INTRODUCTION

 

Foramen magnum tumors, especially those located ventrally, are surgically challenging.¹ The borders of the foramen magnum (FM), as defined by George and colleagues² range anteriorly from the lower third of the clivus, to upper margin of the body of C-2, laterally from the jugular tubercle to the upper margin of the C-2 laminae, and posteriorly from the anterior edge of the squamous occipital bone to the C-2 spinous process.

Tumors of the FM comprise only 1.1–3.8% of all brain and spinal cord tumors. Approximately 30% of these tumors are benign and extramedullary, with a curative potential for surgical interventions. Meningiomas are most frequently seen comprising more than 85% of all FM tumors. Schwannoma and chordomas are also commonly seen.³

Different surgical approaches have been used to facilitate the resection of tumors involving the foramen magnum and to reduce postoperative morbidities. According to the location of the lesion, the FM may be exposed by anterior transoral approach^4,5, the standard midline posterior and the anterolateral or posterolateral

approaches. The posterolateral approach with minor modifications has been described with various names as lateral approach⁶ far lateral inferior suboccipital approach,⁷ transcondylar approach ⁸ and the extreme lateral transcondylar approach.⁹

 

Aim of Work:

To present our experience of management of 10 patients with foramen magnum benign tumors operated upon. We studied clinical and surgical details of intradural extramedullary tumors located in the region of the foramen magnum.

 

PATIENTS AND METHODS

 

This is a retrospective study included 10 patients with foramen magnum intradural extramedullary tumors operated upon.

All patients underwent thorough neurological and laboratory examination. All patients were investigated by MRI. Magnetic Resonance Angiography was done for seven patients. Patients were divided into two groups. Group A included patients with posterolaterally situated lesions (n=3), while group B included patients with lesions located anterolaterally (n=7).

 

The head is secured in a 3-pin Mayfield skull clamp fixation device and the surgical area prepared and draped from the inion to the upper thoracic vertebrae. The attitude of the neck and lateral flexion are tailored to the position of the tumor as related to the brain stem and upper cervical cord. If it is anterior to the stem and cord, head rotation to the side of the lesion maximizes tumor visualization and decreases the need for brain stem retraction.

An inverted hockey-stick incision is made. It begins at the level of the mastoid tip and extends superiorly to the superior nuchal line, curving toward the inion and then inferiorly through the midline to the level of the C-3 spinous process. Dissection is carried down to the fascial plane of the superficial muscle layer, and the skin flap is mobilized inferiorly and laterally.

The superficial and intermediate musculature layers are disconnected from their occipital and mastoid insertions and reflected laterally. The muscles are also detached in the midline from the spinous processes for exposure of the posterior arch of the atlas and the C-2 lamina, if required. After identification of the spinous process of C-2 and the posterior tubercle of C-1, subperiosteal dissection is carried laterally over the posterior arch of C-1 to identify the sulcus arteriosus and the horizontal segment of the VA. A prominent venous plexus surrounds this segment of the vessel and can be the source of profuse bleeding. Careful dissection over the posterior arch of C-1 is performed in a subperiosteal manner from medial to lateral, inferior to superior, to protect the VA and venous plexus from injury and avoid unnecessary bleeding. Once exposed, the VA is displaced superiorly to review the lateral mass of C-1, the lateral margin of the laminectomy at that level. The arch of C-1 is then removed by rongeur resection and/or the use of a diamond drill. Commonly, the inferior extension of the tumor necessitates a complete laminectomy of C-2 as well. Care must be taken to identify the venous plexus that exists between the C-1 and C-2 vertebrae. The suboccipital craniectomy is performed by placing multiple burr holes and rongeuring away the intervening occipital bone or by utilizing a diamond burr.

The lateral extension of bony removal along the arch of C-1 and the rim of the foramen magnum must be taken as far laterally as possible to allow adequate exposure of the tumor and its vascular supply. Extensive lateral exposure also permits decreased manipulation of the medulla and cervical spinal cord as the tumor is evacuated. The dura is opened in a Y-shape with the superior flap reflected upward and secured to the bony margin or muscle. The lateral dural margins are tacked to the paraspinous musculature. After opening the arachnoid, the tumor will be identified as arising primarily from an anterior or posterior origin in relationship to the medulla and cervical spinal cord. Great effort must be made to preserve the vertebral, posterior inferior cerebellar, anterior spinal and posterior spinal arteries. Posteriorly located tumors are approached directly.

However, anteriorly positioned tumors tend to push and rotate the cord and medulla posteriorly; stretching the dentate ligaments, first and second cervical roots, and the eleventh nerve over the tumor. Division of the dentate ligaments allows access to the anterior arachnoidal cisterns as well as providing a means of supporting the spinal cord during the tumor dissection.

The tumor removal is delivered piecemeal by means of sharp dissection and aspiration technique. Large tumors often parasitize blood supply from the pial vasculature and care should be taken to identify these arteries. The dura will necessarily be left intact on anteriorly located meningiomas but must be cauterized to decrease the chances of recurrence. For schwannomas, the nerve root of origin should be sacrificed with adequate proximal and distal margins to encompass the tumor extension along the root. A watertight closure of the dura is mandatory. The most important layer is the ligamentum nuchae inferiorly and the galea superiorly. This layer must be closed meticulously as a single structure. The subcutaneous tissues are then reapproximated and finally the skin is closed.

An MRI with contrast was done 3 months postoperatively to assess the extent of tumor removal.

 

RESULTS

 

Ten patients had surgery (7 females and 3 males). The age ranged from 32 to 59 years with an average of 44.6 years. The average length of hospitalization was 18 days. Follow-up periods ranged from 7 to 55 months (mean 32 months). There were 7 patients with meningiomas and 3 patients with neurofibromas.

The most common symptoms were occipital pain and headaches in 9 patients (90%) followed by gradual progressive weakness with gait disturbance (80%). Swallowing problems and change in voice (40%).The average motor power was II in one patient, III in six patients and IV in three patients. Four patients showed signs of bulbar affection on examination. One patient showed weakness of the hypoglossal nerve. Two patients showed weakness of the spinal accessory nerve. It was more common to observe a combination of two or more symptoms and signs than only one.

Of the 10 patients in whom surgery was performed, 5 underwent gross-total, three near-total, and two subtotal removals of their tumors. All schwannomas were totally excised. Pathologically, seven cases were meningiomas (Figure 1) and three were schwannomas (Figure 2). 

Transient worsening of neurological deficits was occurred in three patients. There were two cases of CSF leakage; one was treated with external lumbar drainage and the other need operative revision. There was no incidence of vertebral artery injury in the ten cases. One patient died due to chest infection.

In the post-operative period, after 6 months two patients achieved a motor power III and seven patients of average grade IV or V. The bulbar symptoms improved in two out of four patients.

 

 

 

 

 

Figure 1. Axial MRI pre- and postoperative of posterolateral meningioma

 

 

 

Figure 2. Sagittal MRI pre- and postoperative of anterolateral schwannoma.

 

DISCUSSION

 

The foramen magnum contains several critical neuroanatomical and vascular structures of which the surgeon must be aware. The neural structures include the cerebellar tonsils, inferior vermis, fourth ventricle, caudal aspect of the medulla, lower cranial nerves (9^th–12^th), rostral aspect of the spinal cord, and upper cervical nerves (C-1 and C-2). Major arterial structures located within the foramen magnum include the vertebral arteries, PICAs, anterior and posterior spinal arteries, the meningeal branches of the vertebral, external, and internal carotid arteries.¹⁰

Most of the intradural extramedullary lesions in the region of foramen magnum are meningiomas and neurofibromas.³ The other reported lesions include aneurysms, chordomas, chondrosarcomas, glomas jugular tumors, arteriovenous malformations, C-V junction anomalies, osseous tumors, hemangioblastomas, melanomas, angiolipomas, epidermoid cysts and metastases.³ Some of these lesions may be both intradural and extradural and may also infiltrate bony structures. In the present study, there were seven patients with meningiomas and three patients with neurofibromas.

There was a classification of the primary tumors of foramen magnum according to their anteroposterior and lateromedial orientations. Most lesions (68-98%) arise anterolaterally; a posterolateral origin is the second most frequent, purely posterior lesions the third, and least common are entirely anterior.¹¹ In the present study, there were seven tumors located anterolaterally (70%) and three (30%)   tumors located  posterolateral.

Suboccipital headache and upper cervical pain are the most common early complaints, with the pain frequently exacerbated by coughing, straining, or Valsalva maneuvers.¹⁰ The classic foramen magnum syndrome is defined by development of unilateral arm sensory and motor deficits, which progress to the ipsilateral leg, then the contralateral leg, and then the contralateral upper extremity.¹² Long tract findings and spastic quadriparesis present later on in the patient’s clinical course. Untreated cases may progress to quadriplegia and respiratory arrest. Lower cranial nerves are infrequently involved. Cranial nerve XI is the most commonly affected, resulting in atrophy of sternocleidomastoids and trapezius muscles.¹⁰ In this study, the most common symptoms were occipital pain and headaches in nine patients (90%); followed by gradual progressive weakness with gait disturbance (80%). Swallowing problems and change in voice occurred in four patients (40%). The average motor power was II in one patient, III in six patients and IV in three patients. Four patients showed signs of bulbar affection on examination. One patient showed weakness of the hypoglossal nerve. Two patients showed weakness of the spinal accessory nerve.

Complete resection rates have varied from 0% to 100%, while recurrence rates have varied from 0% to 33% in surgical studies published in the last couple of decades.^4,11,13-16 The reason behind this wide variability lies in the heterogeneity of the tumors, in addition to the aggressiveness with which these lesions were approached. Many authors have attempted complete resections even at the cost of potential morbidity¹¹, while others argue for a more conservative approach to minimize complications.^13,17 Factors associated with incomplete resections and recurrences include encasement of the vertebral arteries¹⁸, tumor invasiveness (as evident from the extradural component of the tumors)¹⁷ and adherences to vital structures, especially in recurrent lesions.^11,18 In our opinion, all neurosurgeons must remember that they are treating people and not just resecting tumors. Sometimes this requires consideration of subtotal resection or monitoring. Of the ten patients in whom surgery was performed, five underwent gross-total, three near-total, and two subtotal removal of their tumors. All schwannomas were totally excised.

Overall, advances in microneurosurgical techniques over the last 2 decades have resulted in an average mortality rate that has decreased to 6.2% (range 0%-25%), neurological improvement in 70%-100% of patients, neurological stability in 2.5%-20% of patients, and neurological decline in 7.5%–10% of patients in published series.^{4,11,13,19-21} Major morbidity has varied between 0% and 60% of patients and is mainly associated with lower cranial nerve deficits leading to CSF leaks, myelopathy, hydrocephalus and aspiration pneumonia.^13,15,22 These complications are more commonly reported with the transoral transclival approach.^4,5 The far lateral approach^13,16,23 is associated with decreased rates of  lower cranial nerve deficits and overall morbidity, as compared with rates in the extreme lateral and lateral suboccipital approaches.^5,24 An anterior tumor location^17,18, tumor extension into the lower clivus, small tumor size, tumor invasiveness, extradural extension, vertebral artery encasement, absence of an arachnoidal sheath, and adherences in recurrent lesions are all associated with a more challenging surgical procedure and higher procedural morbidity.^13,17,18

In this study, transient worsening of neurological deficits was occurred in three patients. There were two cases of CSF leakage; one was treated with external lumbar drainage and the other need operative revision. There was no incidence of vertebral artery injury in the ten cases. One patient died due to chest infection. 

 

Conclusion

Foramen magnum tumors have long been regarded as difficult lesions both in terms of diagnosis and management. However, with the availability of MR imaging, newer surgical techniques and skull basal exposures, the excision of these lesions is becoming safer.

 

[Disclosure: Authors report no conflict of interest]

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