Anatomy

The foramen magnum is made up of the occipital bone, which forms an oval ring ( Fig. 11.1 ). The anterior part of the ring is made from the lowest extension of the clivus. The occipital condyles lie in the anterior half of the foramen, allowing access to ventrally located tumor if the condyle is partially removed. The hypoglossal canal traverses the occipital condyle approximately halfway and travels in an anterior to posterior, medial to lateral trajectory as it courses from the intracranial compartment outward. Lateral to the occipital condyle is the jugular foramen, which is posterior to the carotid canal. The stylomastoid foramen and process lie lateral to the jugular foramen. The posterior belly of the digastric muscle attaches at the digastric notch and lies over the facial nerve at its exit point. The condylar vein enters the jugular bulb through the foramen posterior and lateral to the occipital condyle.

History

In the past, primary extramedullary tumors of the CVJ have been difficult to diagnose. Their insidious onset and often bizarre symptoms may resemble those of degenerative diseases of the central nervous system (CNS). The clinical course may simulate cervical spondylosis, multiple sclerosis, syringomyelia, or congenital skull base disorders, such as platybasia or Arnold-Chiari malformation. Symptoms may arise only after lesions have grown to a significant size due to the generous subarachnoid spaces of the cervicomedullary junction. The insidious course of these neoplasms can be explained by the anatomical complexity of the area and the decussation of the neural tracts at that level.1,2

The first systematic evaluation of tumors of the foramen magnum was done by Elsberg in 1925.3 Other significant early contributions were made by Abrahamson and Grossman4 and Elsberg and Strauss.5 They emphasized the clinical pattern of compression of the cord at the foramen magnum. Further elaboration was provided by Symonds and Meadows.6 In their 1938 monograph on meningiomas, Cushing and Eisenhardt discussed the compressive nature of spinal cord lesions of the foramen magnum.7 They divided foramen magnum meningiomas into two subgroups: craniospinal and spinocranial. Craniospinal tumors arise above the foramen magnum ventral to the neuraxis and project downward, displacing the medulla and cervical spinal cord. Spinocranial lesions are found dorsal or dorsolateral to the spinal cord and project upward into the cerebellar cisterns. A posterior fossa syndrome caused by tumors of this region was documented by Castellano and Ruggiero.8 Their classification was based on the dural site of attachment. Stein and colleagues described 25 cases of mostly spinocranial meningiomas with a stereotypic clinical syndrome.9

The first large series of intra- and extra-axial tumors of the CVJ was reported in 1941 by Love and Adson.10 Later, Love revealed that 30% of lesions were extramedullary tumors amenable to surgical resection.11 Subsequent reviews limited to benign intradural extramedullary lesions were reported by Dodge and colleagues,12 Martin and Kleyntjens, and others.13–15 Yasuoka and colleagues analyzed 57 cases of neurofibromas and meningiomas of the foramen magnum and reported a ratio of 19 neuromas to 37 meningiomas, which was larger than the previously reported ratio.12,15,16 Meyer reviewed 102 cases of benign extramedullary tumors of the foramen magnum seen between 1924 and 1982.17 He reported that 40% of these patients had a normal neurological examination at first evaluation and that there were no pathognomonic symptoms or physical findings to identify foramen magnum tumors. More recently, Arnautovic and colleagues have supported with their data the efficacy of the transcondylar approach in the resection of foramen magnum meningiomas.18 Wu and colleagues reported the comparative effectiveness of different surgical techniques in the resection of foramen magnum meningiomas in their group of 114 patients.19 In the largest series published to date, Menezes has reported his group′s experience with CVJ neoplasms in a database of 888 patients seen between 1977 and 2003.20

Epidemiology

In an analysis of the University of Iowa Hospitals and Clinics Neurosurgery CVJ registry, 888 patients were evaluated between 1977 and 2003.20 Of these patients, 382 presented with osseous tumors, 426 had neural tumors, and 179 were treated surgically. A review of several series on the incidence of benign intradural extramedullary tumors of the foramen magnum yielded several cases between 1929 and 2010.4,17–22 About 75% of these tumors were meningiomas, occurring in a 3:1 ratio to schwannomas. Rare cases of dermoid tumors, teratomas, lipomas, schwannomas, arachnoid cysts, paragangliomas, and intradural extraosseous chordomas have also been encountered. Intramedullary tumors at the cervicomedullary junction include astrocytomas, ependymomas, and cerebellar region tumors such as hemangioblastomas, medulloblastomas, and choroid plexus papillomas. Chordomas, chondromas, chondrosarcomas, and metastases are common extradural intraosseous tumors.

In adults, meningiomas ( Figs. 11.2 and 11.3 ) are some of the most common tumors of the foramen magnum.20 As with other meningiomas of the neuraxis, there is a definite female predominance for meningiomas of the foramen magnum, with ratios from 2:1 to 3.6:1.8,17,21–23 These lesions usually become symptomatic in the fourth, fifth, and sixth decades of life.20 The average age of diagnosis reported in a large series is 50.1 years.20 Lesions are usually located ventrally or ventrolaterally,20,24 attached to the anterior rim of the foramen magnum, and frequently invade the area of the vertebral artery and the exit of the cervical nerve roots.

The occurrence of schwannomas ( Fig. 11.4 ) at the region of the foramen magnum is less common than meningiomas in adults.24 The literature describes the incidence of schwannomas of the CVJ at ~13%. These benign tumors often grow to considerable size before neurological abnormalities manifest. They are seen with roughly equal frequency in men and women. The distribution of patient ages at the time of presentation ranges from adolescence to the seventh decade. In a large series, the average age of presentation was 38 years.20 Levy and colleagues reported 66 cases of spinal schwannomas, of which 30% were located in the cervical region.25 Approximately 50% of these tumors are located ventral to the craniospinal axis. Tumors such as chordomas ( Fig. 11.5 ), chondrosarcomas, glomus jugulare tumors, and metastases are occasionally observed in this region as well. They are located ventrolateral to the neural structures and are frequently encased in the caudal cranial nerves or vertebral artery.

The epidemiology of the lesions of the CVJ is significantly different in the pediatric population. Menezes has reported chordomas to be the most common tumor of the CVJ in his series of 38 pediatric patients.1,25–30 Chordomas in children have a slight male predominance with a peak occurrence at 121 months of age.31 These tumors appear to be very aggressive, with a 68.5% mortality in 25 months, as reported by Borba.31 By the time chordomas are diagnosed in children, they are usually of a prohibitive size for surgical intervention. However, multidisciplinary treatment of these tumors, including radiation therapy, has resulted in disease-free survival as high as 68% at 5 years.

The second most frequently encountered type of tumor in Menezes’ series was meningiomas.1 In general, meningiomas are rare in children and adolescents, comprising less than 2% of all pediatric intracranial lesions.27,28 Only a few case series of meningiomas in pediatric patients arising in the region of the foramen magnum exist.29,30 The operative management of pediatric patients with foramen magnum lesions via the transcondylar approach is similar to that of adults and presents no additional problems. Schwannomas are also encountered in the CVJ, especially in children with familial syndromes. Children with neurofibromatosis type I tend to become symptomatic with multiple bilateral lesions, including plexiform neurofibromas.1 Most of these lesions occur in a dorsal location, making them easily accessible. Other rare extramedullary lesions of the CVJ encountered in children include fibrous dysplasia of the CVJ, aneurysmal bone cyst, eosinophilic granuloma, primary Ewing sarcoma, and osteoblastoma.1

Clinical Presentation

Extramedullary tumors of the foramen magnum may pose a diagnostic dilemma. Their symptoms can be peculiar and fail to fit classic localized findings. They can be intermittent, progressive, and remitting.1,3–5,7,10,13–19,24,26–52 Diagnosis is often delayed for several years because of these clinical peculiarities. Signs and symptoms can include headaches, occipital radicular pain, cervical pain, phrenic nerve paralysis, lower cranial nerve palsies, hyperreflexia, downbeat nystagmus, atrophy of limb muscles, paralysis of limbs, sensory deficits, cerebellar deficits, sphincter disturbances, Horner syndrome, papilledema, and endocrinopathies.1,3–5,7,10,13–19,26–52 The most frequent symptom, observed in up to 70% of the patients in a large series, is pain in the C2 nerve root distribution, followed by paresthesias (40% of patients) and cranial nerve palsies (30% of patients), usually manifesting as dysarthria or dysphagia.21 Children tend to present with involvement of the lower cranial nerves, brainstem dysfunction, and occasionally cerebellar symptoms. They usually maintain their head in a flexed position, resembling torticollis at times.1

Supported by classical teaching, partial lesions in this area interrupt decussating pyramidal tract fibers destined for the legs. These fibers cross below those of the arms, resulting in a crural paresis of the lower limbs. Compressive lesions near the foramen magnum may produce weakness of the ipsilateral shoulder and arm followed by weakness of the ipsilateral leg, then the contralateral leg, and finally the contralateral arm (an “around the clock” pattern that may begin in any of the limbs). However, this phenomenon was observed only in 22% of the patients in a large series.21

Meningiomas of the foramen magnum are usually symptomatic from involvement of the vertebrobasilar system as well as cranial nerves IX through XII, especially affecting the spinal accessory nerve. Pain and paresthesias are the major symptoms in patients with schwannomas of the foramen magnum. Menezes reports a lower cranial nerve deficit in 38% and myelopathy in 50% of these patients.21

Operative Techniques

The region of the foramen magnum is the site of many intradural and extradural tumors. Most intradural extramedullary lesions are meningiomas and schwannomas, and most are located anterior to the neuraxis. Malignant lesions such as chordomas, chondrosarcomas, metastases, and glomus jugulare tumors are extradural tumors and frequently found anterior or anterolateral to the cervicomedullary junction. Previously, many surgical approaches have been used to gain access to the skull base for tumor removal. These routes have included transoral, transsphenoidal, transcervical, transbasal, infratemporal, transpetrosal, suboccipital, and retromastoid approaches.53–61 However, considering the anatomy of the CVJ, there are three main access points:

1. 
   

   Posterior access with the standard midline posterior approach is the simplest access point and does not destroy important structures, although it is limited to the posterior aspect of the CVJ.

   
   
2. 
   

   Anterior access uses the variants of the transoral approach. At the depth of the mouth, only thin layers of mucosa and muscle are found before reaching the bone. However, anterior access is limited by exposure of the CNS to oral contents and by the difficulty in repairing dura and preventing spinal fluid leakage.

   
   
3. 
   

   Lateral access is directed to the lateral wall of the CVJ and includes the jugular tubercle, occipital condyle, lateral mass of C1, and lateral part of the C2 vertebral body.60

The choice of surgical route is based on multiple factors: tumor location and extent of skull base involvement, tumor histology, tumor consistency, the relationship of the tumor to the dura and neurovascular elements, the goal of the operation (i.e., tissue biopsy for diagnosis versus complete resection), the patient′s age, craniovertebral stability, and the effect of the lesion on the cord (i.e., syringomyelia). The anterior approaches are recommended for midline extradural tumors. Recently, the addition of flexible or rigid endoscopy with anterior techniques has aided diagnosis and treatment of these lesions. The posterior approaches are recommended for dorsal midline tumors, whereas the lateral (transcondylar) approach permits access to the area of the lower clivus and upper cervical spine and is useful for intradural lesions ventral to the foramen magnum.53–61 Both extradural and intradural tumors can be extirpated, leaving neurovascular elements intact. This approach offers the advantage of a direct view to the anterior rim of the foramen magnum without requiring brainstem or cerebellar retraction. In fact, depending on whether the route to the CVJ passes posterior (far lateral approach) or anterior (extreme lateral approach) to the sternocleidomastoid muscle, the surgeon acquires access to the inside or outside part, respectively, of the bony canal. In the pediatric population with torticollis and radiographic evidence of rotary subluxation or atlantoaxial dislocation, the need for stabilization and fusion is obvious.51