Introduction

Surgical expertise in skull base surgery is built upon one′s ability to access challenging areas of the cranial vault while minimizing morbidity and maximizing the degree of tumor extirpation. Therefore, the principle of endoscopic endonasal approaches (EEA) to the skull base is based upon the concept of enhancing a surgeon′s effectiveness in removing tumors by allowing for more direct access to them; a trajectory that avoids traversing vital neurovascular structures is a key element to this modality. To achieve this goal, a keen understanding of the anatomic relationships between cranial nerves, dural sinuses, arterial structures, and the surrounding bony network is critical. To emphasize the relevance of the osseous anatomy, the authors have organized the bony landmarks of the skull base as appreciated from the EEA perspective by discussing them in relation to the internal carotid artery (ICA) as well as to cranial nerves.

The ICA is the most critical arterial structure of the skull base because injuries to this vessel and its branches are associated with the most devastating complications.1–4 When considered from an EEA perspective, the ICA can be divided into six segments: parapharyngeal, petrous, paraclival, parasellar, paraclinoid, and intradural.5

33.1 Parapharyngeal ICA and the Eustachian Tube

The parapharyngeal ICA is relevant in EEA once it is superior to the hard palate. This segment is considered to begin at the common carotid bifurcation and ends at the external orifice of the carotid canal in the petrous bone. From an EEA perspective, this ICA segment is situated in the depth of the posterolateral aspect of the fossa of Rosenmüller, which can be found at the superior–posterior aspect of the torus tubarius ( Fig. 33.1 ). In other words, the Eustachian tube (ET) is anterior and medial to this segment. Typically, the ICA can be found at the junction of the bony and cartilaginous portion of the ET at the level of the carotid canal.6 There are no cranial nerves of significance in this location.

33.2 Petrous ICA and the Vidian Canal

The petrous segment of the ICA begins at the external orifice of the carotid canal in the petrous bone and ends at the posterolateral aspect of the foramen lacerum.5 This segment of the ICA runs in an inferior-to-superior, posterior-to-anterior, and lateral-to-medial direction toward the foramen lacerum. Important landmarks for the localization of this segment of the ICA from anterior-to-posterior and medial-to-lateral spatial orientation are the second and third segments of the trigeminal nerve (V2 and V3) and their respective foramen, as well as the foramen spinosum7; all of these foramina and their respective traversing structures are superior to the petrous segment ( Fig. 33.2 ). In addition, the bony aspect of the ET, found posterior to the carotid canal′s opening in the nasopharynx, shares an osseous wall with the petrous carotid artery; the junction between the cartilaginous and bony aspect of the ET serve as landmark for this segment of the ICA, as it is less than 3 mm from this point.7

The vidian canal serves as the most anterior, superior, and medial bony landmark for the anterior genu of the petrous ICA at the foramen lacerum from an EEA perspective8 ( Fig. 33.3 ). The canal is situated directly in front of the foramen lacerum.9 Therefore, the vidian canal serves as a landmark for the transition of the petrous segment of the ICA to the paraclival segment in the sagittal plane.10 Following the vidian nerve posteriorly allows the surgeon to identify the horizontal position of the petrous ICA. Thus, in the cases where a tumor distorts normal anatomy, the surgeon can navigate toward the vidian nerve to establish a safe area for dissection.

The vidian canal is formed by the intersection of the medial pterygoid plate with the floor of the sphenoid sinus; this beak-shaped area of bone is known as the pterygoid wedge.5 The canal can be found at the immediate superolateral aspect of the pterygoid wedge, which is formed by following the articulation of the vomer with the sphenoid rostrum posterolaterally toward the medial pterygoid plate. The pterygoid plates can be found inferior and lateral to the vidian canal.6 Immediately superior and lateral to the vidian canal is the foramen rotundum and inferior and medial to it is the palatovaginal canal, which is also referred to as the palatosphenoidal canal.8 The foramen ovale and its associated exiting nerve (V3) can be found superiorly and posteriorly after removing the lateral pterygoid muscle from the lateral pterygoid plate.

To access the vidian canal and to visualize its nerve, the pterygopalatine fossa contents must be mobilized. To do so, the periosteum between the palatovaginal canal and the vomer-sphenoidal suture is dissected and the sphenoid process of the palatine bone is removed to transect and/or lateralize the neurovascular contents of the palatovaginal canal, namely the pharyngeal artery branches and the palatovaginal nerve.8 Further dissection allows for the pterygopalatine fossa to be lateralized to expose the medial aperture of the vidian canal.

The distal aspect of the petrous carotid artery and the middle fossa can be reached by beginning to drill the inferomedial aspect of the canal followed by its superior aspect so that the vidian nerve can be transposed superiorly; the inferior and lateral aspect of the canal is then drilled sequentially.11 During transpterygoid approaches, drilling the vidian canal in this fashion protects inadvertent injury to the ICA and allows the vidian nerve to be fully retracted, rather than transected to avoid potential morbidity from keratoconjunctivitis.