Morphometric Data on the Working Angle and Area of Neurosurgical Exposure for Anteromedial Skull Base Lesions Using Extended Endoscopic Endonasal Transclival Approaches: Validation Using Ammirati's Maneuverability Grading System and Neuronavigation

Background: The “Far medial” and “Extreme Medial” are challenging approaches reserved for specifically trained endoscopic skull base surgeons and no published literature is available on the intraoperative maneuverability grading combined with stereotaxic navigation. We offer a comprehensive update on the morphometric data of the working angle and area of neurosurgical exposure for anteromedial skull base lesions using expanded endoscopic endonasal far medial and extreme medial approaches.

Methods: In the current study, stepwise image-guided dissections were performed in eleven colored latex-injected human heads and morphometric measurements on the inferior lateral clivus region ([Fig. 1]). The stereotactic angle of attack and the maneuverability of all relevant neurovascular structures exposed were scored using a modified Ammirati numerical grading system for further comparison and statistical analyses. The authors have performed a comprehensive literature review pertinent to the indications, limitations as well as outcomes along with technical pearls and pitfalls to avoid complications. The working angle is calculated by the degree of exposure, level of manipulability and surgical freedom, while the area of exposure is calculated by quadrangle bounded by the most lateral accessible point at the lacerum level and the most lateral accessible point at the level of the anterior arch of C1, bilaterally.

Results: The comparative results of Ammirati maneuverability score and exposure of most relevant anatomical structures from each compartment were 432 (Mean ± SD 3.92 ± 0.26) for the ventromedial compartment and 285 (Mean ± SD 2.59 ± 0.82) for the dorsolateral compartment (p < 0.05). The surgical freedom of 15.0° ± 1.9° was noted at the level of the hypoglossal nerve and 9.0° ± 1.87° at the level of the glossopharyngeal nerve, respectively (p < 0.05).

Conclusion: The extended endoscopic approaches to the ventral foramen magnum and jugular foramen using endonasal route have distinct advantages obviating brain retraction and the need for crossing major neurovascular structures in the posterior fossa. We have demonstrated cadaveric morphometric data on the working angle and area of neurosurgical exposure for anteromedial skull base lesions using endoscopic endonasal far medial approaches, with validation combining the use of Ammirati maneuverability grading system and neuronavigation.

Fig. 1