The Vagoaccessory Triangle (VAT): The Arena of ELITE

Abstract

Introduction Lesions in the cerebellomedullary angle, anterolateral aspect of lower brainstem, and anterior foramen magnum can be accessed through far lateral approach and extreme lateral infrajugular transcondylar exposure. Detailed anatomical knowledge of this region is of paramount importance to avoid untoward complications.

Objectives Comprehensive anatomy of this microsurgical corridor is sparse. Our aim was to define the anatomy of this region with relevance to the surgical approach, which would enable the surgeon to navigate this region with ease during surgery.

Materials and Methods Cadaveric dissection was performed with focus on the various anatomical structures, which are of relevance to the operating surgeon. The same was correlated with the computerized scan and magnetic resonance imaging brain findings and intraoperative findings. The observations are presented.

Discussion Through the far lateral approach, the vagoaccessory triangle (VAT) and three subtriangles within it are exposed. Lateral boundary of the VAT is formed by the spinal accessory nerve, which follows a superolateral trajectory toward the jugular foramen. Medial boundary is formed by the medulla oblongata. Superior boundary is formed by the vagus nerve, which courses horizontally toward the jugular foramen. Rootlets of the hypoglossal nerve divide the VAT into infra-hypoglossal, supra-hypoglossal, and hypoglossal–hypoglossal subtriangles. The floor is formed by the inferior clivus, anterior foramen magnum, occipital condyle, and jugular tubercle. The roof is formed by the dura covering the lower part of the posterior fossa, biventral lobule, and tonsil of the cerebellum. The VAT contains three cisterns compartmentalized by three membranes, three nerves, and a fourth segment of vertebral artery (V4) with its three branches, in three subtriangles. Cisterns are the premedullary, lateral cerebellomedullary, and posterior cerebellomedullary. Membranes are the medial pontomedullary membrane, lateral pontomedullary membrane, and denticulate ligament. Nerves are the rootlets of hypoglossal, cranial accessory, and first cervical nerve. Posterior inferior cerebellar artery, anterior spinal artery, and posterior spinal artery are the three branches of V4.

Conclusion Detailed knowledge of each anatomical structure and its relationship would enable the surgeon to operate safely within the corridors naturally present in this region.

Keywords

vagoaccessory triangle - cerebellomedullary cistern - foramen magnum meningiomas - anterior brainstem lesions - far-lateral transcondylar approach - lower cranial nerves - hypoglossal nerve

Introduction

The far lateral approach introduced by Heros et al,[1] which was modified to extreme lateral infrajugular transcondylar-transtubercular exposure (ELITE), is the surgical corridor for accessing various lesions in the cerebellomedullary cistern (CMC) including those situated anterior to the lower brainstem and in the foramen magnum.[1] [2] Rodríguez-Hernández and Lawton described the vagoaccessory triangle (VAT) and based upon the relationship of the contents with the hypoglossal nerve divided the region as supra-hypoglossal and infra-hypoglossal triangles.[3] Rootlets of lower cranial nerves create a web that limits microsurgical maneuverability in the CMC.[4] Therefore, comprehensive understanding of microanatomy of CMC and VAT facilitates safer microsurgery.

Objectives

Comprehensive anatomy of the VAT is sparse in the literature. Here, we describe the detailed anatomy of VAT and correlate it with radiological anatomy and intraoperative picture from the surgeon's perspective.

Materials and Methods

Cadaveric dissection was performed with focus on the various anatomical structures, which are of relevance to the operating surgeon. The same was correlated with the intraoperative findings of space-occupying lesions in the region of VAT operated by the surgical team. The anatomical observations that are relevant to the surgeon only are presented here.

Observations and Discussion

Boundaries of VAT ([Fig. 1]) are superiorly the vagus nerve, laterally the spinal root of the accessory nerve, and medially the medulla oblongata.[3] In the far lateral approach, within the surgical field the accessory nerve as it emerges at the lateral aspect of the cisterna magna (CM) follows a superolateral trajectory from its spinal origin and enters the jugular foramen along with the vagus nerve. Its long cisternal segment receives nerve rootlets from the cervical spinal cord and the lower aspect of the posterolateral medullary sulcus (retro-olivary sulcus) ([Fig. 1A, B]).

Fig. 1 Showing the boundaries and three subtriangles of vagoaccessory triangle (VAT) in a far lateral approach/extreme lateral infrajugular transcondylar-transtubercular exposure (ELITE), with its contents. (A) Line diagram. (B) Cadaveric dissection.

The vagus nerve emerges as several rootlets from the posterolateral medullary sulcus (retro-olivary sulcus) at the rostral ventrolateral medulla. It courses through the lateral CMC (LCMC) horizontally and exits through the pars vasculosa of the jugular foramen between the 9th and 11th nerves. The superior border separates the VAT from the glossopharyngeal–cochlear triangle. The lateral border demarcates the lateral CMC corridor between the accessory nerve and the adjacent dura (after division of the denticulate ligament). The superomedial angle is at the point of lowest rootlet of the vagus nerve emerging from the retro-olivary sulcus. The superolateral angle is at its meeting point with the 11th nerve. The inferior angle is at the origin of the lowest rootlet joining the 11th nerve above the 2nd cervical nerve ([Fig. 1B]).

The cerebellomedullary fissure separates the cerebellum from the medulla. This fissure is at the dorsolateral aspect of the inferior half of the roof of the fourth ventricle. Its anterior wall is formed by the posterior surface of the medulla, the inferior medullary velum, and the tela choroidea. Its posterior wall medially is formed by the uvula, nodule, and the vermis, and laterally by the tonsil and biventral lobule.

Floor

Floor is formed by the internal surfaces of inferior clivus, anterior foramen magnum, occipital condyle (OC), and jugular tubercle (JT) ([Fig. 2A]). Basilar plexus runs over the clivus, and the marginal sinus passes along the margin of foramen magnum. A branch of distal vertebral artery (VA) supplies the dura.[5] The inlet of the hypoglossal canal (HC) is at the middle third of the medial surface of the OC.[6] It is approximately 5 mm superior to the inferior condylar surface and approximately 5 mm inferior to the JT[7] ([Fig. 2A]). The distance of the intracranial end of the HC from the posterior margin of the OC is 12 mm. In 20% of skulls OCs protrude significantly to the foramen magnum.[8]

Fig. 2 Cadaveric dissection showing the boundaries and contents of vagoaccessory triangle (VAT): (A) Bony floor, (B) contents of the floor and their relationship (medulla is retracted posteriorly), and (C) roof of the VAT.

JT is a rounded and oval elevation upon superior (internal) surface of the OC. It is located at the junction of the basilar part and lateral part of the occipital bone on either side of the foramen magnum. It is just medial to the jugular foramen, which forms the superolateral angle of the VAT. It is situated superior to the HC and is situated anterosuperior to the internal opening of this canal. The 9th, 10th, and 11th nerves pass across[9] the posterior part of the JT to reach the jugular foramen ([Fig. 2B]). The relationship between the JT, VA, posterior inferior cerebellar artery (PICA), and vertebrobasilar junction (VBJ) is variable.

In the anatomical position, the floor is in three different planes; the clivus sloping posteroinferiorly in the sagittal plane, medial surfaces of anterior foramen magnum and OC placing vertically in the coronal plane, and JT sloping posteroinferiorly and laterally from the superior surface of the OC in axial plane ([Fig. 2B]). In far lateral approach and ELITE, the floor of the VAT is more or less in the same plane, the JT making a prominence on the floor. However, in this approach the floor is not parallel to the VAT but is at an angle to the triangle meeting at the lateral border of the triangle. The OC and the JT are the main bony prominences obstructing the anterolateral surface of the brainstem in far lateral approach/ELITE (green line) ([Fig. 3A]).[10]

Fig. 3 Radiologic anatomy showing the contents of the floor of the vagoaccessory triangle (VAT). (A) Computed tomography (CT) scan showing the contents of VAT. (B) CT coronal section through the odontoid process showing the “double eagle on mountain top” sign. (C) Magnetic resonance imaging (MRI) axial section showing the normal anatomy of VAT. (D) MRI scan showing the tumor displacing the medulla.

Radiological Anatomy of VAT

The bony floor of the VAT can be appreciated in bone window slices of the axial computed tomography (CT) scan ([Fig. 3A]). The OC, JT, and HC can be identified on a coronal section through the odontoid process (OP) by the “double eagle on mountain top” sign[11] ([Fig. 3B]). JT makes the beak, head, and neck of the eagle. HC lies underneath the beak and neck of the eagle. OC forms the torso of the eagle. The OP corresponds to the mountain ([Fig. 3B]). Study of these images greatly assists the surgeon in the ELITE approach. The magnetic resonance (MR) imaging scan images show the soft tissue structures in the VAT, the cerebellum, brainstem, VA, and the course of the cranial nerves ([Fig. 3C, D]).

Roof

Roof is formed by the dura covering the lower part of the posterior fossa, the tonsil (T) and the biventral lobule (BV) of the cerebellum ([Fig. 4A]). The tonsil is separated from the uvula (U) by tonsilouvular fissure (green curve) ([Fig. 2C]), and from the biventral lobule by the tonsilo-biventral fissure (light blue curve) ([Fig. 2C]). Yellow arrow ([Fig. 2C]) shows the cerebellomedullary fissure. The lateral recess of the fourth ventricle and the flocculus of the cerebellum overlap the superior margin. The posterior meningeal artery, which has a tortuous extracranial and straight intracranial segment, is a branch from the third segment of the VA. It courses toward the posterolateral edge of the foramen magnum and enters the intracranial dura across the roof. It ascends parallel to the internal occipital crest to reach the falx cerebelli.[12] [13]

Fig. 4 Intraoperative picture showing the tumor in the vagoaccessory triangle (VAT). (A) Tumor (yellow) anterior to the medulla and upper cervical spinal cord. (B) Meningioma in the cerebellomedullary cistern approached through the far lateral approach as seen in lateral position.

Contents

The VAT contains three cisterns, premedullary, lateral cerebellomedullary, and CM. They are compartmentalized by three membranes, medial and lateral pontomedullary membranes and the denticulate ligament. Rootlets of the hypoglossal, cranial accessory, and first cervical nerve are the three nerves that pass through VAT. PICA and anterior and posterior spinal arteries (ASA and PSA), branches of the VA (V4), traverse through it ([Fig. 1B]).

Premedullary Cistern

The premedullary cistern extends from the pontomedullary junction above to the foramen magnum below, that is, between the lower clivus and the anterior medulla. It is separated from the prepontine cistern by the medial pontomedullary membrane[14]([Fig. 2B]). Posterolaterally, it communicates with the LCMC. It contains the anterior medullary segment of V4, vertebrobasilar confluence, ASA, anterior medullary segment of PICA, and hypoglossal nerve ([Fig. 2B]).

Lateral Cerebellomedullary Cistern

It is located lateral to the medulla and the adjacent cerebellum. It is directly inferior to the cerebellopontine cistern and is separated by the lateral pontomedullary membrane.[14] It merges with the CM. The LCMC receives cerebrospinal fluid (CSF) from the fourth ventricle through the foramen of Luschka. It contains the lateral medullary segment of V4, first four segments of PICA, glossopharyngeal, vagus ([Figs. 4B] and [5]), and cranial accessory rootlets and often choroid plexus of fourth ventricle protruding from the foramen of Luschka.

Fig. 5 Intraoperative picture showing the epidermoid in the cerbellopontine extending downward into the cerebellomedullary cistern (A and B). Between the 9th, 10th, and 12th nerve. (C) Showing the artery over the brainstem after partial excision of the epidermoid.

Cisterna Magna (Posterior CMC)

The CM is located between the suboccipital surface of the cerebellum and the dorsal surface of the medulla oblongata at and above the level of the foramen magnum. It contains cortical branches of the PICA and CSF outflow from the fourth ventricle through the foramen of Magendie.

Denticulate Ligament

The superior most denticulate ligament is attached to the dura of the marginal sinus, superior to the VA as it pierces the dura mater. The ligament is located between the VA and the spinal accessory nerve. In 20% of cases, it is attached to the intracranial VA and histologically blends with its adventitia. In general, this ligament laterally is thicker and medially is often cribriform in nature. The hypoglossal nerve is always superior to the ligament, which always conceals the ventral rootlets of the C1 spinal nerve. The PSA travels posterior to this ligament.[15] The nerve of McKenzie is a communicating branch that can be found between the 11th nerve and the ventral rootlet of the C1. It has a variable course.[16] [17] The medial part of the second denticulate ligament separates the dorsal rootlets of C2 and 11th nerve from the ventral rootlets of C2 at the inferior angle of the triangle.

Surgical importance: The dentate ligament is often cut at its origin at the foramen magnum and at C1 to access the VA anteriorly. The dentate ligament's pure white, fibrous appearance distinguishes it from the spinal accessory nerve, which lies posterior to the dentate ligament and has an off-white color, and has perforating arteries.[18]

The Hypoglossal Nerve

The hypoglossal nerve comprises 10 to 20 rootlets. They emerge as a fan-shaped distribution or in two bundles ([Fig. 1B]) from the ventral surface of the medulla between the medullary pyramid and inferior olive (preolivary sulcus). They converge toward the HC in the subarachnoid space before piercing the dura ([Fig. 2B]). In majority of cases rootlets pierce the dura mater in two separate apertures or less commonly through the same aperture, and in rare cases three individual apertures.[19] Splayed rootlets of the hypoglossal nerve divide the VAT into three separate subtriangles; supra-hypoglossal triangle, which is superior to the hypoglossal nerve, infra-hypoglossal triangle that is inferior to the hypoglossal nerve, and the hypoglossal– hypoglossal triangle located between the lowest and highest rootlets of the hypoglossal nerve. This third triangle was introduced by Meybodi et al.[4] ([Fig. 1A]).

As these rootlets cross the VAT they lie posterior to the V4. Sometimes the V4 can split the rootlets.[18] The anterior medullary segment of the PICA is intimately related to the hypoglossal nerve, which lies anterior and in between the PICA and its parent V4.[20] In some cases, the initial course of the PICA passes between the two bundles of the hypoglossal nerve before ascending toward the lateral border of the fourth ventricle.[19]

Cranial Root of Accessory Nerve

The cranial root fibers of accessory nerve are suggested to originate from the nucleus ambiguus. Four or five rootlets exit the medulla from postolivary sulcus and unites with the spinal portion either before or within the jugular foramen.[21] The VA (V4) traverses anterior to these cranial rootlets ([Figs. 1] and [2B]). The tonsillomedullary segment of the PICA passes posteriorly between the cranial and spinal roots before it descends to the inferior pole of the cerebellar tonsil.[18] [20]

First Cervical Spinal Nerve (C1)

The C1 nerve comprises junctions of ventral and dorsal rootlets that unite within the subarachnoid space and then leave the dura mater between the occipital and posterior arch of the atlas. These rootlets vary in number and thickness. The dorsal rootlets can be absent in the C1 nerve. In the subarachnoid space, the C1 rootlets follow a slightly horizontal to cranial course toward the exit. The anterior rootlets follow an anterior to posterior trajectory, whereas posterior rootlets follow a posterior to anterior trajectory. This upward course of the C1 rootlets is a distinctive feature of the nerve ([Fig. 2B]).[22]

Vertebral Artery

The V3 segment of the VA penetrates the atlanto-occipital membrane and dura behind the OC to enter the cranial cavity through the foramen magnum. It then runs as V4 part of the VA ([Fig. 1B]) upward and medially to terminate at the VBJ ([Fig. 2B]). It forms the basilar artery by joining with its contralateral VA anterior to the medulla oblongata at or near the pontomedullary sulcus.[20] The preolivary sulcus marks the limit between the lateral medullary and anterior medullary segments. The lateral medullary segment passes above the rootlets of the C1 nerve ([Fig. 2B]) and ascends forward, anterior to the denticulate ligament and spinal accessory nerve. The anterior medullary segment courses in front or between the hypoglossal rootlets, crosses the pyramid, and joins the contralateral VA. Thus, the V4 always passes anterior to rootlets of IX, X, and XI cranial nerves ([Fig. 1B]) but can split the rootlets of hypoglossal nerve.[18] The V4 courses from the lower lateral to upper anterior surfaces of the medulla giving off perforators along the way.[18]

Surgical importance: It is advisable to get proximal control of the VA at the level of C1 arch in dealing with highly vascular lesions including the tumors located anterior to the medulla. It is suggested to drill the most lateral aspect of C1 posterior arch and excise it. Delineate the extracranial course of the VA following which it can be retracted to safely drill and excise the OC in the far lateral approach. It is advisable to do either a CT or MR angiogram to study the course of the VA, since variations in VA course can occur.

Posterior Inferior Cerebellar Artery

The PICA typically arises from the V4 as its largest branch, 15 mm proximal to the confluence of vertebral arteries at the VBJ. Classically, the PICA course is divided into five segments; anterior medullary, lateral medullary, tonsilomedullary, telovelo-tonsillar, and cortical segments.[18] [20] The anterior medullary segment (P1) begins at the origin of the PICA, lies anterior to the medulla, and ends at the level of rosto-caudal line that passes through the most prominent part of the inferior olive. It is intimately related to the hypoglossal nerve. The lateral medullary segment (P2) runs around the inferior surface of the olive to the level of origin of IX, X, and XI cranial nerves at the lateral edge of the olive ([Fig. 2B]). The tonsilomedullary segment (P3) begins where the PICA passes posterior to the IX, X, and XI cranial nerves, descends to the inferior pole of the cerebellar tonsil, and reverses course forming caudal loop and ascends along the medial surface of the tonsil up to its midpoint.

The telovelar-tonsillar segment (P4) begins at the midpoint of PICA's ascent along the medial tonsil and continues medial to the tonsil toward the roof of the fourth ventricle lateral to the vermis, turns around again to form a cranial loop ([Fig. 5C]), courses downward, and posteriorly to the tonsilo-biventral fissure. The cortical segment (P5) begins as the PICA emerges from the tonsilo-biventral fissure where the tonsil, vermis, and biventral lobule of the cerebellar hemisphere meet.

The trunk of the PICA gives rise to perforators, which supplies the posterolateral aspect of the medulla, the choroidal, and the cortical branches.[20] The apex of the cranial loop is described as the choroidal point, which marks the roof of the fourth ventricle on the lateral angiogram. It is the distal limit for perforator arteries arising from the PICA trunk. The terminal cortical branches supply the posteroinferior aspect of the cerebellar hemisphere, vermis, choroid plexus, and the fourth ventricle.

Anterior Spinal Artery

The ASA is the last branch of the VA proximal to the VBJ. It joins the contralateral artery to form a single midline ASA that descends through the foramen magnum on the ventral surface of the medulla and the spinal cord in or near the anterior median fissure. It supplies the pyramids, and their decussation, the medial lemniscus, and the hypoglossal nuclei and nerves.[18]

Posterior Spinal Artery

The PSA is the first intracranial branch of the VA, sometimes it can arise extradurally, especially when the PICA originates extradurally from the VA.[23] The PSA courses medially behind the dentate ligament, where it divides into ascending and descending branches that supply the dorsal columns of the cervical spinal cord.[18]

Conclusion

The VAT and its subtriangles provide natural surgical corridors between the lower cranial nerves ([Figs. 4A, B] and [5]), in far lateral approach and ELITE for accessing vascular pathologies and skull base tumors in the cerebellomedullary angle, anterior foramen magnum, and the anterolateral aspect of the brainstem.

Bruneau and George[24] have classified the foramen magnum meningiomas (FMMs). Majority of FMMs are anterolaterally situated. Meningiomas are more often located below the VA. Their growth tends to displace the brainstem in a posterior and contralateral direction. This retraction of the brainstem by the tumor creates an adequate surgical corridor for resection of most of these lesions. In posterior FMMs, the neuraxis is always displaced anteriorly. Lateral FMMs push the neuraxis posterolaterally and thus widely open the lateral surgical corridor. Lower cranial nerves can be displaced in any direction. Anterior and lateral intradural lesions can grow in any direction with respect to VA. Moreover, the dura around the VA penetration may be infiltrated by the tumor.[25] Hence, it is advisable to leave a cuff surrounding the VA to prevent catastrophic injury to the VA. Neuroendoscope and intraoperative neuromonitoring greatly aid in preventing devastating postoperative neurological deficits.

To conclude, thorough knowledge about the microsurgical anatomy of the VAT is mandatory in dealing with the lesions around the foramen magnum and lower brainstem.

Conflict of Interest

None declared.

Acknowledgments

We wish to express our sincere thanks to Dr. Upul Karunarathna, Director, Teaching Hospital, Badulla, Sri Lanka, Dr. Akila Rathnayaka, Medical Officer, Teaching Hospital, Badulla, Sri Lanka for assisting us during cadaveric dissections, Dr. L. Subramanian, Managing Director of Sri Balaji Hospitals, Guindy, Chennai, Tamil Nadu, India, Prof. Dr. Sanjay Behari, Director of Sri Chitra Tirunal Institute of Neurosciences, Thiruvananthapuram, Kerala, India, for providing an intra-operative picture [Fig. 4B], and Prof. Dr. Santhanam, Professor of Neurosurgery, Sri Balaji Medical College, Chennai, Tamil Nadu, India, for permitting to use the details of his surgical case.

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Publication History

Article published online:
23 January 2025