Fig. 6.1
Recurrent chordoma in a 61-year-old male patient. Patient underwent an upper and middle transclival EEA to remove the tumor (red arrows). A small dural invasion and opening into the posterior fossa was observed in the preoperative images. The postoperative MRI shows Duragen occluding the opening and protecting the basilar artery and brainstem (first layer), covered with a free fat graft (second layer) and the nasoseptal flap on top (third layer). Notice in the DWI image pre (upper row)- and post (lower row)-operative that the hyperintensity suggesting a high cellular density is no longer present in the postoperative image
Fig. 6.2
Recurrent chordoma in a 61-year-old male. (a, b) Tumor and scar tissue in front of the clivus being removed. (c) Clivus drilling. (d) Dural opening and posterior fossa invasion. (e) Microsurgical dissection and removal of the tumor from the basilar artery. (f, g) Cranial nerves no longer covered by the tumor. (h, i) Duragen “plug.” (j) Fat-free graft. (k) Nasoseptal flap being placed
The “intradural” posterior fossa tumors that are suitable to EEA resection are represented mainly by meningiomas (Fig. 6.3). In general, EEA for intradural tumors involves more challenging microsurgical dissection and skull base reconstruction . Ventral posterior fossa intradural epidermoid and dermoid cysts may be resected through EEA, but our previous experience suggests that the transnasal corridor may increase the risk of infection and abscess occurrence within the residual tumor, so we do not recommend it. Schwannomas are also another common type of intradural posterior fossa tumor . Nevertheless, the EEA cannot be used to treat the most common posterior fossa schwannomas that arise from CN VIII based on the fact that the cranial nerve VII is anteriorly located and would prevent a ventral approach. Likewise, the EEA is not an adequate approach to less common schwannomas (CN VII, IX, X, XI) since they arise at the lateral aspect of the brainstem with the inferior cranial nerves pushed ventrally. Anatomically, there is a potential application of this approach in the treatment of schwannomas of CN VI and CNXII, but these are extremely rare tumors that even when diagnosed may be managed conservatively.
Fig. 6.3
Ventral foramen magnum meningioma in a 47-year-old female patient. Patient underwent a lower transclival EEA for removal of the lesion (red arrows). Postoperative MRI shows a gross total resection with the multilayered reconstructions of the small clival opening
Endoscopic Endonasal Transclival Approach
The endoscopic transnasal access to the posterior fossa is done through a transclival approach. It may be expanded laterally on the petrous bone depending on the tumor extent and required exposure. The clivus separates the nasopharynx from the posterior cranial fossa. It is composed of the posterior portion of the sphenoid body (basisphenoid) and the basilar part of the occipital bone (basiocciput), and it is further subdivided into upper, middle, and lower thirds:
Upper clivus is at the level of the sphenoid sinus and is formed by the basisphenoid bone including the dorsum sella.
Middle clivus corresponds to the rostral part of the basiocciput, and it is located above a line connecting the caudal ends of the petroclival fissures.
Lower clivus is formed by the caudal part of the basiocciput.
Approaching the posterior fossa through the upper two thirds of the clivus requires wide opening of the sphenoid sinus . When the posterior fossa was approached at the lower clivus, the bone removal may be done solely below the sphenoid rostrum.
The intracranial surface of the upper two thirds of the clivus faces the pons and is concave from side to side. The extracranial surface of the clivus gives rise to the pharyngeal tubercle at the junction of the middle and lower clivus. The upper clivus faces the roof of the nasopharynx that extends downward in the midline to the level of the pharyngeal tubercle.
The upper and middle clivus are separated from the petrous portion of the temporal bone on each side by the petroclival fissure. The basilar venous plexus is situated between the two layers of the dura of the upper clivus and is related to the dorsum sella and the posterior wall of the sphenoid sinus. It forms interconnecting venous channels between the inferior petrosal sinuses laterally, the cavernous sinuses superiorly, and the marginal sinus and epidural venous plexus inferiorly. The basilar sinus is the largest communicating channel between the paired cavernous sinuses [6].
Extradural Posterior Fossa Tumors
Chondrosarcomas and Chordomas
Chondrosarcomas are rare slow-growing malignant bone tumor of chondroid origin cells throughout the axial and appendicular skeleton (Fig. 6.4) [7–9]. They often arise from the lateral aspects of the skull base that house cartilage , including the temporo-occipital synchondrosis , the spheno-occiput , and the sphenoethmoid complex [10]. Most skull base chondrosarcomas involve the clivus (32%), followed by other synchondroses [11]. They can also involve the cavernous sinus, petrous bone, and sphenoid bone. In the skull base, 64% arise in the middle fossa, 14% involve both middle and posterior fossa, 14% occur in the anterior fossa, and 7% originate in the posterior fossa [7].
Fig. 6.4
Right side chondrosarcoma in a 32-year-old female patient (red arrows). Patient underwent an EEA for total resection . Notice in the surgical images the access is limited by the Eustachian tube that is then removed allowing an adequate access to the tumor and gross total resection
Chordomas are rare primary bone tumor thought to arise from transformed notochord remnants with an estimated incidence rate of 0.08–0.09 per 100,000. It occurs most commonly in males with a peak incidence in their fourth to fifth decades of life and rarely affects children and adolescents. Skull base chordomas are essentially midline lesions that occur at the vicinity of the clivus (spheno-occipital bones ) and represent only 0.15% of all intracranial tumors [8]. They are considered a low-malignancy neoplasm with slow-growing pattern that rarely metastasizes. However, chordomas have a local aggressive behavior and high recurrence rates [11–13].
The slow-growing nature of most skull base chordomas and chondrosarcomas often leads to variable presentation of clinical signs and symptoms based on the location of the tumor. Treatment goals should involve complete surgical resection that should be tempered with a judicious effort to avoid neurological compromise. The extent of surgical resection correlates directly with recurrence rates. Among all chordoma patients, median survival is 6.29 years with 5-, 10-, and 20-year survival rates precipitously dropping to 67.6%, 39.9%, and 13.1% across all races and genders underscoring the dismal prognosis of this disease. Nonsurgical management or observation may be reserved for some patients who are high-risk surgical candidates. Chondrosarcomas tend to have a slightly better prognosis and are regarded to be less aggressive with higher recurrence-free survival rates when compared to chordoma. In one study, recurrence-free survival at 5 years was as high as 90%. Surgical and postoperative management goals of chordomas and chondrosarcomas are often very similar, and case series often report results for both these pathologies together because of the rarity of the diseases.
While aggressive gross total resection may be achieved through a variety of skull base approaches including EEA (Fig. 6.4), en bloc resection may not be feasible in the skull base as has been described in the spine due to involvement of critical neurovascular structures, with surgeons most often resorting to piecemeal removal of the tumor. Based on available clinical case series from experienced skull base surgeons, aggressive surgical resection is only possible 48–61% of the time underscoring the technical challenge that surgeons have with treatment of tumors in this location. As a result, radiation treatment has played an important role in the postoperative management of these patients.
Preoperative Radiological Assessment
Radiologic investigation and preoperative planning for skull base approaches of chordomas and chondrosarcomas have greatly improved because of rapidly evolving imaging methods. Computed tomography (CT) and magnetic resonance imaging (MRI) should always be performed in cases of suspected skull base bone lesion for bone and soft tissue assessment, respectively. The combination of these two radiologic modalities permits the definition of important diagnostic and therapeutic characteristics of skull base tumors: radiologic appearance, location, extension, and relation to critical neurovascular structures.
Chordomas and chondrosarcomas have an overall similar appearance on MRI, and sometimes it is impossible to differentiate them without histopathology. In general, chordomas typically present as midline extradural masses originating within bone and tend to expand posteriorly and laterally. They usually present as well-delineated soft tissue masses that may displace and compress adjacent structures. More advanced tumors show local invasiveness and characteristic bone destruction.
The CT scans demonstrate best bone erosion, osteolysis, and intratumoral calcifications. There is typically no surrounding sclerosis. Moderate to marked heterogeneous enhancement following administration of iodinated contrast material can also be depicted. Most chordomas are hypointense or isointense on T1-weighted images. High signal correlates with hemorrhage or mucinous collection. T2-weighted images characteristically demonstrate a high signal. Gadolinium enhancement is mostly heterogeneous and often presents a “honeycomb ” appearance [14].
Chondroid chordomas represent 5–15% of all chordomas and are characterized by the partial replacement of their gelatinous matrix by cartilaginous tissue . Compared with typical chordomas, they normally present in a more lateral position, and intratumoral calcifications are more often evidenced on CT scans. Because of the differences in composition, chondroid chordomas may not appear as bright as typical chordomas on T2-weighted MRIs. These findings are important prognostic factors because of significantly better survival rates of patients with chondroid chordomas.
In chondrosarcomas , CT scan demonstrates bony destruction of the skull base lateral to the midline; the typical appearance is a destructive lesion with scalloped erosive borders. Like in chordomas, the tumor has a low to intermediate signal intensity on T1-weighted images and high signal intensity on T2-weighted images. The enhancement is usually marked, and signal heterogeneity post-contrast is observed frequently because of matrix mineralization and prominent fibrocartilaginous elements within the tumor.
Imaging studies have an important role in defining diagnosis and planning the surgical approach. Angiographic studies (CTA, MRA, or conventional) are important whenever vascular compromise is suspected. The presence of arterial displacement or encasement must be assessed before surgery, so the dissection and debulking of the tumor can proceed safely. Arterial narrowing is highly suggestive of adventitia invasion, which hinders a total resection when the encased artery cannot be sacrificed. Despite not being routinely performed for skull base bone lesions, conventional angiography studies may help to define whether sacrifice of an encased artery is possible or not by defining the patient’s tolerance and collateral flow on balloon test occlusion.
The location and extension of the skull base lesion determines the pattern of CN displacement and involvement, hence the surgical corridors available for tumor resection. New MRI technologies (fast imaging with steady-state precession and fast imaging using steady-state acquisition) now permit clear identification of the CN and its relationship to the skull base lesion, instead of simply assuming it based on the extension and position of the tumor [14].
Approach Selection
The main approaches to the skull base are divided into anterior (transbasal, transsphenoidal, transoral, and EEA), anterolateral (pterional and orbitozygomatic), lateral (subtemporal and anterior petrosal), and posterolateral approaches (posterior petrosal, suboccipital retrosigmoid, and transcondylar). In the past decades, the microsurgical anterior approaches were gradually replaced by the expanded EEA.
Because of the midline origin of the skull base chordomas, the endoscopic endonasal transclival approach is frequently the first and best option when defining the surgical route . As a general rule, a second approach should always be considered in chordomas with lateral extension. Exceptions for not choosing the EEA as the first surgical route are inability to resolve the patient’s main neurologic signs and symptoms through the EEA, need for cranio-cervical junction stabilization/fusion , and impossibility to adequately reconstruct the resulting skull base defect.
Chondrosarcomas tend to have a paramedian origin at petroclival synchondrosis, and the approach selection is based on tumor extension (midline vs. lateral vs. superior) and patient’s symptoms. Since the majority of these tumors start on the petroclival synchondrosis , there is a major advantage on using the EEA route to reach these tumors primarily. The midline location of the tumor extension further facilitates an EEA. The lateral extensions of these tumors are followed from a midline to lateral approach behind the ICA and frequently can be completely resected via EEA (Fig. 6.4).
The first and the best surgical approach for chondrosarcomas is the one that will provide maximal tumor resection and/or improvement of the patient’s symptoms. Most of the time, the EEA is the initial and ideal surgical approach that may be combined to a lateral or posterolateral approach when lateral residual tumor cannot be reached.
The Role of EEA
Endoscopic endonasal approaches provide the most direct access to the ventral skull base. Chondrosarcomas (usually originated at the petroclival region) and chordomas (originated at midline clivus) tend to displace neurovascular elements laterally, superiorly, and posteriorly. For that reason we advocate the use of EEA as the initial surgical corridor. In a single procedure, EEA allows access to multiple skull base compartments avoiding extensive retraction of neurovascular structures. It also allows extensive drilling of the clivus, sphenoid bone, and petrous portions of the temporal bone, which are frequently invaded by tumor.
For lesions located in the upper petroclival region, cavernous sinus, and middle cranial fossa, the transsphenoidal approach with removal of the sphenoid and temporal bony encasement is indicated. Tumors extending in the middle third of the clivus can be approached through a transsphenoidal approach associated to clivectomy and petrosectomy. Lesions in the lower clivus and infratemporal fossa extension require a transpterygoid approach.
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