The epitrigeminal approach to the brainstem

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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.

ABBREVIATIONS CN = cranial nerve; CST = corticospinal tract; HCP = Human Connectome Project; HDFT = high-definition fiber tractography.

Article Information

Correspondence Georgios Zenonos, Department of Neurosurgery, Presbyterian Hospital, 200 Lothrop St., Ste. B400, Pittsburgh, PA 15213. email: zenonosg@upmc.edu.

INCLUDE WHEN CITING Published online August 25, 2017; DOI: 10.3171/2016.12.JNS162561.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.

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Figures

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    Surgically relevant measurements of the CSTs from the epitrigeminal safe entry zone. An axial image at a level close to the rostral end of the trigeminal entry zone approximates the level of the epitrigeminal zone. DD = direct distance of the posterolateral corticospinal fibers from the epitrigeminal zone; ITL = intertrigeminal line extending across the epitrigeminal zones on either side; VD = vertical distance—the distance of the posterolateral CSTs from the intertrigeminal line.

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    Preoperative and postoperative MR imaging. A–C: Preoperative axial, coronal, and sagittal T2-weighted images, respectively, of the cavernous malformation. An 11.2-cm3 multiloculated hemorrhagic mass spanned the entire left ventrolateral pons, extending both above and below the level of the trigeminal nerve, down to the pontomedullary fissure. D: Postoperative axial T2-weighted image showing no evidence of residual cavernous malformation.

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    Key surgical steps for the epitrigeminal approach to the brainstem via a left subtemporal transtentorial corridor. A: After elevation of the temporal lobe, CN III and the superior petrosal sinus (Sup. Petr. Sinus) are visualized. B: Arachnoid dissection in the ambient cistern brings CN IV and the posterior cerebral artery (PCA) into view. C: Tentorial cuts: a horizontal cut in parallel to the superior petrosal sinus starting at the point where CN IV crosses the tentorium, and a vertical cut across the superior petrosal sinus. D–F: The steps of the tentorial cuts in our case. G: The tentorial leaflet is reflected, revealing CN V. The motor fibers of CN V are stimulated. H: The area just above the trigeminal entry zone, the epitrigeminal area (ETA) is coagulated with bipolar electrocautery. I: After a short parenchymal dissection, the cavernous malformation (CM) is exposed and resected in a piecemeal fashion.

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    Pontine anatomy. A: Gross anatomy of the pons. B: Projection of descending fibers of the crus cerebri in the pons. On the left, the corticospinal fibers (or CSTs) are seen occupying the middle two-thirds of the descending fibers. These fibers are surrounded by frontopontine fibers medially, and temporopontine fibers laterally. C: Partial white matter dissection in the pons showing the descending CST intersecting the transverse pontine fibers. D: View from a subtemporal-transtentorial surgical corridor showing the relationship of the descending tracts with the epitrigeminal area. After removing the superficial transverse pontine fibers we see, from rostral to caudal (right to left), the frontopontine, corticospinal, and temporopontine fibers, respectively. E–H: The trapezoid body divides the pons into the tegmentum and basis pontis. The paired medial lemnisci lie in the anterior tegmentum, whereas the spinal and lateral lemnisci lie just lateral and slightly posterior to the medial lemnisci. In the basis pontis, small condensations of gray matter form the pontine nuclei, which give rise to the transverse pontine fibers. E: Internal pontine anatomy at the level just above the trigeminal entry zone. F: Corresponding HDFT image showing the fiber tracts at the same level. G: Internal pontine anatomy at the level just above the pontomedullary fissure. H: Corresponding HDFT image showing the fiber tracts at the same level. ETA = epitrigeminal area; FPF = frontopontine fibers; ICP = inferior cerebellar peduncle; LL = lateral lemniscus; MCP = middle cerebellar peduncle; ML = medial lemniscus; MLF = medial longitudinal fasciculus; MN V = motor nucleus of CN V; SCP = superior cerebellar peduncle; SN = substantia nigra; STT = spinothalamic tract; TPF = temporopontine fibers; TST = tectospinal tract; VN = vestibular nuclei.

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    High-definition fiber tractography studies showing the safe entry zones to the ventrolateral pons. The safe entry zones are shaded in red. A: The peritrigeminal entry zone extends from the trigeminal entry zone above, to the level just anterior to the facial nerve below. B: The lateral transpeduncular approach starts by transgression of the lateral fibers of the brachium pontis. Subsequently, as shown by the red arrow, following a rostro-ventro-medial course within the middle cerebellar peduncle, the surgical corridor passes above the intrapontine segment of the trigeminal nerve to reach the rostral ventrolateral pons. C: The supratrigeminal safe entry zone is located right beneath the oculomotor nerve and disrupts only some of the frontopontine fibers. D: The epitrigeminal entry zone is located just above the posterior margin of the trigeminal entry zone. See Fig. 4 legend for definitions of abbreviations.

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    Angle of attack of safe entry zones and associated surgical corridors to the ventrolateral pons. The epitrigeminal approach through a subtemporal transtentorial surgical corridor (light blue) has an angle of attack that better approximates the long axis of the brainstem, providing good rostrocaudal flexibility. The peritrigeminal approach (light red), and the lateral transpeduncular approaches (light green) through a retrosigmoid surgical corridor are best suited for lesions extending below and above the level of the trigeminal nerve, respectively. The supratrigeminal entry zone has a long surgical corridor provided by an orbitozygomatic craniotomy, and is significantly limited by the proximity to the CSTs.

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