The focus is on the anatomy of acoustic schwannoma because this is the most common tumor to occur in the cerebellopontine angle. These anatomic 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 brain stem. The anatomy of the region offers the opportunity for three approaches to the tumor in the cerebellopontine angle. One is directed through the posterior cranial fossa and posterior meatal lip. Another is directed through the labyrinth and the posterior surface of the temporal bone. The third is directed through the middle cranial fossa and the roof of the internal acoustic canal. The anatomy presented by all three approaches is reviewed.

Acoustic schwannomas, as they expand, may involve a majority of the cranial nerves, cerebellar arteries, and parts of the brain stem. On the lateral side, in the internal acoustic canal, they commonly expand by enlarging the meatus. On the medial side, they compress the pons, medulla, and cerebellum. A widely accepted operative precept is that a nerve involved by tumor should be identified proximal or distal to the tumor, where its displacement and distortion is the least, before the tumor is removed from the involved segment of nerve. Considerable attention has been directed to the early identification of the facial nerve distal to the tumor at the lateral part of the internal acoustic canal, whether the operative route be through the middle fossa, labyrinth, or posterior meatal lip. Less attention has been directed to identification at the brain stem on the medial side of the tumor. These anatomic 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 brain stem.

Meatal Relationships

The nerves in the lateral part of the internal acoustic canal are the facial, the cochlear, and the inferior and superior vestibular nerves. The position of the nerves is most constant in the lateral portion of the canal, which is divided into a superior and an inferior portion by a horizontal ridge, called either the transverse crest or the 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 canal by a vertical ridge of bone, called the vertical crest. The vertical crest is also called Bill's bar in recognition of William House's role in focusing on the importance of this crest in identifying the facial nerve in the lateral end of the canal. The cochlear and inferior vestibular nerves run below the transverse crest, with the cochlear nerve being located anteriorly. Thus, the lateral meatus can be considered to be divided into four portions, with the facial nerve being anterior superior, the cochlear nerve anterior inferior, the superior vestibular nerve posterior superior, and the inferior vestibular nerve posterior inferior.

Because acoustic schwannomas most frequently arise in the posteriorly placed vestibular nerves, they usually displace the facial and cochlear nerves anteriorly. 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 anterior superiorly or anterior inferiorly. Because the facial nerve always enters the facial canal at the anterior superior quadrant of the lateral margin of the canal, it is usually easiest to locate it here, 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 be stretched around the anterior half of the tumor.

Retrosigmoid Approach

The retrosigmoid approach to the meatus is directed through a cranial opening situated just behind the sigmoid sinus, and down the plane between the posterior face of the temporal bone and the anterior surface of the cerebellum. In removing the posterior meatal wall, it is often necessary to sacrifice the subarcuate artery because it passes through the dura on the posterior meatal wall to reach the subarcuate fossa. This artery usually has a sufficiently long stem that its obliteration does not risk damage to the anterior inferior cerebellar artery from which it arises. However, in a few cases, the subarcuate artery and the segment of the anterior inferior cerebellar artery from which it arises will be incorporated into the dura covering the subarcuate fossa. In that case, the dura and artery will have to be separated together from the posterior meatal lip in preparation for opening the meatus.

The posterior semicircular canal and its common crus with the superior canal, both of which are situated just lateral to the posterior meatal lip, should be preserved in exposing the meatal contents if there is the possibility of preserving hearing, because hearing will be lost if they are damaged. Care is also required to avoid injury to the vestibular aqueduct, which is situated inferolateral to the meatal lip, and the endolymphatic sac, which expands under the dura on the posterior surface of the temporal bone inferolateral to the posterior meatal lip. The endolymphatic sac may be entered in removing the dura from the posterior meatal lip. There is little danger of encountering the cochlear canaliculus, which has a more anterior course below the internal auditory canal. An anomaly that may block access to the posterior meatal lip is an unusually high projection of the jugular bulb. The mastoid air cells commonly encountered in the posterior meatal lip must be closed carefully to prevent a postoperative cerebrospinal fluid (CSF) leak.

After the posterior wall of the internal acoustic canal has been removed, the dura lining the canal is opened to expose its contents. The facial nerve is identified near the entrance of the facial canal at the anterior superior quadrant of the canal rather than in a more medial location, where the direction of displacement is variable. It is easy to expose the vestibule if the tumor extends into the vestibule by drilling along the posterior and superior semicircular canals.

Translabyrinthine Approach

In the translabyrinthine approach, the internal acoustic canal and cerebellopontine angle are approached through a mastoidectomy and labyrinthectomy. There are two goals of bone removal in this approach. The first is to remove enough bone to be able to identify the nerves lateral to the tumor as they course through the internal auditory canal and by the transverse and vertical crests. The second is to expose the dura on the posterior aspect of the temporal bone that faces the cerebellopontine angle. The triangular patch of dura facing the cerebellopontine angle. called Trautmann's triangle, extends from the sigmoid sinus laterally to the superior petrosal sinus above and the jugular bulb below.

In the translabyrinthine exposure, the mastoid cortex is opened and the exposure is directed through the triangular gateway between the facial nerve anteriorly. the sigmoid sinus posteriorly and floor of the middle fossa above. Bone is removed to expose the dura covering the sigmoid sinus and middle fossa, the facial nerve, the angle between the sigmoid sinus and middle fossa dura (called the sinodural angle), and the upper surface of the jugular bulb. The mastoidectomy is carried down to the horizontal semicircular canal, which provides the landmark for identifying the other canals and the facial nerve.

The labyrinthectomy portion of the procedure involves removing the semicircular canals and vestibule to expose the dura lining the internal auditory canal. In the process of removing the semicircular canals, the dura of the middle fossa above the internal acoustic meatus is skeletonized and the dura on the posterior fossa plate behind the canal is exposed. After opening and removing the canals, the vestibule is opened and removed, and the dura lining the posterior half of the internal auditory canal is exposed. Care is required to avoid injury to the facial nerve as it courses below the horizontal canal and the ampulla of the posterior canal and around the superolateral margin of the vestibule. Further bone removal at the lateral end of the canal exposes the transverse and vertical crests and the covering of the superior and inferior vestibular and facial nerves. In removing bone behind the internal acoustic canal, it is important to remember that the jugular bulb may bulge upward behind the posterior semicircular canal or internal auditory meatus. The vestibular aqueduct and endolymphatic sac will be opened and removed as bone is removed between the meatus and the jugular bulb. The cochlear canaliculus will be seen deep to the vestibular aqueduct as bone is removed in the area between the meatus and the jugular bulb. The lower end of the cochlear canaliculus is situated just above the area where the glossopharyngeal nerve enters the medial side of the jugular foramen.

The subarcuate artery or the anterior inferior cerebellar artery may be encountered in the dura of Trautmann's triangle. Commonly, the subarcuate artery, which arises from the anterior inferior cerebellar artery, passes through the dura on the upper posterior wall of the meatus as a fine stem but, on occasion, the subarcuate artery along with its origin from the anterior inferior cerebellar artery may be incorporated into the dura and dip into the subarcuate fossa, on the posterior face of the temporal bone.

Middle Fossa Approach

In the middle fossa approach, the internal auditory canal is approached from above, through a temporal craniotomy located anterior to the ear and above the zygoma. The dura under the temporal lobe is elevated from the floor of the middle cranial fossa until the arcuate eminence and greater petrosal nerve are identified. The distance from the inner table of the skull to the facial hiatus, through which the greater petrosal nerve passes, ranges from 1.3 to 2.3 cm (average 1.7 cm). In separating the dura from the floor of the middle fossa, the surgeon should remember that bone may be absent over all or part of the geniculate ganglion. In a previous study of 100 temporal bones, it was found that all or part of the geniculate ganglion and genu of the facial nerve were exposed in the floor of the middle fossa in 15 bones (15 percent). In 15 other specimens, the geniculate ganglion was completely covered but there was no bone extending over the greater petrosal nerve, The greatest length of greater petrosal nerve covered by bone was 6.0 mm. It is also important to remember that the petrous segment of the internal carotid artery may be exposed without a bony covering in the area below where the greater petrosal nerve passes below the lateral margin of the trigeminal ganglion. The foramen spinosum and middle meningeal artery and the foramen ovale and third trigeminal division are situated at the anterior margin of the extradural exposure. Usually the extradural exposure can be completed without obliterating the middle meningeal artery at the foramen spinosum. The tensor tympani muscle and eustachian tube, although not exposed in this approach, are located beneath the floor of the middle fossa roughly parallel to, and in front of the horizontal portion of the petrous carotid artery.

In completing the middle fossa approach, bone is removed over the greater petrosal nerve to expose the geniculate ganglion and genu of the facial nerve. From here the labyrinthine portion of the facial nerve is followed to the lateral end of the internal auditory canal by removing bone. The lateral part of the bone removal is limited posteriorly by the superior semicircular canal, which is located a few millimetres behind and is oriented parallel to the labyrinthine segment of the facial nerve. The anterior edge of the exposure is limited by the cochlea, which sits only a few millimetres anterior to the site of bone removal in the angle between the labyrinthine portion of the facial nerve and the greater petrosal nerve. It is important that the cochlea and semicircular canals not be damaged in this approach because it is used only for removing small tumors in which there is an opportunity to preserve hearing. The vertical crest, which is identified at the upper edge of the lateral end of the internal acoustic canal provides a valuable land­mark for identifying the facial nerve. In the final stage of bone removal, the upper wall of the internal auditory canal is removed to expose the dura lining the entire superior surface of the internal auditory canal from the vertical crest to the porus acusticus.

Nervus Intermedius

The filaments of the nervus intermedius are also stretched around an acoustic schwannoma. The nervus intermedius is divisible into three parts: a proximal segment that adheres closely to the vestibulocochlear nerve, an intermediate segment that lies free between the eighth nerve and the motor root of the facial nerve, and a distal segment that joins the motor root to form the facial nerve. Some nerves are adherent to the eighth nerve throughout their entire course in the posterior cranial fossa and can be found as a separate structure only after opening the internal acoustic meatus. In most instances the nerve is a single trunk, but in some cases it is composed of two to four rootlets. It most frequently arises at the brain stem anterior to the superior vestibular nerve as a single large root and in the meatus, lies anterior to the superior vestibular nerve. When multiple rootlets are present, they may arise along the whole anterior surface of the eighth nerve: however, they usually converge immediately proximal to the junction with the facial motor root to form a single bundle that lies anterior to the superior vestibular nerve.

Brain Stem Relationships

There is a consistent set of neural, arterial and venous relation­ships at the brain stem that facilitate identification of the nerves on the medial side of the tumor.

Neural Relationships

The neural structures most intimately related to the medial side of an acoustic schwannoma are the pons, medulla, and cerebellum. The landmarks on these structures that are helpful in guiding the surgeon to the junction of the facial nerve with the brain stem are the pontomedullary sulcus; the junction of the glossopharyngeal, vagus, and accessory nerves with the medulla, the foramen of Luschka and its choroid plexus and the flocculus.

Pontomedullary Sulcus

The facial nerve arises from the brain stem near the lateral end of the pontomedullary sulcus. This sulcus extends along the junction of the pons and the medulla, and ends immediately in front of the foramen of Luschka and the lateral recess of the fourth ventricle. The facial nerve arises in the ponto­medullary sulcus 1- to 2 mm anterior to the point at which the vestibulocochlear nerve joins the brain stem 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.

Glossopharyngeal, Vagus, and Accessory Nerves

The facial nerve has 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. A helpful way of visualizing the point where the facial nerve will exit from the brain stem, even when displaced by tumor, is to project an imaginary line along the medullary junction of the rootlets forming the glossopharyngeal, vagus and accessory nerves, 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 exits from the brain stem. The glossopharyngeal and vagus nerves are seen and should be protected carefully below the lower margin of the tumor in both the translabyrinthine and retrosigmoid approaches.

Cerebellar-Brain Stem Fissures

Acoustic schwannomas are closely related to the cerebellopontine and cerebellomedullary fissures, the clefts formed by the folding of the cerebellum around the pons and medulla. 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 the cerebellar surface that is compressed by an acoustic schwannoma. 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 called the cerebellopontine angle. The trigeminal, abducens, facial, vestibulocochlear, glosso­pharyngeal. and vagus 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 that 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 and the foramen of Luschka project into the cerebellopontine angle near the facial and the vestibulocochlear nerves.

Foramen of Luschka, Choroid Plexus, and Flocculus

The structures related to the lateral recess of the fourth ventricle that have a consistent relationship to the facial and vestibulocochlear nerves are the foramen of Luschka and its choroid plexus and the flocculus. The foramen of Luschka is situated at the lateral margin of the pontomedullary sulcus, just dorsal to the junction of the glossopharyngeal nerve with the brain stem, and immediately posteroinferior to the junction of the facial and vestibulocochlear nerves with the brain stem. The foramen of Luschka is infrequently well visualized. However, there is a consistently identifiable tuft of choroid plexus that hangs out of the foramen of Luschka and sits on the posterior surface of the glossopharyngeal and vagus nerves just inferior to the junction of the facial and vestibulocochlear nerves with the brain stem.

Another structure related to the lateral recess is the flocculus. It is a fan-shaped cerebellar lobule that projects from the margin of the lateral recess into the cerebellopontine angle. The flocculus, together with the nodule of the vermis, forms the primitive flocculonodular lobe of the cerebellum. The flocculus is attached to the rostral margin of the lateral recess and foramen of Luschka. The flocculus is continuous medially with the inferior medullary velum, a butterfly-shaped sheet of neural tissue that forms on the surface of the nodule and sweeps laterally above the tonsil to form part of the inferior half of the roof of the fourth ventricle. The lateral part of the inferior medullary velum narrows to a smaller bundle, the peduncle of the flocculus, which fuses to the rostral margin of the lateral recess and foramen of Luschka. The flocculus projects from the peduncle of the flocculus into the cerebellopontine angle just posterior to the site at which the facial and vestibulocochlear nerves join the pontomedullary sulcus.

Arterial Relationships

The arteries crossing the cerebellopontine angle, especially the anterior inferior cerebellar artery, have a consistent relationship to the facial and vestibulocochlear nerves, foramen of Luschka and flocculus. The anterior inferior cerebellar artery originates from the basilar artery and encircles the pons near the pontomedullary sulcus. After coursing near, and sending branches to the nerves entering the acoustic meatus and the choroid plexus protruding from the foramen of Luschka, it passes around the flocculus to reach the surface of the middle cerebellar peduncle and terminates by supplying the lips of the cerebellopontine fissure and the petrosal surface of the cerebellum. The anterior inferior cerebellar artery usually bifurcates near the facial and vestibulocochlear nerves to form a rostral trunk and a caudal trunk. The rostral trunk courses along the middle cerebellar peduncle to supply the upper part of the petrosal surface, and the caudal trunk passes near the lateral recess and supplies the lower part of the petrosal surface.

The trunk of the anterior inferior cerebellar artery is divided into three segments based on its relationship to the nerves and the meatus: the premeatal, meatal and postmeatal segments. The premeatal segment begins at the basilar artery and courses around the brain stem to reach the region of the meatus. The meatal segment is located in the vicinity of the internal acoustic meatus. The postmeatal segment begins distal to the nerves and courses medially to supply the brain stem and cerebellum. The meatal segment often forms a laterally convex loop, the meatal loop. directed toward or into the meatus. In a prior study, it was found that the meatal segment was located medial to the porus in 46 percent of 50 cases and formed a loop that reached the porus or protruded into the canal in 54 percent. In opening the canal by the middle fossa, translabyrinthine, or posterior approach, care is needed to avoid injury to the meatal segment if it is located at or protrudes through the porus.

In most cases, the anterior inferior cerebellar artery passes below the facial and vestibulocochlear nerves as it encircles the brain stem, but it may also pass above or between these nerves in its course around the brain stem. In the most common case, in which the artery passes below the nerves, the tumor displaces the artery inferiorly. If the artery courses between the facial and vestibulocochlear nerves, a tumor arising in the latter nerve will displace the artery forward. Tumor growth will displace the artery superiorly if it passes above the nerves.

The branches of the anterior inferior cerebellar artery that arise near the facial and vestibulocochlear nerves are the labyrinthine (internal auditory) arteries, which supply the facial and vestibulocochlear nerves and adjacent structures, the recurrent perforating arteries, which may initially pass toward the meatus but subsequently turn medially and supply the brain stem and the subarcuate artery, which enters the subarcuate fossa. The subarcuate artery usually ends in the bone below the superior canal but it may infrequently supply the distal territory of the labyrinthine arteries.

The superior cerebellar artery, which is separated from the tumor by the trigeminal nerve, is displaced rostrally by the tumor, and the posterior inferior cerebellar artery is displaced caudally with the glossopharyngeal and vagus nerves.

Venous Relationships

The veins on the side of the brain stem that have a predictable relationship to the facial and vestibulocochlear nerves are those draining the petrosal surface of the cerebellum, the pons and medulla, and the cerebellopontine and cerebellomedullary fissures. The identification of any of these veins during removal of the tumor makes it easier to identify the site of the junction of the facial and vestibulocochlear nerves with the brain stem. These veins on the medial side of the tumor are the vein of the pontomedullary sulcus, which courses transversely in the pontomedullary sulcus; the lateral medullary vein, which courses longitudinally along the line of origin of the rootlets of the glossopharyngeal, vagus, and accessory nerves; the vein of the cerebellomedullary fissure, which passes dorsal or ventral to the flocculus before joining the other veins in the cerebellopontine angle; the vein of the middle cerebellar peduncle, which is formed by the union of the lateral medullary vein and the vein of the pontomedullary sulcus and ascends on the middle cerebellar peduncle to join the vein of the cerebellopontine fissure; and the vein of the cerebellopontine fissure, which is formed by the union of the veins that arise on the petrosal surface of the cerebellum and converge on the apex of the cerebellopontine fissure. All of these veins course near the lateral recess and the junction of the facial and vestibulocochlear nerves with the brain stem.

The veins surrounding an acoustic schwannoma terminate by forming bridging veins, called petrosal veins, which empty into the superior petrosal sinus. These veins, which cross the cerebellopontine angle to reach the superior petrosal sinus, are the ones most frequently occluded in the course of operations in the cerebellopontine angle. Bridging veins are more frequently exposed and sacrificed in the rostral part of the cerebellopontine angle during operations near the trigeminal nerve than during operations near the nerves entering the internal acoustic meatus. The exposure of an acoustic schwannoma in the central part of the cerebellopontine angle near the lateral recess can usually be completed without sacrificing a bridging vein. If a vein is obliterated during acoustic tumor removal, it is usually one of the superior petrosal veins, which is sacrificed near the superior pole of the tumor during the later stages of the removal of a large tumor. Small acoustic schwannomas are usually removed without sacrificing a petrosal vein. The largest vein encountered around the superior pole of an acoustic schwannoma is the vein of the cerebellopontine fissure, which passes from the petrosal surface of the cerebellum above the facial and vestibulocochlear nerves to join other tributaries of the superior petrosal sinus.

Summary

Because acoustic schwannomas most frequently arise in the posteriorly placed vestibular nerves, they usually displace the facial and cochlear nerves anteriorly. The facial nerve is stretched around the anterior half of the tumor capsule. The nerve is infrequently found on the posterior surface of the tumor. Because the facial nerve always enters the facial canal at the anterior­superior quadrant of the lateral end of the internal auditory canal, it is usually easiest to locate it here, 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 is most frequently stretched around the anterior half of the tumor. The strokes of the fine dissecting instruments used in removing the tumor should be directed along the vestibulocochlear nerve from medial to lateral rather than from lateral to medial, because traction medially may tear the tiny filaments of the cochlear nerve at the site where these filaments penetrate the lateral end of the meatus to enter the cochlea.

The landmarks that are helpful in identifying the facial and vestibulocochlear nerves at the brain stem on the medial side of the tumor have been reviewed. These nerves, although distorted by tumor, can usually be identified on the brain stem side of the tumor at the lateral end of the pontomedullary sulcus, just rostral to the glossopharyngeal nerve, and just anterior-superior to the foramen of Luschka, flocculus and choroid plexus protruding from the foramen of Luschka. After the facial and vestibulocochlear nerves are identified on the medial and lateral sides of the tumor, the final remnants of the tumor are separated from the intervening segment of the nerves.

In the three operative approaches to the canal and cerebellopontine angle (retrosigmoid, translabyrinthine and middle fossa), a communication may be established between the subarachnoid space and the mastoid air cells that will require careful closure in order to prevent a CSF leak. The retrosigmoid approach is suitable for the removal of both small and large tumors. It does not automatically lead to the loss of hearing because of the structures transgressed in reaching the tumor, as does the translabyrinthine approach, which is directed through the vestibule and semicircular canals. Surgeons who operate by the translabyrinthine route most commonly use it for small or medium-size tumors in which there is no chance of preserving hearing. It may be combined with a retrosigmoid craniectomy for the removal of large tumors. A final disadvantage of this approach is the large communication that is established between the subarachnoid space and the mastoid. The middle fossa approach is the least frequently performed operative approach to an acoustic schwannoma. The middle fossa and retrosigmoid approaches may be used for the removal of small tumors in which there is useful hearing that may be preserved.