Chapter 32 Anteromedial Corridors to the Cranial Nerves
Access to the skull base utilizes four primary approaches: anteromedial, anterolateral, lateral, and posterolateral. The location of the pathology in relation to the cranial nerves (CNs) is the primary determining factor when deciding which approach to utilize. We have previously described the concept of selecting the operative corridor based on the position of the CN.1 In general, pathology of the ventral skull base grows from its basal origin (e.g., meningiomas, chondrosarcomas), often displacing the critical CNs along its dorsal perimeter. In such situations, the traditional lateral approaches provide a trajectory that results in encountering the CNs first, followed by the pathology. The CNs are sensitive to manipulation, and deficits can occur following even the gentlest handling of the nerve. Therefore, the guiding principle in the decision-making process is to avoid crossing the plane of a CN when accessing a tumor or other lesion.
The evolution of minimally invasive techniques has led to a concomitant evolution of minimally invasive approaches to the skull base. The goal of minimally invasive skull base approaches is to minimize surgical morbidity using anatomically-directed corridors. This reduces the need for brain retraction and unnecessary tissue exposure. The endoscopic endonasal approach (EEA) to the skull base provides safe access to the medial anterior, middle, and posterior cranial fossa along both sagittal (median) and coronal (paramedian) planes from an anteromedial trajectory. This median corridor provides the most direct access to the entire ventral skull base, including CNs. The anteromedial approaches to the anterior, middle, and posterior fossa can be grouped into median and paramedian approaches. The median approaches will be limited laterally by critical neurovascular structures, while the paramedian approaches may be limited both medially and laterally. A combination of median and paramedian approaches to the skull base allows access to CNs I–XII.
In the following sections, we describe the common anteromedial corridors to address ventral pathology affecting each of the CNs. We also identify the extradural sinonasal corridor used to access each region. Given that the endoscope requires physical space in the nares, we generally suggest a “cavity and a half” sinonasal corridor. We have previously described this concept, which provides one-half nasal cavity for visualization and manipulation of the endoscope and the remaining one-half nasal cavity and full contralateral nasal cavity (“cavity and a half”) for bimanual dissection.2
32.1 Surgical Steps
32.1.1 Median Approaches to Anterior Cranial Fossa—CN I: Olfactory Nerve
The olfactory nerve and bulb (CN I) can be accessed via an anteromedial transcribriform approach.3 The transcribriform approach is typically combined with the transplanum approach described below. The rostral aspect of the transplanum approach is extended further rostral to the level of the crista galli or the frontal sinus. The attachment of the anterior nasal septum to the skull base is resected. Normally this maneuver would damage the olfactory epithelium at the upper nasal cavity, but it is often performed when olfaction is already compromised. Bilateral ethmoidectomies are performed to expose the medial orbital walls. To gain lateral exposure, the lamina papyracea can be removed. The skull base is then drilled in a rostrocaudal direction starting at the frontoethmoidal recess. The anterior and posterior ethmoidal arteries are identified and transected. The nasal mucosa, olfactory filaments, and ethmoidal artery branches are coagulated and the cribriform plate is removed bilaterally. The crista galli is egg-shelled and fractured. The dura is opened and both olfactory nerves can be seen ( Fig. 32.1 ).
The boundaries of this approach are the frontal sinus anteriorly, the lamina papyracea laterally, and the planum sphenoidale at the level of the posterior ethmoidal arteries posteriorly. A wide bilateral frontal sinus Draf III approach4 provides the half cavity, while bilateral posterior and anterior ethmoidectomies will provide the full cavity for the working corridor. The transcribriform approach and the unique anatomy of the olfactory nerve fibers represent the rare exception to the guiding principle of never crossing the plane of a CN in the approach to a lesion. In this exposure, the nasociliary mucosa and the distal fibers of the olfactory nerve in the cribriform plate are destroyed and olfaction is sacrificed. Therefore, this approach is generally reserved for those conditions in which olfaction has already been lost. We have previously published on the ability to preserve olfaction via a unilateral approach with preservation of the contralateral side, though this is a rare exception.5 The primary indication for the transcribriform approach is a mass that has already created anosmia. This approach offers the ability to minimize retraction and manipulation of the frontal lobe.
32.1.2 Median Approaches to Anterior Cranial Fossa—CN II: Optic Nerve
The optic nerve (CN II) to the optic chiasm can be accessed via the median endonasal transsellar and expanded endonasal transplanum/transtuberculum approaches. The general technique for bilateral exposure using the binasal approach is utilized to gain access to the sphenoid sinus.3 Bilateral sphenoidotomies are completed and widened laterally to the carotid canal, superiorly to the posterior ethmoid sinus, and posteriorly along the floor of the sphenoid to the clivus. With this exposure the sellar floor through the superior intercavernous sinus (SIS) is exposed, and the sellar face, parasellar carotid protuberance, medial opticocarotid recess (mOCR), and bone overlying the SIS are identified. The mOCR is a key anatomic landmark in expanded endoscopic anterior skull base exposures. The superior extent of the carotid protuberance marks the mOCR, specifically, the confluence of the tubercular strut and the middle clinoid (when present). Entry at this level allows for access to the optic canal, which travels obliquely in a posteromedial to anterolateral trajectory. In the transsellar approach, bone removal over the sellar face extends laterally to expose the medial cavernous sinus, and superoinferiorly to expose the superior and inferior cavernous sinus, respectively. This allows downward retraction of the sellar contents and facilitates working space; however, the intrasellar dura mater is opened and an intrasellar dissection is performed only when needed. At the superolateral aspect of the sellae, the optic nerve, internal carotid artery (ICA), and mOCR can be identified.
Extension of the transsellar approach rostrally allows access to the suprasellar cistern and anterior cranial fossa floor via a transplanum/transtuberculum approach without going through the sellae turcica.3 This approach provides access to the optic canals and optic chiasm. Following a general bilateral transsphenoidal approach, the rostral extension begins with wide bilateral posterior ethmoidectomies, extending superiorly to the anterior cranial fossa floor and laterally to the lamina papyracea, which is the medial wall of the orbit. The anterior margin of the exposure is the posterior ethmoidal arteries, which travel in the posterior ethmoidal canals ( Fig. 32.2 ). The posterior ethmoidal canal courses from the lamina papyracea laterally to the fovea ethmoidalis medially. The dissection is limited to the posterior ethmoidal arteries as the anterior margin to avoid injuring the olfactory filaments and creating anosmia. The planum sphenoidale is removed in a caudal to rostral direction with a V-shaped lateral osteotomy to avoid injuring the optic canal ( Fig. 32.3 ). The rostral portion of the sellar floor can be opened to the SIS, and the SIS mobilized to allow access to the suprasellar parachiasmatic cisterns ( Fig. 32.4 ). This approach provides a cranial base opening that extends posteriorly from the junction of the cribriform plate and planum sphenoidale to the clival recess, and to the lamina papyracea bilaterally ( Fig. 32.5 ). The dura mater is opened and the paraclinoid ICA identified as it enters the dura at the level of the mOCR. The ICA can be followed superiorly to identify the optic nerve, which can be followed to the optic chiasm and the contralateral optic nerve. The optic canals at the orbital apex represent the lateral limit of the transplanum/transtuberculum approach.
32.1.3 Paramedian Approaches to Anterior Cranial Fossa—CN II
The optic nerve passes anterolateral through the optic foramen and along the optic canal to enter the orbit, where it can be accessed via the paramedian medial transorbital approach.1,3,6 Following the bilateral sphenoethmoidectomies described above, the medial transorbital approach requires removal of the lamina papyracea and medial optic canal to gain access to the annulus of Zinn. The lamina papyracea is fractured and elevated from the orbital floor to the anterior skull base, and posteriorly to the orbital apex. The bone of the orbital apex to the superior orbital fissure can be removed to expose the intracanalicular optic nerve. To gain access to the intraconal space, the periorbita is opened, and the superior rectus muscle, medial rectus muscle, and/or superior oblique muscle are identified and mobilized. This will often require a transconjunctival incision. Following mobilization of the superomedial extraocular muscles, a corridor between the extraocular muscles provides direct access to the optic nerve in the orbit ( Fig. 32.6 ). The surgeon can then follow the intradural optic nerve from proximal to distal to the annulus of Zinn and the intraorbital optic nerve from distal to proximal to the annulus of Zinn. Care must be taken to avoid manipulating the ophthalmic artery, which is tethered at this point, and the emerging central retinal artery, as either can be avulsed resulting in vison loss.
32.1.4 Median Approaches to Middle Cranial Fossa—CNs III–VI
Cranial Nerve III: Oculomotor Nerve
The oculomotor nerve (CN III) emerges from the ventral midbrain in the interpeduncular fossa, travelling between the posterior cerebral and superior cerebellar arteries and traversing the interpeduncular cistern. CN III occupies a consequential position emerging from the interpeduncular cistern. Its relationship to the P1 and P2 segments, the basilar apex located medially, and the uncus of the temporal lobe positioned laterally along the tentorial edge, is critical. This relationship explains why temporal (uncal) herniation leads to an ipsilateral dilated pupil (CN III compression), followed by P1 perforator compromise, and midbrain Duret hemorrhage in that specific temporal sequence. CN III courses between the borders of the tentorium cerebelli, and pierces the dura to enter the cavernous sinus, where it travels within the lateral wall of the cavernous sinus to enter the orbit through the superior orbital fissure.
Cranial Nerve IV: Trochlear Nerve
The trochlear nerve (CN IV) emerges from the dorsal midbrain, crosses to the contralateral side, and courses around the cerebral peduncle in the cerebellomesencephalic fissure to enter the ambient cistern. It passes between the posterior cerebral and superior cerebellar arteries and along the inferior surface of the tentorium to pierce the dura and enter the cavernous sinus, where it courses in the lateral wall inferior to the oculomotor nerve before entering the orbit through the superior orbital fissure.
Cranial Nerve V: Trigeminal Nerve
The trigeminal nerve (CN V) arises from the lateral pons and courses obliquely superior, passing beneath the tentorial attachment and above the petrous apex to enter Meckel′s cave, where it separates into three sensory divisions. The ophthalmic branch (V1) rises obliquely to enter the cavernous sinus and travel lateral to the abducens nerve. V1 passes in the lateral wall of the cavernous sinus inferior to the trochlear nerve before entering the orbit through the superior orbital fissure. The maxillary branch (V2) functionally forms the upper boundary of Meckel′s cave, entering the pterygopalatine fossa through the foramen rotundum. The mandibular branch (V3) functionally forms the inferolateral boundary of Meckel′s cave, exiting through the foramen ovale into the pterygopalatine fossa.
The quadrangular space ( Fig. 32.7 ) represents an anatomic boundary (i.e., parallel oblique lines) marking the location of Meckel′s cave. The quadrangular space is defined medially by the ascending paraclival ICA, inferiorly by the petrous ICA, laterally, for practical purpose, by V3, and superiorly by the abducens nerve, or by surrogacy V2. The Gasserian ganglion is considered the trunk of the trigeminal nerve, located in the middle of Meckel′s cave, with the sensory divisions V1, V2, and V3 emerging.