The Intracranial and Intracanalicular Optic Nerve as Seen Through Different Surgical Windows: Endoscopic Versus Transcranial

Abstract

Surgically manageable lesions involving the intracranial or intracanalicular portions of the optic nerve (cranial nerve II) can be approached through several different operative windows. Given the complex anatomy of the optic nerve and its surrounding neurovascular structures, it is essential to understand the conventional and topographic anatomy of the optic nerve from different surgical perspectives as well as its relationship with surrounding structures. We describe the intracranial and intracanalicular course of the optic nerve and present an analytical evaluation of the degree of exposure provided by several different transcranial and endoscopic surgical approaches. Using 12 cadaveric specimens (24 sides), pterional, frontotemporal-orbital, supraorbital, unilateral subfrontal, and extended endonasal approaches were performed. The transcranial approaches were extended by removing the anterior clinoid process, unroofing the optic canal, and/or cutting the falciform ligament. The endonasal approach was extended using the transplanum transtuberculum, transmedial optic carotid recess, and transcanalicular modifications. The optic nerve was divided into proximal intracranial, distal intracranial, and intracanalicular segments, which were each divided coronally into quadrants and subquadrants, to evaluate their degree of exposure in each approach. The pterional approach provided 135° of exposure along the superolateral aspects of the entire intracranial optic nerve, and 225° of exposure of the intracanalicular optic nerve. The supraorbital and subfrontal approaches provided similar degrees of exposure, with 225°-270° of superolateral and superomedial exposure of the nerve along its intracranial and intracanalicular segments, depending on the approach extension used, with the subfrontal approach allowing for more medial control of the nerve. The endoscopic endonasal approach provided access to the inferior and medial quadrants of the optic nerve, allowing for 180° of exposure. Although the pterional approach provides the widest degree of surgical exposure of all optic nerve segments, the inferior and medial quadrants of the nerve can be adequately exposed only through an endoscopic endonasal approach. Optimal approach selection based on the intended target quadrant is essential for safe surgical exposure of the optic nerve.