E-ISSN: 1019-5157
ISSN: 2651-5024
Research
REVISITING BRAINSTEM SAFE ENTRY ZONES THROUGH STEPWISE FIBER DISSECTION
Berra Bilgin✉ ,
Mustafa Eren Yüncü ,
Ender Vergili ,
Ali Karadağ ,
Erik H. Middlebrooks ,
Mehmet Şenoğlu ,
Necmettin Tanriover
Neurosurgery, Izmir City Hospital; Neurosurgery, Istanbul University-Cerrahpasa, Cerrahpasa Faculty of Medicine; Neurosurgery, University of Health Sciences; Neurosurgery, Istanbul University - Cerrahpasa
Accepted: 20/08/2026
Article in Press
Corresponding Author:
Berra Bilgin (bilginberra@hotmail.com)
Abstract
Aim
Brainstem (BS) safe entry zones (SEZs) are commonly described as surface-defined corridors; however, their operative relevance is ultimately shaped by the depth-dependent convergence of compact longitudinal tracts and cranial nerve nuclei. We aimed to delineate the internal microsurgical architecture of the BS underlying established SEZs using a stepwise, bidirectional fiber-dissection framework supported by diffusion tractography.
Material and Methods
Fiber microdissections of the BS were performed in anterior-to-posterior and posterior-to-anterior directions, and the results were compared to those of tractography.
Results
The surface-defined permissiveness of BS SEZs rapidly diminished with depth due to the predictable convergence of critical pathways and nuclei. In the mesencephalon, narrow dorsal and lateral corridors were constrained by central longitudinal systems and adjacent tegmental nuclei. In the pons, ventral corridors were limited by the proximity of the corticospinal tract and medial lemniscus, whereas dorsal corridors centered on the facial colliculus converged toward overlapping longitudinal systems at deeper planes. In the medulla, ventrolateral corridors encountered descending motor pathways early, while dorsal and dorsolateral corridors demonstrated progressive vulnerability of sensory pathways and lower cranial nerve nuclei with deepening.
Conclusion
Multidirectional 3Dstepwise fiber-dissection model, supported by tractography, links surface SEZ landmarks to depth-dependent internal constraints and structures at risk. This depth-oriented anatomical framework may assist trajectory selection, depth-limited progression, and anticipatory risk mapping during microsurgical planning for intrinsic and exophytic BS lesions.
Brainstem (BS) safe entry zones (SEZs) are commonly described as surface-defined corridors; however, their operative relevance is ultimately shaped by the depth-dependent convergence of compact longitudinal tracts and cranial nerve nuclei. We aimed to delineate the internal microsurgical architecture of the BS underlying established SEZs using a stepwise, bidirectional fiber-dissection framework supported by diffusion tractography.
Material and Methods
Fiber microdissections of the BS were performed in anterior-to-posterior and posterior-to-anterior directions, and the results were compared to those of tractography.
Results
The surface-defined permissiveness of BS SEZs rapidly diminished with depth due to the predictable convergence of critical pathways and nuclei. In the mesencephalon, narrow dorsal and lateral corridors were constrained by central longitudinal systems and adjacent tegmental nuclei. In the pons, ventral corridors were limited by the proximity of the corticospinal tract and medial lemniscus, whereas dorsal corridors centered on the facial colliculus converged toward overlapping longitudinal systems at deeper planes. In the medulla, ventrolateral corridors encountered descending motor pathways early, while dorsal and dorsolateral corridors demonstrated progressive vulnerability of sensory pathways and lower cranial nerve nuclei with deepening.
Conclusion
Multidirectional 3Dstepwise fiber-dissection model, supported by tractography, links surface SEZ landmarks to depth-dependent internal constraints and structures at risk. This depth-oriented anatomical framework may assist trajectory selection, depth-limited progression, and anticipatory risk mapping during microsurgical planning for intrinsic and exophytic BS lesions.
Keywords
brainstem
safe entry zones
microsurgical anatomy
fiber dissection
diffusion tractography