Selecting the Skull Base Approach to the Anterolateral Midbrain and Pons: Quantitative Three-Dimensional Reconstruction of Operative “Cones” to Cranial Nerve Origins

Introduction: Several skull base approaches were devised to offer wider, shorter and more direct views to different levels of the anterolateral pons and midbrain. Such perspectives were traditionally assimilated to “cones” or “pyramids,” and separate morphometric studies were performed for different quantitative comparisons. Since these approaches overlap and lesions tend to extend beyond a single level, choosing the appropriate surgical strategy may be difficult. We intend to accurately reconstruct these “cones” through a continuum of dissections toward cranial nerve origins, and present illustrative surgical cases.

Methods: Three cadaveric specimens (six sides) were imaged using computed tomography and magnetic resonance imaging frameless navigation. The following approaches were performed in a stepwise fashion: orbitozygomatic pterional (OZP), pretemporal transcavernous (PTC), anterior petrosal (AP), combined petrosal (CP), retrosigmoid (RS), and extended retrosigmoid (ERS). We recorded the Cartesian coordinates of cranial nerve origins and those of the extreme points of exposure (deep working “W” and superficial “S” areas). Such areas were measured and the ratio of surface exposure (S)/depth (d) was deduced. A computer three-dimensional reconstruction of operative views to cranial nerve targets was made on preoperative MRI.

Results: Quantitative three-dimensional operative cones were reconstructed. The transcavernous approach increased the deep working area W around CN3 compared with the OZP from 42.4 to 327.8 mm², even though the superficial area was similar (S/d = 22.8). The AP, which can be added to the PTC, allowed for a short access to CN5 (d = 15.8 mm) but with a limited superficial area (S/d = 6.3). The ERS increased the W around CN5, 7–8 and 9–11 compared with the RS approach. The CP approach further increased the W around CN5 (227–740 mm²), but not significantly for CN7–11. Illustrative cases of posterior circulation aneurysms, cavernomas, and petroclival meningiomas were approached through the PTC and ERS approaches.

Conclusion: The PTC offers a wide working space to the interpeduncular fossa and anterolateral brainstem if the lesion's epicenter is above CN5. Limited inferior extensions can be accessed through an added AP, allowing an exposure down to CN7–8 origin. The ERS is suitable for lesions below CN5, but at the expense of a greater depth compared with the CP. Analyzing the lesion's relationship to cranial nerves and against reconstructed three-dimensional surgical cones, can be helpful in selecting the appropriate approach to the anterolateral brainstem.

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

Publication History

Article published online:
12 February 2021