Expanded Endonasal Approaches to Middle Cranial Fossa and Posterior Fossa Tumors

Skull base lesions that involve the middle and posterior cerebral fossae have been historically managed through extensive transcranial approaches. The development of endoscopic endonasal techniques during the past decade has made possible a vast array of alternative routes to the ventral skull base, providing the ability to expose lesions in difficult-to-access regions of the cranial base in a less invasive manner. In this review, the authors detail the endoscopic surgical anatomy and the operative nuances of the expanded endoscopic endonasal approaches to tumors of the middle and posterior cranial fossae. These techniques offer excellent exposure of the targeted regions yielding optimal resections, while avoiding the morbidity associated with transcranial surgical approaches.

During the past decade, the increasing use of endoscopy in skull base surgery has become an important alternative to traditional transcranial surgery. Anatomic studies coupled with technological advances in endoscopic equipment, surgical instruments, and neurophysiological monitoring have allowed novel and exciting approaches to flourish within the field. What once was only a route to the pituitary gland and sellar region has become a major highway to the entire ventral skull base and craniocervical junction.

Among these innovations stand the expanded endonasal approaches (EEAs). The philosophy behind these approaches is, as in any skull base approach, to tailor the bone removal to gain a direct access to deep regions, thus minimizing manipulation of the cerebrum, blood vessels, and cranial nerves. Although deemed minimally invasive, these techniques provide a wide exposure of the cranial base and its pathologic conditions, often with early control of tumor-nurturing vessels and minimal or no neural tissue retraction. Furthermore, minimally invasive must not be perceived as less effective but rather as a less-aggressive pathway to reach maximum effectiveness.

The EEA to the anterior cranial fossa and sellar/parasellar regions has already been well described and thus it is not discussed in this article. Hence, the authors focus on EEA to the middle cranial fossa (MCF) and posterior fossa (PF).

Middle cranial fossa

Traditionally, neurosurgical procedures for the pathologic conditions of the MCF have been performed through lateral craniotomies. Although lesions located at the most lateral aspect of the temporal fossa are directly reached when a lateral approach is used, more medially located mass lesions require various degrees of temporal lobe manipulation and/or retraction. In this article, the endonasal route is presented as an alternative direct approach to the medial compartment of the MCF.

Indications

Virtually any pathologic process can be accessed through an endonasal route as long as it is located in the medial portions of the middle fossa, dislocating the temporal lobe superolaterally. Several pathologic processes commonly arise in the middle fossa and can be directly reached through an EEA. Of special interest are meningiomas and trigeminal nerve schwannomas. Chordomas and chondrosarcomas can also extend cranially to the middle fossa when they grow to large sizes ( Fig. 1 ). Juvenile angiofibromas grow from the pterygopalatine fossa (PPF) and can expand posteriorly into the middle fossa as well. Aggressive pituitary adenomas can also grow laterally into the cavernous sinus and Meckel cave. Sinonasal malignancies such as adenoid cystic carcinomas can infiltrate perineural tissue and use it as a gateway to reach the middle fossa. All these pathologic conditions are examples of diseases in which the endonasal route can facilitate resection without brain retraction.

Anatomic Considerations

To access the MCF through an endonasal corridor, one needs to completely understand the ventral cranial base anatomy.

The sphenoid sinus lateral recess is a pneumatized projection of the sinus under the middle fossa. Once the bone that covers the lateral wall of the sphenoid sinus is removed, the periosteum of the middle fossa is exposed. The meningeal layer of the dura is located lateral to the gasserian ganglion.

Meckel cave is basically a small compartment within the 2 layers of dura mater on the anteromedial portion of the MCF containing the trigeminal (gasserian) ganglion. The limits of the Meckel cave include the periosteal dural layer that covers the MCF bone at its medial and inferior aspect and the dural meningeal layer at its lateral and superior aspect that separates it from the subarachnoid space. From the gasserian ganglion, arise the 3 trigeminal branches: ophthalmic (V1), maxillary (V2), and mandibular (V3) nerves ( Fig. 2 A). The 3 nerves exit the skull through the superior orbital fissure (SOF), foramen rotundum, and foramen ovale, respectively.

The gasserian ganglion guards the MCF medially. While dealing with benign disease, the trigeminal nerve must be preserved. The pathway to reach the MCF from the sphenoid sinus is through the 2 anterior triangles of the cavernous sinus: the anteromedial and anterolateral triangles (see Fig. 2 B). The anteromedial cavernous sinus triangle is limited superiorly by the ophthalmic branch of the trigeminal nerve (V1), pointing toward the SOF, and inferiorly by the maxillary branch (V2). On the other hand, the anterolateral triangle is enclosed within V2 superiorly and the mandibular branch (V3) inferolaterally. After bone removal, the exploration of these spaces allows the direct exposure of the temporal meningeal dura, which if needed can be followed posteriorly into the lateral component of Meckel’s cave or opened laterally into the subarachnoid space.

The concept of the quadrangular space (QS) is important to deal with gasserian ganglion disease or pathologic conditions located medial to it. This “window” is bordered by the sixth cranial nerve (abducens nerve) superiorly, the maxillary branch of the trigeminal nerve (V2) laterally, and the internal carotid artery (ICA) medially and inferiorly (see Fig. 2 B).

Appreciation of the sixth cranial nerve anatomy is also important for this approach, particularly its trajectory within the cavernous sinus. The sixth cranial nerve pierces the clival dura posterior to the ICA and advances between the dural layers superolaterally toward Dorello canal. Once inside the cavernous sinus, the nerve runs parallel to V1 on its way to the SOF. As the nerve ascends to the SOF, it forms the superior limit of the QS. The abducens nerve must be carefully observed and avoided during the approach to prevent undesirable postoperative sixth nerve palsy.

The medial and inferior aspects of the QS are created by the paraclival (vertical) and petrous (horizontal) portions of the ICA, respectively. Identification of these portions, particularly the paraclival segment, is essential for the proper exposure of the QS. The vidian nerve, formed by the union of the greater superficial petrosal nerve and the deep petrosal nerve, is used as a landmark for identification of the ICA anterior genu, where the petrous ICA turns into paraclival ICA at the foramen lacerum.

Surgical Technique

Preoperatively, computed tomography and magnetic resonance imaging of the head are carried out. Results of both the techniques are combined and fused for comprehensive intraoperative image guidance.

All EEAs are performed with cranial nerve monitoring (using electromyography), with special attention to the third, fourth, motor V3, and sixth cranial nerves ipsilateral to the disease. Also, somatosensory evoked potentials are monitored throughout the surgical procedure.

Once in the operating room, the patient undergoes standard intravenous anesthesia and orotracheal intubation. A 3-pin head holder is placed with a slight neck extension and discreet head rotation to the right and a tilt to the left.

The nasal cavity is decongested with topical 0.05% oxymetazoline; antisepsis is achieved with perinasal and periumbilical povidone solution (in case a fat graft is needed). Intravenous antibiotic, a third- or fourth-generation cephalosporin, is administered at the beginning of the procedure.

The procedure begins with the use of a 0°-lens endoscope. Initially, the inferior turbinates are displaced laterally. Next, the middle turbinate that is ipsilateral to the lesion is removed and its pedicle, a direct branch of the sphenopalatine artery, is identified and coagulated. Constant irrigation with warm saline solution with either an endoscope sheath or a common 60-mL syringe helps to maintain the endoscopic view, avoiding surgical delays.

Before entering the sphenoid sinus, a vascularized flap is elevated for eventual closure of the anticipated skull base defect. A nasoseptal pedicled flap with blood supply from the posterior nasal artery, a branch of the sphenopalatine artery, is elevated from the contralateral side. Once elevated, the flap can be stored in the nasopharynx or in the respective maxillary sinus after an antrostomy, thus still providing the surgeon with direct and detailed visualization of the anterior wall of the sphenoid sinus.

Once the flap is secured, a posterior septectomy is performed to allow the 2-nostrils 4-hands technique. Also, ipsilateral posterior ethmoidectomy and wide anterior sphenoidectomy are performed to improve visualization and achieve an adequate working corridor. Septations within the sinus are then flattened.

The next stage of the approach encompasses the lateral aspect of the ventral skull base. This part of the procedure is based on bone removal and drilling to progressively expose the elements of the QS and middle fossa.

An ipsilateral transpterygoid approach is required, which starts with an uncinectomy and enlargement of the maxillary ostium to obtain an antrostomy. In some patients, removal of the bulla ethmoidalis is necessary for a better operative field view. Within the posterior wall of the maxillary sinus, the distal segment of V2 (infraorbital nerve [ION]) and the terminal branches of the maxillary artery are visualized. Removal of this bony wall provides access to these branches and the PPF. The maxillary nerve (V2) is encountered superiorly and laterally in the PPF.

Next, the terminal branches of the maxillary, posterior nasal, and sphenopalatine arteries are dissected and coagulated. The vidian nerve is also identified, and the surrounding bone is drilled down in an anterior to posterior direction. Whenever possible, the vidian nerve and artery, if present, are transposed from the vidian canal and preserved. These measures allow the surgeon to precisely delineate the path of the vidian canal toward the anterior surface of the petrous carotid at its emergence at the foramen lacerum area. Once the vidian nerve is transposed or sectioned, the soft contents of the PPF are lateralized and the base of the pterygoid plate is drilled, completely exposing the sphenoid sinus lateral recess.

The surgeon then focuses toward the medial and inferior limits of the QS. These limits are represented by the horizontal (petrous) and vertical (paraclival) segments of the ICA. At this stage of the approach, the surgeon should be able to visualize the vidian nerve, V2, and ION laterally. The ipsilateral optic canal, paraclinoid ICA, and lateral optic-carotid recess (LOCR) must be identified. The region that is anterior to the ICA siphon at the paraclinoid area and inferior to the LOCR (optic strut) represents the SOF, which is the superior limit of any approach to the middle fossa. The carotid protuberances, which are bony impressions around the ICA paraclival canals, can be easily recognized in well-pneumatized sinuses.

The entire bone covering the medial aspect of the middle fossa should be drilled, tailoring the specific needs of the pathologic condition. The periosteum of the middle fossa is exposed completely, and the gasserian impression, with all 3 trigeminal branches, is visualized underneath (see Fig. 2 A). Usually there is no need for complete ICA skeletonization, which is only performed when there is direct disease encasement or in cases in which ICA mobilization or proximal control is required.

Finally, the periosteum can be opened accordingly. In general, there are 3 areas that are initially explored: Meckel’s cave and anteromedial and anterolateral cavernous sinus triangles (see Fig. 2 B).

When the disease is posterior, medial, or directly related to the gasserian ganglion, an incision is made at the QS. The periosteal dural layer overlying its anterior surface is opened, and the lesion can be directly accessed. To avoid abducens nerve injury, its neurophysiology should be used before any incision is performed and the opening should not transgress the level of the superior border of V2. The MCF can also be reached through the anteromedial (between SOF/V1 and V2) and anterolateral (between V2 and V3) triangles, which provide direct access to the subarachnoid space at the temporal fossa. Consequently, tumors such as meningiomas can be resected at that level ( Fig. 3 ). Schwannomas are restricted to Meckel’s cave and are reached after the periosteum is opened at the QS and the lateral meningeal dura is preserved ( Figs. 4 and 5 ). These lesions can even be followed into the PF in selected cases (see Fig. 4 ).