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Midline and far lateral approaches to foramen magnum lesions.
Correspondence Address:
Twenty patients with foramen magnum lesions were operated upon in the last 5 years at Postgraduate Institute of Medical Education and Research, Chandigarh. The common presenting features were quadriparesis, quadriplegia, diminished sensations, neck pain and respiratory insufficiency. The lesions encountered were meningiomas, neurofibromas, posterior inferior cerebellar artery aneurysms, neurenteric cyst and chordoma. Patients with posterior or posterolaterally placed lesions were operated by the midline posterior approach while those with anterior or anterolateral lesions were managed by the far lateral approach. All mass lesions were excised completely and the aneurysms were clipped. Seventeen patients made good neurological recovery while three died. The latter three patients presented very late. The merits of various surgical approaches to the foramen magnum are discussed.
The lesions of foramen magnum (FM) can be managed through various surgical approaches. The choice of surgical route is dictated by the anatomic location of the lesion. In the horizontal plane, the lesion may be anterior, lateral or posterior. In relation to the vertebral artery, it may develop above, below or on both the sides. In addition the lesion may be extradural, intradural or of dumbbell shape with intra and extradural extension. Depending upon the location of the lesion, the FM may be exposed by anterior transoral approach, the standard midline posterior and the anterolateral or posterolateral approaches. The posterolateral approach with minor modifications has been described under various names: lateral approach,[1] far lateral inferior suboccipital approach,[2] transcondylar approach[3] and the extreme lateral transcondylar approach.[4] We present our experience of management of 20 patients with foramen magnum lesions operated by either the posterior or the far lateral approach. Clinical Material In the last 5 years (1994 through 1998), 20 patients with foramen magnum lesions were operated upon at the Department of Neurosurgery, Postgraduate Institute of Medical Education and Research, Chandigarh. Their age ranged from 14-75 years (mean 41 years). There were 12 female and eight male patients. The common presenting features were quadriparesis/quadriplegia, diminished sensations and neck pain. Five patients had respiratory insufficiency at admission. It was severe in three cases [Table I]. SAH : Subarachnoid haemorrhage All patients were investigated by MRI. Three patients who presented as subarachnoid hemorrhage (SAH) underwent digital substraction angiography (DSA). Depending upon the location of the lesion, patients were divided into two groups. `Group A' included patients with either posterior or posterolaterally situated lesions (n=5) while `group B' included patients with lesions located either anteriorly or anterolaterally (n=15). The pathological spectrum in both groups is depicted in [Table II]. PICA : Posterior inferior cerebellar artery Surgical Technique : The posterior/posterolateral lesions were approached via the midline posterior approach while anterior/anterolaterally situated lesions were operated through the far lateral approach. Midline posterior approach : The lateral limit of this approach is the medial end of the vertebral artery groove of the atlas, which corresponds to the lateral aspect of the dural sac. Because of this anatomical intradural relationship, it is not feasible to work well laterally and anterior to the spinal cord and the medulla. Far lateral inferior suboccipital approach : The patient is positioned in lateral position and head is tilted up by 30o. This allows the cerebellum to fall away. The incision starts at the level of top of the ear, about 2 finger breadths medial to the tip of the mastoid in the sagittal plane. It extends down towards the mastoid and then curves medially to the midline and straight down to the C2 spinous process. The occipital bone is exposed by subperiosteal dissection. The posterior arch of C1 is exposed by subperiosteal dissection from just on the opposite side of midline to the area of the sulcus arteriosus underlying the vertebral artery laterally. The craniectomy extends from below the transverse sinus superiorly to the sigmoid sinus laterally and through the foramen magnum just to the opposite side of midline inferiorly. The posterior arch of C1 is removed from beyond the midline on the opposite side to underneath the vertebral artery laterally. The perivertebral venous plexus overlies the vertebral artery and venous bleeding is usually encountered before the artery can be exposed. A radical removal of the bone in the area of foramen magnum is necessary, going laterally as far as the condylar fossa posterior to the occipital condyle and just above and behind the entry of vertebral artery into the dura motor. It is helpful to expose the vertebral artery in the region to avoid any injury. Laterally the bone edges become vertical and therefore a high speed drill is required for the last 5-10 mm of exposure. This extreme inferolateral bony removal is the key to approach the front of brainstem with minimal or no retraction. The dura is opened in an oblique line starting superolaterally and coming down towards the midline in the area of the foramen magnum and continuing straight down to the level of C1. Further dissection is done under an operating microscope. The arachnoid covering of the cranial nerves is opened widely and the first dentate ligament is cut to allow the medulla to fall away. The cerebellar tonsil is gently retracted superomedially. This brings into view the vertebral artery entering the dura. The proximal vertebral artery aneurysms are visible now. To reach aneurysms of the vertebrobasilar junction, further retraction of the tonsil and of the inferior aspect of cerebellar hemisphere is required. Depending upon the location of the aneurysm or the tumour, dissection between various lower cranial nerves is undertaken. The space between 9th-10th cranial nerve and 11th nerves is usually wider than the space between 7th-8th and 9th-10th cranial nerves.
Fourteen patients showed complete neurological recovery; two had mild neurological deficit and one had significant neurological deficit (dependent on others). Three patients died. The complications and their management are depicted in [Table III]. Three patients had severe preoperative respiratory insufficiency and needed postoperative ventilation. Only one of the three patients showed improvement. The remaining two died. One patient died of shunt infection.
Most of the intradural extramedullary lesions in the region of foramen magnum are meningiomas and neurofibromas.[5],[6] The other reported lesions include aneurysms, chordomas, chondrosarcomas, glomas jugular tumours, arteriovenous malformations, C-V junction anomalies, osseous tumours, haemangioblastomas, melanomas, angiolipomas, epidermoid cysts and metastases.[1],[6],[7],[8] Some of these lesions may be both intradural and extradural and may also infiltrate bony structures. In the present series 70% of the lesions were either meningiomas or neurofibromas. There were 3 patients with PICA aneurysms. Interestingly, two patients had an anteriorly placed neurenteric cyst. Clinical presentation of the FM lesions may be in form of neck pain, dysasthesiasis in the upper limbs, quadriparesis or quadriplegia, cruciate hemiparesis, impaired pain and touch sensations and occasionally pseudoathetoid movements of the hands.[3],[9] Three of our patients presented in late stage with severe respiratory distress while one patient had severe contractures of the limbs as well. In general, anterior lesions may be reached by the transoral approach, the posterior lesions by the standard midline posterior approach and the lateral lesions by the postero or anterolateral approaches.[10] Five of our patients had a posterior or posterolaterally placed tumour and these were removed totally by the posterior approach, without significant surgical complication. In contrast, the management of lesions situated in the anterior or anterolateral part of FM is challenging as it is associated with high morbidity and mortality.[7] Various surgical routes employed include anterior transoral, anterolateral or lateral approach. For such lesions the posterior approaches are insufficient and may be hazardous.[11],[12] A bilateral or unilateral suboccipital approach with cervical laminectomy has also been used but the results have been disappointing.[13],[14] Guidetti[5] described the transoral approach as the safest route for the removal of ventrally located FM lesions as it provided a direct route to the pathology, without having to work around the neural structures. However, there are many drawbacks of this approach, especially for intradural lesions.[15] The lateral access is limited by the vertebral arteries and jugular vein. Since the tumour is encountered first, the tumour-cord/brainstem surface is approached blindly. There is problem of CSF leak and infection. In addition, instability of the spine may occur requiring a fusion procedure. The extreme lateral transcondylar approach has been described for lesions situated in the anterior foramen magnum and lower clivus.[3],[4],[7],[16] This approach involves the resection of condyle and mobilisation of the vertebral artery after opening of the transverse foramen of C1. According to Babu et al, this approach is absolutely essential for intradural mass lesions located ventral to the spinomedullary junction. The disadvantages of this approach include possible vertebral artery injury and occipitocervical instability, if the entire occipital condyle is resected.[7] Heros[2] described the far lateral inferior suboccipital approach for deep vertebral and vertebrobasilar aneurysms. In this approach, the condyle is not resected. We found this approach adequate for removal of anterior or anterolaterally situated lesions. The vertebral artery was not mobilized. It is concluded that the exposure obtained via the far lateral approach is sufficient for total removal of anteriorly placed well circumscribed lesions without handling of spinomedullary junction which in most cases has already been displaced posteriorly.
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