Fig. 5.1 The grading system for these tumors is included here, as the techniques and results vary markedly between different tumor sizes.

Fig. 5.2 Grade 1: intracanalicular tumor, with a longitudinal diameter of 1–10 mm. Grade 2: intracanalicular and intracisternal tumor, with a longitudinal diameter up to 20 mm. Grade 3: Intracanalicular and intracisternal tumor, with a longitudinal diameter up to 30 mm (reaching the brain stem). Grade 4: intracanalicular and intracisternal tumor, with a longitudinal diameter more than 30 mm (indenting and displacing the brain stem).

Fig. 5.3 The origin of acoustic neurinomas. At the fundus of the internal auditory canal, the inferior vestibular nerve is consumed by tumor in more than 70 % of cases, the superior vestibular nerve in 30 %, and the cochlear nerve in less than 20% of cases. In the majority of cases, “acoustic neurinomas” originate from the vestibular portion of the eighth nerve in its intrameatal course, but may also involve other components of the eighth nerve. All of these tumors originate in the neurilemmal segment of the nerve, lateral to the glial–neurilemmal junction.

Fig. 5.6 The four tumor grades and the corresponding surgical approaches suggested. Grade 1: where hearing is preserved, either an extradural subtemporal approach or a suboccipital transmeatal approach is preferred. Only in cases in which hearing is absent can the translabyrinthine approach be recommended. Grade 2: the same approaches can be used, although the suboccipital approach, with its full visualization of the intracranial course of the nerves, is preferable. Grades 3 and 4: visualization of the brain stem and cranial nerves makes the suboccipital transmeatal approach the procedure of choice here.

Fig. 5.7 Grade 2 tumors are divided into two subtypes, based on the extrameatal size of the tumor. This division has prognostic implications related to facial nerve and hearing outcomes. Grade 2 a tumors do not extend more than 10 mm into the cerebellopontine angle (CPA), measured from the lip of the porus acusticus, and grade 2 b tumors extend 11–18 mm into the CPA from the porus acusticus. This means that grade 2 a tumors provide more space between the tumor itself and the brain stem (5–8 mm).

Fig. 5.9 a–d Type 1 a tumors indent the CN VIII complex from an occipital origin. Preresection and postresection illustrations are shown here.

Fig. 5.10 a–d The lesion indents and displaces nerve fibers from above in type 1 b tumors. As shown in d, these tumors tend to be easily separable from the adherent nerve fibers.

Fig. 5.17 a Intracanalicular right-sided acoustic neurinoma grade 1. An axial postcontrast CT section at the level of the internal acoustic canal. Note the globular contour of the small neurinoma, which does not extend to the fundus of the meatus.

Fig. 5.17 b The right cerebellopontine angle is exposed during a standard suboccipital retromastoid approach. The upper lip of the internal acoustic porus is overlapped by a bony protrusion. No tumor is seen protruding out of the internal meatus.

Fig. 5.17 c The dura overlying the posterior wall of the internal acoustic meatus is coagulated and incised with a blade 11 knife; the small dural flap is separated from the bone and pushed to the posterior lip of the internal acoustic porus.

Fig. 5.18 a Axial contrast-enhanced MRI, demonstrating a grade 1 acoustic neurinoma in a 33-year-old woman.

Fig. 5.18 b A high-resolution bone window CT scan demonstrates relative lack of erosion of the internal auditory canal (IAC). A significant air cell complex is located around the IAC. It appears that exposure to the fundus is possible without fenestration of the labyrinth.

Fig. 5.18 c Initial view of the right cerebellopontine angle (CPA) via a retrosigmoid approach.

Fig. 5.18 d Arachnoid dissection frees the anterior inferior cerebellar artery (AICA), allowing posterior mobilization, away from the porus acusticus.

Fig. 5.19 a An axial MRI with contrast reveals an intracanalicular grade 1 acoustic neurinoma. In such cases, when hearing is good, the patient has a high chance of hearing preservation with removal of the tumor. The recurrence rate is generally quite low for such tumors, and surgery was therefore offered to this patient as a good treatment option.

Fig. 5.19 b Coronal contrast-enhanced MRI at the level of the internal auditory canals, demonstrating the tumor.

Fig. 5.19 c Via a retrosigmoid approach, the typical view is obtained, with no tumor visible at the porus acusticus. To plan removal of the posterior wall of the IAC, the location of the endolymphatic sac is determined, which marks the lateral limit of the dural incision of the posterior petrosal surface.

Fig. 5.19 d The dura is incised in a semilunar shape, with the lateral extent approaching the bony operculum. The operculum is a bony ledge that overhangs the vestibular aqueduct entrance into the endolymphatic sac. Respecting this lateral limit will preserve the integrity of the endolymphatic sac and avoid potential hearing loss due to loss of endolymph.

Fig. 5.20 a A contrast-enhanced axial MRI of a grade 2 a tumor on the right side.

Fig. 5.20 b The intraoperative view of the right CPA shows the tumor extending in an extrameatal direction.

Fig. 5.20 c Limited removal of the posterior wall of the IAC has been carried out to expose more tumor in the canal.

Fig. 5.21 a, bCoronal contrast-enhanced MRI views of a grade 2 a mass on the right side.

Fig. 5.21 c The intraoperative view via a right retrosigmoid approach demonstrates the tumor after arachnoid dissection.

Fig. 5.21 d The petrosal dural flap has been reflected, and the necessary length of the posterior IAC wall has been drilled away.

Fig. 5.22 a A preoperative contrast-enhanced axial MRI view of a right-sided grade 2 a acoustic neurinoma, detected in a 55-yearold man with good hearing.

Fig. 5.22 b, c Intraoperative views at different magnifications demonstrate the tumor. The eighth nerve fibers can be discerned coursing over the top and bottom of the mass, suggesting type 1 a topography.

Fig. 5.23 a, b Coronal (a) and axial (b) contrast-enhanced MRI views in this patient demonstrate the cause of canal erosion, a grade 2 a acoustic neurinoma. Note the lack of bony erosion at the porus acusticus. A constriction of this type can make removal of bone in this area more difficult.

Fig. 5.23 c The axial bone window CT demonstrates an eroded IAC in this 60-yearold woman with symptoms of tinnitus.

Fig. 5.23 d A view via the retrosigmoid approach of the CPA and tumor extending from the porus acusticus.

Fig. 5.23 e The dural flap is elevated before removal of the posterior wall of the IAC.

Fig. 5.24 a An axial MRI image with contrast enhancement shows a grade 2 a lesion.

Fig. 5.24 b Intraoperative view of the CPA. The relationships of the adjacent cranial nerves are clearly demonstrated.

Fig. 5.24 c A dural flap is raised, and the posterior wall of the IAC is removed.

Fig. 5.25 a MRI with contrast demonstrates a grade 2 a tumor in this axial view at the level of the IAC.

Fig. 5.25 b An air-contrast CT scan was also performed, demonstrating the tumor mass in the canal and a relative lack of bone erosion at the porus acusticus.

Fig. 5.25 c The initial operative view after arachnoid dissection in the CPA shows the tumor mass extending out of the porus acusticus. The facial nerve is seen at the brain stem, just inferior to the eighth nerve complex, before it passes on the medial side.

Fig. 5.25 d A higher-magnification view demonstrates the type 1 b morphology, as the tumor is deflecting the eighth nerve from the superior direction.

Fig. 5.25 e The dura is reflected, and limited removal of the posterior canal wall is carried out before tumor dissection. The plane between the tumor capsule and the cochlear division is carefully developed with a micro-dissector after tumor debulking.

Fig. 5.26 a, b Axial and coronal T1-weighted MRIs with contrast, showing a grade 2 a tumor with very limited extension into the IAC in this patient. This is really an ideal case for removal via the retrosigmoid approach.

Fig. 5.26 c Exposure after arachnoid dissection reveals another case of type 1 b morphology, with the tumor mass displacing the nerve bundles from above. It appears that a very fine auditory artery is present, branching from the AICA as it turns toward the cerebellum.

Fig. 5.26 d Gentle elevation of the eighth nerve complex near its brain-stem origin reveals the proximal segment of the facial nerve.

Fig. 5.27 a A contrast-enhanced axial MRI scan at the level of the internal auditory canals, showing a grade 2 a acoustic neurinoma.

Fig. 5.27 b The intraoperative view after arachnoid dissection via a standard retrosigmoid approach demonstrates the type 2 a neurinoma protruding into the cerebellopontine angle from the porus acusticus. This exposure requires only minimal retraction of the cerebellum.

Fig. 5.27 c With the posterior wall of the IAC removed, the tumor is first debulked and then separated away from the eighth nerve complex. After removal, the facial nerve is seen here in full view, medial and superior to the eighth nerve, which is still in place.

Fig. 5.28 a This axial contrast-enhanced MRI shows a grade 2 a acoustic neurinoma. The tumor in the internal auditory canal extends nearly as far as the fundus.

Fig. 5.28 b The initial operative view, after arachnoid dissection, shows what appears to be a type 1 b morphology, with the tumor situated above the eighth nerve bundle.

Fig. 5.28 c The dura lateral to the porus acusticus is incised and elevated, reflecting the dural flap over the tumor.

Fig. 5.29 a After the usual exposure of the meatal region, the eighth nerve, its blood supply, and the tumor emerging from the internal acoustic porus are visualized.

Fig. 5.29 b The dura over the meatus has been opened, and the bone removed with a diamond drill.

Fig. 5.29 c The tumor is carefully mobilized and sharply separated from its attachment to the inferior vestibular nerve.

Fig. 5.30 a An axial contrast-enhanced MRI of a grade 2 a acoustic neurinoma, with extension to the fundus of the internal auditory canal.

Fig. 5.30 b The initial view in a retrosigmoid approach demonstrates the neurinoma extending out of the porus acusticus in the cerebellopontine angle. The double arachnoid membrane remains over the tumor and eighth nerve complex. Superiorly, the arachnoid is seen covering the intact petrosal vein.

Fig. 5.30 c After arachnoid dissection, the full extent of the tumor in the cerebellopontine angle comes clearly into view. At this stage, the topographical relationship of tumor to the eighth nerve complex appears to be type 1 b. The petrosal vein complex remains covered by a layer of arachnoid and is preserved during tumor removal on principle.

Fig. 5.31 a An axial contrast enhanced MRI of a grade 2 a tumor and its extension into the inner meatus.

Fig. 5.31 b The initial intraoperative view after a typical retromastoid approach.

Fig. 5.31 c Higher magnification, demonstrating the vestibular cochlear bundle before the brain stem, representing a typical grade 2 a neurinoma.

Fig. 5.32 a An axial contrast-enhanced MRI of a grade 2 a neurinoma, at the upper end of the size range for the classification.

Fig. 5.32 b The initial operative view demonstrates type 1 b topography.

Fig. 5.32 c The tumor is completely removed, with preservation of the eighth cranial nerve and its vascularity, as well as the facial nerve.

Fig. 5.33 a– c Coronal (a) and axial (b) contrast-enhanced MRI views of a grade 2 a tumor. The bone window CT scan in c shows a relative lack of bony erosion of the internal auditory canal. This section, at the level of the lateral semicircular canal, suggests that exposure of the fundus should be possible by drilling away the compact bone of the posterior wall of the internal auditory canal.

Fig. 5.33 d The initial operative view via the retrosigmoid approach shows the small protrusion of the tumor at the porus acusticus.

Fig. 5.33 e The interface between the tumor capsule and the eighth nerve complex is developed. The small tag of tumor is gently retracted superiorly with suction, and this small piece is removed before drilling over the posterior wall of the internal auditory canal.

Fig. 5.34 a The axial CT demonstrates a grade 2 acoustic neurinoma. The striking feature in this patient was total hearing loss with such a small tumor. The patient also reported constant tinnitus.

Fig. 5.34 b Using a right retromastoid approach, the neurinoma becomes visible. The eighth nerve follows a course along the inner surface of the tumor and appears severely compressed by the labyrinthine artery, which traverses the nerve shortly after its origin from the brain stem.

Fig. 5.34 c The labyrinthine artery is sectioned and mobilized from the eighth nerve.

Fig. 5.35 a, b Preoperative contrast-enhanced coronal (a) and axial noncontrast (b) images of a grade 2 a acoustic neurinoma.

Fig. 5.35 c A microscopic intraoperative view via a retrosigmoid approach, demonstrating what appears to be type 2 a topography, with the mass of the tumor separating the eighth nerve bundles from the facial nerve. It is possible to see the eighth cranial nerve thinly fanned out over the lateral aspect of the neurinoma.

Fig. 5.35 d The fiber bundles of the eighth cranial nerve can be carefully separated from the bulk of the tumor mass. Here, an angled sharp round microdissector is being used to develop the plane at the tumor–nerve interface.

Fig. 5.36 a An axial contrast-enhanced MRI scan, showing a typical grade 2 a neurinoma.

Fig. 5.36 b Using a right retromastoid approach, a large acoustic neurinoma covered with arachnoid is encountered.