The Ventral Perspective: Topographic Neurovascular Anatomy of the Cranial Base From Endoscopic Endonasal Perspective: The Median Sagittal Plane

Introduction: There are two primary considerations in accessing targets within the central skull base:

1. The 360-degree circumferential corridor (CC): anterior medial/lateral, lateral, and posterior lateral) (previously described) and

2. The Radial Corridor (RC): Outer-Radial Corridor (ORC): osseous and soft-tissue envelopes; Inner-Radial (IRC): Neural (cranial nerves and parenchyma) and Vascular.

Endoscopic endonasal approaches (EEA) represent a medial CC to the ventral cranial base, representing viable alternatives to conventional transfacial/transoral ORCs, thereby important to the armamentarium of contemporary skull base surgery. In this report we present a 3D model of the Osseous, Vascular (Internal Carotid (ICA)) and correlative Cranial Nerve (CN) framework. We add in relevant CN and soft-tissue envelopes from the ventral perspective. For the Median plane will focus on the IRCs lateral to the respective ICA segments and the respective ORCs (frontal, ethmoid, sphenoid, and parapharyngeal).

Methods: EEA dissections were performed in 5 cadaveric specimens. Topography of the neurovascular structures within the osseous framework of the cranial base was recorded in 3D and correlated with orthogonal and radially-stacked views using a 3D rendering software (Synaptive Medical, Toronto, ON) with multimodality imaging. These 3D rendered views were then combined with EEA views using a template of ICA coursing through the respective osseous framework to create an anatomic background palate. On this platform, position of CN nerves was superimposed by individually dissecting them and correlating them with 3D rendering. Finally, a soft-tissue envelope of relevant muscles was layered on top of the IRC to provide a comprehensive ventral perspective of EEA median corridor selected.

Results: Based on the ICA and relevant cranial nerves the IRCs for EEAs to the median ventral cranial base may be described as 3 distinct modules with the lateral boundaries of each zone confined by the respective cranial nerve: (1) Zone I – Inferior clival module, extends from the hard palate and the Atlas inferiorly to the level of foramen lacerum superiorly and confined inferior and posterolaterally by the hypoglossal canal, Jugular foramen, and superolaterally by the abducens nerve entering Dorello’s canal, (2) Zone II – Sphenoidal module, extends from the sphenoid sinus floor inferiorly to the tuberculum sellae superiorly, and confined laterally by sphenoid sinus walls. (3) Zone III – Anterior fossa zone – extends from tuberculum sellae posteriorly to the frontal sinus anteriorly, confined laterally by the lamina papyracea.

Conclusion: The median ventral cranial base is directly accessible via anteromedial circumferential endonasal corridor. The proposed classification divides the median region into 3 modules based on the correlative inner radial corridor bound by the position of the respective cranial nerves and the relationship with the ICA. This has proven to translate operatively independent of pathology subtypes. Therefore, this classification provides a practical algorithmic guide that further refines our previously described approaches to median cranial base lesions, respecting the plane of the cranial nerves as the defining anatomic determinant of the IRC.