. The pterion marks the lateral margin of the sphenoid ridge and sylvian fissure. C, the bone flap has been elevated to expose the frontal and temporal dura and the periorbita. The flap includes almost 180 degrees of the orbital rim and provides excellent access to the superior and lateral aspects of the orbit. D, the temporal lobe has been elevated to expose the lateral wall of the cavernous sinus. The oculomotor nerve passes forward between the posterior cerebral and superior cerebellar arteries to enter the roof of the cavernous sinus. E, the sylvian fissure has been opened and the remaining part of the orbital roof, the anterior clinoid, and the roof of the optic canal have been removed. The clinoid segment of the internal carotid artery is exposed in the clinoidal triangle. The optic sheath surrounds the optic nerve in the optic canal. F, some of the dura forming the roof of the cavernous sinus and lining the floor of the middle cranial fossa has been removed, while preserving some of the dura forming the lateral sinus wall. G, enlarged view. The site at which the oculomotor nerve passes through the sinus roof medial to the tentorial edge has been preserved. Removing the dura from the lateral part of the floor of the middle cranial fossa exposes the petrous segment of the carotid artery and the mandibular nerve entering the foramen ovale. H, the dura has been elevated from the lateral wall of the cavernous sinus to expose the trigeminal ganglion and the ophthalmic, maxillary, and mandibular nerves. Bone has been drilled from the floor of the middle fossa to expose the facial nerve and geniculate ganglion. The ophthalmic nerve has been depressed to expose the abducens nerve coursing around the lateral surface of the intracavernous carotid. The meningohypophyseal trunk arises from the posterior bend of the internal carotid artery within the cavernous sinus. A., artery; Car., carotid; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Comm., communicating; Front., frontal; Frontozygo., frontozygomatic; Gang., ganglion; Gen., geniculate; Gr., greater; M., muscle; Men. Hyp., meningohypophyseal; N., nerve; Olf., olfactory; Ophth., ophthalmic; P.C.A., posterior cerebral artery; Pet., petrosal, petrous; Post., posterior; S.C.A., superior cerebellar artery; Seg., segment; Squam., squamous; Sup., superior; Temp., temporal, temporalis; Tent., tentorial; Tr., tract; Zygo., zygomatic. (Images courtesy of AL Rhoton, Jr.)

The nerves in the lateral sinus wall can be exposed using a pterional or orbitozygomatic extradural approach, in which the outer layer of dura in the lateral wall is peeled from the thin, fragile inner layer, investing the nerves that separate easily from the thicker outer layer (25). The peeling is begun laterally and anteriorly near the sphenoid ridge where the dura is elevated from the osseous surface of the greater and lesser wings of the sphenoid. At the lateral edge of the superior orbital fissure, where the middle fossa dura blends into the periorbital, the dura becomes resistant to being separated. A shallow, sharp cut in the fibrous band at the lateral edge of the fissure allows the dural elevation to proceed posteriorly along the lateral wall of the sinus, where the thicker outer layer of dura separates from the thin inner part of the lateral wall that encases the oculomotor, trochlear, and ophthalmic nerves. The upper limit of the dural separation is the anterior petroclinoid dural fold at the lateral edge of the sinus roof, and the posterior limit is the petrous ridge. At the posteromedial aspect of the exposure, the dura is elevated from the middle fossa floor to expose the mandibular and greater petrosal nerves and the lateral wall of Meckel’s cave. The upper part of Meckel’s cave is located lateral to the posterior part of the cavernous sinus. The petrous segment of the internal carotid artery and the greater petrosal nerve may be exposed in the floor of the middle fossa lateral to Meckel’s cave and the trigeminal ganglion.

In the orbitozygomatic approach, a frontotemporal scalp flap is reflected to expose the superior and lateral orbital margins, the zygomatic arch, temporal fascia, and frontozygomatic suture. The upper edge of the temporal fat pad above the zygoma, which overlies the temporal fascia and contains the frontal branches of the facial nerve, comes into the exposure as the scalp flap is being elevated (Fig. 9.11). The superficial layer of temporalis fascia is incised at the upper edge of the fat pad so that the fat pad and the underlying temporal fascia can be reflected downward in a single layer with the scalp flap to protect the branches of the facial nerve. The temporalis muscle deep to the superficial fascial incision is not incised in completing the fascial incision above the fat pad. The pericranium in the frontal region may be elevated with the scalp flap or as a separate layer that can be used in closing the anterior fossa.

In some cases, the orbitozygomatic osteotomy needs to include only the superior and lateral orbital rims and not the zygomatic arch. If an osteotomy of the zygomatic arch is needed, an alternative to elevating it with the superior and lateral orbital rims is to divide its anterior or posterior margin so that it can be reflected inferiorly with the temporalis muscle. In the orbitozygomatic exposure, the periorbita is separated from the anterior part of the roof and lateral orbital wall before completing the orbitozygomatic osteotomy. The one-piece orbitozygomatic bone flap includes part of the upper and lateral orbital rim, and possibly the zygomatic arch with the pterional flap. In the two-piece orbitozygomatic approach the frontotemporal bone flap is elevated as the initial step and the orbitozygomatic osteotomy is performed as the second step. The site of the osteotomy in the roof and lateral orbital wall are more easily completed in the two-piece approach because the site of the orbital bone cuts can be easily visualized from intracranially after the dura has been elevated from the orbital roof and lateral wall (Fig. 9.12). In the one-piece approach, the site of the cut in the orbital roof is visualized in the narrow space between the periorbita and bone, and there is the added risk that the dura is not elevated from the orbital roof before making the bone cuts as it can be with the two-piece approach. An important step is accurate placement of the keyhole burr hole, which has the frontal dura and periorbita exposed in the depths of its upper and lower margin, respectively. The keyhole is located just behind the junction of the anterior end of the superior temporal line with the upper part of the lateral orbital rim just above and behind the frontozygomatic suture. Having the keyhole properly placed facilitates the cuts along the lateral wall and roof of the orbit. A cuff of temporalis fascia is left along the temporal line during the opening to serve as a site for anchoring the temporalis muscle and fascia at the time of closing. Elevating the bone flap exposes the frontal and temporal dura and the periorbita. The orbitozygomatic exposure commonly includes nearly 180 degrees of the orbital rim, and provides excellent access to the superior and lateral aspects of the orbit. The recurrent  meningeal branch of the ophthalmic artery courses along the temporal dura near the junction of the roof and the lateral wall of the orbit. In a rare case, the ophthalmic artery will arise from the anterior branch of the middle meningeal artery in the region of the sphenoid ridge; occlusion of this anomalous artery results in blindness (10).

Figure 9.12 (A–F). Three-piece orbitozygomatic craniotomy. A, the inset shows the site of the skin incision. The first set of osteotomies divides the anterior and posterior end of the zygomatic arch so that the arch can be folded downward with the temporalis muscle. The second bone cut is the pterional bone flap, which is outlined, and the third piece is the orbitozygomatic osteotomy. B, the dura has been elevated from the roof and lateral wall of the orbital roof and the sphenoid ridge has been removed. The third piece, the orbitozygomatic osteotomy, includes the roof and the portion of the lateral wall of the orbit as outlined. The inset shows the orbitozygomatic osteotomy. Completing the bone cuts from intracranially allows more of the roof and lateral wall of the orbit to be preserved than with the one-piece orbitozygomatic craniotomy. C, the dura has been elevated from the lateral sinus wall to expose the oculomotor, trochlear, and ophthalmic nerves entering the superior orbital fissure and the maxillary and mandibular nerves exiting the foramen rotundum and ovale. The extradural exposure extends backward across the greater petrosal nerve to the petrous ridge. D, the ophthalmic nerve has been depressed to expose the abducens nerve and the intracavernous carotid. The anterior clinoid has been removed to expose the clinoid segment. E, the floor of the middle fossa has been removed to expose the mandibular nerve, lateral pterygoid muscle, and terminal part of the maxillary artery in the infratemporal fossa. F, the petrous apex behind the petrous segment of the carotid has been removed to expose the anteroinferior cerebellar artery and the cranial nerves in the cerebellopontine angle. A., artery; A.I.C.A., anteroinferior cerebellar artery; Car., carotid; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Eust., eustachian; For., foramen; Front., frontal; Frontozyg., frontozygomatic; Gr., greater; Infratemp., infratemporal; Lat., lateral; Lig., ligament; M., muscle; Max., maxillary; Men., meningeal; Men. Hyp., meningohypophyseal; Mid., middle; N., nerve; Pet., petrosal, petrous; Petrosphen., petrosphenoid; P.I.C.A., posteroinferior cerebellar artery; Post., posterior; Pteryg., pterygoid; Pterygopal., pterygopalatine; Seg., segment; Temp., temporal, temporalis; Tens., tensor; Tymp., tympani; Vert., vertebral. (Images courtesy of AL Rhoton, Jr.)

Figure 9.12 (G–K). Three-piece orbitozygomatic craniotomy. G, the cervical carotid artery and the branches of the mandibular nerve have been extended in the infratemporal fossa. The eustachian tube and tensor tympani muscle are layered along the anterior margin of the petrous carotid. H, the tensor tympani and eustachian tube have been resected to expose the upper cervical and petrous carotid. I, the carotid artery has been reflected forward to expose the petrous apex and clivus on the medial side of the carotid canal. J, the petrous apex and lateral part of the clivus have been removed and the dura opened, exposing the anterior and posteroinferior cerebellar and vertebral arteries and the facial and hypoglossal nerves. The opening through the petrous apex and clivus is located medial to where the lower cranial nerves exit the jugular foramen. K, the exposure has been carried forward to where the maxillary artery and nerve cross the pterygopalatine fossa. A., artery; A.I.C.A., anteroinferior cerebellar artery; Car., carotid; Cav., cavernous; Clin., clinoid; CN, cranial nerve; Eust., eustachian; For., foramen; Front., frontal; Frontozyg., frontozygomatic; Gr., greater; Infratemp., infratemporal; Lat., lateral; Lig., ligament; M., muscle; Max., maxillary; Men., meningeal; Men. Hyp., meningohypophyseal; Mid., middle; N., nerve; Pet., petrosal, petrous; Petrosphen., petrosphenoid; P.I.C.A., posteroinferior cerebellar artery; Post., posterior; Pteryg., pterygoid; Pterygopal., pterygopalatine; Seg., segment; Temp., temporal, temporalis; Tens., tensor; Tymp., tympani; Vert., vertebral. (Images courtesy of AL Rhoton, Jr.)

Opening the dura and the sylvian fissure, if needed, exposes both optic nerves, the optic chiasm, ipsilateral optic tract, oculomotor nerve, and the carotid bifurcation, in addition to the lateral wall and roof of the sinus. This exposure allows access to the optic nerve from the chiasm to the globe, and permits the orbital contents to be exposed from above or laterally. Elevating the orbital surface of the frontal lobe and displacing the anterior pole of the temporal lobe posteriorly provides a pretemporal exposure of the basal cisterns. The large sylvian veins coursing along the sylvian fissure and emptying into the cavernous sinus should be preserved if possible. Elevating the internal carotid artery so as to open the interval between the artery and the oculomotor nerve exposes the basilar, posterior cerebral, superior cerebellar, and posterior communicating arteries.

Exposing the superior orbital fissure and its sectors requires at least limited exposure of the cavernous sinus posteriorly and the orbit anteriorly (Fig. 9.9). Fortunately, all of the nerves of the cavernous sinus, except the abducens nerve, can be exposed by opening or elevating the outer layer of dura in the lateral sinus wall while leaving the inner layer intact. It is possible to expose the oculomotor, trochlear, and ophthalmic nerves from their entrance into the sinus roof and through the fissure into the orbit without opening into the major venous spaces of the sinus, because these nerves course in the inner part of the dura forming the lateral sinus wall. Exposure of the abducens nerve is more hazardous because it courses medial to the ophthalmic nerve and is adherent to the lateral surface of the horizontal segment of the intracavernous carotid.

Removal of the bone in the margin of the superior orbital fissure is a frequent step in exposing tumors and aneurysms in the region. The anterior clinoid process and the adjacent part of the lesser wing are frequently removed in dealing with ophthalmic and superior hypophyseal aneurysms and in removing tumors involving the cavernous sinus, anterior clinoid process, and medial sphenoid ridge. The greater wing may also be removed in dealing with tumors involving the middle fossa and cavernous sinus. Removing the bony margins of the fissure without exposing the neural structures will often suffice in dealing with tumors, such as meningiomas, that have grown through the bone to compress but not infiltrate the structures in the fissure. In other cases, tumors such as schwannomas and meningiomas may grow along the nerves, requiring that the various sectors of the fissure be opened, as described in Chapter 7.

In removing the anterior clinoid to expose the clinoid segment, it is important to remember that the carotid artery passes not only along the medial edge of the clinoid, but also courses upward against, often grooving, the medial half of the lower surface of the clinoid. The posterior tip of the clinoid may also project medially behind the clinoid segment toward the middle clinoid, to which it may be united by an osseous bridge, thus forming a complete bony ring around the clinoid segment. There may also be an interclinoidal osseous bridge joining the anterior and posterior clinoid process.

Care is required in removing the anterior clinoid process to avoid damaging the optic nerve on its medial side and the oculomotor nerve on its lower side, because these nerves are separated from the clinoid by only the thin layer of dura on the surface of the clinoid (Figs. 9.5–9.8). The segment of the trochlear nerve crossing medially between the upper surface of the oculomotor nerve and the lower surface of the clinoid can also be damaged in removing the anterior clinoid or the upper margin of the superior orbital fissure lateral to the clinoid. Similar care must be exercised when removing the optic strut to avoid injury to the optic nerve, which passes along the upper margin, and the oculomotor nerve, which passes along the lower margin of the strut (Figs. 9.5–9.8). Exposure or removal of the optic strut can be helpful when exposing the internal carotid artery for proximal control when dealing with lesions located at the roof of the cavernous sinus. Both the anterior clinoid and the strut may contain air cells that communicate with the sphenoid sinus and must be repaired, if opened, to prevent cerebrospinal fluid rhinorrhea.

The finding that the clinoid segment is intracavernous, being located within a collar of dura in which venous tributaries of the cavernous sinus course, has important implications for surgery in the region. The lateral margins of the carotid collar and the dural rings are the easiest to expose because these margins are accessed by removing the anterior clinoid process. The anterior and medial part of the carotid collar and rings, on the other hand, are more difficult to access because of their deeper location. It is important to note that the anterior part of the upper dural ring, which forms the roof of the cavernous sinus, does not extend across the top of the optic canal, but is located below the optic nerve on the upper surface of the optic strut, which forms the floor of the optic canal. The dura covering the anterior part of the upper surface of the anterior clinoid process also extends medially to cover the roof of the optic canal. However, it is the dura extending below the optic nerve that forms the anterior part of the upper ring.

Removing the anterior clinoid aids in visualization of the origin of the ophthalmic artery, which arises medial to and below the level of the upper edge of the anterior clinoid. However, removing the clinoid provides only limited access to the part of the upper dural ring formed by the dura extending posteriorly from the upper surface of the optic strut. Gaining access to the upper surface of the optic strut and the anterior part of the upper ring usually requires that the roof of the optic canal and the adjacent posterior part of the roof of the orbit be removed. This allows the falciform dural fold, which extends medially above the optic nerve from the upper surface of the anterior clinoid process to the chiasmatic sulcus at the posterior edge of the planum sphenoidale, and also the posterior part of the optic sheath, to be opened, so that the optic nerve can be elevated for a more adequate exposure of the upper ring at the level of the floor of the optic canal. The falciform ligament usually covers several millimeters of the optic nerve just proximal to where it passes below the intracranial edge of the roof of the optic canal. Elevating the optic nerve aids in visualization of the origin of the ophthalmic artery, which most commonly arises just above the upper dural ring and passes forward below the optic nerve on the superior surface of the dura lining the optic strut.

Opening some part of the upper ring aids in exposing the segment of the internal carotid artery located immediately proximal to lesions involving the origin of the ophthalmic artery. It is best to open the dura just outside the junction of the upper ring with the carotid collar, leaving a small cuff of dura attached to the artery rather than separating the upper ring from the arterial wall, which may create an opening into the venous spaces inside the carotid collar. The incision in the upper ring is usually confined to the area lateral, anterior, and anteromedial to the internal carotid artery, which may yield an exposure adequate to place a clip across the neck of an ophthalmic aneurysm or a temporary clip on the carotid artery below the origin of the ophthalmic artery. Opening the posteromedial part of the upper ring will commonly open into the junction of the anterior intercavernous sinus with the cavernous sinus, with resultant brisk bleeding that can usually be controlled with gentle packing with a hemostatic agent.

The dura forming the upper part of the carotid collar can usually be opened with minimal bleeding because the venous channels within the clinoid venous space are small. However, opening the dura at the lower margin of the carotid collar or the lower dural ring formed by the carotidoculomotor membrane may be associated with more brisk venous bleeding, but it is usually easily controlled with gentle packing unless there is an arteriovenous fistula in the sinus. Removing the anterior clinoid and lateral part of the optic strut may also lead to easily controllable venous bleeding, caused by opening the small veins that drain the diploe of the anterior clinoid, optic strut, and posterior orbital roof and penetrate the carotid collar to enter directly into the venous channels within the collar.

There is the possibility of placing a temporary clip across the clinoid segment of the carotid artery for proximal control without opening the collar, if the extradural plane between the dural collar and the bony surface of the optic strut and carotid sulcus is developed after removing the anterior clinoid process (Fig. 9.6). Developing the plane between the collar and the optic strut on the anterior side and between the collar and the carotid sulcus on the medial side of the clinoid segment will allow a curved clip to be applied, placing one blade of the clip through the opening created by elevating the dura from the optic strut and carotid sulcus and placing the blades of the clip to close on the medial and lateral side of the artery across and outside the collar. Removing the optic strut facilitates this clip placement by creating a space through which one blade of the clip can be advanced to reach the medial side of the artery.

That the collar and the upper ring slope downward as they proceed medially from the upper surface of the anterior clinoid allows lesions located on the medial side of the clinoid segment in the area below the optic chiasm to project below the level of the anterior clinoid on radiological studies, even though they are located intradurally. Aneurysms, such as those arising at the origin of the superior hypophyseal artery, that project medially between the diaphragm and the chiasm and which might be considered to be intracavernous on the lateral angiogram because they project below the upper edge of the clinoid, are actually intradural within the subarachnoid space.

The medial wall of the cavernous sinus wall may also be exposed in the lateral margin of the transsphenoidal approach by extending the bone removal laterally from the anterior sellar wall to the area of the prominences overlying the carotid arteries, superior orbital fissure, and the maxillary nerve (Figs. 9.5 and 9.6) (5, 9). Some of the posterior ethmoid air cells often have to be removed to gain the lateral exposure in the sphenoid sinus needed to see the wall of the cavernous sinus because the posterior ethmoid air cells are located in front of the upper part of the lateral wing of the sphenoid sinus. Removing the posterior part of the middle turbinate may also assist in the exposure. Care is required to avoid injury to the optic nerve in the superolateral part of the sphenoid sinus, the nerves passing through the superior orbital fissure in the midportion, and the maxillary nerve in the lower portion, where they may occasionally be exposed directly under the sphenoid mucosa. The length of carotid artery exposed in the wall of the sphenoid sinus offers the possibility that the intracavernous segment might be exposed by the transsphenoidal approach for trapping procedures, inserting catheters for obliteration of fistulas, or for insertion of material to thrombose arteriovenous fistulas in the sinus.

Contributor: Albert L. Rhoton, Jr., MD

Content from: Rhoton AL, Jr. The cavernous sinus, the cavernous venous plexus, and the carotid collar. Neurosurgery 2002;51(4 Suppl):375-410, 10.1097/00006123-200210001-00010. With permission of Oxford University Press on behalf of the Congress of Neurological Surgeons.

The Neurosurgical Atlas is honored to maintain the legacy of Albert L. Rhoton, Jr., MD.

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