Endoscopic endonasal approach to the ventral brainstem: anatomical feasibility and surgical limitations

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OBJECTIVE

Sporadic cases of endonasal intraaxial brainstem surgery have been reported in the recent literature. The authors endeavored to assess the feasibility and limitations of endonasal endoscopic surgery for approaching lesions in the ventral portion of the brainstem.

METHODS

Five human cadaveric heads were used to assess the anatomy and to record various measurements. Extended transsphenoidal and transclival approaches were performed. After exposing the brainstem, white matter dissection was attempted through this endoscopic window, and additional key measurements were taken.

RESULTS

The rostral exposure of the brainstem was limited by the sella. The lateral limits of the exposure were the intracavernous carotid arteries at the level of the sellar floor, the intrapetrous carotid arteries at the level of the petrous apex, and the inferior petrosal sinuses toward the basion. Caudal extension necessitated partial resection of the anterior C-1 arch and the odontoid process. The midline pons and medulla were exposed in all specimens. Trigeminal nerves were barely visible without the use of angled endoscopes. Access to the peritrigeminal safe zone for gaining entry into the brainstem is medially limited by the pyramidal tract, with a mean lateral pyramidal distance (LPD) of 4.8 ± 0.8 mm. The mean interpyramidal distance was 3.6 ± 0.5 mm, and it progressively decreased toward the pontomedullary junction. The corticospinal tracts (CSTs) coursed from deep to superficial in a craniocaudal direction. The small caliber of the medulla with very superficial CSTs left no room for a safe ventral dissection. The mean pontobasilar midline index averaged at 0.44 ± 0.1.

CONCLUSIONS

Endoscopic endonasal approaches are best suited for pontine intraaxial tumors when they are close to the midline and strictly anterior to the CST, or for exophytic lesions. Approaching the medulla is anatomically feasible, but the superficiality of the eloquent tracts and interposed nerves limit the safe entry zones. Pituitary transposition after sellar opening is necessary to access the mesencephalon.

ABBREVIATIONS AMZ = anterior mesencephalic zone; CST = corticospinal tract; ICD = intercarotid artery distance; PBMI = pontobasilar midline index.

Article Information

Correspondence Theodore H. Schwartz, Department of Neurological Surgery, NewYork-Presbyterian Hospital, Weill Cornell Medical College, 525 East 68th St., Box 99, New York, NY 10065. email: schwarh@med.cornell.edu.

INCLUDE WHEN CITING Published online January 13, 2017; DOI: 10.3171/2016.9.JNS161503.

Drs. Essayed and Singh contributed equally to this work.

Disclosures Dr. Schwartz reports holding stock in VisionSense and consulting for Karl Storz.

© AANS, except where prohibited by US copyright law.

Headings

Figures

  • View in gallery

    Pontine exposure before (left) and after (right) vessels displacement and white matter dissection. a/b = PBMI; IPD = interpyramidal distance; LPD = lateral pyramidal distance; PTW = pyramidal tract width; SCA = superior cerebellar artery; III = oculomotor nerve; V = trigeminal nerve; VI = sixth cranial nerve.

  • View in gallery

    Skull base projection of the lateral limitations of endoscopic exposure. Carotids = carotid arteries.

  • View in gallery

    Left: Axial schematic representation of the medulla at the level of the hypoglossal nerve exit. The red triangle represents the endoscopic tangential view of the olivary body, with the hypoglossal roots in the way. Copyright Theodore H. Schwartz. Published with permission. Right: Endoscopic view of the medulla. XII = hypoglossal nerve roots; Ver = vertebral artery.

  • View in gallery

    Left: Pontomedullary attainable exposure with keyhole anatomical structures. Right: Basilar trunk perforators. AICA = anterior inferior cerebellar artery; B = basilar trunk; C = carotid artery (intrapetrous); Pit. Gd = pituitary gland; SCA = superior cerebellar artery; Vert = vertebral artery; III = oculomotor nerve; V = trigeminal nerve; VI = sixth nerve; VII = facial nerve.

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    Illustrations demonstrating the brainstem exposure (A), the underlining CSTs (B), and deeper tracts and nuclei (C). Copyright Theodore H. Schwartz. Published with permission.

  • View in gallery

    Axial schematic representation of the microscopic and endoscopic exposure of the pons, at the level of the trigeminal nerve exit, with normal anatomy (A), a paramedial tumor (B), and midline tumor (C). Blue triangles represent the microsurgical view; red triangles represent the endoscopic view. Midline tumors restrict the microscopic anterolateral approach window, while they are still accessible through the endoscopic approach. Copyright Theodore H. Schwartz. Published with permission.

  • View in gallery

    Brainstem sagittal section with the principal nuclei and tracts. The blue triangle represents the endoscopic upward dissecting trajectory inside the pons, avoiding the sixth and facial nerve fibers. Copyright Theodore H. Schwartz. Published with permission.

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