Endoscopic Endonasal Transclival Transcondylar Approach for Foramen Magnum Meningioma: Anatomical and Technical Note

Introduction: Among all meningiomas, only 1.8 to 3.2% arise at the foramen magnum region. Surgical resection is always challenging in this region, with increasing surgical complications for tumor located in the ventral aspect of the foramen magnum. The endonasal endoscopic approach has evolved to become an effective route for resecting ventral skull base lesions, and the far medial transclival approach has provided more lateral exposure of the inferior third of the clivus and the foramen magnum region. However, the unique anatomy of occipital condyle and the issue of craniocervical instability usually make resection of the medial condyle confusing. In this report, we describe the anatomic and technical nuances to safely and effectively perform an endoscopic endonasal transcondylar approach. Here, we aim to determine the amount of condyle to be resected in order to obtain appropriate exposure of the lateral wall of the foramen magnum.

Method: A total of colored silicon-injected anatomic specimens were dissected in the Surgical Neuroanatomy Laboratory at the University of Pittsburgh to simulate endonasal access to the foramen magnum region in a stepwise manner. Each specimen was imaged with a CT scan twice, predissection and postdissection. On the basis of previous anatomical work and extensive surgical experience, we define the lateral limit of an effective medial condylectomy by an imaginary line extending inferiorly from the petroclival fissure to the occipital condyle combined with exposure of the anterior cortical bone of the hypoglossal canal. The volume of each condyle was measured by software before and after CT scan, and the resection ratio of each condyle was calculated.

Result: The mean volume of condyle resection in five specimens (10 condyles) was 18% (range, 9.7– 28.3%). This amount of medial condyle resection was efficient in all specimens to expose the lateral wall of the foramen magnum and identify the entrance of the vertebral artery into the posterior fossa. We present several illustrative cases to show the surgical applicability of the proposed medial condylectomy technique.

Conclusion: The endoscopic endonasal transclival approach has the advantage of direct access to ventral foramen magnum meningiomas. Drilling the condyle medially up to the line extending inferiorly from the petroclival fissure and in the depth up to the anterior cortical bone of the hypoglossal canal exposes the foramen magnum widely with minimal resection of the condyle. Current clinical data from our institution have shown no cases of postoperative craniocervical instability when using this technique, but this remains to be confirmed by longer follow-up and further biomechanical study.