28.2.1 Endoscopic Endonasal Transclival Approach

The front door to approach the posterior fossa through an endoscopic endonasal route is represented by the clivus. For surgical purposes, it can be schematically divided into three portions: upper, middle, and lower ( Fig. 28.1 ). The upper third of the clivus is usually approached through the sphenoid sinus. It is composed by the posterior clinoids and the dorsum sellae, medially to the parasellar segment of the carotid artery ( Fig. 28.2a ). The main landmarks of the route to the upper clivus are the same as those routinely adopted for a midline endoscopic transsphenoidal approach, i.e., the tail of the superior turbinate to identify the sphenoethmoid recess and the opening of the sphenoid sinus, the sellar bulge, and the two optic–carotid recesses, which are formed by the optic nerves and parasellar carotid artery protuberances, to localize the course of the carotid artery ( Fig. 28.2b ). The intradural space corresponding to the upper clivus is represented by the interpeduncular cistern, occupied by the tip of basilar artery, posterior communicating artery, superior cerebellar artery, and the third cranial nerve (CN III) in its cisternal portion ( Fig. 28.2c ). To expose the middle clivus it is necessary to extend the previous route, drilling off the floor of the sphenoid sinus in the space comprised by the two internal carotid arteries (ICA) ( Fig. 28.3b ). An useful landmark for these vessels is represented by the vidian nerve, which points out the genu between the petrosal and paraclival arteries and then allows identifying a safe area between the two ICAs ( Fig. 28.4 ). Intradurally, the middle clivus corresponds to the pons, the basilar trunk, anterior inferior cerebellar arteries, and the cisternal portion of CN VI ( Fig. 28.3c ). Finally, the inferior third of the clivus can be identified detaching the rhinopharyngeal mucosa and muscles (longus capitis and rectus capitis anterior), below the floor of the sphenoidal sinus ( Fig. 28.3a ). This route gives access to the lower part of the clivus and the craniovertebral junction till, usually, the body of C2. Indeed the lowest point that can be reached is depicted by an imaginary line passing through the anterior border of nasal bone and the inferior border of the hard palate.18 The surgical landmarks are represented by the inferior turbinate and posteriorly by the choana, while the eustachian tube is of crucial relevance to point out the parapharyngeal tract of internal carotid artery. Thus, the detachment of the rhinopharyngeal mucosa and the parapharyngeal muscles should be performed medially to the tube to avoid injuries to the vessel. This approach gives access to a trapezoid space, wider superiorly and limited by the condyles inferiorly. The main anatomic landmark of the condylar area is represented by the hypoglossal canal, which divides the inferolateral portion of clivus in two segments:

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  The superior or tubercular compartment represents the ventral portion of jugular tuberculum. Indeed, its lateral limit is represented by the medial portion of the jugular foramen. Intradurally, it corresponds to the tracts of CN IX, CN X, and CN XI toward the jugular foramen and medially to the vertebral artery.

  
  
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  The condylar compartment is composed by the condyle itself. Inferiorly limited by the articular surface and laterally by the CN XII. Intradurally, it corresponds to first segment after the piercing of the dura of the vertebral artery, which is crossed anteriorly by the cisternal segment of the hypoglossal nerve.

To perform the endoscopic endonasal approach to the clivus, we prefer that the patient is placed in a semisitting position, with the thorax slightly elevated on the operating table. An orotracheal intubation is needed and the laryngopharynx is packed with gauzes to avoid blood and fluid leakage. Routinely, we use a neuronavigation system (StealthStation S7 MEDTRONIC), based on a computed tomography angiogram (CTA), processed through the StealthMerge software (MEDTRONIC).

The surgical approach should be selected depending on the localization of the lesion in the upper, middle, or lower clivus and on its lateral extension. For cases limited to the upper clivus, we start displacing the middle and inferior turbinates laterally. When necessary, the tail of the latter is resected to widen the surgical corridor. Surgery is performed through both nostrils and with a four-hand technique, after detaching of the vomer from its insertion and resecting the posterior aspect of the septum. Afterward, we perform an anterior sphenoidectomy, and the posterior wall of the sphenoid sinus is exposed ( Fig. 28.5a ). Except in presellar or conchal types of sphenoid sinus, for which a neuronavigation system is mandatory, the upper clivus can be easily identified and accessed through its anatomic landmarks in the posterior wall of the sphenoidal sinus ( Fig. 28.6a ). The dorsum sellae lies behind the pituitary gland; thus, the approach to this superior part of the upper clivus might require a pituitary gland transposition. The latter consists in a superior hypophysopexy,19 which permits to progressively expose the floor and the dorsum of the pituitary fossa. Chordomas can usually be identified after drilling the upper clivus as much as necessary, and eventually the dorsum sellae ( Fig. 28.7 ), while intradural neoplasias require the opening of the dura wall. For extradural tumors in this segment of the clivus, the lateral limit of this approach is represented by the paraclival carotid artery, while for intradural extension this limit is given by a sagittal plane passing at the level of the CN III, which cannot be trespassed to avoid injury to the nerve. To extend this approach more laterally, an ethmoid- pterygoidsphenoidal route is necessary. For this, an ethmoidectomy, followed by a medial maxillectomy and drilling of the tip of the vertical process of the palatine bone and of the medial pterygoid process, is performed. This route allows facing frontally the cavernous sinus and reaching its compartments lateral to the carotid artery. We adopt this extension for paramedian tumors with an extensive cavernous sinus involvement, in particular when the tumor occupies the anteroinferior and/or lateral compartments or encases the internal carotid artery. The lateral limit of this extension is given by the lateral wall of cavernous sinus, where CN III, CN IV, and CN V1 run. To expose the middle clivus, we perform an inferior extension of the approach to the upper clivus by drilling off the sellar floor between the paraclival carotid ( Fig. 28.5b ). Anatomic landmarks, eco-Doppler, and neuronavigation system are routinely adopted to avoid injuries to the internal carotid artery ( Fig. 28.6b, c ). Once the dural plane has been exposed, its opening to address intradural tumors should be performed carefully to avoid injuries to the CN VI ( Fig. 28.4c ). This nerve pierces the dura medially to the carotid artery above the level of the vertebrobasilar junction. Many techniques have been proposed to avoid damages to the CN VI, including intraoperative electrophysiologic stimulation of the dura to recognize its location, as well as eco-Doppler and/or neuronavigation to localize the vertebrobasilar artery and thus start opening the dura inferiorly. For extradural lesions, the main limitation of this approach is given by the paraclival carotid artery, while for intradural tumors, a sagittal plane passing through CN VI represents the lateral limit of this route. Often, tumors of the middle clivus can spread to the surrounding regions, such as the pterygopalatine of the infratemporal fossa. To follow the tumors extending to these areas, a transmaxillo-pterygoid approach is useful. It requires a medial maxillectomy (or an anterior and medial maxillectomy according to Denker for tumors located more laterally), followed by the opening of the posterior wall of maxillary sinus and drilling of the vertical process of the palatine bone and of the pterygoid process. In this approach, particular care should be paid to the management of the internal maxillary artery, which usually requires a clipping, to avoid intraoperative or postoperative bleeding. The posterior limit of this approach is given by V3, which is the landmark for the petrosal portion of the carotid artery, running behind the nerve. To reach the lower clivus, a further inferior extension of the middle clivus approach is necessary. Sometimes, for lesions located far inferiorly, we prefer to retract the soft palate with two thin rubber nasogastric probes inserted through the nose and extracted through the mouth. This maneuver is performed at the beginning of the surgery.

Many techniques to skeletonizing the rhinopharynx mucosa and dissecting the muscular planes have been proposed. We prefer an inverted U-shaped flap from one Rosenmüller fossa to the other. Before surgery, an angio-CT scan to study the location of the parapharyngeal carotid artery is of great help and can be recommended. It is not uncommon to observe a midline loop of the carotid, which could represent a major caveat for this surgery. The neuronavigation system and eco-Doppler are valid tools to identify the vessel while skeletonizing the lower clivus ( Fig. 28.6b ). As in the other segments of the clivus, the carotid artery in its parapharyngeal tract represents the main lateral extradural limit of this approach, while for intradural lesions, this limit is represented by the plane passing for the CN IX, X, XI at the jugular tuberculum level and CN XII at the condylar level. To expand this access laterally, some authors propose a transcondylar and/or transjugular tuberculum approach (so-called “far medial”).11,12 It consists in the removal of condylar and tubercular regions, respectively. The transcondylar approach allows maximizing the exposure of the foramen magnum area. The transjugular–tuberculum approach allows obtaining an exposure of the medial border of the jugular foramen. Once the approach has been allowed to face the tumor as frontally and straightforward as possible, its removal is performed through the usual microsurgical technique: the lesion is dissected bimanually from the surrounding anatomic planes (we prefer to fix the endoscope on the holder to allow both surgeons to operate with four hands), and avoiding tractions, it is progressively debulked and finally resected ( Fig. 28.7b–d ). When it presents a soft consistency, tumor removal can be progressively performed with suction, following the chordomas’ typical “termite-like” erosion of the clivus ( Fig. 28.7b ). Conversely, for tumors with a hard consistency, curettes and dissectors are useful, and for calcified tumors, a diamond drill can be necessary. Debriders are useful to reduce the tumor mass, but it should be used carefully, after identifying the vessels and nerves in the surgical field. We do not suggest using debriders intradurally to avoid an injury to arteries or nerves. Here, the CUSA (ultrasound) can be used carefully to debulk the tumor.

The relationship of the tumor with the dura is of crucial importance for the surgical strategy. When there is no invasion, the tumor should be dissected by this layer and then resected. In case of extensive dura invasion by a chordoma for example, it is not always possible to resect the tumor completely because of their invasiveness. When the dura is trespassed by the tumor, it is possible to follow its extension, entering in the posterior fossa. If the dural dimple is of small dimension, a bigger opening can be performed to visualize all the tumor extension. Furthermore, the tumor should be carefully dissected from nearby vascular, nervous, and parenchymal structures, avoiding any tractions, which could lead to injuries ( Fig. 28.7c ). Especially for lesions involving or compressing the brainstem, an electrophysiologic monitoring of somatosensory-evoked potentials and motor-evoked potentials is recommended to monitor the brainstem.

Cerebrospinal fluid (CSF) leaks need to be repaired. We usually use mucoperiosteum from middle turbinate and fascia lata, positioned in a multilayer fashion. The fascia lata is placed intracranial intradurally and the mucoperiosteum externally. Abdominal fat is suitable to fill the “dead space” between both, if needed. In case of approaches to the lower clivus, the U-inverted muscular and mucosal flap from the nasopharynx can be repositioned with some stitches to repair the CSF leak. The nasal cavity is filled with Gelfoam and Merocel is kept in place for the first 3 days.