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|NI FEATURE: THE EDITORIAL DEBATE II-- PROS AND CONS
|Year : 2019 | Volume
| Issue : 1 | Page : 61-64
Far-lateral skull base approaches: Shades of grey
Vinayak Narayan1, Pankaj Agarwalla2, Fareed Jumah1, Anil Nanda3
1 Department of Neurosurgery, Rutgers–Robert Wood Johnson Medical School and University Hospital, New Brunswick, Newark, New Jersey, USA
2 Department of Neurosurgery, Rutgers–New Jersey Medical School and University Hospital, Newark, New Jersey, USA
3 Department of Neurosurgery, Rutgers–Robert Wood Johnson Medical School and University Hospital, New Brunswick; Department of Neurosurgery, Rutgers–New Jersey Medical School and University Hospital, Newark, New Jersey, USA
|Date of Web Publication||7-Mar-2019|
Dr. Anil Nanda
Department of Neurosurgery, Rutgers–Robert Wood Johnson Medical School, Rutgers–New Jersey Medical School, Neurosurgical Services, RWJ Barnabas Health, New Jersey
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Narayan V, Agarwalla P, Jumah F, Nanda A. Far-lateral skull base approaches: Shades of grey. Neurol India 2019;67:61-4
In the January–February issue of Neurology India, Srinivas et al., present an interesting retrospective analysis of 20 consecutive patients with foramen magnum meningiomas who were surgically treated at the National Institute of Mental Health and Neurosciences over a 10-year period spanning from 2005 to 2015. The characteristic feature of the study is that all the patients underwent a modified far-lateral approach, emphasizing condylar preservation. The most common clinical presentation noted in the study was spastic weakness (100%) followed by lower cranial nerve symptoms (55%). All patients were evaluated preoperatively with high-resolution computed tomography and magnetic resonance imaging to assess the bony details and precisely evaluate the adequacy of the surgical corridor. Preoperative digital subtraction angiogram was done in 45% of cases where MRI could not yield optimal information on the exact anatomy of the vertebral artery, particularly in tumor-encased scenarios. Some of these tumors usually show increased tendency to bleed and it is interesting to note that none of the aforementioned patients underwent preoperative arterial feeder embolization.
The authors then adopted a “tailored” surgical approach to the ventral craniovertebral junction (CVJ) by drilling only a part of the occipital condyle based on tumor extension. The authors claim to achieve Simpson Grade 2 excision in 80% of tumors and Grade 3 excision in 10% of tumors. Neurologic improvement was noted in 65% of cases. There were no major surgical vascular complications. There was one intraoperative complication of inadvertent vertebral artery injury, the details of which were not clear. The authors recommend the modified far-lateral approach in foramen magnum meningiomas, with tailored resection of the occipital condyle, which does not hinder operative visualization, handling of critical neurovascular structures, and CVJ stability. Furthermore, the current world literature is strongly in line with achieving the surgical goals of foramen meningioma resection through the modified far-lateral approach by minimizing the condylar resection.,,,,
The article brings up a very important topic of discussion in cranial base surgery. Frequently, a neurosurgeon encounters this kind of lesion, and to establish a wide corridor, one must take a decision regarding whether the condyle needs to be drilled out completely or partially. Unfortunately, there is insufficient literature to help guide the surgeon, and the neurosurgical community has paid little attention to the issue. Because the consequences of completely removing the condyle may be disastrous by creating significant instability, the prevailing trend is to modify the extent of condylar resection based on tumor morphology and tumor extension and never electively remove it as a whole, except in a very few selected indications. This, of course, is generally a salutary surgical attitude, but the debatable issue of surgical judgment comes into action not infrequently when preserving the most of it may lead to excessive manipulation of the neurovascular structures, increase the risk of tumor resection, enhance the chance of iatrogenic major arterial injury, or reduce the grade of tumor excision because of the “limited” exposure allowed by the preservation of condyle. In the subsequent section, we discuss the intradural anatomy of foramen magnum region and general guidelines on the surgical techniques of far-lateral approach giving due emphasis on the extent of occipital condyle preservation, based on the senior author's (A.N.) personal experience, and it is not intended to devalue the importance of sound surgical judgment on a case-to-case basis.
| » Microsurgical Anatomy of the Intradural Compartment of Foramen Magnum Region|| |
The initial anatomical structure visualized while entering into the intradural compartment is the thick arachnoid membrane covering the cisterna magna. Opening the cistern exposes the rootlets of the spinal accessory nerve crossing the foramen magnum. The vertebral artery can be seen in relation to the ligamentum denticulatum and its course varies superiorly or inferiorly based on the first tooth of ligamentum denticulatum. Interestingly, the spinal accessory nerve roots are the only motor rootlets to pass dorsal to the denticulate ligaments. The spinal accessory nerve courses toward the jugular foramen along with glossopharyngeal and vagus nerves, while the rootlets of hypoglossal nerve enter into the hypoglossal canal. The vertebral artery usually runs anterior to the lower cranial nerve complex, and the posterior inferior cerebellar artery courses between the rootlets of the lower cranial nerves, which further loops around the cerebellum. The vestibulocochlear bundle can also be visualized in this approach along with anterior inferior cerebellar artery. The superior-most extent of visualization is generally the trigeminal nerve, which also depends on the extent of suboccipital craniotomy.
| » Technical Nuances of “modified” Far-Lateral Approach in Foramen Magnum Meningioma|| |
After induction of general anesthesia, the patient is placed in a lateral position and the head is fixed on a Mayfield three-point fixation device (Ohio Medical Instrument Co., Inc, Cincinnati, OH, USA). Intraoperative monitoring, using somatosensory evoked potentials and auditory evoked responses, is an important adjunct for the procedure. The incision is usually a retroauricular curvilinear “S”-shaped one, extending from the mastoid to just below the C2 level. The gradual modification from the initial “C”-shaped incisions in many of our early cases to the “S”-shaped incision in the current surgeries is based on our personal experience of better exposure and improved visualization of the suboccipital and paracondylar regions, better approximation on closure, as well as saving the surgery duration. The sternocleidomastoid muscle and fascia are dissected from the mastoid process and reflected forward with the skin flap whenever possible [Figure 1]a. However, the former structure needs to be reflected inferiorly and separated from the skin flap to get exposure below C2. The muscular dissection has to be extended deep down upto the suboccipital muscular triangle after detaching and reflecting the splenius capitis and longissimus capitis inferomedially. After reflecting the muscle layers, the suboccipital triangle and occipital artery are exposed. The vertebral artery, dorsal ramus of C1 nerve root, and the suboccipital venous plexus are exposed in the triangle. The vertebral artery is completely dissected off the fatty and areolar tissue around it in the triangle. It is located anterior to the anterior ramus of C2 ganglion as it emerges from the foramen transversarium of C2 and enters the foramen of C1. The rectus capitis posterior major, superior oblique, and inferior oblique need to be divided and vertebral artery needs to be traced upto its entry into the intradural compartment [Figure 1]b and [Figure 1]c. The dura will be exposed after resecting the atlanto-occipital membrane.
|Figure 1: Anatomical demonstration of varying stages of modified far-lateral approach in a cadaveric specimen. (a) Exposure of sternocleidomastoid, spinal accessory nerve, and splenius capitis muscle. (b) Division of superior and inferior oblique in anatomical relation to vertebral artery and C2. (c) Anatomical view of the suboccipital triangle after removal of the muscles. (d) Image showing the anatomy of vertebral artery in relation to occipital condyle, C1 arch, and C2 nerve root. (e) Image depicting the close relationship of vertebral artery, marginal sinus, and occipital condyle. (f) Intradural view of foramen magnum region demonstrating the vertebral artery, lower cranial nerves, and ligamentum denticulatum|
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A standard suboccipital craniotomy is performed extending up to the foramen magnum and the C1 posterior arch is drilled. We suggest the removal of the lateral margin of the foramen magnum that corresponds with the posterior edge of occipital condyle to make the anterolateral view better. Drilling of the occipital condyle varies from case to case; however, total removal is not indicated in most cases [Figure 1]d. This “modified” far-lateral approach provides an excellent visualization of the anterosuperior spinal canal, anterior portion of foramen magnum, jugular foramen, and middle-inferior clivus. The surgical angle of approach to the ventral foramen magnum lesion does not quantitatively differ significantlybetween one-third and one-half condylectomy exposures (15.9° vs 19.9°). Dura needs to be opened and tacked up. Care must be taken when extending across the marginal sinus [Figure 1]e, where we will often use vascular ligation clips (hemoclips), as necessary. The cerebellomedullary cistern and the cisterna magna are opened and cerebrospinal fluid is released. The sharp cutting of the ligamentum denticulatum creates a vivid spacious corridor in the tumor–spinal cord interface. Tumor debulking can be attempted using an ultrasonic aspirator, and dissections are continued from lateral to medial direction and performed around the 10th, 11th, and 12th cranial nerves and the vertebral artery [Figure 1]f. The gradual debulking opens up more space in the tumor–cord–brainstem planes, and thereby a complete resection can be achieved. After achieving the hemostasis, the standard dural and skin closure can be done. We use fibrin glue as an aid for the dural closure and obliterate the mastoid air cells using fat and wax. The radiological and intraoperative images of two cases of foramen magnum meningioma from the senior author's personal experience are demonstrated in [Figure 2].
|Figure 2: Radiological and intraoperative images of foramen magnum meningioma. (a) Preoperative sagittal MR image showing the foramen magnum meningioma in a 45-year-old male patient. (b) Intraoperative view of the meningioma. (c) Postoperative axial MR image showing the gross total resection of the lesion. (d) Preoperative sagittal MR image showing a foramen magnum meningioma in a 52-year-old lady. (e) Intraoperative view of the meningioma. (f) Postoperative sagittal MR image showing the gross total resection of the lesion|
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| » Discussion|| |
Foramen magnum meningioma constitutes around 2.5% of all intracranial meningiomas and most of these lesions are located ventrally and ventrolaterally. The gross total resection of a ventrally located foramen magnum meningioma is a major operative challenge in cranial base surgeries. The unique characteristics of the lesion apart from the intricate location include relapsing–remitting symptomatology, prominent vascularity, firm-to-hard consistency, entangling of neurovascular structures, and spinal cord or brainstem compression (with or without edema). The senior author (A.N.) has a vast experience of more than 600 meningioma surgeries, of which the majority were skull base meningiomas (n = 448). The ventrally located foramen magnum meningioma tumors comprised a significant proportion (n = 25).
The surgical complexity of the foramen magnum meningioma varies from case to case and careful planning is essential. Preoperative embolization was attempted in a limited number of our cases; however, the routine preoperative angiograms revealed significant information for surgical planning. The position of vertebral artery varies and we recommend the optimal use of preoperative angiogram for an early tackling of tumor feeders and for avoiding devastating vertebral artery injury. The advantage is more pronounced in situations of minimal or “no” condyle drilling approaches. Based on our experience, a majority of the tumors were firm-to-hard in consistency and significantly vascular too. In addition, the venous information can be extremely useful.
The surgical approach towards ventral foramen magnum has evolved over the past decades. The senior author's personal series of modified far-lateral approach and management challenges in unilateral, bilateral (mirror) foramen magnum meningiomas and posterior circulation aneurysms have been reported before.,, Safe maximal resection should be the goal in any skull base tumor surgery, especially while dealing with lesions like a giant foramen magnum meningioma. As the surgical intervention should not compromise patient survival and postoperative quality of life, even achieving partial resection followed by stereotactic radiosurgery is a valid treatment strategy in difficult cases. The basic principles of any skull base meningioma surgery, such as proper positioning to avoid venous hypertension, avoid neural tissue injury, meticulous dissection, early identification of the critical neurovascular structures, identification of perforators and venous channels, maintain the arachnoid plane, and achieve proper hemostasis, are applicable to foramen magnum meningioma resection also.
Using the “modified” approach, we noted that the prognostic outcome was relatively better with tumor surgeries than aneurysm surgeries, indirectly explaining the added benefit of the surgical technique in the former one. The senior author has used this approach for various other complex skull base tumors such as chondrosarcoma, petroclival meningioma, and high cervical neurofibroma. The benefit of achieving a little added working space, wider lateral angle, or shorter surgical depth after a condylectomy needs to be weighed against the morbidity of CVJ instability (with an added CVJ instrumentation surgery), statistical risk of hypoglossal nerve injury, vascular complications (vertebral artery, posterior condylar vein, or sigmoid sinus injury) in posterior cranial fossa, higher risk of cerebrospinal fluid leak, and increased operating time. The same purpose of surgical freedom after a condylectomy can be well served by the division of ligamentum denticulatum also. Another valid justification to avoid a condylectomy in such cases is the “tumor-induced” brainstem/spinal cord mobilization leading to the development of better natural surgical corridor and thereby achieve a higher rate of tumor resection. However, a condylectomy may appear necessary in certain surgical scenarios such as the resection of aggressive bony tumors like chondrosarcoma or tumors like giant meningioma invading the bone. We have previously reported the surgical challenges and management of giant intracranial meningiomas. In our opinion, elective condylar resection should be considered upfront with planned, staged craniovertebral stabilization when there is significant preoperative bony invasion and longstanding hypoglossal dysfunction. In these cases, tumor resection itself on top of an already damaged condyle can lead to significant instability. Transcondylar approach may also be required in certain vascular pathologies such as complex aneurysms of the vertebral artery, where skeletonization of the vertebral artery is an indispensable part of surgery for either obtaining proximal vessel control prior to aneurysm clipping, or in rare scenarios, preparing for arterial bypass procedures.
The debate between the two concepts – maximal bony resection to achieve minimal brain retraction in conventional skull base surgery versus maximal preservation of anatomy in minimally invasive surgery – is gaining momentum in the current era of advanced neurosurgery. The aforementioned complications arising out of aggressive bony removal and extensive skull base surgery might be worse than the natural history of the existing lesion, even in safe surgical hands. Therefore, we recommend the preservation of occipital condyle whenever possible for a better safety profile of the patient and to optimize the surgical outcome.
| » Conclusion|| |
The “modified” far-lateral approach is a safe and effective approach providing adequate visualization and manoeuverability for foramen magnum surgeries. The sound surgical judgment and individualization of the optimal surgical approach are the keystones to handle the management dilemma in such difficult situations, while the literature “for or against the condyle preservation” are mere guidelines. As the entire field of cranial base surgery moves toward less invasive and less aggressive maneuvers, the growing trend is to modify the extent of condylar resection based on tumor morphology and tumor extension. Although the approach has a steep learning curve, it provides a unique operative corridor to the skull base surgeons for safe maximal resection of ventral and ventrolateral foramen magnum meningiomas.
| » References|| |
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[Figure 1], [Figure 2]