Chapter 16 The Anteromedial Corridor via the Expanded Endonasal Approach: The “Front Door to Meckel′s Cave”
Meckel′s diverticulum, or Meckel′s cave, as the name suggests, represents a hidden pyramidally shaped den that is situated deep in the skull base, harboring a carefully guarded anatomic treasure of vascular and neural structures. The cave is no larger than the size of a thumbnail, vulnerable to pathologic entities that can infiltrate from within, namely schwannomas, or other lesions that can intrude along the perimeter and eventually invade the protective dural walls. To best preserve the highly prized treasures contained within, it is imperative to understand the intricacies of the intrinsic and adjacent anatomic structures, such as the gasserian ganglion and its pre- and postbranches, the abducens nerve and internal carotid artery (ICA), and the protective osseous and soft-tissue frameworks.
Over the past two decades, we have come to the realization that the most common primary long-term morbidity with skull base surgery is associated with cranial nerves. Therefore, every effort should be taken to minimize their manipulation. To better understand this crowded space, we will review, in significant detail, the anatomic boundaries of Meckel′s diverticulum and its treasures contained within. This represents the essential truth by which a strategy of modular approaches can be designed and executed to minimize impairment. The goal of this chapter is to describe a step-by-step approach to the endoscopic endonasal anteromedial corridor—“the front door to Meckel′s cave.” The reader must not forget, however, that this represents all but one corridor for surgical access. The role of the traditional transcranial antero- and posterolateral corridors, and their variants, while beyond the scope of this chapter, must be a part of the surgeon′s armamentarium. Our guiding principle in selecting the specific corridor has been to let the truth of the anatomy, rather than the agenda of the surgeon, guide the selection of corridor. Explicitly stated, when possible, avoid crossing the plane of a cranial nerve.
Every surgeon requires a complete understanding of the anatomy, the anatomic landmarks, and the surrounding structures, to surgically address a region appropriately. Meckel′s cave, or the trigeminal cave,1 is often described as a challenging area of the skull base to access, involving complex anatomy and requiring technical finesse. Various abnormalities can affect Meckel′s cave, including epidermoid cysts, chondrosarcomas, chordomas, and sinonasal malignancies, which often experience perineural invasion and spread. The most common lesions in this area are meningiomas and schwannomas originating from the trigeminal nerve.2,3
16.2 Anatomic Considerations
Meckel′s cave represents a potential space in the middle cranial fossa where the two layers of dura mater (periosteal and meningeal) split, forming a diverticulum4; therefore, we prefer to refer to it as Meckel′s diverticulum. At this point, the trigeminal nerve travels from the brainstem to the Gasserian ganglion.5 From there, the nerve divides into its three primary divisions that exit through their respective foramina in the middle cranial fossa. More precisely, the boundaries of the trigeminal cave are formed superolaterally by the meningeal layer of the dura mater that covers the middle cranial fossa. Inferomedially, Meckel′s diverticulum is bounded by the periosteal layer of the temporal fossa and petrous carotid canal.6–9
In 2009, we described the key anatomic boundaries of Meckel′s diverticulum as a quadrangle (parallel oblique lines).4 The quadrangular space is defined medially by the ascending paraclival ICA, inferiorly by the petrous ICA, laterally by V3, the mandibular division of CN V, and superiorly by the abducens nerve or, by surrogacy, V2, the maxillary division of the trigeminal nerve.
Four approaches have been described to access this corridor that can generally be categorized into the respective trajectories: (1) anteromedial, (2) anterolateral, (3) lateral, and (4) posterolateral. We have previously reported that all four are valuable trajectories in avoiding crossing the plane of the nerve, to better guide the operator. The location and nature of the pathology will, respectively, displace the cranial nerves within and juxtaposed around Meckel′s diverticulum. Specifically, the appropriate corridor should ideally be selected to avoid crossing the path of the nerve, to allow direct access to the lesion, and to leave the primary nerves along the perimeter. Given the ventral nature of many of the pathologies that afflict this region, such as meningiomas, the nerves are often displaced posterolaterally. This situation lends itself ideally to an anteromedial approach to the diverticulum. In this chapter, we describe, in stepwise detail, accessing Meckel′s diverticulum through the anteromedial corridor via the expanded endonasal approach.
16.3 Surgical Steps
Each patient obtains a preoperative computed tomography angiography to precisely (CTA) locate the ICA (petrous and paraclival segment), defining critical inferior and medial boundaries of the quadrangular space, and to be used for image-guidance during the procedure.
Ideally, the patient should have a high-resolution magnetic resonance imaging (MRI) such as a FIESTA sequence to provide a general perspective of the location of the juxtaposed cranial nerves defining the superior and lateral quadrangular boundaries.
Once the images have been set up in the room, the patient is placed under general anesthetic and orotracheally intubated.
The head is fixed in a three-pin head holder positioned with the neck extended slightly and the head turned slightly to the right, extended and rotated to allow for the nares to be in an ergonomic position for the operating surgeon. We prefer fixed pining positions to allow for drilling without the risk of moving.
The nose is decongested with 0.05% topicalxylometazoline or oxymetazoline using ½” x3” cottonoid pledgets.
Povidone solution is then applied perinasally. It is also applied to the periumbilical areas as well in preparation for the possibility of using a fat graph during the reconstructive phase of the procedure. The right thigh can also be prepared in preparation for possibly using a tensor fascia lata graft as part of the reconstruction. Once the solution has been placed, the patient is draped, exposing the nose and eyes in one field and keeping the lower abdomen and leg completelyseparate.
16.3.2 Nasal Cavity Approach
After the patient is appropriately positioned, prepped, and draped, and the image-guidance system has been accurately registered, we can begin with our endonasal exposure. Our preferred instrumentation is a 0-degree rod lens endoscope for direct visualization, with an irrigation system for lens cleaning—either the Medtronic Endo-Scrub (Medtronic Corporation, Minneapolis, MN) or Infiniti syringe irrigation (Nico Corporation, Indianapolis, IN). We recommend a four-handed technique for these approaches, and we find the Infiniti is best with the team approach. This helps maintain our visualization. We then begin with our exposure.
We have espoused the concept of a “cavity and a half” to allow for adequate visualization. We separate one-half of one nasal cavity for endoscope insertion and the other half as a working cavity for the bimanual binasal dissection. In the case of the front door exposure to Meckel′s cave, this consists of creating a vertical rectangle that extends from the posterior ethmoid through the nasopharynx and laterally, incorporating wide bilateral sphenoid/ethmoidectomies (lamina papyracea to lamina papyracea) with extension to the ipsilateral maxilla. This process is described in the following.
Phase I: Rostrocaudal Binasal Rectangular Midline Nasal Exposure
Ipsilateral nasal cavity preparation: The inferior turbinate is out-fractured and lateralized to maximizevisualization. This is done using either a Goldman freer or caudal elevator, in conjunction with decongesting the nose with oxy- or xylometazoline ( Figs. 16.1 and 16.2 ).
The middle turbinate is then injected in the axilla with 1% lidocaine and epinephrine, and allowed to set for 3 minutes.
Ipsilateral nasal cavity preparation (continued): On the ipsilateral side of the lesion, the lower third of the middle turbinate is resected, leaving a stump. One can use either endoscopic scissors or a Colorado needle tip monopolar cautery to make the initial incision of the middle turbinate. Hemostasis of the residual middle turbinate stump is achieved with either an endoscopic bipolar or a suction cautery to control the middle turbinate branch of the sphenopalatine artery (SPA) ( Fig. 16.3a, b ). Lateralization of the inferior turbinate and partial middle turbinate creates a wide endonasal working corridor ( Fig. 16.4 ; the inferior turbinate is lateralized and still in place in this figure).
Contralateral nasal cavity preparation: On the contralateral side of the lesion, the middle turbinate is in-fractured and medialized ( Fig. 16.5 ). This allows for both a binasal approach to the sphenoid sinus and greater exposure to raise the nasoseptal mucosal flap ( Fig. 16.9 ). With our lateralization, we are able to visualize the natural sphenoid ostium ( Fig. 16.6 ).
Contralateral nasoseptal flap harvest: A nasoseptal flap (NSF) is harvested on the contralateral side. This is based on the need to expose the paraclival and genu segments of the ICA (boundaries of the quadrangular space) which would put the base of the NSF at risk ( Figs. 16.7–16.10 ). We raise the flap by making a superior incision inferior to the olfactory epithelium (this is identified as thicker and with a more yellowish hue in comparison to the normal nasal mucosa) on the septum and extending it anteriorly to the limen nasi (we use a Covidien Bovie with an arthroscopic needle tip; Fig. 16.7 ). The inferior cut extends posteriorly from the inferior aspect of the nasal choana on the nasal floor. One can create an extended width of the NSF by making the inferior cut lateral and inferior to the inferior turbinate ( Fig. 16.8 ). However, to decrease the risk of postoperative dental hypoesthesia, we recommend decreasing the power on the cautery needle or to make the incisions with an extended-length beaver blade. We then raise the flap in a subperichondrial plane using a suction Cottle elevator ( Fig. 16.9 ). Once fully elevated, we then place the NSF in the nasopharynx to keep it out of the way during the remainder of the exposure ( Fig. 16.10 ). Alternatively, one can create a contralateral maxillary antrostomy as a storage point for the NSF. A posterior septectomy is then performed to allow for our binasal approach to the posterior nasal corridor ( Fig. 16.11 ).
Upper rectangular corridor: Posterior and, when needed, anterior ethmoidectomies are performed, including an ipsilateral uncinectomy. The uncinectomy is completed using a back-biting rongeur and a microdebrider, Stammberger downbiter, and/or a through-cutting Blakesley ( Fig. 16.12 ).
The sphenoid sinus ostium on the ipsilateral side of the lesion is identified posteroinferior to the superior turbinate. The posteroinferior aspect of the superior turbinate is resected to allow for sufficient exposure. Using a high-speed drill, the sphenoid sinus face is resected and opened superiorly to the skull base and inferiorly to the sphenoid sinus floor. Once the ipsilateral side is completed, the same process is repeated on the contralateral side, providing the critically needed wide bilateral exposure through the lateral recess of the sphenoid sinus ( Fig. 16.12 ).
At this stage, posterior septectomy and wide bilateral sphenoidotomies have been completed, creating the superior and the initial lateral working corridor. The lateral wall can be widened via an ipsilateral maxillectomy later. Next, the floor of the sphenoid sinus is drilled to the level of the clival recess to allow the inferior boundary to be extended ( Fig. 16.13 ).