Chapter 55 Endoscopic Endonasal Approaches to the Skull Base and Paranasal Sinuses

Paul A. Gardner, Amin B. Kassam, Carl H. Snyderman, Ricardo L. Carrau, Daniel M. Prevedello

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INTRODUCTION/BACKGROUND

Approaches to the skull base via the paranasal sinuses were introduced in the late 19^th and early 20^th century. Tumors of the sellar region were first approached through various incisions in the forehead to gain access to the sphenoid sinus via the ethmoid sinuses. Cushing introduced what became the standard for transsphenoidal surgery in 1910 when he first used a sublabial incision to gain access to the sinuses.¹ coincidentally, on the exact same day, June 4, Oscar Hirsch used an endonasal approach to gain access to the sphenoid sinus. This would eventually replace the sublabial approach, but Cushing’s influence delayed this for decades.

This transsphenoidal approach to the sella turcica was conceptually unchanged for approximately 70 years. However, the operation and its outcomes were changed greatly during this period by advances in technology which provided improved visualization, intraoperative image-guidance and instrumentation. Dott, Guiot and Hardy added pneumoencephalocisternography, radiofluoroscopic guidance, and the operative microscope. All of this revolutionized the approach and improved outcomes. A similar change in technology began in the 1980s with the development of early intraoperative CT and MR image-guidance systems and the introduction of the endoscope by Carrau.^2,³ At the same time, otolaryngologists were developing and refining functional endoscopic sinus surgery (FESS).⁴^–⁹ It was the collaboration between otolaryngologists and neurosurgeons that led to the expansion of the standard transsphenoidal approach to include the rest of the anterior skull base and beyond.

BASIC ENDOSCOPIC ENDONASAL CONCEPTS

The application of endoscopy to the endonasal approach has allowed the expansion of this approach while improving visualization. This is due to one basic optic difference when compared to a microscope. A microscope visualizes from a distance and delivers light in a cone whose apex is at the target. This requires a superficial exposure which is wider than the deep exposure (“ice cream cone” effect). An endoscope delivers light and provides a view in a cone whose apex is at the tip of the scope. This allows a smaller exposure superficially while allowing a much wider working field in the depth (“flashlight” effect). This becomes a distinct advantage when approaching deep lesions with complex surroundings, such as those involving the skull base and paranasal sinuses. This does create a problem with instrumentation and the modification of existing and development of new instrumentation was necessary and is still ongoing.

The loss of three-dimensional visualization is easily overcome using an active, handheld endoscope and proprioceptive cues obtained by keeping one instrument on or near the object of focus. Three-dimensional sensation is recreated via propioception and triangulation of instruments. This is a part of the learning curve, but one which is relatively easy to overcome.

Despite these differences, endonasal techniques should not differ from standard microsurgical techniques. Two-handed dissection by the operating surgeon is required and therefore two surgeons are needed. Identification of critical structures, central debulking followed by extracapsular dissection and fine, sharp dissection are all critical components of microsurgery and should be directly translated to endoscopic neurosurgery (endoneurosurgery).

One of the main reasons these approaches hold such promise is that many skull base lesions are medial and anterior to the surrounding neurovascular structures. This provides a distinct advantage to a direct anterior, midline approach, as these structures do not have to be displaced or transgressed in order to access them. This principle guides the selection of tumors for EEA. Vascularity, tumor consistency, and size represent important surgical considerations but do not constitute contraindications to an EEA.

EXPANDED ENDONASAL APPROACHES: ANATOMIC MODULES

The key to the development of endonasal approaches to skull base pathology was the collaboration between otolaryngologists and neurosurgeons. The knowledge of the paranasal sinuses developed by otolaryngologists melded with the work neurosurgeons had done in the sella turcica for pituitary tumors. Both were supplemented by knowledge of skull base anatomy as learned through performing traditional, open approaches.

Endoscopic endonasal approaches (EEAs) can be divided into the sagittal or rostral-caudal plane (between the carotids) and coronal or paramedian plane (lateral to the carotid). The sagittal plane can be divided from rostral to caudal, anterior to posterior into the transfrontal, transcribriform, transplanum/transtuberculum, transsellar, transclival and transodontoid approaches (Fig. 55-1). The coronal plane is somewhat more complex in that the lateral expanded approaches vary based upon which fossa is involved (Fig. 55-2). The anterior fossa, lateral approaches consist of the supra- and transorbital approaches. The middle fossa is the most complex and is broken into five “transpterygoid” anatomic zones of approach: medial pterygoid (petrous apex), petroclival junction, quadrangular space or Meckel’s cave, superior cavernous sinus, and the infratemporal fossa (Fig. 55-3). These are all critically dependent upon their relationship with the internal carotid artery (ICA). Finally, the posterior fossa or inferior expanded approaches consist of the transcondylar and parapharyngeal space approaches.

Figure 55-1 A, Lateral view of the sagittal plane modular endoscopic endonasal approaches (EEAs) as shown in a rostral-caudal axis. B, View of the anterior skull base from above showing sample tumor locations for the various rostral-caudal modules.

Figure 55-2 Superior view of skull base with general regions of approach by fossa. 1) anterior, 2) middle, 3) posterior.

Figure 55-3 Coronal plane, lateral EEAs by anatomic zones. Zone 1 is the petrous apex, Zone 2 the petroclival junction, Zone 3 the quadrangular space/Meckel’s cave, Zone 4 the superior lateral cavernous sinus, and Zone 5 the infratemporal fossa.

Sagittal Plane

This represents the region between the internal carotid arteries (ICA) as it rises and courses along the ventral skull base from a caudal to rostral direction as well as the rostral extension of this midline region. The sagittal plane is also referred to as the rostral-caudal plane.

Transfrontal Approach

The transfrontal approach represents the most anterior expanded endonasal approach. A transfrontal approach starts with identification of the nasofrontal recesses and continues until bilateral frontal sinusotomies are performed with creation of a superior septal defect and removal of the floor of the frontal sinuses (Draf 3 or Lothrop procedure) if needed. The anterior attachment of the middle turbinate provides a posterior landmark to preserve olfaction and avoid violation of the cribriform plate with resultant cerebrospinal fluid (CSF) leak. This approach can be used in isolation for rare lesions such as nasal dermoid cysts or fibro-osseous tumors and more commonly for frontal sinusitis or mucoceles. In pediatric patients with dermoid cysts, the cyst is drained endonasally but the septum should be resected up to the skull base. The transfrontal approach is also used in conjunction with the transcribriform approach for accessing the anterior extent of lesions such as olfactory groove meningiomas. A key anatomic landmark for this region is the frontoethmoidal recess.

Transcribriform Approach

The transcribriform approach is a commonly used approach for anterior skull base tumors. Most often used in our practice for resection of olfactory groove meningiomas and esthesioneuroblastomas, it is also used for repair of post-traumatic and iatrogenic CSF leak repairs and has potential for any subfrontal lesion.

A complete sphenoethmoidectomy is performed on each side and the nasofrontal recesses are visualized. If needed, exophytic tumor within the nasal cavity can be debulked. The superior nasal septum is transected from the crista galli to the sphenoid rostrum as needed. As mentioned above, a transfrontal approach can be performed to establish an anterior margin. The transcribriform approach provides direct access to the vascular supply of tumors such as olfactory groove meningiomas, allowing early devascularization. The anterior and posterior ethmoidal arteries should be identified early and cauterized or clip ligated. The lateral bony margins are drilled to form “gutters” or osteotomies for the length of the tumor or planned resection as needed. If necessary, the lamina papyracea can be removed as well to allow retraction on the periorbita, thereby displacing the orbital contents for even more lateral access. In fact, the lateral access can extend all the way to the midorbital line at the level of the superior rectus muscle. After the lateral margins are drilled, the planum or tuberculum can be drilled as needed for a posterior margin. Even if necessary for tumor resection, it may be safest to leave the bone of the optic canals intact as long as possible. Finally, the dura is dissected from the crista galli to allow its removal. The remaining bone of the cribriform plate can now be dissected free (Fig. 55-4). If necessary for purely extradural lesions, the dura should be left intact. This may not be possible over the olfactory filaments, which can be cauterized to minimize CSF leakage, but should nonetheless undergo formal repair (see below). Obviously, olfaction (when present) is sacrificed if a bilateral approach is undertaken.

Figure 55-4 Intraoperative, endoscopic view of the entire anterior skull base following exposure and removal of necessary bone for approach to an anterior base lesion. (CG = crista galli; ICA = internal carotid artery; lOCR = lateral opticocarotid recess; mOCR = medial opticocarotid recess; Oflm = olfactory filament; ON = optic nerve protuberance; Pl = planum; PO = periorbita; tub = tuberculum.

Rights were not granted to include this figure in electronic media. Please refer to the printed book.

(Reprinted with permission Gardner PA, Kassam AB, Thomas A, Snyderman CH, Carrau RL, Mintz AH, Prevedello DM. Endoscopic endonasal resection of anterior cranial base meningiomas. Neurosurgery 2008;63:36-52.Copyright 2008, Lippincott, Williams & Wilkins.)

The dura is opened in the same fashion as with microsurgery, with a fine blade and scissors. The durotomies are paramedian on both sides of the falx. This is done in order to minimize bleeding from branches of the anterior falcine artery and the inferior sagittal sinus. Bilateral durotomies are made either at the lateral extent of desired resection or overlying tumor. The durotomy is extended to the midline both anteriorly and posteriorly. The falx and inferior sagittal sinus must be systematically addressed next. Pistol-grip, endonasal bipolars are used with one blade on either side of the falx/sinus to coagulate it prior to transecting it to allow anterior tumor or dural release. In addition, anterior falcine branches may be encountered during tumor resection for access to the falx, providing additional devascularization. Dura can be resected en bloc as needed for tumors such as esthesioneuroblastoma. All tissues in such cases must be evaluated both intraoperatively and postoperatively for margins.

Intradural tumor resection must be performed with caution, especially when approaching the interhemispheric fissure, as there are frequently anterior cerebral artery branches or even the anterior communicating artery on the surface of the tumor (Fig. 55-5). An endonasal ultrasonic aspirator or two suctions can be used depending upon tumor consistency and proximity of involved structures. Microsurgical concepts of preserving arachnoid planes when possible and sharp dissection of critical structures is maintained throughout. All of this may require the use of angled scopes (45° or 70°).

Figure 55-5 A, Preoperative CT angiography sagittal reconstruction showing the anterior cerebral artery (ACA) involvement with an olfactory groove meningioma (OGM). B, Postoperative, sagittal MRI following endoscopic endonasal approach (EEA) showing complete resection of the tumor with patent and intact ACA.

Transplanum/Transtuberculum Approach

Also described as an “extended” approach, the transplanum approach was the first expansion of a traditional transsphenoidal approach.¹⁰^–¹³ This approach provides a natural corridor for many suprasellar tumors such as craniopharyngiomas, tuberculum meningiomas and large pituitary adenomas. It may also be used for biopsy or resection of infundibular lesions such as metastases or hypophysitis. This approach is often an integral addition to transsellar and transcribriform approaches.

A transsphenoidal exposure (see transsellar approach below) is augmented by a posterior ethmoidectomy. The posterior ethmoidal arteries are a good landmark for anterior extent to preserve olfaction while providing adequate access. The optic canals are obviously critical to identify in order to avoid damage to the nerves. Whenever drilling over or near the optic canals, it is important to constantly irrigate to avoid heat transmission from the drill to the nerves. After the sella is exposed, the planum can be drilled and thinned. This can require an angled endoscope for adequate visualization depending upon the slope of the anterior skull base. Though somewhat counterintuitive, the planum is most easily removed in an anterior to posterior direction after the bone has been adequately thinned, exposing dura at the anterior extent and laterally. The lateral margins are actually the optic nerves which form the sides of a trapezoid which include the anterior planum and tuberculum sellae. Often there is no need to expose the optic nerves. However, in many tuberculum meningiomas, there is extension of disease into the medial optic canals (Fig. 55-6). This disease is not easily accessed via a craniotomy and requires release and retraction of an often already compromised optic nerve. In these cases, the bone overlying the tumor in the optic canal should be thinned (“blue-lined”) with a drill and carefully removed with a dissector.