Gmaan Alzhrani, Ph.D. Department of Neurosurgery, Center for Clinical Neurosciences, University of Utah, Salt Lake City, UT, USA. Couldwell, MD, PhD Department of Neurosurgery, Center for Clinical Neurosciences, University of Utah, Salt Lake City, UT, USA.
Surgical Anatomy of the Posterior Fossa
The cochlea is immediately in front of the fundus of the internal auditory canal (Fig. 1.9c). The superior and inferior cerebellar peduncles contribute to the lateral walls of the fourth ventricle.
Approaches
Retrosigmoid Craniotomy and Its Variants
2.5 (a, b) A C-shaped retroauricular incision two finger widths medial to the mastoid process, extending from the pinna to the tip of the mastoid. One recent meta-analysis of the three approaches to acoustic neuroma surgery (translabyrinthine, middle fossa, and retrosigmoid) found that in >5,000 patients in 35 studies, the only significant complication differences between the approaches were higher rates of CSF leakage and postoperative headache with the retrosigmoid approach [18].
Middle Fossa
Care must be taken to avoid damage to the mastoid segment of the facial nerve here. The facial nerve is identified in the anterior lateral part of the internal auditory canal.
Posterior and Combined Petrosal Approaches
Linear dural openings are made along the floor of the middle fossa (TD) and into the presigmoid dura (PFD). A full 3D reconstruction prior to craniotomy showing the location of the dural sinuses (blue), tumor (green), and arterial system (red).
Far Lateral Approach and Its Variants
Elevation of the muscles laterally exposes the rim of the foramen magnum and lamina C1 and the vertebral artery. The occipito-transcondylar variant involves removal of the posterior third of the condyle up to the hypoglossal canal. The condyle and lateral mass of C1 are then drilled to the depth of the medial hypoglossal canal.
Further lateral exposure or access to the posterior aspect of the jugular foramen can be achieved with a transjugular version. Removal of the jugular process reveals the transition between the sigmoid sinus and the jugular bulb (Figure 5.7a).
Endoscopic Endonasal Approach for Posterior Fossa Tumors
New MRI technologies (fast imaging using steady-state precession and fast imaging using steady-state acquisition) now allow clear identification of the CN and its relationship to the skull base lesion. The expanded EEA in its current form is not a substitute for posterior skull base approaches in the treatment of ventral posterior fossa meningiomas. Extradural defects are defined by the resection of the skull base bone with an intact dura and therefore no CSF leak.
Complete endoscopic endonasal transclival approach: meningioma attached to the ventral surface of the brainstem. Comparative analysis of the anterior transpetrosal approach with the endoscopic endonasal approach to the petroclival region.
Specific Diseases
Petroclival Meningiomas
SPS ligation distal to the drainage of the vein of Labbé can lead to catastrophic sequelae, especially in the dominant lobes. The tentorium is then divided posteriorly to the entry point of the trochlear nerve to complete the exposure. The superior petrosal sinus is then ligated in place to ensure drainage.
After ligating the superior petrosal sinus, the tentorium is then divided along the region of the petrous apex. Interpositional carotid artery bypass strategies in the surgical management of aneurysms and tumors of the skull base.
Meningiomas
MPFM
A 43-year-old woman presented with decreased hearing, dizziness, and fullness in her right ear. She could not use the phone with her right ear due to reduced auditory perception. The study showed a contrast-enhancing mass arising above the IAC, extending across the internal acoustic meatus with enhancement in the auditory canal (Fig. 8.3a).
Gross total resection of the CPA mass and extension to the first 5 mm of the IAC was achieved (Fig. 8.3b). Postoperative imaging showed residual IAC enlargement; however, at 118 months postoperatively, there was no recurrence (Fig. 8.3d).
PPFM: Small-Sized Tumor
PPFM: Large-Sized Tumor
CPA meningiomas are the second most common tumors in this location after vestibular schwannomas. Preoperative axial T1 postcontrast MRI showing the tumor in the area of the vestibular aperture. Functional preservation is one of the keys to resection, with facial and auditory outcomes most important for facial stone tumors.
Retrosigmoid approach for cerebellopontine angle meningiomas: surgical results and site of adjunctive treatments. Transpetrosal approaches for meningiomas of the posterior aspect of the petrous bone results in 43 consecutive patients.
Tentorial Meningiomas
Retrosigmoid Approach (Fig. 9.2)
The tumor slightly compressed the brainstem, and there was no edema in the brainstem. The tumor had a well-defined plane of dissection from the brainstem and cranial nerves. The tumor could be totally removed with preservation of the petrosal veins and there were no postoperative complications.
Retrosigmoid Approach with Drilling of the Petrous Apex
Anterior Transpetrosal Approach (Fig. 9.4)
Combined Transpetrosal Approach (Fig. 9.5)
Postoperative axial (c) and coronal (d) T1-weighted gadolinium magnetic resonance images showing total removal of the tumor via an anterior transpetrosal approach. Postoperative axial (c) and coronal (d) T1-weighted gadolinium magnetic resonance images showing near-total resection of the tumor via a right combined transpetrosal approach. The distal portion of the right SCA and brain stem are peeled from the tumor (i).
Almost complete resection of the tumor is achieved with a small residual tumor along the right oculomotor nerve (j). Depending on the ratio of the tumor to the large vein of Galen, tumors are divided into.
Posterior Interhemispheric Transtentorial Approach (Fig. 9.7)
Resection of the tumor in such cases leads to damage to the surrounding structures with additional neurological deficits. In cases where the galenic venous system is patent, the surgical procedure is more difficult because the tumor adheres firmly to the venous system. To prevent injury to the deep veins, a small portion of the tumor can be left around the deep veins to prevent injury in the context of an inferior type of tumor.
Considering the surgical risk involved in removing the inferior type of meningioma, a combination of subtotal resection of the tumor and stereotactic radiotherapy may be recommended. As a surgical strategy, the superior-type tumor is accessed using a posterior interhemispheric transtentorial approach, and there may be some who prefer a posterior interhemispheric transtentorial approach for inferior-type tumors.
Retrosigmoid Approach (Fig. 9.8)
With inferior tumors located below the vein of Galen, a supracerebellar approach may be beneficial because the vein of Galen would not be directly at risk. Preoperative CT venography shows stenosis of the .. and sagittal g) gadolinium-weighted T1-weighted magnetic resonance imaging showing subtotal tumor resection via a posterior interhemispheric transtentorial approach. The tumor is removed from the deep venous system (l) and subtotal resection of the tumor is performed.
Combined Presigmoid and Retrosigmoid Approach
Aggressive resection is contraindicated for meningiomas located in the posterior tentorial region invading the sinus partially with sinus patency [5]. After a conservative approach, the residual tumor can be observed or irradiated stereotactically either initially or at the time of recurrence [6]. In the case of total occlusion of the sinus, especially if collateral venous channels have developed, complete removal of the tumor, including the segment of the occupied sinus, can be safely performed without restoration of venous flow [7].
An aggressive removal may be necessary for a higher-grade meningioma with reconstruction of the sinus, especially in younger patients [4].
Suboccipital Approach with Resection of Invasion
Preoperative axial (a) and coronal (b) T1-weighted magnetic resonance images with gadolinium showing the posterior type of tentorial meningioma. Postoperative axial (d) T1-weighted magnetic resonance images with gadolinium showing total resection of the tumor, including the portion invading the sinus, via a suboccipital approach.
Aggressive Resection with Reconstruction of the Superior
Postoperative axial (d) and sagittal (e) T1-weighted magnetic resonance images with gadolinium show total resection of the tumor. A detailed discussion of the far-lateral approach is described in the chapter devoted to that subject. Patient positioning, location of the incision and drilling of the foramen magnum are addressed there.
Once the dentate ligament is divided, the rostrocaudal extent of the tumor must be defined. When part of the base is reached, the tumor can be devascularized, leading to increased.
Vestibular Schwannomas
This then enables identification of the location of the facial nerve at the start of the dissection. It is verified with stimulants. m) Continuous dissection of the tumor from the facial nerve to the IAC. n). The tumor capsule is detached from the facial nerve from the brainstem in a medial-lateral direction. o).
The origin of the facial nerve is located ventral and caudal to the origin of the vestibulocochlear nerve. The dissection is now focused on the lateral extent of the tumor in the IAC.
Epidermoid Cyst
In the next section, we describe a keyhole retrosigmoid transtentorial approach for a CPA epidermoid cyst extending into the supratentorial (middle fossa). Unnecessary traction on the ipsilateral shoulder and arm should be avoided to prevent injury to the brachial plexus. A neuronavigation system can be used to help localize the position of the skin incision and bone removal relative to the sigmoid and transverse sinuses.
A line extending from the asterion, inferiorly, just posterior to the apex of the mastoid, marks the posterior border of the sigmoid sinus. Epidermoid cysts of the fourth ventricle: very long follow-up in 9 cases and literature review.
Metastasis to the Posterior Fossa
Prior to the work of Harvey Cushing, tumor resection of posterior fossa lesions was rarely attempted due to high morbidity and mortality. The incision and craniotomy/craniectomy are made relative to the mastoid notch, but should be adjusted depending on the exact location of the lesion. Finally, the neurosurgical oncologist must be aware of the varying tumor biologies caused by metastatic lesions in the brain.
Metastatic lesions of the posterior fossa are often well suited for SRS intervention, yet SRS is not without complications. Brain edema after SRS is a well-described phenomenon, and the small space of the posterior fossa amplifies these effects.
Microsurgical Management
The vascular anatomy of the posterior circulation consists of the paired vertebral arteries (VAs), the basilar artery (BA), the posterior inferior cerebellar arteries (PICAs), the anterior inferior cerebellar arteries (AICAs), the superior cerebellar arteries (SCAs ), and the posterior cerebral arteries (PCAs) [2]. The p1 segment (the anterior medullary segment) arises from the VA and travels anteriorly to the medulla to the hypoglossal roots at the medial border of the olive. The P2a segment is the portion of the PCA from the insertion of the PCoA to the posterior border of the peduncle.
P2p is the portion of the vein from the posterior peduncle to the calcarine fissure. Aneurysms of the posterior circulation have a more aggressive course and natural history than their counterparts in the anterior circulation [21, 22].
