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.