Asymptomatic Intradural Schwannoma: Surgery Versus Radiosurgery Versus Observation

Solitary nerve sheath tumors are the most commonly occurring intradural spinal tumors in adults. They represent approximately 10% to 30% of all primary spinal tumors depending on the patient series. They typically occur in the fourth and fifth decades of life and affect women and men equally. A small percentage of patients with this tumor have neurofibromatosis type 1 (NF-1) or NF-2. These tumors are thought to originate from the Schwann cell and are almost uniformly benign. A subset of patients with NF-2 may have malignant schwannomas.

The growth of these tumors is usually very slow and is typically concentric along the spinal nerve. The tumors can demonstrate various configurations depending on their anatomic location. They can be spherical or hourglass shaped secondary to growth constriction at the neural foramen or dural aperture. Approximately 80% to 90% of these tumors are intradural and 10% to 20% are dumbbell shaped with foraminal or extraforaminal components. Approximately 10% of all tumors arise from extradural components of the nerve and have proximal growth within or into the foramen. The majority of these tumors arise from the sensory root.

Increasingly, asymptomatic patients are diagnosed with incidental spinal tumors. Management considerations for these patients can be complex and depend on many factors, such as patient age and comorbid conditions, size and location of the tumor, and patient preference. This chapter discusses some of the management considerations for a patient with a schwannoma.

Case Presentation

A healthy 35-year-old woman was referred for evaluation of severe right-sided leg pain of 9 weeks’ duration.

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PMH: Unremarkable
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PSH: Unremarkable
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Exam: The pain was in a typical S1 nerve root distribution. Neurologic examination revealed a positive finding on right straight leg raising at 20 degrees, numbness on the lateral aspect of the right foot, 4/5 strength in plantar flexion and ankle external rotation, and an absent right Achilles reflex.
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Imaging: Magnetic resonance imaging (MRI) revealed a large extruded right L5-S1 disk fragment with caudal migration and severe compression of the right S1 nerve root ( Figure 9-1 , A and B ). An intradural abnormality was identified at the level of the L3 vertebral body on the initial MRI, so contrast-enhanced MRI was performed. This revealed a small, well-defined, uniformly enhancing intradural mass on the left side of the spinal canal ( Figure 9-1 , C and D ) consistent with the diagnosis of nerve sheath tumor. The patient had no signs or symptoms related to this tumor.

FIGURE 9-1

T1-weighted sagittal ( A ) and axial ( B ) MRI scans of the lumbosacral spine show a right-sided L5-S1 herniated disk with caudal migration and compression of the right S1 nerve root. T1-weighted contrast-enhanced sagittal ( C ) and axial ( D ) MRI scans show a uniformly enhancing mass at the level of the L3 vertebral body.

Management Considerations

Any decision making and management planning should first and foremost be predicated on the nature of the condition that is potentially to be treated. Most importantly, the specific pathologic diagnosis and natural history should be considered. Obviously, the consequences of the different management strategies, including no treatment, must also be considered, as should the patient’s preference. For the patient described in the Case Presentation, the diagnosis of an L5-S1 disk herniation was made with a high degree of confidence based on the clinical presentation and the MRI scan appearance. What is not as certain is the individual natural history for this patient in terms of the probability of improvement without surgical intervention, both for the relief of pain and for the recovery of neurologic function. Although nonoperative management could result in improvement, there is also a possibility of worsened neurologic function as well as a concern that the longer the S1 nerve root remains compressed, the less likely that improvement will occur after successful surgical removal. On the other hand, although microdiskectomy is highly effective in relieving the pain from nerve root compression, it can be associated with complications, and there is no guarantee of return of neurologic function.

The management considerations for the intradural mass at the L3 level are quite different for several reasons. First, the specific pathologic diagnosis is less certain. Although a benign nerve sheath tumor is likely based on location and appearance, the correct diagnosis cannot be known for sure. Moreover, even if one could be assured that this was a schwannoma, the natural history in terms of biology and growth rate can vary considerably. Finally, unlike the disk herniation, this lesion is asymptomatic, and the risks associated with surgical resection are higher.

Due to the uncertainties of this case there are a number of options that can reasonably be considered. Because of the persistence of severe pain for over 2 months and the presence of objective neurologic deficit from the L5-S1 disk herniation, microdiskectomy is probably the preferred option. Continued nonoperative management remains a rational option, but just as the surgeon must discuss the risks and complications of surgical intervention, the potential implications of nonoperative management must also be considered. Ultimately, the patient must make a decision under conditions of uncertainty, so it is the responsibility of the surgeon to ensure that the patient has as much information as possible to choose the most rational option for him or her.

The benefit of surgical treatment for the intradural mass is primarily prophylactic, since the patient is currently asymptomatic. Surgical resection would also provide a precise histopathologic diagnosis. Presumably, the tumor will never be smaller and the risk of surgery will never be lower than at the present time. On the other hand, surgery does carry some risk for either neurologic deficit or lower back dysfunction that could be permanent.

In the patient described here, the tumor is relatively small, is not associated with any significant mass effect, and is below the level of the spinal cord. It is also more laterally situated, although not dura based, which strongly supports the diagnosis of a Schwann cell tumor arising from a root of the cauda equina. Although the specific root of origin with respect to level and dorsal or ventral root cannot be known for certain, dorsal root schwannomas are more common than ventral root tumors (there are more fascicles in the dorsal root); however, dorsal root tumors are more likely to present with radicular pain. Based on the lateral location, the nerve root level of origin is most probably L4 or L5, due to the somatotopic organization of the cauda equina. In either case, therefore, this tumor arises from a critical lumbar nerve root. Resection of a critical root of origin may be associated with a neurologic deficit, so this must be considered.

In the end, the recommendation for this patient was surgical microdiskectomy for removal of the extruded right L5-S1 disk fragment, but no treatment of the incidental cauda equina lesion. It was certainly an option to remove the tumor following microdiskectomy, but it was felt that this was a significantly different additional procedure (different level, intradural) that would have added measurable risk to an otherwise extremely safe microdiskectomy. Periodic clinical and radiographic surveillance of the tumor was recommended. Repeat MRI at 3 months postoperatively was recommended to ensure that this was not a biologically aggressive tumor. If no growth was apparent, then MRI would be performed annually. If the patient became symptomatic or the tumor showed significant growth, then surgery would be recommended. In many cases little or no growth is apparent after even several years. Furthermore, experience indicates that this tumor could more than double in size without risking neurologic deficit or adding to the risk or technical complexity of tumor resection. There are numerous exceptions to this expectant management strategy. For example, due to the often slow growth rate of these tumors, they can have considerable size and mass effect on the spinal cord or cauda equina at presentation. One patient recently had an incidental tumor at the T3 level with significant spinal cord compression at presentation ( Figure 9-2 ). In these cases resection is typically recommended due to the concern for potentially rapid neurologic deterioration or an increasingly more technically challenging surgical resection. On the other hand, early surgery is also recommended for even small, centrally located asymptomatic cauda equina tumors. Although such lesions may be schwannomas from sacral nerve roots, they may also represent filum terminale myxopapillary ependymomas. These latter unencapsulated tumors may exhibit dissemination via the cerebrospinal fluid (CSF) or sheetlike extension along the roots of the cauda equina that precludes safe total resection. Early surgery in these situations seems prudent.

An alternative treatment is focused radiotherapy or stereotactic radiosurgery. Various modalities have been described, including proton beam radiosurgery, linear accelerator radiosurgery (CyberKnife, Accuray Corp., Sunnyvale, Calif.; Synergy system, Elekta AB, Stockholm, Sweden; Novalis system, Brainlab AG, Feldkirchen, Germany), and intensity-modulated radiation therapy (IMRT). Although most reports on spinal radiosurgery involve treatment of malignant primary and metastatic disease of the vertebral spine, there is accumulating experience with radiosurgery for benign intradural spinal lesions. Benefits of such treatment include the possibility for long-term tumor control without the associated risks of open surgery. Limitations of this strategy are numerous. First, as with nonoperative management, no tissue is obtained to verify the histologic characteristics of the tumor. Second, since these tumors are slow growing, it may be difficult to accurately assess the effectiveness of the treatment in individual cases. Third, there is a risk of radiation injury, particularly at spinal cord levels. In addition, subsequent operative intervention may be compromised by prior radiosurgery. In the end, for reasonably healthy patients with a single intradural spinal lesion surgery is such a highly effective and safe treatment that radiosurgery is generally not considered as a primary treatment modality. Patients with significant comorbid conditions, recurrent or residual tumors, or multiple lesions, particularly patients with von Hippel–Lindau disease or neurofibromatosis, are currently the most likely to be considered for radiosurgery as a primary or adjunctive treatment option. This is an evolving technology, however, and treatment considerations are likely to change in the future.

Surgical Considerations

Understanding the anatomic relationships of these tumors is essential to successful surgery. The optimal goal of surgery is gross total resection with preservation of nerve function. Complete visualization of these tumors usually can be achieved with posterior or posterolateral approaches to the spine. Microsurgery is invaluable for opening of the dura and arachnoid layers covering the tumor and for debulking and dissection of the tumor capsule. Use of intraoperative neurophysiologic monitoring is a useful tool to determine if there is functional nerve worth preserving within the tumor.

Surgical Options

Schwannomas may be completely resected from a variety of approaches depending on location. Often with dumbbell tumors there are advantageous changes within the surrounding bony anatomy of the spinal column from the slow-growing tumor that may aid in the dissection. Often, the bony elements of the foramen are attenuated, expanded, or nonexistent. The need for stabilization and bony fusion depends on the degree of preexisting biomechanical instability secondary to tumor-related changes and the amount of bone and ligament that must be removed for adequate tumor exposure and excision.

Expected rates of neurologic improvement after surgical resection range from 80% to 95% of patients based on large surgical series. A secondary goal of surgery is to preserve the function of the involved nerve root. Preservation of nerve function may not require anatomic preservation of the affected nerve root. Some degree of anatomic preservation is usually possible with small, intradural tumors, but complete removal of larger tumors may require sacrifice of rootlets or the entire nerve. Dorsal nerve roots can be cut during surgery for these tumors with minimal adverse effect. Hypesthesia has been documented in a small percentage of patients. Enhanced motor weakness usually is not a complication of surgery, even in situations in which the ventral nerve root is sacrificed, because the affected nerve roots are often nonfunctional before surgery. Motor weakness occurred in approximately 10% to 33% of patients who underwent sectioning of the affected nerve in functional nerve roots (C5 through T1 and L1 through S1). The mechanism of this phenomenon is unclear. Presumably, gradual functional denervation of the involved nerve root allows reinnervation to occur by means of sprouting at the anterior horn cell or muscle end-plate level. The delicate intradural nerve fascicles of the entire motor or sensory root of tumor origin usually disappear into the substance of the tumor, because they do not have a well-developed interfascicular connective tissue matrix. A thin arachnoid layer separately ensheathes the motor and sensory roots. Although this sheath might allow anatomic preservation of the corresponding motor root for intradural sensory root tumors, or vice versa, sacrifice of the entire motor and sensory root is required if the tumor extends through the root sleeve, distal to the dorsal root ganglion ( Tips from the Masters 9-1 ).

Tips from the Masters 9-1

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