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|Year : 2019 | Volume
| Issue : 1 | Page : 142-148
”Tailored” far lateral approach to anterior foramen magnum meningiomas – The importance of condylar preservation
Dwarakanath Srinivas1, Pragyan Sarma2, Harsh Deora1, Manish Beniwal1, V Vikas1, K V L N Rao1, BA Chandramouli1, Sampath Somanna1
1 Department of Neurosurgery, National Institute of Mental Health and Neurosciences, Bengaluru, Karnataka, India
2 Department of Neurosurgery, Guru Teg Bahadur Hospital, New Delhi, India
|Date of Web Publication||7-Mar-2019|
Dr. Dwarakanath Srinivas
Department of Neurosurgery, National Institute of Mental Health and Neurosciences, Bengaluru, Karnataka
Source of Support: None, Conflict of Interest: None
Introduction: Anterior and anterolaterally situated foramen magnum meningiomas are a technically complex subgroup of meningiomas. The need for an extensive exposure and bone work and their complex anatomy make them a difficult and challenging group of tumors to resect. The bone work has ranged from an extensive condylar resection to condylar preserving exposures. In this paper, we present our experience with condylar preserving or minimal condylar resection based approaches to these tumors.
Materials and Methods: All patients who underwent surgical resection of anterior and anterolaterally situated foramen magnum meningiomas were included in the analysis. The study period was more than 10 years from 2005 to 2015 at our institute; a tertiary referral centre in India. The records along with demographic profile, clinico-radiological features, surgical strategies, outcomes as well as mortality and morbidity were analysed.
Results: There were a total of 20 patients (9 males and 11 females) who were operated during the study period. The average age was 36.7 years. In 16 patients, gross-total or near-total resection could be achieved, four patients underwent subtotal resection. Eight patients had fresh morbidity in the form of new motor deficits, pseudomeningocele formation, worsening of the lower cranial nerve functions or post-operative adhesions leading to syrinx formation. The follow-up ranged from 6 months to 140 months.
Conclusion: Foramen magnum meningiomas are an eminently treatable group of tumors. Condylar preservation provides a good visualization, while helping to preserve joint stability and in avoiding instrumental stabilization.
Keywords: Far lateral, foramen magnum, meningioma, skull base, surgery
Key Message: In the far lateral approach to anteriorly placed foramen magnum meningiomas, following the natural corridor created by the mass of the tumor between the cervicomedullary neuraxis and the clivus, and a conservative occipital condylar resection (thus obviating the need for an occipital condylar-C1 lateral mass stabilization), helps in achieving a satisfactory excision of these complex lesions with a very low morbidity and mortality.
|How to cite this article:|
Srinivas D, Sarma P, Deora H, Beniwal M, Vikas V, Rao K V, Chandramouli B A, Somanna S. ”Tailored” far lateral approach to anterior foramen magnum meningiomas – The importance of condylar preservation. Neurol India 2019;67:142-8
|How to cite this URL:|
Srinivas D, Sarma P, Deora H, Beniwal M, Vikas V, Rao K V, Chandramouli B A, Somanna S. ”Tailored” far lateral approach to anterior foramen magnum meningiomas – The importance of condylar preservation. Neurol India [serial online] 2019 [cited 2020 Jun 4];67:142-8. Available from: http://www.neurologyindia.com/text.asp?2019/67/1/142/253609
Meningioma of the foramen magnum (FMM) are unusual and comprise only 0.3%–3.2% of the overall incidence of meningiomas. However, they are the most common benign intracranial, extra-axial tumors arising at the craniocervical junction., Initially described by Hallopeau in 1872 in a post-mortem of a patient with rapidly progressing quadriparesis leading to death within a short span of 5 months, they are often diagnosed late (especially when they have attained a large size), in view of their slow indolent course.,, The management is often challenging in view of the very limited space available between the cervicomedullary junction and the clivus at the foramen magnum; and, the propensity of the tumor to involve the lower brainstem, the spinal cord, the lower cranial nerves and the vertebral artery (VA). Over the past decades, various surgical approaches have been described and refined by different authors. Since the landmark article by Sen and Sekhar, extensive skull base exposures have been the mainstay in their microsurgical removal. These include extensive condylar removal, mobilization of the VA and an occipitocervical fusion. However, these extensive approaches carry their own mortality and morbidity. In this article, we describe our technique of “tailored resection” of these lesions with the stress on condylar preservation in our series of these extremely complex tumors. The importance of condylar preservation (>50%), and minimal VA handling have been emphasized, thus obviating the need for extensive skull base approaches in order to resect these tumors.
| » Materials and Methods|| |
This is a retrospective analysis of all anterior and anterolaterally situated FMMs operated over a period of 10 years (2005–2015) at our institute, a tertiary referral center in India. All of them underwent a modified far lateral approach (FLA) with stress on condylar preservation. The patient records were scanned and the demographic profile, clinico-radiological features, surgical strategies, and outcome, along with the mortality and morbidity were analysed. The pathology of the tumors was reconfirmed via re-examination of the histological slides. The follow-up data was collected from the outpatient department records. The range and average duration of follow-up were noted.
| » Results|| |
Demographics and clinical presentation
There were a total number of 20 patients (9 male and 11 female patients) with an average age of 36.7 years. The duration of the symptoms varied from 6 to 22 months. The presence of pyramidal tract sign was the commonest presentation (present in all the included patients) followed by lower cranial nerve involvement (present in 11 patients). Three patients had dysesthetic neck pain and two patients presented with headache, which was holocranial in nature that worsened in the morning and was suggestive of raised intracranial pressure secondary to hydrocephalus [Figure 1]. Pre-operatively, three patients were in Nurick grade 2, 10 in Nurick grade 3 and 7 in Nurick grade 4. The detailed patient characteristics have been outlined in [Table 1]. Eighteen cases were operated primarily and two of them underwent surgery for recurrence. One patient was operated for both residual and recurrent tumor, to excise the extradural part extending up to cervical (C)2 level, which was left behind at the time of first surgery.
|Figure 1: Compostite image showing T1 weighted post-gadolinium enhanced sagittal image and correspoding FLAIR image of a patient with an anterior foramen magnum meningioma with hydrocephalus|
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|Table 1: Demographics and clinico-radiological characteristics of the patient cohort|
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All the patients underwent a pre-operative computed tomographic (CT) scan with thin posterior fossa cuts (1 mm) and also an magnetic resonance imaging (MRI) of the head. The assessment of high-resolution, thin-slice CT sections of the craniocervical junction was an integral part of our pre-operative planning protocol to clearly demarcate the bone margins and also to accurately measure the surgical corridor. An MRI image is often deceptive in assessing these parameters. A CT scan was also useful in evaluating the cases associated with an intratumoral calcification or a bony hyperostosis, or in those with osteolysis of the surrounding bone. It was also used for the planning of bony resection required for tumor removal. Additional findings noted on the CT head scan of the head included hyperostosis of the bone in 15 patients. The tumor extended inferiorly into the cervical cord with extension up to the lower border of C2 in two cases. One patient had both a significant intra- and extradural component and hence was operated in two settings. On MRI, the FMMs appeared as well-delineated dural-based extra-axial lesions, which were isointense or mildly hypointense on T1-weighted and T2-weighted MR images with homogenous-to-heterogenous contrast enhancement with a 'dural tail' sign often being present. Information about the vertebrobasilar system and of the presence of vessels adjacent to the tumor were also obtained from the MRI [Figure 2]a, [Figure 2]b, [Figure 2]c, [Figure 2]d. Digital subtraction angiogram of the head was performed only in 9 out of the 20 cases (45%). Angiography was done in only the cases where MRI did not provide useful inputs about the VA; and, whether or not the artery was encased by the lesion. Alternatively, a CT angiography was done in rest of the cases and provided a similar information. None of our patients underwent a preoperative embolization of the tumor feeders.
|Figure 2: (a-c) T1-weighted post contrast sagittal, axial and coronal magnetic resonance images showing a contrast enhancing dural-based lesion on the anterior lip of foramen magnum with dural tail. (d) DSA showing tumor blush due to feeders from the vertebrobasilar system|
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The patients were placed in the lateral position on the head holder and their head was fixed with pins. We do not flex the head laterally or rotate the head. We use prophylactic antibiotics and mannitol at induction of anesthesia. Mannitol was given only at induction as a single dose in order to reduce the intracranial pressure and prevent inadvertent dural tears during the performance of the craniotomy. Administration of mannitol prevented dural bulging and its coming in contact with the drill due to the prevalence of chronically raised intracranial presssure. A lazy-S or a C-shaped incision was given extending up to the C5 level in the midline [Figure 3]a. The muscles were cut in layers and the C1 and C2 posterior elements were exposed along with the suboccipital region. A cuff of muscle was left at the nuchal line for better approximation. The exposure also included the lateral mass of C1 along with the atlanto-occipital joint. A suboccipital craniotomy was performed extending from the level of the inferior nuchal line down to the foramen magnum. A sub-periosteal dissection was performed at the C1 and C2 level followed by a laminectomy. While the C1 lamina was always excised, C2 lamina was removed depending on the extent of tumor. The V3 segment of the VA, including the horizontal extradural segment was exposed and dissected carefully. We followed the VA up to the transverse foramen of C1 but did not unroof it. The VA was identified over the superior surface of C1 and traced and skeletonized up to its point of entry into the dura [Figure 3]b and [Figure 3]c. There was usually a brisk and troublesome bleeding from the vertebral venous plexus, which, however, could easily be controlled by surgicel or by fibrin glue. We drilled a part of the occipital condyle, depending only upon the tumor morphology. The only indication that necessitated an upfront condylar drilling was the inadequate visualization of the whole lesion. Neither complete skeletonizing of the VA nor its mobilization was indicated in any of our cases. Thus, we “tailored” the condylar drilling depending on the extent of the tumor. Care was taken never to drill more than one-third of the occipital condyle. The dura was opened in a curvilinear shape. The denticulate ligaments were cut and the C1 root was sacrificed if it obstructed the view to the tumor. The tumor was identified and the rootlets of the lower cranial nerves were carefully dissected away from the surface of the tumor [Figure 3]d. The tumor was coagulated, decompressed (using an ultrasonic aspirator or micro- scissors, depending upon the tumor consistency) and was dissected away from the surrounding structures. It was important to remember that the VA was closely related and extreme caution was necessary while working upon the lower and anterior surface of the tumor. Once the tumor had been excised, hemostasis was attained and the dura closed in a watertight manner.
|Figure 3: (a) Intraoperative photograph showing the positioning of the patient and a lazy-S or a C-incision being given extending up to the C5 level in the midline. (b and c) The VA is identified over the superior surface of C1 and traced and skeletonized up to its point of entry into the dura. (d) The tumor is identified and the rootlets of the lower cranial nerves are carefully dissected away from the surface of the tumor|
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| » Results|| |
We were able to achieve Simpson Grade II excision in 16 cases [Figure 4]a and [Figure 4]b. Grade III excision was performed in two cases, and only partial decompression could be achieved in two cases. The details of the patients operated as a part of this study are given in [Table 2]. Encasement of the dominant VA in three cases, a high vascularity in one case and an ill-defined arachnoidal plane in two cases, were the reasons for incomplete excision of the lesion. Re-exploration was done in two cases, among whom one had been operated at another center.
|Figure 4: (a) T1-weighted contrast enhanced magnetic resonance images at follow-up showing a Grade II excision of the meningioma. (b) CT axial bone section shows the extent of bony removal needed for exposure and removal of the lesion|
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In one case, there was VA injury, which was controlled with packing. However, the symptoms related to lower cranial nerve paresis worsened in three cases; among these patients, two recovered, whereas the third patient required a tracheostomy. The follow-up duration varied from 6 months to 140 months (average follow -up duration: 61 months). Significant morbidity included worsening of hemiparesis in one case, a pseudomeningocele formation in three cases, and post-operative dural adhesions leading to syrinx formation in one case. The Nurick grade improved in 13 patients, deteriorated in 1 patient, and remained same in 6 patients. Post-operative MRI was obtained 6 months after surgery and at a one-year interval subsequently. Only two patients had a residual tumor for which stereotactic radiosurgery (SRT) was administered. Both these patients had Grade II meningiomas.
| » Discussion|| |
FMM pose a significant challenge for the operating surgeon in view the difficulties that arise in accessing these lesions, their close proximity to the lower brain stem, and the frequent involvement of the lower cranial nerves and the VA. These lesions have a slow growth rate and the patients often have a propensity to remain asymptomatic for a long duration. The insidious growth of the tumor has been partly contributed by the wide subarachnoid space present at this level. Thus, usually these tumors are detected at an advanced stage.,, Frazier and Spiller, way back in 1922, for the first time attempted surgical excision of FMM, and 7 years later, Elsberg and Strauss performed the first successful excision of this benign entity. However, the surgery of anterior and anterolaterally placed lesions operated by the posterior approach continues to be hazardous due to the very high complication rates., the incipient occipito-cervical instability, the prolonged duration of surgery, the high incidence of operative site pain. The possibility of injuring the VA and the XII nerve have also posed significant challenges during surgery of these benign lesions by the posterior approach. Mortality has been recorded due to respiratory failure, VA injury, brainstem infarction and pneumonia due to aspiration. An en-plaque growth of the tumor, a hard tumor consistency and the presence of strong adhesions of the tumor capsule to adjacent neurovascular structures are potential risk factors leading to lower cranial nerve deficits and subsequent morbidity.
Surgery of anterior and anterolaterally placed FMM was revolutionized in 1988 by George, et al. He described the FLA for these tumors, which was similar to that proposed by Heros  for vertebral and vertebrobasilar lesions. This paved the way for the FLA and its variants to become an important arsenal for the operating surgeon approaching the lesions involving the lower clivus or the foramen magnum. Thus, these complex lesions could be resected with minimum complications. Several modifications of this approach have been described in literature till now. The backbone of the FLA is resection of the occipital condyle. The other documented variations of this approach are the excision of the superior facet of C1, removal of the jugular tubercle, a more convenient exposure of jugular foramen, a mastoidectomy and dissection of the hypoglossal canal with preservation of its contents., However, there was no consensus on the extent of condylar removal. Açikbaş, et al., studied the effect of condylectomy during the FLA and concluded that bony removal of the postero-medial part of the condyle beyond the hypoglossal canal significantly increased the angle of exposure and decreased the working distance. There are several proponents of the modified FLA without the removal of occipital condyle; prominent among them are studies by Koos and Spetzler  and Samii, et al.
In 1990, Sen and Sekhar in their study emphasized the need for a total condylectomy, which was revalidated in 1991 by further endorsement of partial or complete condylectomy. It was followed by many different studies which affirmed and agreed with Sekhar.,, In 1996, Yasargil, in his study, was able to achieve complete excision of FMM by the lateral suboccipital approach with very low complication rates. But Salas et al., was not able to achieve similar results by the transcondylar approaches. Nanda, in his cadaveric study, suggested that resection of the occipital condyle should be tailored to the individual cases and should not be considered technically imperative for all the cases in whom surgical resection is being performed. They also showed that the degree of improved exposure between one-third removal of the ipsilateral occipital condyle and removal of half the condyle was only 4 degrees. In recommendations published to date, it is advised that the occipitocervical joint (OCJ) should be stabilized when >70% of the occipital condyle has been removed. However, Vishteh et al., analyzed the effect of occipital condyle removal on joint mobility and found that greater than 50% removal causes major instability. They recommended an occipito-cervical fusion for this extent of condylar resection. Till date, this study continues to provide the only scientific evidence for the assessment of instability at the CVJ. A review of the various series of FMMs focusing on the extent of condylar resection shows a decreasing trend towards the the extent of condylar resections being done, thus further cementing our belief in a less aggressive resection strategy [Table 3].
|Table 3: Review of literature of series focusing on the extent of condylar resection, as well as the morbidity and mortality in these series|
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Despite these various studies published in literature, there is no consensus till now regarding the extent of condylar drilling required for safe excision of these tumors. Although complete removal of the occipital condyle improves visualization and helps in the dissection of the tumor-nervous tissue interface, the same may also be achieved by cutting of the denticulate ligament and mobilization of the cord. Also, more extensive bone removal is usually not necessary because displacement of the brainstem by the lesion provides an enhanced visualization.
In our technique, we do not remove the occipital condyle completely. Instead the condyle is exposed and depending on the visualization, the occipital condyle is drilled (the “tailored” resection technique). In all these cases, we have not found the necessity to remove more than one-third of the occipital condyle or mobilize the VA from the C1 foramen. Employing this technique, we found that the operative exposure was adequate, stability of the occipito-cervical joint was not compromised, and chances of hypoglossal nerve injury and VA were minimized. This strategy is a safe alternative to the FLA or the extreme lateral exposure for the anterior and anteriolaterally placed FMM. During our surgery, to improve the exposure, we sometimes sacrificed the C1 root, whenever it obstructed visualization of the tumor-neuraxial interface. To reach the tumor, we follow the naturally existing trajectory between the lower cranial nerve rootlets and the C2 nerve. The most important and challenging step was to avoid the injury to the VA. There are reports in the literature of partial drilling of the occipital condyle and the jugular tubercle for clipping of VA and vertebrobasilar aneurysms, without medial mobilization of the intradural VA. Our technique was to identify the VA from the superior surface of the C1 and then skeletonize it up to its point of entry into the dura. The dura was opened in a C-shaped manner. We neither drilled the entire occipital condyle upfront (thus avoiding instability) nor did we mobilize the VA medially. The tumor could be identified and provided the natural path through which removal could be performed. A watertight dural closure has been considered as mandatory to avoid the troublesome incidence of pseudomeningocele.
| » Conclusion|| |
FMMs though rare are an eminently treatable group of tumors which require a great degree of skill by the operating skull base neurosurgeon. The modified FLA with condylar preservation provides a good visualization, while helping to preserve joint stability and to avoid an instrumented stabilization of the occipito-atlantal joints. With the introduction of recent advances in microsurgical techniques and the application of novel principles of surgery, the outcome of surgery has been excellent with an overall very low rate of morbidity and the occurrence of negligible mortality.
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Conflicts of interest
There are no conflicts of interest.
| » References|| |
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3]