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Georgios Andrea Zenonos, David Fernandes-Cabral, Maximiliano Nunez, Stefan Lieber, Juan Carlos Fernandez-Miranda and Robert Max Friedlander

OBJECTIVE

Surgical approaches to the ventrolateral pons pose a significant challenge. In this report, the authors describe a safe entry zone to the brainstem located just above the trigeminal entry zone which they refer to as the “epitrigeminal entry zone.”

METHODS

The approach is presented in the context of an illustrative case of a cavernous malformation and is compared with the other commonly described approaches to the ventrolateral pons. The anatomical nuances were analyzed in detail with the aid of surgical images and video, anatomical dissections, and high-definition fiber tractography (HDFT). In addition, using the HDFT maps obtained in 77 normal subjects (154 sides), the authors performed a detailed anatomical study of the surgically relevant distances between the trigeminal entry zone and the corticospinal tracts.

RESULTS

The patient treated with this approach had a complete resection of his cavernous malformation, and improvement of his symptoms. With regard to the HDFT anatomical study, the average direct distance of the corticospinal tracts from the trigeminal entry zone was 12.6 mm (range 8.7–17 mm). The average vertical distance was 3.6 mm (range −2.3 to 8.7 mm). The mean distances did not differ significantly from side to side, or across any of the groups studied (right-handed, left-handed, and ambidextrous).

CONCLUSIONS

The epitrigeminal entry zone to the brainstem appears to be safe and effective for treating intrinsic ventrolateral pontine pathological entities. A possible advantage of this approach is increased versatility in the rostrocaudal axis, providing access both above and below the trigeminal nerve. Familiarity with the subtemporal transtentorial approach, and the reliable surgical landmark of the trigeminal entry zone, should make this a straightforward approach.

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Georgios A. Zenonos, Samir Sur, Maximiliano Nuñez, David T. Fernandes-Cabral and Jacques J. Morcos

In this 3D video we review the case of a lower pontine cavernous malformation in a 31-year-old man who presented with hemiparesis and an abducens palsy. The cavernous malformation was completely resected through a far lateral approach and a peritrigeminal brainstem entry zone, with a significant improvement in the patient’s hemiparesis. The relevant anatomy is reviewed in detail through multiple anatomical brainstem dissection specimens, as well as high-definition fiber tractography images. The rationale for the approach is analyzed relative to other possible options, and a number of technical pearls are provided.

The video can be found here: https://youtu.be/fH2Q7RjlBKQ.

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Osamu Akiyama, Ken Matsushima, Maximiliano Nunez, Satoshi Matsuo, Akihide Kondo, Hajime Arai, Albert L. Rhoton Jr. and Toshio Matsushima

OBJECTIVE

The lateral recess is a unique structure communicating between the ventricle and cistern, which is exposed when treating lesions involving the fourth ventricle and the brainstem with surgical approaches such as the transcerebellomedullary fissure approach. In this study, the authors examined the microsurgical anatomy around the lateral recess, including the fiber tracts, and analyzed their findings with respect to surgical exposure of the lateral recess and entry into the lower pons.

METHODS

Ten cadaveric heads were examined with microsurgical techniques, and 2 heads were examined with fiber dissection to clarify the anatomy between the lateral recess and adjacent structures. The lateral and medial routes directed to the lateral recess in the transcerebellomedullary fissure approach were demonstrated. A morphometric study was conducted in the 10 cadaveric heads (20 sides).

RESULTS

The lateral recess was classified into medullary and cisternal segments. The medial and lateral routes in the transcerebellomedullary fissure approach provided access to approximately 140º–150º of the posteroinferior circumference of the lateral recess. The floccular peduncle ran rostral to the lateral recess, and this region was considered to be a potential safe entry zone to the lower pons. By appropriately selecting either route, medial-to-lateral or lateral-to-medial entry axis is possible, and combining both routes provided wide exposure of the lower pons around the lateral recess.

CONCLUSIONS

The medial and lateral routes of the transcerebellomedullary fissure approach provided wide exposure of the lateral recess, and incision around the floccular peduncle is a potential new safe entry zone to the lower pons.

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Sukhdeep S. Jhawar, Maximiliano Nunez, Paolo Pacca, Daniel Seclen Voscoboinik and Huy Q. Truong