Endoscopic Endonasal Approach to the Ventral Midbrain for Brainstem Cavernous Malformations: An Anatomical and High-Accuracy Fiber Tractography Study

Background: The endoscopic superior transclival approach provides access to the ventral midbrain, and has been used for resection of cavernous malformations arising in this area. In this study, we sought to define the anatomical limitations of this approach using cadaveric specimens, the relevant vascular anatomy, as well as study the variations in anatomy of the eloquent white matter tracts in the ventral midbrain using high-accuracy fiber tractography (HAAFT).

Methods: With the aid of image guidance, we dissected 10 human head specimens. Endoscopic endonasal anatomical dissections were performed in a stepwise technique: step 1, standard endoscopic endonasal approach with extensive sellar and parasellar exposure; step 2, unilateral transcavernous posterior clinoidectomy; step 3, division of the ipsilateral diaphragm sella; and step 4, contralateral posterior clinoidectomy following the same steps. Additionally, the corticospinal tracts and the oculomotor nerves of 32 normal individuals from the Human Connectome Project were dissected in the DSI studio and the distance between them was studied.

Results: There was significant difference (p < 0.001) in the drilled area of the upper clivus between steps 2 (66.4% ± 10), 3 (81.5% ± 5.6), and 4 (100%). With regard to midbrain exposure, significantly greater exposure (p < 0.001) of the ipsilateral peduncle was observed in step 3 (85.8 mm² ± 23.8) compared with step 2 (61.4 mm² ± 19.9). We also observed significantly greater exposure (p < 0.001) of the contralateral peduncle after each step (step 2: 27.8 mm² ± 14.3; step 3: 46.9 mm² ± 9.9; and step 4: 75.5 mm² ± 18.8). Consecutive greater exposure of the interpeduncular fossa was also noticed among the steps. We obtained maximal length of the ipsilateral oculomotor nerve after step 2 (17.1 mm ± 2.3) and significant difference (p = 0.001) between steps 2 (5.8 mm ± 3.2), 3 (10.4 mm ± 2.3), and 4 (16.2 mm ± 2.2) regarding contralateral oculomotor nerve was observed. The average length of the P1 segments was 6.1 mm ± 1.3 and P1 was larger than PComA in 75% of cases. Thalamoperforating arteries were identified in all P1 segments (100%; mean 2.35), originated from the posterosuperior aspect of the P1. Both circumflex branches (35%; 0.7) and medial posterior choroidal arteries (20%; 0.2) arose from either the posterior or posteroinferior surface of P1. Based on HAAFT, the average distance between the two corticospinal tracts is 22.1 mm at the midlevel of the midbrain, 21.1 mm at the level of the pontomesencephalic sulcus, and 4.4 mm at the level of the midpons. The average distance of the two oculomotor nerves at the site of emergence from the midbrain is 2.1 mm.

Conclusion: From an anatomical perspective, this study quantitatively describes the advantages of the endoscopic posterior clinoidectomy for access the ventral midbrain and analyzes the importance of each particular surgical step in increasing the exposure to the ventral midbrain. Most of perforating branches originated from the posterior aspect of P1, not complicating the ventral access to the midbrain. The distance of the corticospinal tracts allows safe entry to the ventral midbrain.

Fig. 1 Stepwise dissection.

Fig. 2 Intradural anatomy.

Fig. 3 HAAFT.

No conflict of interest has been declared by the author(s).

Publication History

Article published online:
12 February 2021