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.

FIGURE 187-1 A, Artist’s drawing demonstrates a typical type I intradural spinal schannoma arising from a dorsal nerve root. Note that the corresponding ventral nerve root is tightly applied to the tumor capsule. Although the dorsal root of origin is usually nonfunctional, the corresponding ventral root is functional. B, Artist’s drawing of a type II dumbbell tumor. Once the tumor grows beyond the dorsal root ganglion, neither the dorsal root nor the ventral root can be salvaged during a complete resection. Usually, neither the dorsal root nor the ventral root component is functional in these type II dumbbell tumors.

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.

FIGURE 187-2 T1-weighted sagittal (A) and axial (B) MRI studies of the lumbosacral spine demonstrate a well-defined, uniformly enhancing mass at the L5 level. C, Intraoperative photograph shows a nerve sheath tumor. Note the congested appearance of the afferent component of the nerve root of origin, in this case the left L5 sensory root. D, Intraoperative photograph following tumor resection. Note that the sensory afferent root has been divided (black arrowhead), whereas the functional ventral motor root has been preserved (blue arrowhead).

Tumors that arise from the very proximal portion of the nerve root may not have a definable afferent nerve root attachment. Instead these tumors may abut and be adherent to the spinal cord at the root entry zone. In some cases, proximal tumor growth may actually elevate the pia, where it is reflected at root entry zones, and occupy a subpial location. Great care must be taken to safely remove this subpial tumor component to avoid injury to the spinal cord or fragile epipial vascular network. In these cases, microsurgical dissection directly on the tumor surface usually allows safe, complete removal. This dissection can be difficult for removal of ventral root tumors, even with gentle retraction of a detached dentate ligament. Some portions of the pia may have to be incised to follow the subpial tumor component. If uncertainty regarding the margin of the tumor remains, then further dissection may need to cease. Conversely, tumors arising more distally along the root of origin may have their distal margin near or just beyond the dural nerve root sleeve. In these cases, internal decompression, as well as early identification and division of the afferent attachment, allows adequate visualization and mobilization of the distal tumor component to allow preservation of a critical corresponding functional nerve root (Fig. 187-2D). Nerve root stimulation can be useful during this part of the dissection.

Following tumor resection, the subarachnoid space is irrigated with warm saline. The dura is closed with a running 4-0 silk or 5-0 PROLENE suture (Ethicon, Inc., Somerville, NJ, USA). A Valsalva maneuver to 35 mm Hg is then performed to verify watertight dural closure. DuraGen (Integra LifeSciences Corp., Plainsboro, NJ, USA) may also be placed over the suture line. The paraspinal muscles, deep and superficial fascia, and skin are closed separately in layered fashion. A deep subfascial Hemovac drain is infrequently used. The patient is kept on bedrest for lumbosacral lesions until the morning of postoperative day 2 and then progressively mobilized.

Dumbbell Tumors

Dumbbell spinal schwannomas arise from the intradural segment of a dorsal or ventral nerve root. Thus many of the intradural anatomic features and surgical considerations for purely intradural schwannomas and dumbbell schwannomas are similar. The main difference is the extended lateral growth of dumbbell tumors through the dural root sleeve and neural foramen, beyond the dorsal root ganglion, and into the paraspinal region (Fig. 187-1B). The presentation of dumbbell tumors with respect to the spinal level, as well as to the size of the spinal and paraspinal tumor components, vary considerably. Additional surgical considerations for dumbbell tumors include adequate exposure for removal of the intraspinal and paraspinal tumor components; the logistics and sequencing of tumor removal, particularly if separate surgical exposures are utilized; and possible spinal instability.

Most dumbbell tumors are removed through a standard posterior approach with the patient in a prone position. There are several benefits of using this approach. For example, the vast majority of dumbbell tumors can be adequately accessed for safe removal through a single exposure. Contiguous access of intraspinal and paraspinal tumor components preserves the variety of surgical options and techniques for removal of these often complex tumors. With the use of anterior or lateral approaches, the intradural portion of the tumor is not immediately visualized and may be accessed only following removal of the paraspinal component. This can limit surgical options, create a less secure operative field, or even require a separate surgical exposure. Posterior stabilization with lateral mass or pedicle screws can be effectively achieved through the standard midline exposure with the patient in the prone position. There are specific advantages to midline posterior exposure at each spinal region. For instance, in the cervical spine, though both the vertebral artery and the proximal portion of the brachial plexus may need to be identified, neither of these structures needs to be formally exposed, as is often the case with more standard anterior or lateral approaches. Intrapleural transthoracic approaches can be associated with substantial perioperative morbidity, as can transdiaphragmatic exposures to the throacolumbar junction and retroperitoneal exposures for lumbar paraspinal tumor extension. Retroperitoneal approaches to paraspinal tumors are additionally problematic, because these tumors are usually buried in the psoas muscle. Retroperitoneal exposures require the surgeon to dissect through the psoas muscle to gain access to the tumor capsule. This can put the components of the lumbar and lumbosacral plexus, which have a variable course through the psoas muscle, at risk. In addition, it can be difficult to confidently differentiate the tumor capsule from blanched, compressed psoas muscle fibers. At all levels, the large dead space from anterior or anterolateral exposures can be difficult to close and therefore creates a greater risk of pseudomeningocele formation. This is particularly problematic if the sub-atmospheric intrapleural space has been opened.

The midline incision may be longer in the case of dumbbell lesions than with purely intradural tumors to allow more lateral exposure of the involved paraspinal region. The paraspinal muscles are elevated laterally. A standard laminectomy is performed to allow adequate exposure for safe removal of the intradural tumor component. A hemilaminectomy is used if the tumor does not extend beyond the midline. A complete unilateral facetectomy is then performed on the side of the tumor. This extended posterior approach allows contiguous exposure of intradural, foraminal, and paraspinal tumor components that extend up to 3 cm lateral to the dural root sleeve origin at cervical levels. At thoracic levels, removal of the facet complex, transverse process, and proximal rib allows considerable access into the retropleural space. Similarly, removal of the lateral lumbar spinal elements provides contiguous access into the paraspinal retroperitoneal region.

The sequence and techniques of dumbbell tumor removal once the exposure has been achieved vary according to a number of factors, including surgeon preference as well as tumor characteristics. Initial removal of the intradural tumor component through a midline longitudinal dural opening allows early, direct decompression of the spinal cord or cauda equina, identification and division of the afferent nerve of origin, and direct nerve stimulation for verification of a nonfunctioning corresponding motor root for sensory root tumors at critical cervical and lumbosacral levels. This sequence is preferred for most tumors, but especially for tumors at these important levels. The intradural tumor is ultimately amputated at the distal dural root sleeve margin. Following intradural tumor resection, the laterally everted dural edge is retracted medially to expose the paraspinal component. For large tumors, the tumor capsule may be incised along the axis of the spinal nerve and internally decompressed with a Cavitron device (Dentsply International Inc., York, PA, USA). Ultimately, the margins of the tumor are identified. A robust epidural and foraminal venous plexus is typically tightly applied and compressed on the tumor surface, particularly in the foramen. It is important to stay directly on the tumor capsule to avoid problematic epidural and foraminal bleeding and to maintain the correct plane as the dissection progresses ventrally. This is particularly important at cervical levels where the tumor may abut the vertebral artery and perivertebral venous plexus. The experience at my institution has been that benign nerve sheath tumors displace, rather than invade or engulf, the vertebral artery. Similarly, in the thoracic spine, the tumor capsule often abuts the pleural membrane. If the dissection is not carried out precisely on the tumor capsule, then the pleural cavity may be inadvertently opened, often necessitating the need for a chest tube and increasing the risk of a postoperative CSF pleural fistula. A well-developed plane between the tumor capsule and the perivertebral plexus serves as an effective dissection plane as long as the dissection is carried out directly on the tumor capsule. Often several layers of epidural vessels are compressed on the tumor surface. These pseudocapsule venous layers must be cauterized and sequentially peeled away to reach the true tumor capsule. The medial aspect of the tumor should be freed of its medial dural root sleeve attachment. This may require a circumferential incision around the dural root sleeve. This allows the medial paraspinal tumor margin to be elevated laterally to facilitate circumferential access. This maneuver also allows the distal or most lateral tumor component to be pulled medially into the surgical field. More internal decompression may be required until the efferent attachment can be identified and divided. Following resection, the midline dura is closed first with a running 4-0 silk or 5-0 PROLENE suture. The dural root sleeve usually requires a muscle or fascial patch. A Valsalva maneuver to 35 mm Hg is used to test the integrity of the dural closure. DuraGen is typically placed over both suture lines. In the experience at my institution, postoperative CSF leaks usually occur at the dural root sleeve.

Alternatively, the epidural foraminal and paraspinal component may be removed first. This can be helpful when there is a large epidural intraspinal component that interferes with intradural access. This sequence allows the dura to remain closed while the often bloody foraminal and extradural component of the procedure is performed. The tumor should be amputated medially earlier in the procedure to avoid undue traction on the tumor that could be transmitted to the spinal cord or cauda equina nerve roots. Once the paraspinal tumor component is removed, all epidural bleeding is secured and the intradural microsurgical removal may commence. The sequence and strategy of dural closure after complete tumor resection is identical, regardless of which tumor component is removed first.

The issue of instability following laminectomy and unilateral facetectomy has not been resolved. Postlaminectomy instability and resultant kyphosis most likely depend on numerous factors, such as the involved spinal level, the degree of bone and ligament removal, and individual patient characteristics and factors. In most cases, however, the placement of posterior segmental instrumentation with lateral mass or pedicle screws has evolved into a reliable, secure, and efficient method of stabilization. At my institution, therefore, we have opted for stabilization that extends one level rostrally and caudally beyond the operative segment following resection of nearly all dumbbell tumors.

Sacral dumbbell tumors are unique because of the lateral sacroiliac articulations and the ventral exit of the spinal nerves through the sacral foramina. Sacroiliac stability may be compromised during the removal of large sacral tumors. Although even giant sacral tumors can be removed, as long as they remain intrasacral, any significant degree of presacral extension will generally require a separate anterior retroperitoneal approach.

Most often the dura can be closed primarily with a running 4-0 silk or 5-0 PROLENE suture. If a dural patch is necessary, bovine pericardium or synthetic material may be used. An onlay dural substitute is typically placed over the durotomy and covered with a thin layer of fibrin glue. A loose, running, absorbable monofilament suture is used to appose the paraspinal muscles. The fascia is closed with figure-of-eight, interrupted, braided, absorbable sutures. Interrupted, inverted absorbable sutures are placed in the dermis. The skin is closed with a running nylon suture.

Meningiomas

Meningiomas make up 13% to 19% of intracranial tumors, and spinal meningiomas account for 12% of all meningiomas.¹ Within the spinal canal, these lesions show a preference for the thoracic region. They occur predominantly in the cervical (28%) and thoracic cord (64%) and only rarely in the lumbar spine (8%). Overall, meningiomas are found to be evenly distributed around the circumference of the spinal cord. Anterior meningiomas have been observed to be more common in the cervical spine (up to 48% of cervical meningiomas), lateral meningiomas are the most common in the thoracic spine, and posterior lesions are the most common in the lumbar region. In one study, extradural tumor extension was identified in 6.3% of patients.²

The peak incidence for spinal meningiomas is between the sixth and eighth decades of life. Patients younger than age 50 years have a higher rate of tumors located in the cervical spine (39%) and a greater number related to a predisposing factor, such as neurofibromatosis 2, radiation, or trauma. These patients are also more likely to require reoperation for residual tumor (22% vs. 5% in one case series).⁵ As with intracranial meningiomas, there is a higher incidence of spinal meningiomas in women, with a rate ranging from 3:1 to 4.2:1.¹ This rate is even higher for meningiomas in the thoracic spine, with women presenting with significantly more spinal meningiomas than their male counterparts.

The imaging features and characteristics of intradural spinal meningiomas are well established. MRI is the imaging modality of choice for the diagnosis and evaluation of virtually all intradural pathology. Computed tomography (CT) scanning may provide important complementary information, especially for mineralized or calcified tumors. Myelography is rarely needed, although the spatial resolution or myelographic CT can be more sensitive than MRI for the identification of intradural extension of dumbbell tumors.

Spinal meningiomas are usually isointense to the spinal cord on both T1- and T2-weighted sequences and demonstrate uniform enhancement following gadolinium administration (Fig. 187-3A, B). A dural attachment is also usually identified, although a dural “tail” is less common. Significant intratumoral mineralization or calcification alters MRI characteristics. The differential diagnosis of spinal meningiomas includes predominantly nerve sheath tumors, which more commonly show cystic regions or heterogeneous enhancement, and ependymomas.

FIGURE 187-3 T1-weighted sagittal (A) and axial (B) MRI scans of the lumbosacral spine demonstrate a well-defined, unifromly enhancing lesion at the L1 level. Note the broad-based dural attachment on the left side. C, Intraoperative photograph shows a well-circumscribed tumor ventral to the dentate ligament, which is splayed out on the tumor’s dorsal surface. D, Intraoperative photograph following tumor resection. Note that the dural origin of the tumor has been extensively cauterized to prevent tumor recurrence.

The appropriate approach varies, depending on the level of the tumor, the location of the tumor in relation to the spinal cord, and surgeon preference. Tumor vascularity and consistency can also influence the choice of operative exposure, particularly for ventrally located lesions, although these characteristics are difficult to determine pre-operatively. Anterior or anterolateral tumors may require a more complicated anterior approach, which could be contraindicated in a medically compromised patient. Approaches described in the literature for the cervical region include posterior, posterolateral, and anterior cervical corpectomy and fusion.¹¹ Approaches described for the thoracic region are more varied and carry a higher risk of complications due to the associated difficulty of accessing the anterior spinal cord in this region. Posterior approaches for this area include posterior, posterolateral, and the posterolateral endoscopic techniques.^10,11,13 Anterior and anterolateral approaches should be divided on the basis of upper thoracic versus lower thoracic locations. Upper thoracic lateral and anterior approaches include the trap-door approach and the parascapular extrapleural approach. The lower thoracic spine is accessible through the lateral extracavitary approach and retropleural exposures. The lumbar spine is reached predominantly by using the posterior, posterolateral, and lateral retroperitoneal approaches.

Fortunately, most meningiomas of the spinal cord arise posterior, posterolateral, or lateral to the cord and therefore can be accessed by using a posterior approach. Patient positioning and the initial techniques of intradural exposure are similar to those utilized for nerve sheath tumors. The exposed tumor surface should be visualized, including the rostral and caudal tumor poles. A small cottonoid is placed in each lateral gutter above and below the tumor margins. Division of one or more of the dentate ligament attachments to the lateral dura may improve access to ventral tumor extension (Fig. 187-3C). A well-defined arachnoid plane typically exists between the spinal cord and tumor capsule. Depending on the size and consistency of the tumor, internal debulking with an ultrasonic aspirator or laser facilitates visualization and development of the tumor margins. All traction on the tumor should be away from the spinal cord. The tumor surface may be quite friable and of variable vascularity. Cauterization of the dural base may reduce bleeding from highly vascular and/or friable lesions. Fortunately, due to a well-developed spinal epidural space, bony involvement does not typically occur. Management of the dural attachment depends on practical considerations. For dorsal tumors, resection of the dural origin facilitates resection. For more lateral and ventral tumors, cauterization of the tumor origin is preferred because of the difficulty of dural reconstruction in these locations (Fig. 187-3D). In either case, the rate of recurrence is quite low.

Filum Terminale Tumors

The vast majority of filum terminale tumors are myxopapillary ependymomas. Paragangliomas are less common, whereas astrocytomas, hemangioblastomas, and cavernous malformations are rare. The basic principles of intradural surgical exposure also apply to filum terminale tumors. Ependymomas and paragangliomas should ideally be resected en bloc, as there is some evidence that piecemeal resection of these tumors increases recurrence rates.⁸ Paragangliomas can be particularly problematic because of their high vascularity. The tumor is carefully freed from adjacent nerve roots, and the filum is identified visually and tested with a neurostimulator. The filum is cauterized above and below the tumor and divided, and the tumor is carefully rotated out of the canal. En bloc resection is often not possible to achieve safely in larger tumors for various reasons (Fig. 187-4). For instance, the tumor may lack sufficient internal integrity and fall apart with even gentle manipulation. The tumor may also be too large to tease out without putting unacceptable amounts of traction on overlying nerve roots. Large tumors may exhibit sheetlike growth along arachnoid fenestrations. Functional roots may appear to course directly through the substance of the tumor. Safe resection can be impossible in these cases because the lack of supportive connective tissue matrix (i.e., epineurium) in the cauda equina nerve roots does not allow safe dissection of tumor off the involved roots. Only subtotal resection may be possible in these patients.

FIGURE 187-4 A, T1-weighted sagittal MRI study of the lumbosacral spine shows a well-defined, uniformly enhancing midline lesion at the L3 level. B, Intraoperative photograph shows a midline myxopapillary ependymoma of the filum terminale. The roots of the cauda equina have been displaced laterally and dorsally off the tumor surface. C, Intraoperative photograph of tumor specimen following en bloc resection.

Postoperative Management

If a watertight dural closure is achieved, a subarachnoid lumbar spinal drain is unnecessary. The patient is kept flat in bed for 48 to 72 hr with T.E.D. (thromboembolic deterrent) stockings, sequential compression devices, and a Foley catheter in the case of lumbosacral lesions. If a spinal drain has been placed, it is generally clamped and removed prior to mobilizing the patient. Perioperative antibiotics are continued until all drains have been removed. Skin sutures are removed no earlier than 10 days following surgery. Early CSF leaks through the skin may be initially treated with recumbent and augmented skin sutures, but return to the operating room for repair is performed for persistent drainage. Delayed pseudomeningoceles are followed over time because many spontaneously resolve. Long-term clinical and radiographic follow-up is performed as needed.