Endoscopic Approaches to the Paramedian Skull Base: Quantitative Analysis and Comparison of Exposure and Surgical Freedom between the Endonasal and Contralateral Sublabial-Transmaxillary Approaches

Background Exposure of deep-seated lesions requires complicated endoscopic maneuvers. The expanded endonasal approach (EEA) to the paramedian skull base is limited laterally by the petrous and paraclival segments of internal carotid arteries (ICAs). The endoscopic contralateral sublabial-transmaxillary approach (CTMA) provides a direct surgical corridor to the paramedian skull base and affords navigation around the ICA and foramen lacerum.

Objective To quantify and compare the exposure, surgical freedom, angles of attack, turning angles, and angular diameters of the corridors afforded by EEA and CTMA for various targets at the paramedian skull base.

Methods Five colored-injected cadaveric specimens were dissected bilaterally (10 sides). Dissections of the middle and inferior transclival corridor were combined with infrapetrous and supracondylar lateral expansion. Targets of interest included the contralateral root entry zones (REZs) and dural entry points (DEPs) of the sixth, seventh/eighth, and lower cranial nerves (CNs). In addition, we measured the distances among the foramen lacerum, REZs, anterior inferior cerebellar artery origin, and vertebrobasilar junction. The latter were used to determine the angular diameter of the surgical corridor to brain stem targets in each approach. Furthermore, for each target in each approach, the surgical freedom, angles of attack in the horizontal and vertical planes, and the turning angles or angular change of the trajectory required to circumvent the ICA and foramen lacerum were documented using frameless stereotaxy. Outcomes were statistically compared between approaches.

Results Compared with EEA, the CTMA offered significantly greater surgical freedom to all targets except for the REZ of CN XII, where the difference between approaches was not statistically significant. CTMA afforded a maximum allowable surgical freedom at the DEP of CNs VII/VIII (CTMA = 888.63 cm² vs. EEA = 449.33 cm²; p = 0.0002). By contrast, EEA achieved a maximum allowable surgical freedom at the DEP of CN VI; yet, remained inferior to CTMA. The angles of attack were significantly different between approaches, with CTMA offering superior vertical and horizontal attack angles to all the targets of interest. Moreover, except for the REZ of CN XII, CTMA provided significantly wider turning angles to each of the targets in question. CTMA also granted significantly wider angular diameters when brain stem targets were approached.

Conclusion CTMA provides a minimally invasive option and complements EEA for the dissection of deep-seated lesions of the paramedian skull base. In contrast to EEA, the CTMA provides greater surgical freedom and angles of attack to structures of the paramedian skull base, precluding the need for ICA lateralization and dissection of the foramen lacerum.