Endoscopic Endonasal Approach to the Ventral Brainstem

Objectives: Some cases of endonasal intra-axial brainstem surgery have been reported in the recent literature. We tried to assess the feasibility and limitations of endonasal endoscopic surgery for approaching lesions in the ventral brainstem.

Methods: Five human cadaveric heads were used to assess the anatomy and to report various measurements. Extended transsphenoidal and transclival approaches were performed. After exposing the brainstem, white matter dissection was attempted through this endoscopic window. Key measurements were reported though out the approach.

Results: The exposure was limited rostrally by the sella, laterally by the intra-cavernous and intra-petrous carotids. Partial of the anterior C1 arch and the odontoid process resection was necessary to achieve a satisfying caudal extension. The midline pons and medulla were exposed in all specimens. Trigeminal nerves were barely visible without angled endoscopes. Access to the peri-trigeminal safe zone for gaining entry into the brainstem is medially limited by the pyramidal tract, with a lateral pyramidal distance (LPD) of 4.8mm±0.8mm. The mean inter-Pyramidal distance was 3.6mm±0.5mm, and progressively decreased toward the pontomedullary junction. The cortico-spinal tracts (CSTs) coursed from deep to superficial in a cranio-caudal direction. The small caliber of the medulla with very superficial CSTs left no room for ventral safe dissection. The mean ponto-basilar midline index (PBMI) averaged at 0.44 ± 0.1.

Conclusions: Endoscopic endonasal approaches to the brainstem are best suited for midline pontine intra-axial tumors that are strictly anterior to the cortico-spinal tracts, or for exophytic lesions. Approaching the medulla is anatomically feasible, but the superficiality of the eloquent tracts and interposed nerves limit the safe entry zones. Pituitary transposition after sellar opening is necessary to access the mesencephalon.”