Microsurgical Anatomy of the White Matter Tracts of the Ventral Brainstem through the Expanded Endonasal Approach

Introduction: Expanded endonasal approach )EEA) is a minimally invasive surgical technique that has the potential to access lesions of the ventral skull base and corresponding extracranial compartments, including tumors, aneurysms, compression syndromes )i.e., rheumatoid pannus) and brainstem cavernomas. This approach can expose two planes in the skull base: the median and the paramedian. The median plane can be accessed via the following approaches: )1) transfrontal, )2) transcribriform, )4) transplanum, )5) transsphenoidal, )6) transclival, and )7) transodontoid. We here describe three EEA modules )transsphenoidal, transclival and transodontoid) to reach the ventral brainstem. Based on these modules, radial corridors, or anatomic constructs, were designed in an effort to locate safe working areas within the brainstem and also locate important neurovascular structures.

Method: Six cadaver heads, injected with blue and red silicon, were used. The cadavers were also CT-scanned and images were coregistered to allow for real-time navigation of osseous structures. The internal anatomy of the ventral brainstem was studied by Klingler’s technique under an endoscopic vision )0o and 45o). The areas studied were in the mesencephalon )perioculomotor zone), in the pons )the peritrigeminal zone), and the medulla )supraolivary fossette).

Result: The outer radial corridor )ORC) consisted of the superior, middle and inferior turbinates and the sella and the inner radial corridor )IRC) comprised of neurovascular structures. The relationship was as follows: )1) the sellar region was involved with the basilar bifurcation, the third nerve and the perioculomotor zone; )2) the superior turbinate was involved with the basilar trunk and the peritrigeminal zone; )3) the middle turbinate was involved with the VBJ and the VI, VII and VIII; and 4) the inferior turbinate was involved with the vertebral artery and the supraolivary fossae. The use of a 30-degree endoscope allowed for adequate visualization.

Conclusion: Understanding the ORC and IRC via EEA to the ventral brainstem can aid the surgeon in planning a safe entry zone. There are certain limitations in entering the ventral brainstem, as certain lesions such as cavernomas can alter surrounding morphology, especially neurovascular structures. Thereby, the concomitant application of the ORC and IRC with tractography and intraoperative neuronavigation can potentially solve this problem.