7 Endonasal Corridors and Approaches

10.1055/b-0040-177063

7 Endonasal Corridors and Approaches

Harminder Singh, Jeffrey P. Greenfield, Gustavo J. Almodovar, Vijay K. Anand, and Theodore H. Schwartz

Abstract

Numerous endonasal surgical approaches to the anterior skull base have been described in the literature. These utilize a variety of terminologies and trajectories, which can sometimes be difficult to comprehend for the novice practitioner. In this chapter, we present a straightforward algorithm to conceptualize these approaches. It is based upon five different nasal corridors that must be transgressed to reach various anterior skull base targets.

7.1 Transsphenoidal Corridor

The sphenoid sinus has been called the “gateway to the anterior skull base.” Via the transsphenoidal corridor, one can access the sella, tuberculum, planum, upper clivus, and the medial cavernous sinus (CS; ▶ Fig. 7.1).

Fig. 7.1 The transsphenoidal corridor is the most commonly used corridor and permits approaches to the sella, suprasellar cistern, medial cavernous sinus, and superior clivus.

The transsphenoidal approach begins by lateralizing the middle (▶ Fig. 7.2) and superior turbinates and identifying the sphenoid ostium, which is located in the sphenoethmoid recess posterior to the superior turbinate (▶ Fig. 7.3).

Fig. 7.2 Right nostril. The middle turbinate (MT) is lateralized to gain access to the superior turbinate. The septum (S) is seen on the right.

Fig. 7.3 The ostium (Os) of the sphenoid sinus is seen in the sphenoethmoid recess, posterior to the superior turbinate (ST).

The sphenoid ostium can be enlarged with a mushroom punch to gain entry into the sphenoid sinus, either unilaterally or bilaterally, depending on the pathology. Care must be taken not to punch inferolaterally into the territory of the sphenopalatine artery (SPA), which can cause unwanted bleeding. The SPA provides blood supply to the nasoseptal mucosa. In cases where a nasoseptal flap (NSF) must be harvested for skull base repair, it is critical to preserve this blood supply (▶ Fig. 7.4).

Fig. 7.4 Location of the sphenopalatine artery (SPA) in relation to the sphenoid ostium and choanae, and the surgical cuts (dotted line) that need to be made to harvest the nasoseptal flap (NSF). In pediatric patients, the olfactory epithelium (OE) and the maxillary crest growth plate (MCGP) need to be preserved. (a) Sagittal view. (b) Coronal view.

The anterior wall of the sphenoid sinus is removed for a panoramic view of the sphenoid sinus region (▶ Fig. 7.5). The posterior one-third of the nasal septum (consisting of the perpendicular plate of the ethmoid bone superiorly and the vomer inferiorly) can be removed for a binarial approach to the sphenoid. The posterior inferior portion of the vomer (the “Keel”) should also be removed if an NSF is used to reconstruct the skull base. This maneuver prevents kinking of the vascular pedicle of the NSF and allows the flap to sit flush with the skull base, promoting healing ¹ .

Fig. 7.5 View inside the sphenoid sinus. From superior to inferior, the following structures are identified: S, sella; TS, tuberculum sella; PS, planum sella; C, clivus.

In the pediatric population, there can be incomplete pneumatization of the sphenoid sinus. In the sagittal plane, five different pneumatization patterns have been described ² (▶ Fig. 7.6):

Type 1: conchal (completely missing or minimal sphenoid sinus).
Type 2: presellar (posterior wall of sphenoid sinus is in front of the anterior wall of the sella).
Type 3: sellar (posterior wall of sphenoid sinus is between anterior and posterior wall of sella).
Type 4A: postsellar (posterior wall of sphenoid sinus is behind the posterior wall of sella).
Type 4B: postsellar (posterior wall of sphenoid sinus is behind the posterior wall of sella, with air dorsal to the sella).

Fig. 7.6 Sphenoid sinus pneumatization patterns in the sagittal plane. From left to right: type 1, conchal; type 2, presellar; type 3, sellar; type 4A, postsellar; type 4B, postsellar, with air dorsal to the sella.

Conchal pneumatization patterns are a relative limitation in pediatric endoscopic endonasal surgery, because the cancellous bone can be easily drilled down to approach the sella (▶ Fig. 7.7).

Fig. 7.7 **(a)** Type 1: conchal pneumatization of the sphenoid sinus. The cancellous bone can be easily drilled to reach the sella. **(b)** The cancellous bone in the sphenoid sinus has been removed with a high-speed drill to reveal the PS (planum sphenoidale), TS (tuberculum sphenoidale), and the S (sella).

Laterally, the bone between the optic nerve and the carotid artery, or medial opticocarotid (mOCR) recess can be removed to expose the superomedial aspect of the CS. This opening can be extended inferolaterally to expose the carotid siphon in the medial CS. In order to access the lateral inferior portion of the CS, the medial pterygoid plate (mPP) will have to be removed for better exposure (▶ Fig. 7.8).

Fig. 7.8 Endoscopic view of the left lateral aspect of the sphenoid sinus, showing the medial opticocarotid recess (mOCR) and the medial pterygoid plate (mPP). The bone over the medial aspect of the cavernous sinus has been scored with a drill (CS). In order to access the lateral superior aspect of the cavernous sinus (*), the ethmoid air cells lateral and anterior to the sphenoid sinus have been removed. In order to access the lateral inferior aspect of the cavernous sinus, the superior portion of the medial pterygoid plate (mPP) has to be removed.

7.2 Transethmoidal Corridor

The transethmoidal corridor allows access to the lateral anterior skull base, from the frontal sinus anteriorly to the sphenoid sinus posteriorly. The orbital apex, the lateral CS, and the anterior fossa (though the fovea ethmoidalis) can be reached through this corridor. The anterior and posterior ethmoidal arteries are important landmarks, which must be cauterized and transected (▶ Fig. 7.9a; ▶ Fig. 7.9b).

Fig. 7.9 **(a)** The transethmoidal corridor exposes the orbital apex, the lateral cavernous sinus, and the anterior fossa though the fovea ethmoidalis. **(b)** Complete ethmoidectomy is performed on both sides of the superior turbinate. Note the location of the anterior and posterior ethmoidal arteries.

A total ethmoidectomy is performed in an anterior to posterior fashion, lateral to the superior turbinate after first removing the uncinate process (UP) to expose the bulla ethmoidalis (▶ Fig. 7.10). This will lead to the exposure of the fovea ethmoidalis superiorly (roof of the ethmoid cells, which leads into the anterior cranial fossa) and the lamina papyracea laterally (leading into the medial orbit; ▶ Fig. 7.11; ▶ Fig. 7.12).

Fig. 7.10 A total ethmoidectomy is performed, lateral to the superior turbinate (ST) and cephalad to the middle turbinate (MT). Et, ethmoid cells

Fig. 7.11 A coronal CT scan showing the relationship of the ethmoid air cells to the turbinates, the fovea ethmoidalis superiorly, and the lamina papyracea laterally.

Fig. 7.12 The lamina papyracea (LP) is removed revealing the medial orbital dura (Od). The fovea ethmoidalis (FE) superiorly is being obscured by the superior turbinate (ST).

The lateral superior aspect of the CS can be accessed via the transethmoidal corridor, in conjunction with the transsphenoidal corridor. In order to access the lateral inferior aspect of the CS, the superior portion of the mPP has to be removed (▶ Fig. 7.8).

Individual variations in the posterior ethmoidal sinus anatomy must be studied prior to utilizing this approach corridor. Onodi cells are posterior ethmoidal cells extending into the sphenoid bone, abutting the optic nerves (▶ Fig. 7.13). There might be variations in the course of the carotid artery in the lateral wall of the sphenoid sinus, as well as pneumatization variations of the sphenoid sinus, ³ placing the neurovascular structures at risk (▶ Fig. 7.14).