Intradural Extramedullary Tumors

Published on 13/03/2015 by admin

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Chapter 187 Intradural Extramedullary Tumors


The main indication for surgery is the presence of an intradural extramedullary spinal tumor in a symptomatic patient. Symptoms vary according to tumor location, type, and biology. Pain localized to the tumor level is common. The pain may worsen with activity, particularly in the case of lesions within the cauda equina. Classic night pain may also occur with these tumors, but this symptom may also occur with extradural primary or metastatic tumors of the spine. Alternatively, pain may be radicular in nature as a result of irritation or involvement of a local nerve root. Larger tumors ultimately cause local nerve root dysfunction and myelopathy or cauda equina syndrome.

Increasingly, however, these tumors are diagnosed in asymptomatic patients. This can present a management dilemma, since any value of surgery must be considered prophylactic in these circumstances. These lesions are often biologically indolent and may exhibit only a very slow rate of growth. In these patients, a number of other issues need to be considered, such as patient age, comorbidities, and personal preferences. Periodic surveillance with serial imaging may be an appropriate treatment option for many asymptomatic patients. One potential exception is the patient with a midline tumor of the cauda equina, since benign myxopapillary ependymomas can exhibit cerebrospinal fluid (CSF) dissemination. Early surgery to avoid this complication should be reasonably considered. Prophylactic surgery is rarely recommended in syndromic patients with multiple lesions.

Surgical Considerations

Planning for resection of intradural spinal tumors includes numerous considerations such as tumor type, sagittal and axial location, and surgeon preference. Tumor-specific considerations such as the presence of extradural tumor extension, function of the root of origin for nerve sheath tumors, resection of the dural attachment in the case of meningiomas, or en bloc resection for myxopapillary ependymomas are also relevant. Despite these numerous factors, the vast majority of these lesions can be safely removed through a standard posterior exposure with a nondestabilizing laminectomy or osteoplastic laminoplasty. Modifications of the standard posterior exposures may be required to adequately access ventral and/or paraspinal tumor extension.9 More formalized posterolateral or even anterior exposures may occasionally need to be considered in some patients, particularly for midline ventral intradural tumors.10,11 Minimally invasive exposures for resection of intradural extramedullary spinal tumors have also been described. In experienced hands, safe resection may be achieved by using these techiques.12

Nerve Sheath Tumors

Nerve sheath tumors are the most common intradural extramedullary spinal tumors in adults. Most are sporadically occurring solitary schwannomas. Men and women are equally affected, with a peak occurrence in the fourth and fifth decades of life. They are evenly distributed throughout the spinal canal, although involvement of the dorsal root is more common. Solitary spinal nerve sheath tumors represent a heterogeneous group of neoplasms with respect to size, location, and nerve root of origin. Surgical considerations can be complex and include adequate exposure for safe tumor resection, spinal stability, and function of the nerve root of origin. The techniques of surgical removal of intradural spinal schwannomas are well-established. After routine endotracheal intubation, perioperative steroids and intravenous antibiotics are administered. Intraoperative motor- and sensory-evoked potential monitoring is often utilized. Nerve root stimulation is recommended in patients with tumors arising from critical cervical (C5-T1) or lumbosacral (L2-S1) levels.

The patient is placed in the prone position on chest bolsters, a Wilson frame, or an open-frame spine table. For tumors at or above the T4 level, the head is held in a three-point Mayfield head fixator or in Gardner-Wells tongs with 15 lb of traction. The arms are tucked alongside the patient for cervical and thoracic lesions at or above T6; for more caudally located lesions, the shoulders are abducted 90° and the arms are placed on padded armboards.

Most patients are approached through a standard laminectomy or osteoplastic laminoplasty that extends past the rostral and caudal tumor poles. The facet joints are completely preserved in most cases. A hemilaminectomy is performed if the tumor does not extend past the midline. With this exposure, the vast majority of intradural spinal schwannomas can be safely removed without compromise of spinal stability in adult patients. Some modification of this standard posterior approach may be needed for tumors located ventral to the dentate ligaments. Most ventral schwannomas occupy a unilaterally eccentric ventral location and produce some degree of spinal cord rotation and lateral displacement. In these circumstances, additional removal of portions of the facet joint and suture retraction of detached dentate ligaments usually provide adequate exposure for safe removal. Spinal stability may be compromised in some patients, particularly when significant removal of the lateral vertebral bone and joint components occurs. Pure ventral lesions without spinal cord rotation or lateral displacement may require a more formal anterior or anterolateral approach through a corpectomy or transforaminal exposure.

A midline or paramedian longitudinal dural opening is performed. The dural opening should extend just beyond the polar margins of the tumor to facilitate tumor removal and precise identification of the afferent and efferent nerve origin attachments. The dural edges are everted laterally and sutured to the paraspinal muscles to maximize intradural exposure and prevent the introduction of blood from the epidural space or paraspinal muscles into the dependent intradural surgical field. The intermediate arachnoid layer is sharply opened over the dorsal tumor surface. A second arachnoid layer is usually tightly applied to the tumor surface. This layer effectively compartmentalizes and ensheaths individual dorsal and ventral roots. Although the proximal portions of corresponding segmental dorsal and ventral nerve roots remain separate, they become compartmentalized within a common arachnoid sheath as they course toward the dural root sleeve. Identification and opening of the arachnoid nerve sheath is important for two reasons. First, it allows the dissection to take place directly on the tumor surface. This layer is ultimately reflected off the tumor surface at its margins and can make mobilization and visualization of tumor margins difficult if the dissection is performed outside this layer. This is particularly important with regard to nonvisualized tumor margins that abut the spinal cord. Second, the corresponding nerve root is usually tightly applied to the tumor capsule within this arachnoid layer (Fig. 187-1A). Upon initial inspection, this nerve root may appear to be the nonfunctional nerve root of origin because of its tight attachment to the tumor surface. Upon opening this layer, however, it becomes clear that this root may be dissected off the tumor capsule and preserved. The same is not the case for the actual nerve of origin. Although a portion of the afferent and efferent components of the nerve of origin may be dissected and separated from the tumor capsule, eventually this dissection plane disappears as the nerve root becomes incorporated into the tumor capsule.

Once the tumor surface is identified, the polar margins are defined. Direct orthogonal visualization of both rostral and caudal tumor poles facilitates tumor removal. For large tumors, the dorsal tumor capsule is entered, and internal decompression with an ultrasonic aspirator or laser is performed. Sufficient internal decompression allows progressive delivery of initially nonvisualized tumor into the resection bed. Division of the lateral dentate ligament attachment facilitates ventral access. Ultimately, the afferent and efferent tumor attachments need to be divided to achieve removal. Identification of these attachments depends on tumor size, origin, and location. In some cases, the afferent and efferent components may be immediately apparent on the dorsal surface of the tumor. Early division of these attachments allows easy removal of the tumor, particularly at the thoracic levels. More commonly, however, the afferent and efferent tumor attachments are not visualized on initial tumor exposure. The afferent root is often identified by its enlarged, congested, and hypervascular appearance (Fig. 187-2C). In contrast, the efferent root component usually appears normal. Progressive internal decompression allows delivery of tumor margins into the resection bed until the attachments are visualized. The dorsal and ventral nerve roots may already be contained within a common arachnoid sheath at the proximal origin of cauda equina tumors. At these levels, the functional corresponding nerve root may appear to be part of the afferent root of origin. However, fascicles from the corresponding root will be reflected onto the tumor surface and can be dissected and preserved. Occasionally, some of the fascicles from the actual nerve root of origin may also be reflected onto the tumor surface and may be separable from the tumor capsule over much of, or occasionally the entire, tumor surface. Unless these fascicles arise from critical cervical or lumbosacral levels and demonstrate intraoperative stimulation, they need not be preserved, as such futile dissection unnecessarily prolongs the resection.

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