Vestibular schwannomas (VSs), as they expand, may involve a majority of the cranial nerves and cerebellar arteries, and the midbrain, pons, and medulla. An understanding of microsurgical anatomy of the cerebellopontine angle and internal auditory canal provides the basis for optimizing surgical results with these tumors. This chapter reviews this anatomy basic to understanding and treating VSs.
30.2 Microsurgical Anatomy
30.2.1 Neural Relationships
An understanding of microsurgical anatomy is especially important in preserving the facial and cochlear nerves, which are the neural structures at greatest risk during VS removal. A widely accepted operative precept is that, when possible, a nerve involved by a tumor should be identified both proximal and distal to the tumor, where its displacement and distortion are least, before the tumor is removed from the involved segment of nerve. This operative principle has received only a limited application in operations for VS removal. Considerable attention has been directed to the early identification of the facial and vestibulocochlear nerves distal to the tumor at the lateral part of the internal auditory canal,s. Literatur,s. Literatur,s. Literatur,s. Literatur,s. Literatur,s. Literatur,s. Literatur,s. Literatur,s. Literatur but less attention has been directed to identification at the brainstem on the medial side of the tumor. The neural considerations are divided into sections dealing with the relationships at the lateral end of the tumor in the meatus and those on the medial end of the tumor at the brainstem.
30.2.2 Meatal Relationships
The four nerves in the lateral part of the internal auditory canal are the facial, the cochlear, and the inferior and superior vestibular nerves (Fig. 30‑1 ). The position of the nerves is most constant in the lateral portion of the meatus at the fundus, which is divided into a superior and an inferior portion by a horizontal ridge, the transverse or falciform crest. The facial and the superior vestibular nerves are superior to the crest. The facial nerve is anterior to the superior vestibular nerve and is separated from it at the lateral end of the meatus by a vertical ridge of bone, called the vertical crest (a.k.a. “Bill’s bar” after William House). The cochlear and inferior vestibular nerves run below the transverse crest with the cochlear nerve located anteriorly. Thus, the lateral meatus can be divided into four portions, with the facial nerve being anterosuperior, the cochlear nerve anteroinferior, the superior vestibular posterosuperior, and the inferior vestibular nerve posteroinferior.
Fig. 30.1 Posterior (retrosigmoid) view of the nerves in the right cerebellopontine angle. (a) The vestibulocochlear nerve enters the internal auditory canal with a labyrinthine branch of the AICA. The posterior inferior cerebellar artery (PICA) courses around the glossopharyngeal, vagus, and accessory nerves. The abducens nerve ascends in front of the pons. A subarcuate artery enters the subarcuate fossa superolateral to the porus of the internal auditory canal. Choroid plexus protrudes into the cerebellopontine angle behind the glossopharyngeal and vagus nerves. (b) The posterior wall of the internal auditory canal has been removed. The cleavage plane between the upper bundle, formed by the superior vestibular nerve, and the lower bundle, formed by the inferior vestibular and cochlear nerves, was begun laterally where the nerves have separated near the meatal fundus and extended medially. The nervus intermedius arises on the anterior surface of the vestibulocochlear nerve, has a free segment in the cistern and/or meatus, and joins the facial nerve distally. The facial nerve is located anterior to the superior vestibular nerve and the cochlear nerve is anterior to the inferior vestibular nerve. (c) The cleavage plane between the cochlear and inferior vestibular nerves, which is well developed in the lateral end of the internal auditory canal, has been extended medially. Within the cerebellopontine angle, the superior vestibular nerve is posterior and superior, the facial nerve anterior and superior, the inferior vestibular nerve posterior and inferior, and the cochlear nerve anterior and inferior. (d) The superior and inferior vestibular nerves have been divided to expose the facial and cochlear nerves. A labyrinthine branch of the PICA enters the internal auditory canal. (Reproduced with permission from Rhoton AL Jr. The cerebellopontine angle and posterior fossa cranial nerves by the retrosigmoid approach. Neurosurgery 2000;47(3 Suppl):S93–S129.)
Because VSs most frequently arise in the posteriorly placed vestibular nerves, they usually displace the facial and cochlear nerves anteriorly (Fig. 30‑2 ). The facial nerve is most commonly stretched around the anterior half of the tumor capsule. Variability in the direction of growth of the tumor arising from the vestibular nerves may result in the facial nerve being displaced, not only directly anteriorly, but also anterosuperiorly or anteroinferiorly. Because the facial nerve always enters the facial canal at the anterosuperior quadrant of the lateral margin of the meatus, it is usually easiest to locate it here after the internal auditory canal has been exposed, rather than at a more medial location where the degree of displacement of the nerve is more variable. The cochlear nerve also lies anterior to the vestibular nerve and will usually be stretched around the anterior margin of the tumor.
Fig. 30.2 Posterior view of right internal auditory canal with the posterior lip removed to show variable direction of facial nerve displacement by VS. (a) Normal neural relationships with the eighth nerve dividing into its three parts in the lateral internal auditory canal. The facial and superior vestibular nerves are above the transverse crest, and the cochlear and inferior vestibular nerves are below. The facial nerve occupies the anterosuperior quadrant of the fundus. (b) The facial nerve is displaced directly anteriorly. This is a frequent direction of displacement with VS. (c) Another frequent direction of displacement of the facial nerve is anterior and superior. (d) The facial nerve is displaced anteriorly and inferiorly by tumor, which erodes the superior wall of the internal auditory canal above the nerves and grows into the area above the nerves, displacing them inferiorly. (Reproduced with permission from Rhoton.s. Literatur)
30.2.3 Brainstem Relationships
The importance of early identification of the facial nerve proximal to the tumor at the brainstem has received less attention, even though there is a consistent set of relationships at the brainstem that facilitates identification of the facial nerve at this location.s. Literatur
The neural structures most intimately related to the medial side of a VS are related to the junction of the pons, medulla, and cerebellum (Fig. 30‑3 a, Fig. 30‑4 a, and Fig. 30‑5 a). The landmarks on these structures that are helpful in guiding the surgeon to the junction of the facial nerve with the brainstem are: the sulcus between the pons and medulla, junction of the glossopharyngeal, vagus, and accessory nerves with the medulla; the flocculus and choroid plexus protruding from the foramen of Luschka; and the inferior olive.
Fig. 30.3 Neurovascular relationships on the brainstem side of a VS. Anterolateral view of the right cerebellopontine angle. (a) Neural relationships. The facial (VII) and vestibulocochlear (VIII) nerves arise from the brainstem near the lateral end of the pontomedullary sulcus (Pon. Med. Sulc.), anterosuperior to the choroid plexus (Chor. Plex.) protruding from the foramen of Luschka (F. Luschka), anterior to the flocculus, rostral to a line drawn along the junction of the rootlets of the glossopharyngeal (IX), vagus (X), and accessory (XI) nerves with the brainstem, and slightly posterior to the rostral pole of the inferior olive. The abducens nerve (VI) arises in the medial part of the pontomedullary sulcus. The hypoglossal rootlets (XII) arise anterior to the olive. The cerebellopontine fissure (Cer. Pon. Fiss.) formed by the cerebellum wrapping around the lateral side of the pons and middle cerebellar peduncle (Mid. Cer. Ped.) has a superior limb (Sup. Limb) that passes above the trigeminal nerve (V) and an inferior limb (Inf. Limb) that extends below the foramen of Luschka. The cerebellomedullary fissure (Cer. Med. Fiss.), which extends superiorly between the medulla and cerebellum, communicates in the region of the foramen of Luschka with the cerebellopontine fissure. The trochlear nerve (IV) is above the trigeminal nerve. (b) Arterial relationships. The anterior inferior cerebellar artery (AICA) arises from the basilar artery (Bas. A.) and divides into rostral (Ro. Tr.) and caudal (Ca. Tr.) trunks. The rostral trunk, which is usually the larger of the two trunks, courses below the facial and vestibulocochlear nerves, and then above the flocculus to reach the surface of the middle cerebellar peduncle. The posterior inferior cerebellar artery (PICA) arises from the vertebral artery (Vert. A.) and passes first between the hypoglossal rootlets, and then between the vagus and accessory nerves on its way to the cerebellar hemisphere. The superior cerebellar artery (S.C.A.) passes above the trigeminal nerve. The cerebellar arteries give rise to hemispheric branches (He. A.).(c) Venous relationships. The veins that converge on the junction of the facial and vestibulocochlear nerves with the brainstem are the veins of the pontomedullary sulcus (V. of Pon. Med. Sulc.), cerebellomedullary fissure (V. of Cer. Med. Fiss.), middle cerebellar peduncle (V. of Mid. Cer. Ped.), and the retro-olivary (Retro-olivary V.) and lateral medullary veins (Lat. Med. V.). The vein of the cerebellopontine fissure (V. of Cer. Pon. Fiss.), which passes above the flocculus on the middle cerebellar peduncle, is formed by the anterior hemispheric veins (Ant. He. V.) that arise on the cerebellum. Transverse pontine (Trans. Pon. V.) and transverse medullary (Trans. Med. V.) veins cross the pons and medulla. The median anterior medullary (Med. Ant. Med. V.) and median anterior pontomesencephalic veins (Med. Ant. Pon. Mes. V.) ascend on the anterior surface of the medulla and pons. The veins of the middle cerebellar peduncle and the cerebellopontine fissure and transverse pontine vein join to form a superior petrosal vein (Sup. Pet. V.), which empties into the superior petrosal sinus. A bridging vein (Br. V.) passes below the vagal rootlets toward the jugular foramen. (d) Neurovascular relationships of a VS. The tumor arises from the vestibulocochlear nerve and displaces the facial nerve anteriorly, the trigeminal nerve superiorly, and the vagus and glossopharyngeal nerves inferiorly. The facial nerve, even though displaced by the tumor, enters the brainstem along the lateral margin of the pontomedullary sulcus, rostral to the glossopharyngeal and vagus nerves, anterior to the flocculus, and rostral to the choroid plexus protruding from the foramen of Luschka. The rostral trunk of the anterior inferior cerebellar artery, after passing below the tumor, returns to the surface of the middle cerebellar peduncle above the flocculus. The veins displaced around the medial side of the tumor are the veins of the middle cerebellar peduncle, cerebellomedullary fissure, cerebellopontine fissure and pontomedullary sulcus, and the retro-olivary and lateral medullary veins. (Reproduced with permission from Rhoton.s. Literatur)
Fig. 30.4 Neurovascular relationships on the brainstem side of a VS. Anterosuperior views. (a) Neural relationships. The cerebrum and tentorium cerebelli have been removed, and the trigeminal (V), trochlear (IV), and oculomotor (III) nerves have been divided to allow the brainstem to be displaced posteriorly in order to expose the cerebellopontine angle from above. The facial (VII) and vestibulocochlear (VIII) nerves arise at the lateral end of the pontomedullary sulcus (Pont. Med. Sulc.) anterior to the flocculus, rostral to the glossopharyngeal (IX), vagus (X), and accessory (XI) nerves, and anterosuperior to the choroid plexus (Chor. Plex.) protruding from the foramen of Luschka (F. Luschka). The hypoglossal nerve (XII) arises anterior to the inferior olive. The abducens nerve (VI) arises from the medial part of the pontomedullary sulcus and ascends to pierce the dura mater on the clivus. The facial and vestibulocochlear nerves pass laterally to enter the internal auditory canal. The glossopharyngeal, vagus, and accessory nerves converge on the medial side of the jugular foramen (Jug. Foramen). The cerebellopontine fissure, formed where the cerebellum wraps around the lateral side of the pons and middle cerebellar peduncle (Med. Cer. Ped.), has superior (Sup. Limb) and inferior (Inf. Limb) limbs. The foramen of Luschka opens into the inferior limb near the facial and vestibulocochlear nerves. (b) Arterial relationships. The anterior inferior cerebellar artery (AICA) arises from the basilar artery (Bas. A.), passes below the facial and vestibulocochlear nerves, gives rise to the subarcuate artery (Subarc. A.), and divides into a rostral (Ro. Tr.) and a caudal (Ca. Tr.) trunk. The rostral trunk passes above the flocculus to course on the middle cerebellar peduncle, and the caudal trunk supplies the area below the flocculus. The posterior inferior cerebellar artery (PICA) arises from the vertebral artery (Vert. A.) and passes below the hypoglossal nerve. The superior cerebellar artery (S.C.A.) passes above the trigeminal nerve.(c) Venous relationships. The veins converging on the junction of the facial nerve with the brainstem are the lateral medullary (Lat. Med. V.) and retro-olivary (Retro-olivary V.) veins, and the veins of the pontomedullary sulcus (V. of Pon. Med. Sulc.), cerebellomedullary fissure (V. of Cer. Med. Fiss.), and middle cerebellar peduncle (V. of Mid. Cer. Ped.). The median anterior pontomesencephalic vein (Med. Ant. Pon. Mes. V.) ascends on the anterior surface of the brainstem, and the transverse pontine (Trans. Pon. V.) and transverse medullary (Trans. Med. V.) veins cross the pons and medulla. The vein of the cerebellopontine fissure (V. of Cer. Pon. Fiss.) passes above the flocculus. The transverse pontine vein and the veins of the middle cerebellar peduncle and cerebellopontine fissure join to form one of the superior petrosal veins (Sup. Pet. V.) that empty into the superior petrosal sinus. A bridging vein (Br. V.) passes from the side of the brainstem to the jugular foramen. The anterolateral marginal vein (Ant. Lat. Marg. V.) crosses the anterolateral margin of the cerebellum. The vein of the pontomesencephalic sulcus (V. Pon. Mes. Sulc.) courses in the pontomesencephalic sulcus below the oculomotor nerve. (d) Neurovascular relationships of a VS. The tumor arises from the vestibulocochlear nerve and displaces the facial nerve anteriorly, the trigeminal nerve superiorly, and the glossopharyngeal and vagus nerves inferiorly. The vestibulocochlear nerve disappears into the tumor. The facial nerve enters the brainstem along the lateral margin of the pontomedullary sulcus, rostral to the glossopharyngeal nerve, anterior to the flocculus, and rostral to the choroid plexus protruding from the foramen of Luschka. The anterior inferior cerebellar artery is usually displaced around the lower margin of the tumor. The veins displaced around the medial side of the tumor are the veins of the pontomedullary sulcus, middle cerebellar peduncle and cerebellomedullary fissure, and the lateral medullary and retro-olivary veins. (Reproduced with permission from Rhoton.s. Literatur) Fig. 30.5 Neurovascular relationships on the brainstem side of a VS. Posterior view through a retrosigmoid craniectomy. (a) Neural relationships. The orientation, skin incision (solid line), and craniectomy site (interrupted fine) are shown in the inset. The retractor is on the petrosal surface of the cerebellum. The facial (VII) and vestibulocochlear (VIII) nerves arise at the lateral end of the pontomedullary sulcus (Pon. Med. Sulc.), anterior to the flocculus, rostral to the glossopharyngeal (IX), vagus (X), and accessory (XI) nerves, and anterosuperior to the choroid plexus (Chor. Plexus) protruding from the foramen of Luschka (F. Luschka). The hypoglossal nerve (XII) arises anterior to the olive. The abducens nerve (VI) arises from the medial part of the pontomedullary sulcus. The trigeminal nerve (V) arises in the upper part of the exposure. The cerebellopontine fissure, formed where the cerebellum wraps around the lateral side of the pons and middle cerebellar peduncle (Mid. Cer. Ped.), has superior (Sup. Limb) and inferior (Inf. Limb) limbs. (b) Arterial relationships. The anterior inferior cerebellar artery (AICA) arises from the basilar artery (Bas. A.) and divides into a rostral trunk (Ro. Tr.), which passes above the flocculus to reach the surface of the middle cerebellar peduncle, and a caudal trunk (Ca. Tr.), which supplies the area below the flocculus. The posterior inferior cerebellar artery (PICA) arises from the vertebral artery (Vert. A.) and passes dorsally between the vagus and accessory nerves. The superior cerebellar artery (S.C.A.) courses above the trigeminal nerve. (c) Venous relationships. The veins that join near the junction of the facial and vestibulocochlear nerves with the brainstem are the lateral medullary (Lat. Med. V.) veins and the veins of the cerebellomedullary fissure (V. of Cer. Med. Fiss.), pontomedullary sulcus (V. of Pon. Med. Sulc.), and middle cerebellar peduncle (V. of Mid. Cer. Ped.). The vein of the cerebellopontine fissure (V. of Cer. Pon. Fiss.) passes above the flocculus along the superior limb of the cerebellopontine fissure and joins the vein of the middle cerebellar peduncle and a transverse pontine vein (Trans. Pon. V.) to form a superior petrosal vein (Sup. Pet. V.), which empties into the superior petrosal sinus. A bridging vein (Br. V.) passes behind the vagus nerve. The lateral anterior pontomesencephalic vein (Lat. Ant. Pon. Mes. V.) ascends on the pons. (d) Neurovascular relationships of a VS. The tumor arises from the vestibulocochlear nerve and displaces the facial nerve anteriorly, the trigeminal nerve superiorly, and the glossopharyngeal and vagus nerves inferiorly. The vestibulocochlear nerve disappears into the tumor. The facial nerve enters the brainstem at the lateral margin of the pontomedullary sulcus, anterior to the flocculus and rostral to the choroid plexus protruding from the foramen of Luschka. The rostral trunk of the anterior inferior cerebellar artery courses below the tumor and above the flocculus to reach the surface of the middle cerebellar peduncle. The veins displaced around the medial side of the tumor are the lateral medullary veins and the veins of the middle cerebellar peduncle, cerebellomedullary fissure, and pontomedullary sulcus. The vein of the cerebellopontine fissure passes above the tumor. A recurrent perforating branch (Rec. Perf. A.) of the anterior inferior cerebellar artery passes across the tumor and supplies the brainstem. (Reproduced with permission from Rhoton.s. Literatur)
Pontomedullary Sulcus
The facial nerve arises from the brainstem near the lateral end of the pontomedullary sulcus. This sulcus extends along the junction of the pons and the medulla and ends just medial to the foramen of Luschka and the lateral recess of the fourth ventricle (Fig. 30‑3 a, Fig. 30‑4 a, Fig. 30‑5 a). The facial nerve arises in the pontomedullary sulcus 1 to 2 mm anterior to the point at which the vestibulocochlear nerve joins the brainstem at the lateral end of the sulcus. The interval between the vestibulocochlear and facial nerves is greatest at the level of the pontomedullary sulcus and decreases as these nerves approach the meatus. When in the upright position, the pontomedullary sulcus is roughly horizontal. In the exposure provided by a retrosigmoid craniotomy, the junction of the facial nerve with the pontomedullary sulcus will be hidden directly anterior to the vestibulocochlear nerve, and in some cases the facial nerve can be seen only by gently elevating, depressing, or dividing the vestibulocochlear nerve.
Glossopharyngeal, Vagus, and Accessory Nerves
The facial nerve enjoys a consistent relationship to the junction of the glossopharyngeal, vagus, and accessory nerves with the lateral side of the medulla. The facial nerve arises 2 to 3 mm above the most rostral rootlet contributing to these nerves. In the suboccipital operative exposure, the rootlets of these three nerves are seen entering the brainstem below the tumor. A helpful way of visualizing the point where the facial nerve will exit from the brainstem, even when displaced by tumor, is to project an imaginary line along the medullary junction of the rootlets forming the glossopharyngeal, vagal, and accessory nerves, and upward through the pontomedullary junction. This line, at a point 2 to 3 mm above the junction of the glossopharyngeal nerve with the medulla, will pass through the pontomedullary junction at the site where the facial nerve exists from the brainstem.
Cerebellar-Brainstem Fissures
VSs are closely related to the cerebellopontine and cerebellomedullary fissures, the clefts formed by the folding of the cerebellum around the pons and medulla (Fig. 30‑1 , Fig. 30‑2 , Fig. 30‑3 ). The cerebellopontine fissure is a V-shaped fissure formed by the folding of the petrosal surface of the cerebellum around the lateral side of the pons and middle cerebellar peduncle. The petrosal surface is the cerebellar surface that faces the posterior surface of the petrous bone and is retracted to expose the nerves entering the internal auditory canal. The cerebellopontine fissure has a superior limb situated between the rostral half of the pons and the superior part of the petrosal surface, and an inferior limb located between the caudal half of the pons and the inferior part of the petrosal surface. The apex of the fissure is located laterally where the superior and inferior limbs meet. The V-shaped area between the superior and inferior limbs, which has the middle cerebellar peduncle in its floor, corresponds to the area that is referred to as the cerebellopontine angle. The trigeminal, abducens, facial, vestibulocochlear, and glossopharyngeal nerves arise between the superior and inferior limbs of the fissure. The facial and vestibulocochlear nerves arise just anterior to the inferior limb of the fissure, and just below the middle cerebellar peduncle. The trigeminal nerve arises near the superior limb of the fissure.
The cerebellomedullary fissure—the cleft between the cerebellum and medulla, which extends upward between the cerebellar tonsil and the medulla—communicates with the inferior limb of the cerebellopontine fissure near the lateral recess of the fourth ventricle. Several structures related to the lateral recess project into the cerebellopontine angle near the facial and the vestibulocochlear nerves.
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28 Intraoperative Eighth Nerve Monitoring During Vestibular Schwannoma Microsurgery
32 Management of the Facial Nerve During Vestibular Schwannoma Microsurgery
33 Translabyrinthine Approach for Vestibular Schwannoma Resection
38 Transotic Approach for Vestibular Schwannoma Resection
44 Salvage Microsurgery After Radiation Treatment of Vestibular Schwannoma
48 Postoperative Counseling, Restrictions, and Care
