The Endoscopic Endonasal Approach to the Inferior Clivus and Foramen Magnum: Anatomical Landmarks in the Surgical Window

Introduction: With the advent of the endoscopic era, the endoscopic endonasal approach (EEA) has become a viable option in the management of intradural and extradural skull base lesions involving the lower clivus and foramen magnum, particularly, for pathology ventromedial to the brain stem. Given the novelty of this approach, we sought to characterize practical landmarks in the surgical corridor through radiographic analysis.

Methods: A total of 10, high-resolution (1.0 mm slice) computed tomography (CT) scans with contrast were acquired retrospectively from the authors' database of patients with normal sinonasal, skull base, and vascular anatomy. Cephalometric measurements were obtained using the AGFA Healthcare IMPAX Software.

Results: At the superior aspect, the mean distance between the paraclival segments of the internal carotid artery (ICA) was 17.21 mm (range, 8.7–24.1 mm). The mean distance from the inferior wall of the sphenoid sinus to the ring of the C1 bone was 32.83 mm (range, 29.4–41.8 mm). On the posterolateral aspect, the mean distance between the vertebral arteries at the level of the foramen magnum and the mean distance between the intradural aspect of the hypoglossal canal was 18.95 mm (range, 13.8–24.8 mm) and 32.03 (range, 28.6–35.5 mm), respectively. Regarding the vertebrobasilar junction (VBJ), the mean distance from the foramen magnum to the VBJ was 26.03 mm (range, 22.8–29.8 mm). In the anteroposterior axis, the mean distance from the dorsal clivus to the VBJ was 2.88 mm (range, 1.7–7.5 mm).

Conclusion: The EEA provides a direct surgical window to the lower clivus and foramen magnum while improving visualization and minimizing tissue displacement. Through understanding the dimensions achieved in this potential corridor, surgical planning can be improved when considering the management of skull base pathology. With radiographic analysis, we were able to understand the limits of intradural and extradural dissection for lesions involving the compartment ventromedial to the brain stem.