Microsurgical Anatomy of the Endoscopy-Assisted Retrosigmoid Intradural Suprameatal Approach to the Meckel's Cave : Operative Neurosurgery

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NEUROSURGICAL ATLAS SERIES

Microsurgical Anatomy of the Endoscopy-Assisted Retrosigmoid Intradural Suprameatal Approach to the Meckel's Cave

Xu, Yuanzhi MD; Hendricks, Benjamin K MD; Nunez, Maximiliano Alberto MD; Mohyeldin, Ahmed MD, PhD; Fernandez-Miranda, Juan C MD; Cohen-Gadol, Aaron A MD, MSc, MBA

Author Information
Operative Neurosurgery 21(2):p 41-47, August 2021. | DOI: 10.1093/ons/opab096

Abstract

The lesions in the Meckel's cave that are typically treated via the anterior petrosal or extended middle fossa approaches are invasive and remain technically challenging. In recent years, suprameatal tubercle drilling for gaining access to the Meckel's cave has been adopted in rare studies for trigeminal schwannomas and petroclival meningiomas.1 Understanding the microsurgical anatomy of the retrosigmoid intradural suprameatal approach (RISA) is critical for surgeons treating Meckel's cave tumors. Since the pioneering investigation of Rhoton,2 several studies have focused on this approach.1,3,4 However, a more detailed and clearer image regarding relevant surgical landmarks, especially when assisted by endoscopy, which provides more illumination and deeper/wider-angle views, is still lacking.

The objective of this study was to evaluate the increased exposure of the Meckel's cave in the RISA and assess the surgical landmarks under microscopy assisted by endoscopy. Our goal is to provide an updated description of the relevant microsurgical anatomy that can be used intraoperatively in a safe manner while reducing the incidence of neurovascular injury.

METHODS

Four colored-silicon-injected human cadaveric specimens (8 hemispheres) were lightly fixed in a 95% ethyl alcohol solution and prepared for dissection. Anatomical dissection of these heads was performed using a standard retrosigmoid craniotomy to the cerebellopontine region. Once the suprameatal tubercle and petrous apex were identified, resection was performed from lateral to medial until the petrous apex was resected to expose the Meckel's cave by using a high-speed surgical drill system (Medtronic, Dublin, Ireland).

The area of drilling is limited superiorly by the superior petrosal sinus, inferiorly by the cranial nerve (CN) VII/VIII complex, medially by the internal carotid artery (ICA) (petrous segment), and laterally by the posterior and superior semicircular canals. Once the surgical approaches were completed, the cerebrum was removed, and the temporal bones were dissected to investigate the length, depth, and height of the drilled area and the surgical landmarks for this approach (Figure 1). To provide maximal visualization, an ×3 to ×40 magnification microscope (Haag-Streit Surgical, Wedel, Germany), 0° and 30° rod-lens endoscopes attached to 4K cameras (4-mm diameter, 18-cm length [Hopkins II, Karl Storz, Tuttlingen, Germany]), and a high-definition camera (Canon, Tokyo, Japan) were used in all steps.

fig1
FIGURE 1.:
Measurement of the RISA. A, Posterior surface of the right petrous bone. The suprameatal tubercle is located on the superior wall of the IAM. The yellow shadow indicates the drilling area in the RISA, and the double-headed arrow indicates the length of the region to be drilled. B, Posterolateral view of the left petrous ridge. The double-headed arrow shows the height of the suprameatal tubercle, which is from the superior margin of the IAM to the superior petrous sinus. C and D, Superolateral views of the left dissected petrous bone reveal the length (from the posterior semicircular canal to the Meckel's cave C) and depth (from the posterior surface of the petrous apex to the petrous segment of the ICA D) of the drilling area. A., artery; Ac., acoustic; A.I.C.A., anteroinferior cerebellar artery; Ant., anterior; Car., carotid; Clin., clinoid; Fiss., fissure; For., foramen; Hypogl., hypoglossal; Int., internal; Jug., jugular; Lat., lateral; Lig., ligament; Pet., petrosal/petrous; Petroclin., petroclinoid; Petrocliv., petroclival; Post., posterior; S.C.A., superior cerebellar artery; Semicirc., semicircular; Sig., sigmoid; Sup., superior; V., vein. (Used with permission from The Neurosurgical Atlas by Aaron A. Cohen-Gadol, MD.)

One illustrative case of a trigeminal schwannoma resection with extension into the Meckel's cave is also discussed later to demonstrate application of the RISA. All data were recorded as continuous variables and are therefore presented here as means (± standard deviation). SPSS 25 (IBM, Armonk, New York) was used for all analyses.

Institutional Review Board/Ethics Committee approval and patient consent were neither required nor sought for this study.

RESULTS

Microsurgical Anatomical Features for the RISA

The suprameatal tubercle, which is resected in the RISA, is located above the location at which the CN VII/VIII bundle enters the internal acoustic meatus (IAM) and below and lateral to the site at which the trigeminal nerve enters the porus of the Meckel's cave. The Meckel's cave was exposed adequately, and the trigeminal root could be gently mobilized after drilling. The superior and lateral walls of the internal acoustic canal were resected, and the CN VII/VIII bundle was exposed (Figures 2-4).

fig2
FIGURE 2.:
Microsurgical anatomy of the RISA. A, Posterior view of dual relationships with nerves preserved in petrous bone. The trigeminal nerve (CN V) passes above the petrous apex to enter the porus of the Meckel's cave. The abducens nerve (CN VI) pierces the dura mater and enters Dorello's canal. The facial (CN VII) and vestibulocochlear (CN VIII) nerves pass into the IAM laterally, and the glossopharyngeal (CN IX), vagus (CN X), and accessory (CN XI) nerves enter the jugular foramen. The hypoglossal nerve (CN XII) enters the hypoglossal canal. B, Further dissection in the same specimen reveals the Gasserian ganglion in the Meckel's cave and the petroclival region. The abducens nerve ascends to enter Dorello's canal at the tip of the petroclival fissure, which is superior to the inferior petrosal sinus. C, Superior view of the dissected petrous bone showing the anatomical structures in the RISA route. The suprameatal tubercle limits access to the trigeminal nerve, and the posterior and superior semicircular canals, internal auditory canal, superior petrous sinus, and ICA (petrous segment) serve as landmarks in this approach. D, Anterosuperior view to the dissected petrous bone in a separate specimen. The petrous apex has been partially resected. E, Fiber dissection of the trigeminal nuclei to display its relationship in the brainstem. F, Superior view of the dissected petrous bone showing the drilling area in the specimen shown in E. The area of drilling is limited superiorly by the superior petrosal sinus, inferiorly by the CN VII/VIII complex, medially by the ICA (petrous segment), and laterally by the posterior and superior semicircular canals. A., artery; Bas., basilar; Dors., dorsal; Gen. Gang, geniculate ganglion; Gr., greater; Inf., inferior; Lat., lateral; Mid., middle; M., muscle; Men., meningeal; N., nerve; Pet., petrosal, petrous; Post., posterior; Semicirc., semicircular; Sup., superior; Tens., tensor; Tymp., tympani. (Used with permission from The Neurosurgical Atlas by Aaron A. Cohen-Gadol, MD.)
fig3
FIGURE 3.:
Stepwise dissection during the RISA under microscopy assisted by endoscopy. A, The cerebellum is partially resected to expose the cerebellopontine angle. The trigeminal nerve is observed in the upper part of the cerebellopontine angle, entering into the Meckel's cave medial to the suprameatal tubercle. The superior cerebellar artery can be seen looping above the posterior root of the trigeminal nerve. B, Increased exposure is achieved after drilling the suprameatal tubercle and the medial, superior, and lateral walls of the IAM (270° open). C, Assisted endoscopy shows that the drilling is from the posterior surface of the petrous apex to the ICA (petrous segment), which is indicated by a double-headed arrow. D, The ruler indicates the increased length of the trigeminal nerve as approximately 1 cm. E, Resection of the tentorium to reveal the adjacent neurovascular structures above the RISA. The superior petrosal sinus and the posterior inferior temporal artery are in close proximity superiorly to the drilling area. F, Superior view of the dissected petrous bone displaying all the landmarks in this approach, including the posterior and superior semicircular canals, internal auditory canal, superior petrous sinus, and ICA (petrous segment). The operative route from the posterior fossa is shown via the large yellow arrow. A., artery; Ac., acoustic; A.I.C.A., anteroinferior cerebellar artery; Car., carotid; Eust., Eustachian; Ext., external; Gen. Gang, geniculate ganglion; Gr., greater; Inf., inferior; Int., internal; Jug., jugular; Lat., lateral; Men., meningeal; Mid, middle; Pet., petrosal, petrous; P.I.C.A., posterior inferior cerebellar artery; Post., posterior; S.C.A., superior cerebellar artery; Semicirc., semicircular; Sig., sigmoid; Sup., superior; Trans., transverse. (Used with permission from The Neurosurgical Atlas by Aaron A. Cohen-Gadol, MD.)
fig4
FIGURE 4.:
Stepwise dissection during the RISA under endoscopy. A, Posterolateral endoscopic view through a retrosigmoid craniotomy into the right cerebellopontine angle. In this specimen, the suprameatal tubercle is not obvious, and a superior petrosal vein passes above the posterior root of the trigeminal nerve and empties into the superior petrosal sinus. B, The suprameatal bone has been drilled to expose the additional length of the trigeminal nerve (CN V) into the Meckel's cave. C, Further drilling is completed to reach the petrous ICA as the medial boundary in this approach. D, The ruler shows that removing the suprameatal tubercle and opening the Meckel's cave adds approximately 1 cm to the increased exposure length along the trigeminal nerve. A., artery; A.I.C.A., anteroinferior cerebellar artery; Car., carotid; Pet., petrosal, petrous; Post., posterior; S.C.A., superior cerebellar artery; Sup., superior; V., vein. (Used with permission from The Neurosurgical Atlas by Aaron A. Cohen-Gadol, MD.)

The depth from the posterior surface of the petrous apex to the ICA (petrous segment) that represents the increased length of the trigeminal nerve that was exposed via this approach was 10.1 ± 1.3 mm. The length of increased exposure from the posterior semicircular canal to the Meckel's cave was 21.4 ± 3.2 mm. The height from the superior margin of the internal auditory canal to the superior petrosal sinus was 6.4 ± 0.6 mm. The posterior and superior semicircular canals, internal auditory canal, superior petrous sinus, and ICA (petrous segment) served as significant landmarks for this approach (Table).

TABLE. - Microsurgical Anatomic Parameters for the RISA
Landmark
Increased exposure From To Measurement (mm)
Depth Posterior surface of petrous apex ICA (petrous segment) 10.1 ± 1.3
Length Posterior semicircular canal Meckel's cave 21.4 ± 3.2
Height Superior margin of IAM Superior petrous sinus 6.4 ± 0.6

CASE ILLUSTRATION

The patient presented with trigeminal neuralgia that was resistant to medical treatment. Imaging revealed a schwannoma along the right trigeminal nerve projecting posteriorly from the posterior fossa into the Meckel's cave (Figure 5).

fig5
FIGURE 5.:
The RISA for mass resection in the Meckel's cave. A and B, Preoperative contrast-enhanced magnetic resonance images show a tumor in the right cerebellopontine angle and extending into the Meckel's cave. C, The suprameatal tubercle, CN V, and CN VIII were exposed in the retrosigmoid approach, and the tumor can be observed medial to CN V. D, The same microsurgical perspective to the right cerebellopontine region is demonstrated in a cadaveric specimen. E, Endoscopy is used during tumor resection and to enhance visualization during suprameatal tubercular drilling. F, Similar anatomical view within a cadaveric specimen. G and H, Postoperative magnetic resonance image showing gross-total resection of the tumor all the way through the Meckel's cave. A., artery; A.I.C.A., anteroinferior cerebellar artery; Post., posterior; S.C.A., superior cerebellar artery; Sup., superior. (Used with permission from The Neurosurgical Atlas by Aaron A. Cohen-Gadol, MD.)

The configuration of the tumor appeared favorable for resection via the RISA; therefore, the patient underwent surgery with this approach (Video). The technique was efficient and permitted gross-total resection. After surgery, the patient demonstrated some mild right-sided V1, V2, and V3 distribution numbness, which had resolved significantly by the first postoperative visit.

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DISCUSSION

A variety of skull base approaches have been performed to access lesions in the Meckel's cave, and they can be divided into 2 categories, supratentorial and infratentorial approaches. The supratentorial approaches are subtemporal, extended middle fossa, and anterior petrous (Kawase) routes that are quite suitable for treating lesions around the Meckel's cave that extend into the middle fossa. Infratentorial approaches, including the retrosigmoid and translabyrinthine approaches, are ideally appropriate for lesions in the Meckel's cave with an extension into the posterior fossa.4 The optimal surgical approach for treating a Meckel's cave or petroclival tumor depends on the size, location, extension, bone invasion, and consistency of the tumor and the surgical expertise available.5

The RISA delineated here was performed for the first time in 1983,1 and Cheung et al6 reported this approach for treating trigeminal schwannomas in 1995. Since that time, the literature has been devoid of a detailed definition of the anatomical constraints of this approach. The increased exposure length of the trigeminal nerve created after drilling the suprameatal tubercle was 10.1 ± 1.3 mm in our study. For better visual field and surgical freedom, the wall of the IAM, including the medial, superior, and lateral walls, is usually removed (approximately 270°). Drilling on the medial side of the IAM extends the resected region through the petrous apex and into the lateral clivus. The posterior and superior semicircular canals should be critically relevant landmarks when drilling the lateral wall of the IAM.

The variation of Meckel's cave, the size of the suprameatal tubercle, and petrous apex pneumatization should be take into consideration when approaching lesions in Meckel's cave. The porus trigeminus, which serves as the dural foramen at the posterior Meckel's cave, is a potential channel for tumor that extends from the prepontine cistern of the posterior cranial fossa into the Meckel's cave of the middle cranial fossa. It is vital to understand the variation of the porus trigeminus and estimate its location during operation. Arslan et al7 indicated that the mean distance between the porus trigeminus and the anterior wall of the IAM was 7.19 mm, and the mean distance between the porus trigeminus and the inferior petrosal sinus was 12.4 mm. The size of the suprameatal tubercle determines the complexity of drilling. The probability of a prominent suprameatal tubercle was reported as 85.5%, among which a small size (0-1 mm) occurs in 41%, a medium size (2-3 mm) was found more frequently in 49.5%, and a large size (>3 mm) was found in 9.5%.8

A study that enrolled a total of 481 samples9 revealed that 15% of the people had bilateral pneumatization of the petrous apex, which means that an increasing incidence of cerebrospinal fluid leakage happens during suprameatal tubercle drilling. In our medical center, the risk of cerebrospinal fluid leakage induced by petrous apex drilling was evaluated by preoperative high-resolution computed tomography. The opened air cells are replaced by autologous tissues (fat or muscle tissue) and covered with fibrin glue to prevent a cerebrospinal fluid leak.10

The drilling area in the RISA in an inferoposterior view is approximately the same region targeted during the Kawase approach. In that approach, the petrous apex is drilled as a rhomboid in an anterolateral view, which is limited by the mandibular division of the trigeminal nerve anteriorly, the petrous carotid artery anterolaterally, the greater superficial petrosal nerve laterally, the internal acoustic canal lateroposteriorly, and the superior petrosal sinus medially. The main differences between these 2 approaches are as follows. In the Kawase corridor, there is no important neurovascular structure obstructing the surgeon's view, and the drilling procedure is more straightforward than that in the RISA, in which the CN VII/VII complex often obstructs the surgical trajectory. The brainstem is very close to the operating area in the RISA, whereas the brainstem is the farthest region in the Kawase approach. The petrous apex in the RISA is deeper than it is in the Kawase route.11,12 The greater superficial petrosal nerve is regarded as an exact and accurate landmark for the carotid artery, which is one of the greatest risks in the Kawase approach. However, it is hard to find a similar landmark for the carotid artery on the posterior surface of the petrous bone. In our experience, the depth from the posterior surface of the petrous apex to the petrous segment of the carotid artery (10.1 ± 1.3 mm) is a useful estimated value for its location.

In the current era, endoscopic-assisted skull base microsurgeries that involve microvascular decompression, an intracranial aneurysm, or a tumor are prevalent; these procedures are performed with high-resolution and stereoscopic-vision microscopy and corner observation, which reduce retraction and bone drilling under endoscopy. In the endoscope-assisted RISA, the microscopic procedure is performed first with a straight-line visual trajectory to dissect the critical landmarks and neurovascular structures, including the suprameatal tubercle, posterior root of the trigeminal nerve, CN VII/VIII complex, and tumor in the cerebellopontine region. When the tumor has been largely resected and the Meckel's cave has been opened via suprameatal drilling, endoscopy is performed in addition to tailored suprameatal tubercle drilling anteriorly along bony or dural corners to remove occult tumor. Because the suprameatal drilling step carries risk for causing cerebrospinal fluid leakage, performing endoscopy reduces unnecessary exploration and plays a critical role in total resection of the tumor. In addition, a small residual tumor can be treated with stereotactic radiosurgery.

Special limitations with the RISA, however, also exist. The surgical corridor between the trigeminal nerve and the facial-vestibulocochlear nerve complex is very narrow, which calls for professional and skilled microsurgical techniques and patience. The petrosal vein complex is at risk of being sacrificed during the suprameatal tubercle drilling step because of its close vicinity,13 which increases the likelihood of complications related to venous occlusion. Carotid artery injury during drilling is another potential risk of using this route. Finally, the angle of dissection into the Meckel's cave can be very acute, and operative blind spots are present despite the use of angled endoscopes.

CONCLUSION

The endoscopy-assisted RISA is suitable for lesions primarily in the posterior fossa that extend into the Meckel's cave. This approach provides optimal visualization and accessibility to the petrous apex region. We believe that better realization and recognition of the microsurgical landmarks and parameters of this approach are crucial for successful outcomes.

Funding

This study did not receive any funding or financial support.

Disclosures

The authors have no personal, financial, or institutional interest in any of the drugs, materials, or devices described in this article.

REFERENCES

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COMMENT

This is a nice and well-written report on the surgical anatomy and clinical use of the endoscopic assisted retrosigmoid intradural suprameatal approach (RISA) to remove a small trigeminal schwannoma that was primarily located at the petrous apex and invaded the Meckel's cave. The RISA technique is nicely illustrated with didactic photographs of anatomical dissections. Although a middle fossa epidural (Kawase's) approach to the Meckel's cave could have been another option in this illustrative case, the RISA is a straight-forward technique for those who have experience in posterior fossa surgery, which has great advantage in comparison with the middle fossa approach of sparing temporal lobe compression. This is particularly important for left-sided lesions, where temporal lobe hyperintensities as observed postoperatively at T2-weighted images is closely related to postoperative neuropsychological disturbances, mood changes and depression. In our experience with several similar cases, endoscopic-assisted RISA is suitable even for large lesions of the petrous apex that invade the supratentorial spaces, such as trigeminal schwannomas, epidermoid cysts or some petrous apex meningiomas.

Marcos S. Tatagiba

Tübingen, Germany

Keywords:

Retrosigmoid intradural suprameatal approach; Meckel's cave; Microsurgical anatomy

Congress of Neurological Surgeons 2021.