Sella and Beyond: Approaches to the Clivus and Posterior Fossa, Petrous Apex, and Cavernous Sinus
Summary
The addition of the endoscope to endonasal approaches has allowed access rostrally to the entire clivus and laterally out to the cavernous sinus, petrous apex, and beyond. This requires a different understanding of the relevant anatomy from a unique perspective. This chapter will discuss this anatomy, associated approaches, and cases with emphasis on preservation of standard microsurgical dissection techniques.
Introduction
Endoscopic endonasal approaches (EEAs) are increasingly accepted for addressing sellar and suprasellar pathologies. One of the greatest advantages of the endoscopic approach is in accessing regions lateral and inferior to the pituitary gland. The maneuverability of the rod lens endoscope within the sinuses is much greater than any speculum. This allows access to the nasopharynx and paranasal sinuses and associated skull base regions without the need for facial incisions. Classification of endonasal approaches to the ventral skull base is shown in Table 39.1
Terminology: Definition of Parasellar Areas
The parasellar area is lateral to the pituitary gland/sella and includes the contents of the cavernous sinus, both anterior and posterior to the cavernous or parasellar segment of the internal carotid artery (ICA). Superiorly and laterally, the parasellar area is bounded by the orbital apex and optic nerve. The contents of the lateral cavernous sinus include cranial nerves (CN) III, IV, V1, and VI (see Figs. 38.2b and 38.10b ).
Inferiorly, the sphenoid cavity immediately below the sella is the clival recess. This roughly corresponds to the middle third of the clivus and extends from the floor of the sella to the floor of the sphenoid sinus. The middle third of the clivus is bounded laterally by the paraclival segment of the ICA. The petrous apex is deep to the paraclival ICA, and CN VI courses superolaterally through the Dorello canal, which lies approximately halfway between the anterior genu of the petrous ICA and the parasellar ICA. Posteriorly lie the basilar artery and pons.
The superior third of the clivus is accessible posterior to the pituitary gland and extends from the posterior clinoids to just below the floor of the sella. Corresponding neurovascular structures for this segment of the clivus include the carotid siphon superficially and laterally, CN III, basilar apex, the posterior cerebral, superior cerebral, and posterior communicating arteries, and the mammillary bodies, all of which lie just deep to the dorsum sellae and posterior clinoids.
The lower third of the clivus extends just below the floor of the sphenoid to the foramen magnum. Laterally, the parapharyngeal ICAs, just below the petrous ICA, are not a limitation as long as they are not ectatic. Deep to the fossa of Rosenmüller, the medial jugular tubercle and medial occipital condyle are separated by the hypoglossal canal and nerve.
Principles of Endoscopic Endonasal Surgery of the Skull Base
Endonasal approaches have been around since before Cushing and Hirsch began using them for sellar and suprasellar tumor removal in the early 1900s.1–3 Cushing, however, abandoned the approach largely due to the poor visualization provided by a headlight applied through a speculum. Dott′s addition of a lighted speculum to the approach was a significant improvement,4 but it was not until Hardy added the operative microscope that the transsphenoidal approach became widely accepted.5,6 This was then extended to the suprasellar space by Laws, Maira, Fahlbusch, Honegger, and coworkers.7–10 The microscopic approach was further expanded by Fatemi et al via modifications of the nasal speculum.11 However, significant limitations remained anteriorly, inferiorly, and especially laterally due to the inherent physical limitations of the microscope and speculum. It was not until the introduction of the endoscope to the transsphenoidal approach, mainly through the efforts of otolaryngologists comfortable with its use with functional endoscopic sinus surgery (FESS), that the approach was extended to nearly all areas of the ventral skull base.
To take full advantage of the endoscope, exposure must be widened beyond that needed for traditional transsphenoidal approaches. This allows the endoscope to move freely within the paranasal sinuses without becoming contaminated with blood or secretions. The endoscope allows close visualization that provides a level of detail not offered by the microscope from afar. This also opens up access to regions not directly in line with the initial approach, thus allowing the resection of tumors that extend beyond the initial skull base exposure. In addition, to safely work beyond the confines of the sphenoid and well into the cranial base, exposure must be widened to allow for the application of standard microsurgical techniques of dissection.
The overriding principle of endonasal skull base surgery is to choose the surgical approach that provides optimal exposure with the least manipulation of neural and vascular tissues. The endonasal approaches work within the confines of the ICA, vertebrobasilar vessels, and cranial nerves and avoid retraction of brain tissue.
Classification of Skull Base Approaches
Key anatomical structures form the basis for the surgical approaches and delimit the boundaries of the surgical modules. The sella is at the crossroads for endonasal skull base approaches. In the sagittal plane, endonasal approaches extend superiorly to the suprasellar area (transplanum approach). Inferiorly, the transclival approach provides access to the middle and lower thirds of the clivus. The pituitary gland can be transposed superiorly to provide access to the upper third of the clivus and the posterior clinoids (pituitary transposition). In the coronal plane, the cavernous sinus may be approached medial (medial transcavernous approach) or lateral (lateral transcavernous approach) to the parasellar ICA. The petrous apex is accessible medial and deep to the paraclival ICA (medial petrous apex approach). Lateral to the paraclival ICA, the suprapetrous approach provides access to Meckel cave and the middle cranial fossa.
Operative Preparation
Patients are evaluated radiographically with both computed tomography (CT) angiography and magnetic resonance imaging (MRI) for parasellar pathologies. This facilitates a differential diagnosis and delineates the relationship of the pathology to key anatomical structures, such as the ICA. Scans are obtained using a skull base protocol compatible with image guidance.
All surgeries are performed by a team consisting of a neurosurgeon and an otolaryngologist/head and neck surgeon. This takes advantage of the specific skill sets and anatomical knowledge of both subspecialties while providing dynamic endoscopy and bimanual dissection.
Note
The team concept is critical in the development and successful application of EEAs.
Patients are placed supine on the operating table with their head usually in three-pin fixation to ensure lack of intraoperative movement during critical dissections and assist with accurate image guidance. The nose is decongested with pledgets soaked in oxymetazoline (0.5%), and the midface is prepped with Betadine solution. Antiseptic solutions are not used intranasally. The otolaryngologist begins by performing an initial inspection of the nasal cavity, followed by elevation of a nasal septal flap when indicated (see the discussion on reconstruction below). The paranasal sinuses (sphenoid and/or maxillary) are then opened widely as needed.
Surgical Approaches
Transclival Approach
Indications
The EEA is applicable to midline pathologies that displace neurovascular structures laterally. Tumors such as chordomas are particularly well suited for this midline approach due to their central location and infiltration of bone. Other osseous and dural tumors, such as chondrosarcomas and meningiomas, often involve the midclivus, as well as other parasellar regions. In addition, intradural pathologies, such as neurenteric cysts, can be drained via EEA when symptomatic.
Many tumors in the midclivus cause abducens nerve palsies. As a result, it is important to fully evaluate patients preoperatively both clinically and radiographically to determine the degree of involvement.
Relevant Anatomy
The midclivus is bounded by the paraclival ICA. In addition, the abducens nerve enters the Dorello canal at the midpoint of the vertical/paraclival ICA ( Fig. 39.1 ). Technically, the uppermost point of the clivus is the dorsum sella. However, the pituitary gland creates a natural limitation that needs to be displaced to access this rostral boundary.
Technique
The EEA to the middle and lower clivus is initiated with a wide sphenoidotomy extending from the planum superiorly to the floor of the sphenoid sinus inferiorly (see Video 67, Cerebellopontine Angle Epidermoid, Transclival Approach ). If a nasal septal flap is needed for reconstruction, the vascular pedicle on one side must be preserved (for a more extensive discussion of reconstruction of the skull base, see Chapter 41). Laterally, the sphenoidotomy extends to the lateral wall of the sphenoid sinus and provides access beyond the course of the ICA to the middle cranial fossa. The lateral recess between the vidian (pterygoid) canal and foramen rotundum is exposed with preservation of the vidian nerve. Inferiorly, the rostrum of the sphenoid should be removed completely to connect the nasopharynx and clival recess.
After exposure, all of the bone between the paraclival and parasellar ICAs can be removed with a high-speed drill with a hybrid cutting/diamond drill bit. The bony removal can be extended as needed all the way down to the foramen magnum by resection of the nasopharyngeal mucosa and basopharyngeal fascia/musculature medial to the eustachian tubes. The lateral exposure can be extended by removing the bone over the vertical/paraclival ICA. This allows the ICA to be displaced, thereby providing access to the distal petrous apex. If drilling posterior to the paraclival ICA is performed, great care is necessary to avoid injury to the deep surface of the artery.
Tips and Tricks
Care must be taken to constantly visualize the shaft of the drill when working near or behind the ICA to avoid inadvertent injury with the spinning shaft.
One of the greatest difficulties when addressing pathology with a transclival approach is dealing with copious bleeding from the basilar venous plexus. This usually requires multiple packings with morselized Gelfoam during the bony and dural resection.
Case Example
Figure 39.2 shows the transclival approach for a clival chordoma with intradural invasion. In this case, a complete resection was done via EEA. A vascularized nasal septal flap was used for reconstruction.
Risks/Complications
Potential complications of the extradural transclival approach are injuries to the ICA and CN VI. The risk of injury to the ICA is increased if the bone covering the paraclival segment of the ICA is removed to improve lateral access.
In addition, this lateral extension carries the risk of damage to the abducens nerve as it exits the dura in the Dorello canal at approximately the midpoint of the paraclival ICA. Intradural work at the level of the midclivus is associated with a risk of injury to the basilar artery and, laterally, the abducens nerve.
Pituitary Transposition (Superior Clivus)
Indications
The most rostral clivus is made up of the sella, bounded by the dorsum sellae and posterior clinoid processes. Accessing this via an endonasal approach (see Video 62, Endoscopic Removal of Retrochiasmatic Craniopharyngioma, Pituitary Transposition ) requires mobilization of the pituitary gland, a concept first described by Kassam et al.12 This can be done either intra- or extradurally. Intradural tumors, such as some craniopharyngiomas and granular cell tumors, may require varying degrees of gland mobilization to safely access tumors that originate on the posterior infundibulum. Most craniopharyngiomas are accessible through a suprasellar approach as they originate from the infundibulum and displace it posteriorly, laterally, or circumferentially. However, some craniopharyngiomas and other rare suprasellar tumors (e.g., granular cell tumors) displace the stalk anteriorly. A suprasellar approach to such a tumor would require splitting the stalk. In addition, if the tumor extends into the perimesencephalic or prepontine cisterns, adequate visualization and control of the vascular structures, such as the posterior cerebral arteries, can only be achieved by elevating the gland.
Extradural tumors, such as chordomas and chondrosarcomas, usually originate in the midclivus or petroclival synchondrosis. As a result, they frequently extend to the dorsum sellae and posterior clinoids. If the dura of the sella is not involved, an extradural pituitary transposition will give adequate access to the involved bone and/or dura.
Relevant Anatomy
The sella and pituitary are bounded by the dorsum sellae and posterior clinoids posteriorly ( Fig. 39.3 ), the cavernous sinus laterally, the sellar floor and inferior intercavernous sinus inferiorly, and the diaphragma and superior intercavernous sinus superiorly. The gland is secured in the sella by multiple fibrous connections with the medial cavernous and dorsum dura. In addition, the inferior hypophysial arteries feed portions of the anterior gland from the parasellar ICA. Finally, the infundibulum pierces the diaphragma through a small aperture that must be opened to allow the stalk to be mobilized.
Technique
An extradural pituitary transposition is usually not a full mobilization of the gland from the sella. Rather, it is the elevation of the dura of the sellar floor and removal of the bony sellar floor with possible ligation of the inferior intercavernous sinus to allow greater mobilization. These maneuvers allow access to the dorsum sellae and even the posterior clinoids. The main difficulties with this are controlling the venous plexus in the dorsum sellae, inferior intercavernous sinus, and medial cavernous sinus and maintaining visualization. The venous bleeding is managed with morselized Gelfoam packing. The working area is small without complete transposition of the gland and may require an angled endoscope to maintain a view. The inferior-to-superior view provided by the 45-degree endoscope when positioned inferiorly in the nostril (the 6ʺo clock position) greatly improves visibility for effective surgical dissection.
Once exposed, the dorsum sellae can be removed with a combination of high-speed drill and Kerrison rongeur. The bone is often thin and may require very little drilling. Once the dorsum is removed, the posterior clinoids remain superolaterally. They can be dissected free from their dural attachments. This must be done with care, as these dural attachments include the interclinoidal and posterior petroclinoidal ligaments, which are continuous anteriorly with the distal and proximal dural rings around the ICA. Rarely, these rings can be calcified, making complete removal of the posterior clinoid impossible.
A full, intradural pituitary transposition requires detachment of the gland from its dural attachments in the sella. The sellar dura is opened widely and the superior intercavernous sinus coagulated and split. The diaphragma is then incised to the aperture through which the infundibulum enters the sella. Care must be taken to preserve the superior hypophysial arteries that run with the stalk. Loss of this blood supply results in hypopituitarism, especially because the inferior hypophysial arteries (IHAs) will be sacrificed.
Once the stalk is freed, the lateral gland must be dissected free from its dural attachments to the medial cavernous wall. Once this plane is established, the dissection is relatively simple. However, the IHAs must be identified, coagulated, and cut so that they are not accidentally avulsed during this dissection. Once the lateral dural attachments are released, the gland can be lifted out of the sella with relative ease. There are still usually a few superior dorsal attachments that must be released for full transposition. The gland can be placed in the suprasellar space or against the planum dura and held in place with fibrin glue if needed.
After the gland has been completely mobilized, full access to the dorsum sellae, posterior clinoids, and subsequently perimesencephalic cistern and prepontine cistern is achieved. Depending on the pathology and its extension, a complete transposition can be performed, or the gland can be partially disconnected and retracted laterally to give access to part of the dorsum and one posterior clinoid. Leaving the connections on one side may lead to greater preservation of gland function, although this has never been evaluated. Complete mobilization allows for unparalleled access to the posterior infundibulum and any associated pathology.
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