Summary
Endoscopic endonasal surgery (EES) of the skull base has become accepted for the treatment of most ventral skull base lesions. EES is applicable to all patient populations and can be applied to a wide variety of benign and malignant pathologies. Multidisciplinary teamwork is a key feature of EES, and an incremental program for training is proposed. Endonasal approaches can be divided into surgical modules that provide flexibility in tailoring the approach to the patient. They are organized along sagittal and coronal (anterior, middle, and posterior) planes, with the sphenoid sinus at the epicenter. Each module is associated with specific anatomical landmarks. Anatomical limitations of endonasal approaches are established by major neural and vascular structures. The risks of EES are similar to those of transcranial surgery. The most common complication, cerebrospinal fluid leak, has diminished with the use of vascularized flaps (local and regional) for reconstruction.
6 Endoscopic Endonasal Skull Base Surgery
6.1 Principles of Endoscopic Endonasal Surgery
The endonasal corridor is just one of multiple corridors that provide access to the skull base. Although the endoscope has enabled surgeons to accomplish more via the endonasal corridor, endoscopic endonasal surgery (EES) of the skull base is predicated on choosing the best approach for the patient, one that accomplishes the goals of surgery with the least chance of morbidity for the patient. Multiple factors are considered in choosing the best approach: tumor size and extent, location, patient comorbidities, risks, morbidity, reconstruction, duration, prior treatment, experience and training of surgical team, and resources.
For many tumors, the endonasal corridor provides the most direct access without displacement of normal neural and vascular tissues. The golden rule of EES is to avoid crossing the plane of nerves and vessels that surround the ventral skull base. If a tumor is on the other side of a major nerve or vessel, an alternative approach should be considered. For some tumors, a multicorridor approach will be necessary to access all parts of the tumor while minimizing manipulation of neurovascular structures.
Unlike other types of collaborative surgery, EES is true team surgery. For much of the operation, surgeons work concurrently rather than sequentially, with one surgeon driving the endoscope and the other surgeon performing bimanual dissection. The benefits of team surgery include superior visualization, increased efficiency, enhanced problem solving, and modulation of enthusiasm. Although the highest-quality endoscopes currently provide a 2D image, 3D visual cues result from the relative movement of instruments. Dynamic endoscopy facilitates the accurate passage of instruments and optimizes the surgical view without hindering bimanual dissection. An experienced cosurgeon is especially critical in a crisis such as a vascular injury for maintenance of a surgical view and active problem solving.
Bimanual access and dissection is key for performing microsurgical resection. The same techniques as would be used with an “open” surgery must be applied to realize the advantages of EES. These include early devascularization, internal debulking, microsurgical blunt and sharp dissection, and early vascular control. In addition, wide exposure allows for freedom of movement and early bony decompression of critical structures. Oncological principles can be preserved with EES for malignant sinonasal tumors.1 Although en bloc excision is not feasible for many tumors, piecemeal resection does not have an adverse effect on outcomes so long as the final margins are negative. Superior visualization with endoscopy may facilitate complete tumor removal with improved oncological outcomes.
6.2 Indications
EES is indicated for the management of a wide variety of benign and malignant tumors of the nasal cavity, paranasal sinuses, skull base, pituitary, brain and meninges, orbit, and spine in all patient populations, including young pediatric patients.2 Examples of tumor types are provided in the following descriptions of endonasal approaches.
The limitations of EES are determined by multiple factors, including the experience of the surgical team.3 Anatomical constraints include major neural and vascular structures: internal carotid artery (ICA), vertebrobasilar arterial system, and cranial nerves (CNs). Further advances in technology and techniques continue to extend the limits of the endoscopic endonasal approach.
6.3 Classification of Endonasal Approaches
Endoscopic endonasal approaches are divided into surgical modules that can be combined like building blocks to create a tailored surgical approach for each tumor (Table 6.1).4 The classification of endonasal approaches is designed to be intuitive, with orientation in sagittal and coronal anatomical planes. The sphenoid sinus is the starting point for many of these modules, because the sphenoid contains the most critical anatomical structures (carotid arteries and optic nerves), is easily accessible and familiar, and is at the junction of the sagittal and coronal planes.
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6.3.1 Sagittal Plane Modules
The sagittal plane is a midline corridor that extends from the frontal sinus to the craniovertebral junction (Fig. 6.1).
Transfrontal Approach
The transfrontal approach provides access to the frontal sinus, the posterior table of the frontal sinus, and the crista galli. A Draf 3 frontal sinusotomy is performed to remove the floor of the frontal sinus. The opening is limited posteriorly by the crista galli in the midline, which extends to the anterior cribriform plates. With drilling of the bone anteriorly at the level of the nasion and the use of angled endoscopes and instruments, the posterior table of the frontal sinus is accessible. Lateral access is enhanced by using a binarial approach with creation of a superior septal window.
Common pathologies for this approach include nasal dermoids, fibro-osseous lesions such as osteoma and juvenile ossifying fibroma, and superior extension of sinonasal tumors such as inverted papilloma (Fig. 6.2). Tumor involvement of the anterior table or lateral recesses of the frontal sinus may necessitate an open approach. A transfrontal approach provides the anterior limit of resection for tumors that involve the cribriform plate (esthesioneuroblastoma).
Transcribriform Approach
The transcribriform approach extends from the frontal sinus anteriorly to the planum sphenoidale posteriorly and is accessed through the ethmoid sinuses and bounded by the orbits laterally. The transcribriform approach provides access to the anterior cranial fossa for tumors that arise intracranially (olfactory groove meningioma), tumors that arise from the olfactory sulcus (hamartoma, olfactory schwannoma, and olfactory neuroblastoma), and sinonasal cancers that extend to the anterior cranial base (Fig. 6.3).
The endonasal version of the classic craniofacial resection combines transfrontal, transcribriform, and transplanum approaches and extends from the frontal sinus anteriorly to the planum sphenoidale posteriorly, to the medial walls of the orbit laterally, and to the nasal septum inferiorly.5 On the lateral aspect of the tumor, the medial wall of the orbit (lamina papyracea) and periorbita may be resected to provide a margin. The cribriform plates are bounded laterally by the anterior and posterior ethmoidal arteries. These may be cauterized and transected on the orbital side of the skull base to devascularize tumors and provide access to the orbital roof.
Transplanum/Transtuberculum (Suprasellar) Approach
The transplanum approach provides access to the suprasellar area through the roof of the sphenoid sinus and is limited by the optic canals posterolaterally and the cribriform plates anteriorly. It is often combined with a transsellar approach for the removal of extrasellar pituitary adenomas, craniopharyngiomas, and tuberculum meningiomas (Fig. 6.4).6 , 7 The bone of the sella, tuberculum, and planum is removed as needed by thinning the bone and dissecting it from the underlying dura using dissectors or a Kerrison rongeur. Between the optic canals and parasellar ICAs, access is narrowed but can be improved by removing the bone of the medial opticocarotid recess (mOCR) and even laterally to decompress the medial optic canals. Removal of the tuberculum exposes the superior intercavernous sinus, which must be cauterized and ligated to provide maximal exposure of the pituitary stalk.
A transplanum approach is also used for decompression of the optic canal. The optic canal is separated from the cavernous ICA and superior orbital fissure by the lateral optic–carotid recess (lOCR). The lOCR represents pneumatization of the optic strut and connects with the anterior clinoid. The optic canal may be decompressed in an anterograde or retrograde direction. With an anterograde approach, a medial orbital decompression is followed posteriorly to the orbital apex and optic canal. With a retrograde approach, the tuberculum is drilled at the sphenoid limbus and removal of bone continues laterally to the optic canal. The ophthalmic artery is inferior to the optic nerve and is at risk for injury intradurally. Drilling of bone superior to the optic canal leads to the anterior clinoid, which cannot be well accessed endonasally. Indications for optic nerve decompression include tuberculum meningiomas that extend laterally to the optic canal, fibrous dysplasia, trauma, and tumors in the orbital apex. Tumors that extend superolateral to the optic nerve or that involve the anterior clinoid may require an alternate or combined approach.
Transsellar Approach
The transsellar approach provides access to tumors of the pituitary gland (pituitary adenomas, Rathke’s cleft cysts; Fig. 6.5).8 The sella is bounded by the cavernous sinus in all directions and by the cavernous segment of the ICA laterally. Bone overlying the cavernous ICA may be thinned and removed to allow lateral displacement of the ICA and access to the anterior–inferior, superior, and posterior compartments of the cavernous sinus for extrasellar pituitary adenomas.9
Transclival Approach
The clivus extends from the posterior clinoids to the foramen magnum. Transclival approaches are divided into three segments (superior, middle, and inferior) (Fig. 6.6).
Superior (Pituitary Transposition)
The superior clivus comprises the dorsum sella and extends from the posterior clinoids to the floor of the sella. Access to the superior clivus requires transposition of the pituitary gland.10 This is best performed as an interdural dissection through the medial cavernous sinus, leaving the pituitary gland ensheathed in the meningeal layer of dura. The inferior hypophyseal arteries may be sacrificed on one or both sides without loss of pituitary function. Removal of the tuberculum sellae provides additional room for superior displacement of the gland. Care must be taken to avoid damage to the unprotected neurohypophysis, but the gland can be elevated from the fossa with preservation of the superior hypophyseal arterial supply as well as superior and lateral dural venous drainage, thereby preserving function.
The superior clivus is bounded laterally by the parasellar cavernous and clinoidal segments of the ICA. Removal of the dorsum sellae and posterior clinoid processes and subsequent dural opening provides access to the basilar apex. Cranial nerve III (oculomotor) is located between the posterior cerebral artery and the superior cerebellar artery and forms the lateral anatomical boundary.
A pituitary transposition may be necessary for the removal of craniopharyngiomas and meningiomas that arise posterior to the dorsum sellae, infundibulum, or gland. A superior transclival approach provides access to the posterior clinoids for infiltrative tumors such as chordomas or extrasellar pituitary adenomas and intradural access for tumors such as petroclival meningiomas (Fig. 6.7).
Middle
The middle clivus extends from the floor of the sella to the floor of the sphenoid sinus (clival recess) and is bounded by the pituitary gland superiorly and the paraclival cavernous segments of the ICA laterally. The medial petrous apex is located deep to the paraclival ICA.
Removal of the bone posteriorly and dural opening provides access to the basilar artery. This requires packing of the significant interdural basilar plexus. Cranial nerve VI (abducens) originates from the brainstem at the level of the vertebrobasilar junction inferiorly and courses obliquely deep to the paraclival ICA, where it enters Dorello’s canal. It is at risk of injury with drilling of bone deep to the paraclival ICA in the upper third of the midclivus.
The middle transclival approach is most commonly used for chordomas, meningiomas, and extrasellar pituitary adenomas (Fig. 6.7).11 Chordomas often grow between the periosteal and meningeal layers of the basilar plexus into Dorello’s canal. Nasopharyngeal carcinomas can also extend into the midclivus, which can be drilled for a margin.
Inferior
The inferior clivus extends from the floor of the sphenoid sinus to the foramen magnum and is most often combined with a middle transclival approach for infiltrative tumors such as chordomas and chondrosarcomas, as well as for access to meningiomas of the posterior cranial fossa (Fig. 6.8).12 It can also be combined with a transodontoid approach to access craniocervical junction tumors such as chordomas and foramen magnum meningiomas. Drilling of the inferior clival bone to the underlying dura can provide a margin of resection for nasopharyngeal cancer, for both primary resection and following radiation therapy.
The inferior clivus is bounded laterally by the lacerum segment of the ICA. The vidian nerve within the pterygoid canal is a landmark for localization of the lacerum and petrous ICA. Removal of the bone posteriorly and dural opening exposes the vertebral and anterior spinal arteries. The hypoglossal nerve courses laterally dorsal to the vertebral arteries.
Transodontoid
The transodontoid approach provides access to the craniovertebral junction from the lower clivus to the body of C2. The nasopalatine line (a line tangential to the inferior edge of the nasal bones and posterior edge of the hard palate) roughly demarcates the inferior limit of access to the spine.13 Caudal access can be improved by drilling the posterior aspect of the maxillary crest flush with the hard palate. Laterally, ectatic parapharyngeal ICAs may limit dissection of the retropharyngeal soft tissues, especially at the level of C1. Removal of the soft tissues of the nasopharynx between the Eustachian tubes and from the sphenoid rostrum to the plane of the hard palate exposes the anterior arch of C1, which can be drilled to access the odontoid and body of C2. Removal of bone laterally (lateral mass of C1) is limited so as to avoid instability of the spine and avoid injury to the vertebral arteries. Intradurally, access is limited laterally by the vertebral arteries, and care must be taken to protect the caudal continuation of the anterior spinal artery.
A transodontoid approach is most commonly used for the treatment of degenerative or inflammatory bony compression, basilar invagination, or, rarely, significant pannus with brainstem compression but also provides access for tumors at foramen magnum such as meningiomas and chordomas of the craniovertebral junction (Fig. 6.9).14
6.3.2 Coronal Plane
The coronal plane is divided into three coronal planes corresponding to the cranial fossae (anterior, middle, and posterior; Fig. 6.10). The transpterygoid approach is a prerequisite for many of the modules of the middle and posterior coronal planes.
Anterior Fossa
Supraorbital
Removal of the medial wall of the orbit (lamina papyracea) with sacrifice of the ethmoidal arteries and displacement of the orbital contents (subperiosteal plane) reliably provides access to the orbital roof to the midpoint of the orbit. The most common bony tumors in this region are benign fibro-osseous lesions (Fig. 6.11). Removal of the orbital roof allows extended resection of dural margins for tumors such as meningiomas (dural tail or lateral extension) and olfactory neuroblastomas. Access is limited anterior to the anterior ethmoidal artery. Intradurally, there are no critical neurovascular structures in this location, which is lateral to the fronto-orbital branches and olfactory tract.
Transorbital
Intraconal orbital tumors can be removed using a medial transorbital approach so long as they are inferior and medial to the optic nerve (Fig. 6.12).15 Following removal of the lamina papyracea, the periorbita is incised and the medial and inferior rectus muscles are identified. Dissection between the muscles provides access to tumors that are medial and inferior to the optic nerve without damaging the nerve supply to the extraocular muscles that lie immediately deep/lateral and deep/superior to the muscles. A smaller dissection window is also available between the medial and superior oblique muscles. A simultaneous transconjunctival approach can facilitate identification of the extraocular muscles, retraction of orbital tissues, and anterior tumor dissection.
The transorbital approach is typically reserved for benign tumors such as vascular tumors (hemangioma, cavernoma) and schwannomas. It also provides an avenue for bony decompression, biopsy, or partial (rarely complete) resection of infiltrative neoplasms (pseudotumor, lymphoma, optic glioma) or optic nerve sheath tumors (meningioma, though only in the presence of an already blind eye). Care is usually taken to preserve the ethmoidal arteries when possible in these cases, for they provide important collateral circulation to the optic nerve. The ophthalmic and central retinal arteries generally run lateral and inferior to the optic nerve, respectively.
Middle Fossa
Medial Transcavernous
A medial approach to the cavernous sinus provides access to the medial (superior and posterior) compartments of the cavernous sinus.9 Access is enhanced with removal of bone over the cavernous ICA and lateral displacement of the ICA. Depending on tumor extension and patient anatomy, the anterior compartment can sometimes be partially addressed as well. Pituitary adenomas with extension into the cavernous sinus are best followed from the sella, taking the same route as the tumor (Fig. 6.13). The CNs are invested in the dura of the lateral cavernous sinus wall and are generally protected from injury unless this dura is invaded. Recurrent pituitary adenomas may be isolated in a cavernous sinus compartment. Rarely, other tumors (hemangiomas, meningiomas, chordomas) may occur or extend here.16 Nonadenomas are at much higher risk for resection than softer, less invasive adenomas are.
Medial Petrous Apex
A midclival approach between the floor of the sella and the floor of the sphenoid sinus at the level of the clival recess provides access to the medial petrous apex, which is bounded anteriorly by the petrous and paraclival segments of the ICA and posteriorly by the posterior cranial fossa dura. The medial triangle (Gardner’s triangle) for accessing the petrous apex consists of the window between the paraclival ICA anteriorly, the abducens nerve superiorly, and the petroclival fissure inferiorly. Expansile lesions such as cholesterol granulomas often expand this area, providing easy access, and can even extend into the clival recess (Fig. 6.14).
If additional access is needed, the bone overlying and lateral to the paraclival ICA can be removed to allow lateral displacement of the vessel. Cranial nerve VI (abducens) courses deep to the upper paraclival ICA in Dorello’s canal and is susceptible to injury where it runs between the periosteal and meningeal layers of the dura.
In addition to cholesterol granulomas, a medial petrous apex approach can be used for the treatment of petrous apicitis, biopsy of petrous apex tumors (primary and metastatic), and resection of clival tumors that extend laterally (meningioma, chordoma, chondrosarcoma).17
Related posts:
5 Anesthesia and Intraoperative Monitoring of Patients with Skull Base Tumors
4 Head, Neck, and Neuro-otologic Assessment of Patients with Skull Base Tumors
10 Management of Benign Skull Base Tumors in Neuro-oncology: Systemic Cytotoxic and Targeted Therapies
9 Clinical Applications of Radiotherapy for Skull Base Tumors
8 Radiotherapy for Skull Base Tumors: Principles and Techniques
15 Pediatric Skull Base Surgery
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