CHAPTER 309 Spinal Cord Tumors in Adults
Spinal cord tumors account for about 15% of central nervous system neoplasms,1 and most intradural tumors arise from the cellular constituents of the spinal cord and filum terminale, nerve roots, or meninges. Metastatic involvement of the spinal intradural compartment is rarely manifested as a mass lesion. Intradural spinal cord tumors are broadly categorized according to their relationship to the spinal cord: intramedullary tumors arise within the substance of the spinal cord, whereas extramedullary tumors are extrinsic to the spinal cord. However, a small number of neoplasms may have both intramedullary and extramedullary components that usually communicate either through a nerve root entry zone or through the conus medullaris–filum terminale transition. Similarly, some intradural tumors may extend through the nerve root sleeve into the extradural compartment.
Extramedullary Tumors
About two thirds of spinal cord tumors in adults are extramedullary (Table 309-1). Nerve sheath tumors, meningiomas, and filum terminale ependymomas account for most extramedullary neoplasms,2 with metastases, inclusion tumors and cysts, paragangliomas, and melanocytic neoplasms being rare. With few exceptions, extramedullary tumors are histologically benign and amenable to complete surgical resection.
Incidence and Etiology
Nerve Sheath Tumors
The histologic appearance of neurofibromas consists of an abundance of fibrous tissue and the conspicuous presence of nerve fibers within the tumor stroma.3 Grossly, the tumor produces fusiform (plexiform) enlargement of the involved nerve, which makes it impossible to distinguish between tumor and nerve tissue. Multiple neurofibromas establish the diagnosis of neurofibromatosis (NF), but this syndrome should be considered even in patients with apparent solitary involvement. Both NF1 and NF2 are associated with nerve sheath tumors. Although neurofibromas predominate in NF1, schwannomas are more common in NF2.4
Schwannomas appear grossly as smooth globoid masses that do not enlarge the nerve but are suspended eccentrically from it with a discrete attachment. Their histologic appearance consists of elongated bipolar cells with fusiform, darkly staining nuclei arranged in compact interlacing fascicles with a tendency toward palisade formation (Antoni-A). A loosely arranged pattern of stellate-shaped cells (Antoni-B) is less common.5 Multiple schwannomas, or “schwannomatosis,” can occur in patients without NF, and these tumors currently have no known genetic basis.6,7
Nerve sheath tumors account for about 25% of intradural spinal cord tumors in adults2,8 with an annual incidence of 0.3 to 0.4 per 100,000.7 Most are solitary schwannomas that occur proportionally throughout the spinal canal (Fig. 309-1). The fourth through sixth decades of life represent the peak incidence of occurrence. Men and women are affected equally.
Most nerve sheath tumors arise from a dorsal nerve root, although neurofibromas represent a higher proportion of ventral root tumors.9 The majority of nerve sheath tumors are entirely intradural, but 30% extend through the dural root sleeve, which results in a “dumbbell”-shaped tumor with both intradural and extradural components (Fig. 309-1).7 About 10% of nerve sheath tumors are epidural or paraspinal in location. Transdural growth is common in cervical tumors because the intradural root segment is short.7 One percent of nerve sheath tumors are intramedullary and are thought to arise from the perivascular nerve sheaths that accompany penetrating spinal cord vessels.7 Centripetal growth of a nerve sheath tumor may also result in subpial extension, most often with plexiform neurofibromas. In these cases, both intramedullary and extramedullary tumor components are apparent. Brachial or lumbar plexus neurofibromas may extend centrally into the intradural space along multiple nerve roots. Conversely, retrograde intraspinal extension of a paraspinal schwannoma usually remains epidural.
About 2.5% of intradural spinal nerve sheath tumors are malignant.10 At least half occur in patients with NF. Malignant nerve sheath tumors carry a poor prognosis; survival rarely extends beyond 1 year. These tumors must be distinguished from the rare cellular schwannoma that displays aggressive histologic features but is associated with a favorable prognosis.
Meningiomas
Meningiomas and nerve sheath tumors occur with about equal frequency in adults. They usually arise from arachnoid cap cells embedded in the dura near the root sleeve, which accounts for their predominantly lateral location. Meningiomas may also arise from pia or dural fibroblasts, probably as a result of their mesodermal origin.3
Meningiomas can develop in any age group, but the majority occur in individuals between the fifth and seventh decades of life. Seventy-five percent to 85% occur in women, and about 80% are thoracic (Fig. 309-2).2,11–14 The upper cervical spine and foramen magnum are also common sites.15 Here, meningiomas often occupy a ventral or ventrolateral position and may adhere to the vertebral artery near its intradural entry and initial intracranial course (Fig. 309-3). Low cervical and lumbar meningiomas are rare. Most spinal meningiomas are entirely intradural, but about 10% may be both intradural and extradural or entirely extradural.2
Their gross characteristics range from smooth and fibrous to the more frequent variegated, fleshy, and friable appearance. Microscopic calcification may occur. The dural attachment is often broader than expected. En plaque examples are unusual but have been described.16 The well-defined epidural space of the spine precludes bony involvement. Unlike intracranial meningiomas, spinal meningiomas do not penetrate the pia. This feature simplifies surgical resection and has been attributed to the presence of an “intermediate leptomeningeal layer” between the pia and arachnoid.17 Another explanation is that spinal meningiomas manifest signs of spinal cord compression early in their course and are therefore treated surgically before they have a chance to penetrate the pia.17
Filum Terminale Ependymomas
About 40% of spinal canal ependymomas arise within the filum terminale,1 most in its proximal intradural portion. Astrocytomas, oligodendrogliomas, and paragangliomas can also originate in the filum terminale but are rare. Filum terminale ependymomas occur throughout life but are most common in the third to fifth decades. They occur in men slightly more often than in women. Filum ependymomas and cauda equina nerve sheath tumors occur with about equal frequency.
Myxopapillary ependymomas are by far the most common histologic type encountered in the filum terminale. Their histologic appearance consists of a papillary arrangement of cuboidal or columnar tumor cells surrounding a vascularized core of hyalinized and poorly cellular connective tissue.5 Almost all are histologically benign.18 These tumors, however, tend to be more biologically aggressive in younger age groups.19
Miscellaneous Pathologic Processes
Numerous neoplastic and non-neoplastic processes are occasionally manifested as an extramedullary mass lesion. Dermoids, epidermoids, lipomas, teratomas, and neurenteric cysts are inclusion tumors and cysts that result from disordered embryogenesis.20,21 These lesions can occur throughout the spinal canal but are more common in the thoracolumbar and lumbar spine. They can be intramedullary or extramedullary. Associated anomalies such as metameric cutaneous lesions, sinus tracts, occult anterior or posterior rachischisis, or split cord malformations may be present.20,22 Inclusion tumors and cysts are most often manifested as a mass lesion, but recurrent meningitis, tethered cord syndrome, or congenital deformity may represent the dominant clinical feature. In most cases, treatment consists of excision. A tethered spinal cord is released or a sinus tract is excised if necessary. In some cases, dense adherence of the lesion to neural structures precludes total extirpation.
Paragangliomas are rare tumors of neural crest origin and may arise from the filum terminale or cauda equina.23 They are benign, nonfunctioning tumors and histologically resemble extra-adrenal paraganglia (i.e., carotid body and glomus jugulare). Grossly, they appear as well-circumscribed vascular tumors that are indistinguishable clinically or radiographically from filum terminale ependymomas. Identification of dense core neurosecretory granules on electron microscopy establishes the diagnosis. Complete resection is usually possible. Cavernous malformations, hemangioblastomas, and ganglioneuromas may involve an intradural nerve root and appear as an extramedullary mass lesion. Clinically, these tumors can occur as nerve sheath tumors with early radicular symptoms. Ganglioneuromas often take the form of dumbbell tumors in pediatric patients. Subarachnoid hemorrhage has been associated with nerve root cavernous malformations. These lesions are benign and are removed surgically. The involved nerve root is usually sacrificed, although it can occasionally be spared with small tumors.
Non-neoplastic lesions are also manifested as extramedullary masses. Arachnoid cysts are a well-known example. They are most commonly found in the thoracic spine dorsal to the spinal cord.24 Intraspinal aneurysms are exceedingly rare. They generally occur in conjunction with arteriovenous malformations or coarctations of the aorta. Most isolated cases occur in the region of the foramen magnum and arise from the vertebral or posterior inferior cerebellar arteries. An anomalous vessel origin or course (i.e., kinking) is frequently described. Isolated spinal aneurysms have also been reported to arise from the anterior spinal artery, posterior spinal artery, and medullary arteries.25 Most involve the anterior spinal artery. Patients with a spinal aneurysm may initially be evaluated for subarachnoid hemorrhage or compressive myelopathy. A definitive diagnosis is achieved with selective spinal angiography. Rarely, a herniated intervertebral disk transgresses the dura to occupy an intradural location.26
An inflammatory pathologic process such as sarcoidosis, tuberculoma, or subdural empyema is occasionally manifested as an intradural mass lesion.26,27 Although spinal carcinomatous meningitis often complicates systemic cancer, secondary metastatic involvement of the spinal canal is rarely manifested as a mass lesion. Malignant intracranial neoplasms that appose the subarachnoid space or ventricles are the most likely intracranial tumors to demonstrate cerebrospinal fluid (CSF) drop metastases into the spinal subarachnoid space.28 Systemic cancer gains access to the subarachnoid space by direct penetration of the dural root sleeve or, more commonly, through the choroid plexus.29,30 Surgical removal of a significant secondary neoplastic deposit may be appropriate in some cases.
Clinical Features
The clinical features of most extramedullary tumors reflect a slow-growing intraspinal mass. Specific clinical signs and symptoms are variable and largely determined by tumor location (i.e., local pain and signs of compression of adjacent neural structures). Most spinal tumors cause radicular pain or a dull axial pain, depending on whether the initial neural compression or infiltration involves the nerve roots or the spinal cord. Neurological symptoms gradually arise. Upper cervical and foramen magnum tumors are often located ventrally and cause symptoms of suboccipital pain and distal arm weakness with atrophy and clumsiness of the intrinsic hand muscles.15 The cause of this well-known syndrome is uncertain but it most likely results from venous insufficiency. Increased intracranial pressure and hydrocephalus rarely occur with extramedullary tumors at any level but are most common with upper cervical tumors.31 This syndrome is probably caused by elevated levels of CSF protein, which impairs CSF flow and absorption. Segmental motor weakness and long-tract signs are hallmarks of midlevel and lower cervical tumors. Asymmetric early signs and symptoms are typical and reflect the predominantly lateral location of most intradural tumors. A Brown-Séquard type of syndrome characterized by dysfunction of the corticospinal tract, posterior column, and contralateral spinothalamic tract is common. Rarely, schwannomas are accompanied by subarachnoid hemorrhage.32
Diagnostic Imaging
On T1-weighted images, most intradural tumors are isointense or slightly hypointense with respect to the spinal cord. On T2-weighted images, nerve sheath tumors are more likely than meningiomas to be hyperintense with respect to the spinal cord, but exceptions exist. On both T1- and T2-weighted images, the signal intensity of cauda equina tumors is usually greater than that of CSF. Small cauda equina tumors, however, are easily overlooked on nonenhanced images.33
Almost all spinal cord tumors demonstrate some degree of contrast enhancement. Meningiomas (see Figs. 309-2 and 309-3) typically exhibit intense uniform enhancement, although nonenhanced calcifications or intratumoral cysts occur occasionally. Enhancement of the adjacent dura (i.e., dural tail) strongly supports the diagnosis of meningioma.34 Although most nerve sheath tumors and filum ependymomas demonstrate uniform uptake of contrast media, heterogeneous enhancement from intratumoral cysts, hemorrhage, or necrosis is common. A peritumoral hypointense rim is often present around meningiomas and corresponds to a well-formed peritumoral CSF space.17
Treatment
Nerve Sheath Tumors
Treatment of benign nerve sheath tumors is complete surgical excision. In almost all cases, resection can be accomplished through a standard posterior laminectomy with partial or complete unilateral facetectomy as needed.35,36 Hemilaminectomy and unilateral facetectomy are options that can reduce postoperative pain and preserve spinal stability.37 Recurrences are rare after gross total resection. Because these tumors grow slowly, complete resection should not be pursued if it carries a high risk of incurring a significant neurological deficit. This is particularly relevant for neurofibromas, which can be admixed with functional neural tissue. The rate of clinical progression after subtotal resection is 50%.7 Recurrences can be treated by reoperation or radiosurgery.
Once exposure is adequate, the correct plane of dissection (directly on the tumor surface) must be identified. The arachnoid membrane generally adheres to the tumor surface tightly. This is the fenestrated arachnoid layer, which separately ensheathes each dorsal and ventral nerve root within the subarachnoid space.38 This layer is incised sharply and reflected off the surface of the tumor. The tumor capsule is cauterized to diminish its vascularity and to shrink the volume of the tumor. Removal of the tumor requires identification and division of the proximal and distal nerve root where the origin of the tumor attaches. With large tumors, this site may not be immediately apparent. Internal decompression with a laser or ultrasonic aspirator is used in such cases. The nerve root of origin must usually be sacrificed to remove the tumor. Occasionally, some fascicles of the nerve root may be preserved, especially with smaller tumors. Neurostimulation is a useful tool to establish the functionality of tumor-associated nerve roots before sacrifice. Once identified, the corresponding intradural nerve root can be preserved because the fenestrated arachnoid sheaths allow anatomic separation of the dorsal and ventral nerve roots to a point just distal to the dorsal root ganglion. In a typical tumor of dorsal root origin, it is possible to preserve the ventral root, which is tightly adherent to the ventral tumor surface, but this may not be feasible with larger tumors.
Dumbbell extension through the nerve root sleeve, however, generally necessitates resection of both nerve roots because the tumor has invaded the corresponding spinal nerve where sensory and motor fibers are admixed.38 Such resection rarely causes a significant deficit, even at the cervical and lumbar enlargement levels. Presumably, the adjacent roots have already compensated for the function of this root. A tumor with a very proximal origin may be partially embedded in the epipial tissue or may elevate the pia to occupy a subpial location. In such cases, the interface between the tumor and spinal cord may be difficult to develop. It may then be necessary to resect a segment of the pia to remove the tumor completely.
Significant tumor extension into the paraspinal region through an enlarged foramen increases the surgical considerations (see Fig. 309-1C). Surgical approaches are influenced by the surgeon’s preference, size and location of the paraspinal tumor component, and intradural tumor extension. Preoperative determination of intradural tumor extension is particularly important. Although MRI adequately identifies various tumor components and relationships to both intraspinal and paraspinal structures, myelography-CT provides greater spatial resolution and more sensitive identification of intradural involvement of dumbbell tumors.
The cervical paraspinal region is difficult to access anteriorly because of the narrow confines of the neck and the numerous neurovascular structures, such as the brachial plexus, descending lower cranial nerves, and vertebral artery. The mandible and skull base musculoskeletal attachments further limit upper cervical exposure. Fortunately, most cervical dumbbell tumors can be removed completely through an extended posterior exposure. A midline incision and standard laminectomy permit safe removal of both intradural and epidural intraspinal tumor components. Complete unilateral facetectomy allows paraspinal access up to 4 cm from the lateral dural margin.36 The vertebral artery is consistently displaced anteromedially and is separated from the tumor capsule by periosteum and an extensive venous plexus. Some authors, however, prefer the anterolateral approach for cervical schwannomas below C2 because the facet is preserved and the vertebral artery can be controlled early.39
When complete unilateral cervical facetectomy is required, tumor resection should be accompanied by lateral mass arthrodesis.36 In cases in which extensive partial facetectomy is required, a unilateral laminectomy may reduce the risk for instability.37 This may be performed after a traditional unilateral subperiosteal muscle dissection or with a muscle-dilating approach and a tubular retractor.40
Paraspinal extension from thoracic tumors can cause a large mass in the cavity. Standard posterior approaches provide inadequate exposure to the anterior paraspinal region, but an anterior transpleural or extrapleural thoracotomy permits excellent visualization of the paraspinal region. Intraspinal access is more limited, however, and the spinal cord is not visualized until most of the tumor has been removed. Because of negative intrathoracic pressure and chest tube drainage, postoperative CSF pleural fistulas can occur if intradural exposure is required. A combination of both anterior and posterior exposure, either staged or consecutive, can be used. Thoracoscopic procedures may be useful for tumors located peripherally within the intercostal nerves or those that extend from the neural foramen into the chest without intradural extension.41
The lateral extracavitary approach is useful as a single-stage operation for tumors that require concomitant complex exposure of the intraspinal and paraspinal compartments.42,43 This exposure is achieved through a hockey stick incision and allows the surgeon to work on either side of the mobilized paraspinal muscles. The superficial thoracic scapular muscles (i.e., trapezius, rhomboid) are detached at the midline and rotated laterally with the skin flap to expose the longitudinally oriented paraspinal muscles. These muscles are mobilized off the posterior spinal elements and ribs. Rib resection and depression of the pleura provide extensive extrapleural paraspinal exposure lateral to the paraspinal muscles. Intraspinal exposure is achieved through a standard laminectomy medial to the paraspinal muscles. Circumferential stabilization, although rarely necessary after removal of dumbbell thoracic tumors, can also be performed through this exposure. CSF fistulas are not a significant problem because the pleural cavity is not entered.
Lumbar dumbbell tumors are also well exposed through the lateral extracavitary approach.42,43 At this level, the thoracodorsal fascia is incised in line with the skin incision and elevated laterally with the skin flap. Paraspinal tumor components in the lumbar spine are buried deep within the psoas muscle. Safe removal of such components through a retroperitoneal exposure is complicated because it is difficult to distinguish the tumor margins from the overlying elongated and blanched fibers of the psoas muscle. The lumbar nerve roots and their branches, including the femoral nerve, course through the psoas muscle. They are difficult to identify and subject to injury during retroperitoneal dissection of the psoas muscle.
Dumbbell sacral tumors generally require both anterior and posterior exposure, which can be staged or performed simultaneously with the patient in a lateral position.44
Severe long-term adverse sequelae from surgical excision of schwannomas are rare. Unlike patients with neurofibromas, the life expectancy of patients with schwannomas parallels that of the general population.7 Symptomatic arachnoiditis and cystic myelopathy may occur in 6% of patients within a few years after tumor removal.7 More than half report some long-term local or radiating pain, but less than 10% seek medical attention.7
Meningiomas
Complete surgical removal is the treatment of choice for spinal meningiomas and can be achieved more than 90% of the time.11 Favorable features relative to intracranial meningiomas include less difficult requirements for ventral exposure, absence of bony involvement because of the well-defined spinal epidural space, lack of venous sinus or major blood vessel involvement, and the presence of a peritumoral hypointense rim on MRI.17,38 Despite these favorable features, the recurrence rate of spinal meningiomas 10 years after gross total or nearly total removal is 10% to 15%.45
Posterior laminectomy provides adequate exposure in most cases. Unilateral laminectomy and facetectomy can be used for eccentrically located or ventral tumors. Large ventral tumors may also be approached satisfactorily through standard posterior exposures because the tumors have already retracted the spinal cord. Suture retraction on a divided dentate ligament or a noncritical dorsal root provides additional ventral exposure (see Fig. 309-3D
