Sellar Region and Cavernous Sinus: Superior View (▶Fig. 9.1)

Anatomy: Superior view of the sellar region and the cavernous sinus. In the left half of the image, the dura has been removed from the floor of the middle fossa. Furthermore, in the left half of the picture, both layers of the dura of the lateral wall of the cavernous sinus have also been removed, exposing the nerves coursing along the cavernous sinus. (1) Anterior clinoid process (ACP); (2) optic nerve; (3) internal carotid artery (ICA; supraclinoid portion); (4) pituitary gland (hypophysis); (5) pituitary stalk; (6) diaphragm; (7) posterior intercavernous sinus; (8) posterior clinoid process; (9) dorsum sellae; (10) basilar sinus (in the right half of the image, it is located below the dura); (11) oculomotor nerve (cranial nerve [CN] III); (12) abducens nerve (CN VI); (13) trochlear nerve (CN IV); (14) anterior petroclinoid dural fold (part of tentorial edge); (15) petrosphenoid ligament (ligament of Gruber); (16) trigeminal nerve (CN V); (17) mandibular nerve (third branch of CN V, V3); (18) maxillary nerve (second branch of CN V, V2); (19) ophthalmic nerve (first branch of CN V, V1); (20) foramen ovale; (21) foramen rotundum; (22) edge of tentorium; (23) clivus; (24) Dorello’s canal; (25) Meckel’s cave; (26) middle cranial fossa; (27) tuberculum sellae; (28) falciform ligament; (29) cavernous sinus; (30) ICA (cavernous portion); (31) anterior intercavernous sinus; (32) interclinoid dural fold; (33) posterior petroclinoid dural fold; (34) oculomotor triangle.

Clinical information and surgical tips: (a) When drilling ACP, beware of CN III deep to the ACP. Furthermore, beware of pneumatized clinoid, which may communicate with sphenoid air sinus. (b) Use optic nerve to identify the ICA laterally. (c) In case of compressing masses, cut falciform ligament to decompress optic nerve. (d) There are two layers of dura covering the cavernous sinus. To achieve hemostasis, peel one layer and then inject Floseal into triangles to control venous bleeding. (e) When entering tentorial edge, beware of the trochlear nerve (CN IV). CN IV enters the tentorium approximately 10 mm posterior to the point where CN III touches the tentorium. (f) CN III enters the roof of the cavernous sinus through the oculomotor triangle (34). (g) Stop bleeding from the superior petrosal sinus by packing it with Gelfoam. The bleeding is usually caused by avulsion of the origin of Dandy’s vein (superior petrosal vein). (h) Dorello canal, through which CN VI enters into the cavernous sinus, is not a bony canal. It is bounded by the petrous apex laterally and the superolateral clivus medially and its roof is formed by the petrosphenoid ligament (15). (i) oculomotor triangle (Hakuba’s triangle): a triangular area of the dura formed by the anterior petroclinoid dural fold laterally (14), the posterior petroclinoid dural fold posteriorly (33), and the interclinoid dural fold medially (32). Through this triangle, CN III enters the roof of cavernous sinus.

Posterior View of Brainstem and Floor of Fourth Ventricle (▶Fig. 9.2)

Anatomy: Posterior view of the brainstem and the floor of the fourth ventricle. Notice the different origin of the two posterior inferior cerebellar arteries (PICAs) from the vertebral arteries: on the left side, the origin is intradural (usual), whereas on the right side it is extradural (5–20%). On the left side, the dentate ligament is preserved. The left tonsil has also been preserved, so that its relationship with the PICA is better demonstrated.

(1) Pineal body (epiphysis), (2) superior colliculus, (3) inferior colliculus, (4) inferior brachium, (5) medial geniculate body, (6) superior medullary velum, (7) trochlear nerve (CN IV), (8) posterior cerebral artery (PCA), (9) superior cerebellar artery (SCA), (10) superior cerebellar peduncle, (11) medium cerebellar peduncle, (12) inferior cerebellar peduncle, (13) medial eminence, (14) dorsal median sulcus, (15) locus coeruleus, (16) vestibular area, (17) facial colliculus, (18) striae medullaris, (19) hypoglossal trigone, (20) vagal trigone, (21) area postrema, (22) lateral recess, (23) dentate nucleus, (24) flocculus, (25) tonsil, (26) posterior median sulcus, (27) gracile fasciculus, (28) posterior intermediate sulcus, (29) cuneate fasciculus, (30) posterolateral sulcus, (31) gracile tubercle, (32) cuneate tubercle, (33) internal acoustic meatus, (34) facial nerve (CN VII), (35) vestibulocochlear nerve (CN VIII), (36) jugular foramen, (37) glossopharyngeal nerve (CN IX), (38) vagus nerve (CN X), (39) cranial root of the accessory nerve (CN XI), (40) spinal root of the accessory nerve (CN XI), (41) hypoglossal nerve, (42) dentate ligament, (43) superior articulating facet of C1, (44) dorsal root of the C1 spinal nerve, (45) rootlets of the dorsal root of the C2 spinal nerve, (46) C1 spinal nerve, (47) dorsal root spinal ganglion of C2 spinal nerve, (48) groove for vertebral artery, (49) vertebral artery, (50) extradural origin of the PICA, (51) intradural origin of the PICA, (52) caudal loop of the PICA (belongs to the tonsillomedullary or posterior medullary segment of the PICA), (53) junction of tonsillomedullary with telovelonsillar segment of the PICA, (54) cranial loop or choroidal point (belongs to telovelonsillar segment of the PICA, (55) lateral trunk, (56) medial trunk, (57) posterior meningeal artery, (58) occipital condyles, (59) obex, (60) taenia, (61) superior fovea, (62) sulcus limitans, (63) inferior fovea, (64) trigeminal tubercle.

Clinical information and surgical tips: (a) In the far lateral approach, the hypoglossal canal obstructs access to the ventral brainstem. (b) Dentate ligament (42) separates rootlets of dorsal (sensory) root from rootlets of the ventral (motor) root. The intradural vertebral artery (49) courses anteriorly to the dentate ligament, whereas the spinal root of the accessory nerve (CN XI; 40) courses posteriorly to the dentate ligament. If the dentate ligament is divided, it can be used to rotate the spinal cord. (c) If there is significant venous bleeding, the C2 root (47) can be sacrificed for easier bleeding control. (d) The choroidal point (cranial loop) is defined as the superior-most point of the PICA. There are no perforators to brainstem distal to this. Occlusion of the PICA at or distal to the choroidal point does not usually result in a functionally significant neurological deficit. (e) Intraoperative irritation of vagal trigone (20) can cause bradycardia. (f) Bilateral damage to the superior cerebellar peduncle (10) is thought to be the anatomical basis of mutism in children after posterior cranial fossa tumor sugery. ¹ (g) Palpate the groove for the vertebral artery (48) on the posterior arch of C1 with a Penfield no. 4 to estimate the location of the vertebral artery. May use Doppler. (h) Lesion to the dentate nucleus (23) can cause intention tremor of the ipsilateral extremities with voluntary movement. The dentate nucleus is located just superior to the rostral pole of the tonsil and is wrapped around the superolateral recess of the roof of the fourth ventricle forming a prominence (dentate tubercle, not depicted). The inferior medullary velum separates the rostral pole of the tonsil from the dentate tubercle. (i) Lesion to facial colliculus (17) can cause facial palsy. (k) Telovelar approach to the fourth ventricle ² : In the telovelar approach to the fourth ventricle, the medullotonsillar and the uvulotonsillar space of the cerebellomedullary fissure are dissected and the tonsils are released from the uvula and the medulla oblongata and retracted superolaterally. In this step, care should be taken to avoid injury to the PICA. An oblique incision is made to the tela choroidea starting near the foramen of Magendie and extended superolaterally through the telovelar junction to the inferior medullary velum (dashed green line). The telar incision can also be extended laterally toward the foramen of Luschka. If needed, the approach can be bilateral for greater exposure. This approach was developed in order to replace the older transvermian approach (splitting of inferior vermis), which was associated with high incidence of postoperative cerebellar mutism.

Annulus of Zinn, Cavernous Sinus, and Middle Fossa: Lateral View (▶Fig. 9.3)

Anatomy: Lateral view of the left orbit, cavernous sinus, and middle fossa. The lesser sphenoid wing has been removed and the superior orbital fissure is open. The ACP has been removed. The annulus of Zinn can be seen. (1) orbital roof (drilled); (2) optic nerve; (3) supraclinoid portion of the ICA; (4) clinoid segment of the ICA; (5) tentorial edge; (6) upper or distal dural ring; (7) lower or proximal dural ring; (8) optic strut; (9) optic nerve covered by optic sheath; (10) oculomotor nerve (CN III); (11) trochlear nerve (CN IV); (12) superior division of CN III; (13) inferior division of CN III; (14) trigeminal ganglion (gasserian ganglion); (15) ophthalmic nerve (first branch of CN V, V1); (16) maxillary nerve (second branch of CN V, V2); (17) mandibular nerve (third branch of CN V, V3); (18) pericavernous venous plexus; (19) abducens nerve (CN VI); (20) nasociliary nerve (branch of V1); (21) annulus of Zinn (annular tendon); (22) lacrimal nerve (branch of V1); (23) ophthalmic artery; (24) lateral rectus muscle (pulled inferiorly); (25) superior rectus muscle (pulled superiorly); (26) medial rectus muscle; (27) residual greater sphenoid wing after drilling (lesser wing has been removed, thus unroofing the superior orbital fissure); (28) cavernous sinus; (29) superior ophthalmic vein (SOV); (30) frontal nerve (branch of V1); (31) falciform ligament.

Clinical information and surgical tips: (a) There are two layers of the dura covering the cavernous sinus. For hemostasis, peel one layer and inject Floseal into triangles to control venous bleeding. (b) The optic strut (8) is the posterior root of the lesser sphenoid wing and forms the floor of the optic canal. Thus, it separates the optic nerve from the superior orbital fissure. (c) The trochlear nerve (CN IV; 11) crosses over the oculomotor nerve (CN III; 10). Beware when dissecting on CN III not to injure CN IV. (d) When you manipulate optic nerve, first incise the falciform ligament (31). (e) The SOV (29) communicates directly to the cavernous sinus. That is the reason there is eye chemosis, pulsatile proptosis, and ocular bruit with carotid–cavernous fistula (CCF). The superior ophthalmic vein appears enlarged on MRI with CCF. (f) The annulus of Zinn (annular tendon; 21) is formed by fibrous tissue and serves as the origin of the four rectus muscles. The following structures pass through the annulus of Zinn: optic nerve, superior and inferior division of CN III, CN VI, nasociliary nerve (from V1), and ophthalmic artery. (g) Clinoidal triangle: this triangle is located between the optic nerve and oculomotor nerve and contains from anteriorly to posteriorly the optic strut (8), the clinoid segment of the ICA (4), and the cavernous sinus. Removal of the ACP is required for exposure of this triangle. (h) Anterior clinoidectomy is indicated in surgery for paraclinoid and ophthalmic aneurysms, as well as for resection of tumors involving the medial sphenoid ridge, the ACP, and the cavernous sinus (meningioma, craniopharyngioma, pituitary adenoma). The aim is to expose the ICA, the origin of the ophthalmic artery, and optic nerve, and to reduce the need for brain retraction. Furthermore, the ICA and the optic nerve are easier manipulated and mobilized after anterior clinoidectomy. Beware of possible pneumatization of ACP communicating with the sphenoid sinus, which could result in post-op cerebrospinal fluid (CSF) rhinorrhea after clinoidectomy. Beware not to injure CN III (deep to the ACP) and optic nerve (medial to ACP) during clinoidectomy. Also, CN IV can be injured. ICA maneuverability increases further if a dural incision around the distal dural ring (leaving a dural cuff) is made to release it from the surrounding dura. (i) Supratrochlear triangle: the triangular space formed between the lower margin of CN III (10) superiorly, the upper margin of CN IV (11) inferiorly, and an imaginary line connecting the dural entry points of these nerves (see ▶Fig. 9.4). (k) infratrochlear triangle (Parkinson’s triangle): the triangular space formed between the lower margin of CN IV (11) superiorly, the upper margin of V1(15) superiorly, and an imaginary line connecting the dural entry point of CN IV and the entry point of CN V into Meckel’s cave. This triangle usually contains the origin of the meningohypophyseal trunk from the cavernous portion of the ICA.

Cavernous Sinus and Middle Fossa: Lateral View (▶Fig. 9.4)

Anatomy: Lateral view of the right cavernous sinus area and the middle fossa. The middle fossa dura has been removed. Furthermore, the two layers of the dura of the lateral wall of the cavernous sinus have also been removed, exposing the nerves coursing along the cavernous sinus. (1) trigeminal nerve (posterior root); (2) trigeminal ganglion (gasserian ganglion); (3) mandibular nerve (third branch of CN V, V3); (4) maxillary nerve (second branch of CN V, V2); (5) ophthalmic nerve (first branch of CN V, V1); (6) abducens nerve (CN VI; medial to V1); (7) trochlear nerve (CN IV); (8) petrosphenoid ligament (ligament of Gruber); (9) Dorello’s canal; (10) Meckel’s cave; (11) petrous portion of the ICA (C2 portion); (12) carotid canal; (13) greater superficial petrosal nerve (GSPN) entering the middle cranial fossa through hiatus fallopii; (14) foramen ovale; (15) middle meningeal artery and vein in foramen spinosum; (16) petrolingual ligament (deep to trigeminal ganglion and over petrosal portion of the ICA); (17) foramen rotundum; (18) superior orbital fissure; (19) cavernous portion of the ICA (C3); (20) clinoid segment of the ICA (part of the cavernous portion between the upper and lower dural rings medial to the ACP); (21) supraclinoid portion of the ICA (C4); (22) optic nerve; (23) oculomotor nerve (CN III); (24) ACP; (25) tentorial edge; (26) petrous part of the temporal bone.

Clinical information and surgical tips: (a) CN VI is located most medially of all cranial nerves in the cavernous sinus. (b) peel the middle fossa dura posterior to anterior. (c) Cauterize the middle meningeal artery above the foramen spinosum, and then cut high enough, so it does not retract and bleed. It may be required to drill around the foramen spinosum to expose the middle meningeal artery. If the middle meningeal artery retracts and bleed, first remove all soft tissue around the osseous foramen and then use bone wax. (d) Beware of dehiscence of carotid canal roof during middle fossa dissection. (e) Trochlear nerve (CN IV) crosses over the oculomotor nerve (CN III). Beware when dissecting on CN III not to injure CN IV. (f) Unilateral injury of the GSPN (13) causes reduction of tear production ipsilateral to the injury. (g) Supratrochlear triangle: the triangular space formed between the lower margin of CN III (23) superiorly, the upper margin of CN IV (7) inferiorly, and an imaginary line connecting the dural entry points of these nerves (see ?Fig. 9.3). (h) infratrochlear triangle (Parkinson’s triangle): the triangular space formed between the lower margin of CN IV (7) superiorly, the upper margin of V1 (5) inferiorly, and an imaginary line connecting the dural entry point of CN IV and the entry point of CN V into Meckel’s cave. This triangle usually contains the origin of the meningohypophyseal trunk from the cavernous portion of the ICA (see ?Fig. 9.3). (i) The posterolateral middle fossa triangle (Glasscock’s triangle): this refers to an area of the middle fossa between the lateral margin of V3 (3) anteromedially (distal to its intersection with GSPN) and the anterior margin of GSPN (13; proximal to its intersection with GSPN) posterolaterally. It contains the middle meningeal artery and drilling of the middle fossa in this area leads to infratemporal fossa. (k) The posteromedial middle fossa triangle (Kawase’s triangle): a triangular space defined by the lateral margin of CN V (proximal to its intersection with GSPN) medially, the GSPN (proximally to its intersection with CNV), and an imaginary line that connects the hiatus fallopii with the Meckel’s cave (10) laterally. It contains the petrous portion of the ICA anteriorly and the cochlea laterally. Drilling of the petrous bone in the medial part of this triangle leads to posterior fossa. (l) The anteromedial middle fossa triangle: a triangular area defined by the lower margin of V1 medially (6), the upper margin of V2 laterally (4), and an imaginary line connecting the entry point of V1 into the superior orbital fissure (18) and the foramen rotundum (17) anteriorly. Drilling the middle fossa in this triangle leads to sphenoid sinus. (m) The anterolateral middle fossa triangle: a triangular area defined by the lower margin of V2 medially (4), the upper margin of V3 laterally (3), and an imaginary line connecting the foramen rotundum (17) with the foramen ovale anteriorly. Drilling the middle fossa in this triangle leads to the lateral wing of the sphenoid sinus.

Frontal Horn and Body of Lateral Ventricle and Third Ventricle: Sagittal Section (▶Fig. 9.5)

Anatomy: Parasagittal section, some millimeters lateral to midline to the left through corpus callosum (CC) and the body of the fornix. The septum pellucidum is removed, exposing the frontal horn and body of the right lateral ventricle. We also see the right lateral wall of the third ventricle. (1) Straight gyrus (gyrus rectus); (2) cingulate gyrus; (3) cingulate sulcus; (4) subcallosal area; (5) right optic nerve; (6) optic chiasm; (7) cervical portion of the right ICA (C1); (8) petrous portion of the right ICA (C2); (9) cavernous portion the of right ICA (C3); (10) supraclinoid portion of the right ICA (C4); (11) anterior communicating artery (ACom); (12) A1 segment of the anterior cerebral artery (ACA; A1: from origin to ACom); (13) infracallosal segment of the ACA (A2, from ACom to junction of rostrum–genu of CC); (14) precallosal segment of the ACA (A3; anterior to genu of CC); (15) supracallosal segment of the ACA (A4, above the CC, from junction of genu–body of CC up to a point just posterior to the coronal suture); (16) postcallosal segment of the ACA (A5, above the CC, from a point just behind the coronal suture to the end); (17) inferior sagittal sinus; (18) falx; (19) clivus; (20) sphenoid sinus; (21) dorsum sellae; (22) pituitary gland (hypophysis); (23) pituitary stalk (infundibulum); (24) basilar artery; (25) SCA; (26) oculomotor nerve (CN III); (27) PCA; (28) thalamoperforating arteries from the P1 segment of the PCA terminating mostly to the interpeduncular fossa, posterior perforating substance, and the cerebral peduncles; (29) pons; (30) midbrain; (31) lamina terminalis; (32) rostrum of CC; (33) genu of CC; (34) body of CC; (35) splenium of CC; (36) septum pellucidum (removed); (37) anterior commissure; (38) body of fornix; (39) columns of fornix; (40) left mammillary body; (41) chiasmatic recess; (42) infundibular recess; (43) medial surface of the right thalamus; (44) massa intermedia; (45) hypothalamic sulcus; (46) habenular commissure; (47) pineal body (epiphysis); (48) pineal recess; (49) posterior commissure; (50) aqueduct; (51) superior colliculus; (52) inferior colliculus; (53) striae medullaris thalami; (54) choroid plexus of the third ventricle; (55) internal cerebral vein; (56) foramen of Monro; (57) great cerebral vein of Galen; (58) basal vein of Rosenthal (cut); (59) precentral cerebellar vein (aka vein of the cerebellomesencephalic fissure); (60) straight sinus; (61) anterior caudate vein; (62) posterior caudate vein; (63) confluence of the anterior caudate vein, anterior septal vein thalamostriate vein, and superior choroidal vein to form the internal cerebral vein passing through the foramen of Monro into the third ventricle; (64) caudate nucleus; (65) lingula; (66) superior medullary vellum; (67) cerebellum; (68) habenula; (69) pontomesencephalic sulcus; (70) tuber cinereum; (71) suprapineal recess; (72) posterior perforated substance.

Clinical information and surgical tips: (a) The body of the lateral ventricle extends from the foramen of Monro (56) anteriorly up to the point where the body of the fornix meets CC. It has a floor (thalamus), a roof (body of CC; 34), lateral wall (body of caudate nucleus; 64), and a medial wall (septum pellucidum and body of fornix; 38). (b) The third ventricle has (1) a roof (body of fornix anteriorly and the crura of fornix with the commissure of fornix posteriorly); (2) a floor (from anterior to posterior, chiasm, infundibulum, tuber cinereum, mammillary bodies, posterior perforated substance, midbrain up to aqueduct); (3)an anterior wall (from superior to inferior, columns of fornix, foramen of Monro, anterior commissure, lamina terminalis, chiasmatic recess, chiasm); (4) a posterior wall (from superior to inferior, suprapineal recess, the habenular commissure, the pineal body and pineal recess, the posterior commissure, and the aqueduct); and (5) two lateral walls (thalamus superiorly and hypothalamus inferiorly). The roof of the third ventricle apart from the fornix is also formed by two layers of tela choroidea below the fornix, a space between these membranes called velum interpositum, where the vessels course and the choroid plexus. (c) The floor of the third ventricle is fenestrated in the area between the mammillary bodies and the infundibular recess in the endoscopic third ventriculostomy. Beware of the basilar artery while fenestrating the floor. (d) The pericallosal arteries are defined as the portion of ACAs distal to ACom around and on the corpus callosum. For identification of midline during callosotomy, find both pericallosal arteries. The midline lies in between. (e) The precentral cerebellar vein (aka vein of the cerebellomesencephalic fissure; 59) can be sacrificed during the supracerebellar infratentorial approach. This vein drains directly (like in the illustration) or indirectly via the superior vermian vein into the great cerebral vein of Galen. (f) Veins draining into the great cerebral vein of Galen (usually): the two internal cerebral veins (55), the two basal veins of Rosenthal (58), superior vermian vein (which receives occasionally the precentral cerebellar vein), the tectal veins, the posterior pericallosal veins, the medial occipitotemporal veins, and the internal occipital veins. Additional veins draining usually in the internal cerebral veins have been described to drain into the great cerebral vein of Galen and vice versa. (g) When you are about to finish the callosotomy, just before entering the ventricle, look for a small midline groove. (h) Opening of the lamina terminalis via the subfrontal approach leads to the third ventricle. (i) Excessive bilateral cingulate gyrus retraction may cause postoperative transient akinetic mutism. ³ Furthermore, during the interhemispheric approach to the corpus callosum, have in mind that below the falx the two cingulate gyri may be adherent to each other and can be mistaken for corpus callosum. Identify corpus callosum by its bright white color and its relative hypovascularity.