11 Endoscopic Endonasal Transclival Approach to the Posterior Fossa
Jose Luis Porras, Jr., Debraj Mukherjee, Carl H. Snyderman, Eric W. Wang, and Paul A. Gardner
Abstract
The endoscopic endonasal transclival approach (EETCA) facilitates direct access to lesions of the posterior fossa with no cerebral retraction and minimal neurovascular manipulation. Through the clivus, one may access a range of structures including the basilar apex, floor of the third ventricle, ventral pons, ventral medulla, and the vertebral arteries. Although this approach is used most commonly for extradural midline or paramedian lesions including chordoma and chondrosarcoma, it may also be employed for intradural lesions such as meningioma. With careful preoperative planning that includes high-resolution imaging, lesion resection may be achieved either with or without an adjunctive approach, depending upon its extent beyond the confines of the internal carotid artery (ICA) and lower cranial nerve boundaries. Resultant skull base defects are typically large, and can feature high-flow cerebrospinal fluid (CSF) leaks, and therefore require careful skull base reconstruction emphasizing use of a vascular flap and multilayer reconstruction as well as adjuvant CSF diversion.
Keywords: Keywords: endoscopic endonasal approach, transclival, posterior fossa, chordoma, chondrosarcoma, meningioma
11.1 Introduction
The endoscopic endonasal transclival approach (EETCA) facilitates direct access to lesions of the posterior fossa while minimizing cerebral retraction and neurovascular structure manipulation (Fig. 11.1).1 Although this approach is used most commonly for extradural midline lesions, it may also be employed for intradural lesions such as meningioma.
Fig. 11.1 A parasagittal view of a cadaver dissection showing the region of clival access via an endoscopic endonasal transclival approach (dashed lines).
11.1.1 Extradural Lesions
The most commonly encountered extradural clival tumors include chordoma and chondrosarcoma. Chordomas are primary bone tumors thought to arise from notochord remnants. They occur more commonly in men with a peak incidence in the fourth and fifth decades of life. Chordomas rarely metastasize but do exhibit aggressive local behavior and have high recurrence rates.2 Chondrosarcomas are primary bone tumors originating from chondroid cells of the axial and appendicular skeleton with 32% arising from the clivus.3
Recurrence rates for both chordoma and chondrosarcoma are correlated to the extent of surgical resection; therefore, the surgical approach should maximize odds of gross total resection while also minimizing associated morbidity. These tumors most commonly originate from the midline or paramedian regions (petroclival junction) making the endoscopic endonasal approach (EEA) the most direct route for access. Even chondrosarcomas, which may extend laterally toward the internal carotid artery (ICA), may still be resected completely via an EEA.1 Given the bone origin of these tumors, much of the tumor resection can be completed during the initial transclival approach, which minimizes risk of cerebrospinal fluid (CSF) leak.4 Other extradural lesions including cholesterol granulomas have been resected successfully via the transclival approach.5 Adjuncts to the EETCA, such as the contralateral transmaxillary (CTM) approach, can dramatically increase the degree of resection when tumors extend into the petrous apex.6 , 7 The EETCA can be used for nasopharyngeal carcinomas with clival involvement, and is also useful for biopsy of radiographically indeterminate lesions (fibrous dysplasia, plasmacytoma, aneurysmal bone cyst, clival extension of adenoma, etc.), though every attempt should be made to understand the nature of the lesion so that appropriate resection can be planned.
11.1.2 Intradural Lesions
The most common intradural tumor resected by the EETCA is meningioma. Those most amenable to resection by the transclival approach arise medial to cranial nerves (CNs) III, VI, and XII.1 Intradural epidermoid or dermoid cysts can also be resected by this approach; however, concern for infection including abscess formation may limit this application in cases where gross total resection cannot be achieved. Vestibular schwannomas are generally poor candidates as the facial nerve is most commonly located anterior to the tumor, hence violating the basic tenet of the approach.1 Other lesions addressed via the transclival approach include rare posterior circulation aneurysms (especially mid-basilar or adjacent), brainstem gliomas, neuroenteric cysts, and brainstem cavernous malformations.8 , 9 , 10 , 11
11.2 Anatomy
The nasopharynx is separated from the posterior cranial fossa by the clivus. The clivus is comprised of the basisphenoid (the posterior portion of the sphenoid body) and the basiocciput (the basilar portion of the occipital bone). The clivus may also be conceptualized as being divided into anatomic thirds (Fig. 11.2). The upper clivus is formed by the basisphenoid bone and includes the dorsum sella. The middle clivus is the rostral portion of the basiocciput and is located above the line connecting caudal end of the petroclival fissures. The lower clivus is formed by the caudal aspect of the basiocciput.4 These are traditionally divided by the open approach used for their access, namely, orbitozygomatic, transpetrosal, and far lateral, respectively. However, with an EETCA, the entire clivus can be accessed, making it extremely versatile (Fig. 11.2).
The upper two-thirds of the clivus faces the pons intracranially and is concave. The extracranial surface gives rise to the pharyngeal tubercle at the junction of the middle and lower clivus. The petroclival fissure separates the upper and middle clivus from the petrous portion of the temporal bone on each side.4
The basilar venous plexus is located on the inner surface of the clivus between endosteal and visceral/meningeal layers of the dura. This plexus interconnects the cavernous and superior petrosal sinuses superolaterally, the inferior petrosal sinuses laterally, and the marginal sinus and vertebral venous plexuses inferiorly.4 These epidural plexus drainage pathways must be understood to achieve hemostasis, and it is also an important space for spread of tumors like chordoma.
Several CNs may be encountered during an EETCA, the most relevant being the abducens nerve (CN VI) (Fig. 11.3). This nerve reliably originates from the ventral brainstem at the vertebrobasilar junction and has an oblique course laterally where it creates the lateral boundary of the mid clivus. The neural lateral boundaries include CNs III, VI, and XII at the super, middle, and inferior clivus, respectively. Petroclival meningiomas often displace CNs V, VII, VIII, IX, X, and XI posteriorly. Displacement and involvement of CN VI is variable, but it may be medial or even engulfed by the tumor, which adds difficulty to the surgical approach. When a meningioma originates superiorly or inferiorly, it may displace CN VI inferiorly or superiorly, respectively, making for a safer approach with a more reliable course.4 Should CN VI be found to be coursing through the tumor, a combined endoscopic and open approach should be considered.1 At the level of the foramen magnum, anteriorly arising lesions will generally displace CN laterally and posteriorly. Those arising from the lateral borders of the foramen magnum may displace CN anteriorly, in which case consideration should be given to an open approach such as a far or extreme lateral. Even with a paramedian origin (e.g., jugular tubercle), if the CNs are displaced posteriorly, an EEA is still a safe approach.1
