Endoscopic Endonasal Approach to Intrinsic Brainstem Lesions: Anatomical, Radiological, and Clinical Study

Background: Intrinsic brainstem lesions represent a surgical challenge. When surgery is indicated, the selection of surgical approach is based on the relative location of the lesion to surrounding fiber tracts. Typically, the ideal approach is the one providing direct access to the most superficial aspect of the lesion to minimize damage to normal brainstem tissue. For lesions that are located predominantly ventrally, the endoscopic endonasal approach (EEA) has been proposed as a potential alternative.

Objectives: To describe the surgical anatomy of the ventral brainstem fibers using white-matter dissection and high-definition fiber tractography (HDFT) to understand the brainstem regions that can be approached through endoscopic endonasal route determining the potential surgical indications for this ventral access.

Method: Five formalin-fixed brainstem where prepared for white-matter dissection using a modification of the Klingler’s technique. HDFT was performed on 10 healthy volunteer-subjects. The clinical case of a 56-year-old woman presenting with a partially exophytic pontine lesion was reviewed. She underwent pre and post-operative HDFT scan and transclival EEA.

Results: The main white-matter tract on the ventral aspect of the brainstem is the corticospinal tract (CST). It travels as a compact bundle at the middle 3/5 of each cerebral peduncle continuing its trajectory through the pons. Here, the pontine nuclei, the VI Cranial Nerves (CNs) on each side of the midline and the transverse fibers of the middle cerebellar peduncle (MCP) intercalate with its fibers making it “loose” until the pontomedullary junction where it continues descending as a compact tract on the medullary pyramids. The MCP represents along the CST the most important white-matter structures occupying the ventrolateral pons surface. The closest CST relationship with the CNs is in the midbrain the III CNs which run medially to it. At the pons the sensory and motor roots of the V CN and the intrapontine segment of the VII CN are posterolateral to the CST, the VI CN as described above. At the medulla the XII CNs are laterally to the CST. Additionally, important vascular structures over the ventral brainstem surface should be taken into consideration. A case of a 56-year-old female with a pontine glioma and right-side hemiparesis is presented. HDFT revealed posteromedially displaced and partially disrupted right-CST. The MCP was severely disrupted, serving as an entry point. Transclival EEA was performed with the aid of image-guided navigation and intraoperative-neuromonitoring of motor-evoked potentials. Postsurgical scan showed complete resection of the exophytic component, with residual diffuse tumor within the brainstem. HDFT study showed recovery of the normal trajectory and preservation of the right-CST. Patient developed transient worsening of her right-side hemiparesis which improved at 3-month follow-up and right VI CN palsy with partial improvement. Pathology revealed a grade 4 glioma and patient passed-away months later due to diseases progression.

Conclusion: The integrated use of new technologies into the skull base field may allow to further increase the safety of this minimal-invasive approach. Although further studies are warranted, we believe that this is a promising combination of advanced techniques, contributing to innovate the skull base surgery.