39 Current Surgical Techniques in the Treatment of Spinal Meningiomas
Introduction
Spinal meningiomas are slow-growing, benign lesions that occur in the intradural, extramedullary space. They are predominantly treated with surgical resection. Despite their slow rate of growth, they can cause permanent neurological symptoms through progressive compression of the spinal cord or cauda equina, and therefore necessitate careful debulking and resection. Contemporary surgical techniques provide definitive treatment, with low recurrence rates, limited morbidity, and preservation or return of neurological function in the vast majority of patients. Surgical challenges lie mainly in access and vary depending on the location of the lesion throughout the spinal canal. For most spinal meningiomas, a posterior approach is appropriate. Ventrolateral and ventral lesions, however, may necessitate a more complicated and nuanced anterior approach, which will vary depending on the region of the spinal canal in which the tumor is found.
This chapter presents current knowledge of the epidemiology and histopathology of spinal canal meningiomas in general, and then describes the surgical approaches to spinal meningiomas that are found within different regions of the spinal canal.
Epidemiology
Epidemiological studies have reported that intradural tumors of the spine occur at an incidence of three to 10 per 100,000 people, of which two thirds are extramedullary. 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 portion of the canal. They occur predominantly in the cervical (28%) and thoracic cord (64%), and only rarely in the lumbar spine (8%). Meningiomas are dural-based lesions that can arise from any aspect of the inner dural layer. Anterior meningiomas have been observed to be more common in the cervical spine (up to 48% of cervical meningiomas), whereas 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.1,3,4 Younger patients (< 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 (NF2), radiation, or trauma. Young patients are also more likely to require reoperation for residual tumor (22% vs 5% in one particular case series).5 As with intracranial meningiomas, there is a far higher incidence of spinal meningiomas in women. The rate has been recently quoted as 3–to 4.2:1.1 This rate is reportedly even higher in the thoracic spine, with women presenting with significantly more spinal meningiomas than their male counterparts.
Clinical Features and Treatment Considerations
The clinical features of spinal meningiomas are variable and depend on numerous factors, such as tumor size, location, and biology. Due to the sensitivity of contemporary imaging, as well as its widespread access, incidental or asymptomatic intradural spinal tumors are now commonly identified on magnetic resonance imaging (MRI). In the absence of any “red flags,” surgical treatment is usually not routinely recommended for incidental spinal meningiomas unless there is significant spinal cord compression. Small tumors can be followed over time with periodic imaging to document any expansion. Individual treatment decisions, however, are largely a matter of clinical judgment and patient preference.
Pain is the most frequent initial symptom of intradural tumors. Back pain is more typical in patients with meningiomas, but radicular pain, particularly thoracic, is also common. Night or recumbent pain is also common, although neither universal nor pathognomonic for intradural tumors, especially for larger tumors. Ultimately, features of myelopathy or cauda equina syndrome commence, the pattern and progression of which depend on tumor location. Significant deficits by the time of diagnosis have become less common, but relatively rapid acceleration of previously subtle or minimally functional or progressive neural dysfunction occurs with some frequency, presumably as the spinal cord reaches its tolerance for external compression.
Radiological Features
The imaging features and characteristics of intradural spinal meningiomas are well established.6,7 MRI is obviously the imaging procedure for the diagnosis and evaluation of virtually all intradural pathology. Computed tomographic (CT) scanning may provide important complementary information, especially for mineralized or calcified tumors. Myelography is rarely needed, although the spatial resolution of myelo/CT can be more sensitive than MRI for identification of intradural extension of dumbbell tumors.
Spinal meningiomas are usually isointense to the spinal cord on both T1 and T2 sequences and demonstrate uniform enhancement following gadolinium administration. A dural attachment is also usually identified, although a dural tail is less common. Significant intratumoral mineralization or calcification will obviously alter MRI characteristics. Differential diagnosis includes mainly nerve sheath tumors, which more commonly show cystic regions or heterogeneous enhancement, or ependymomas.
Histopathology
Spinal meningiomas are generally benign low-grade lesions that demonstrate slow growth with limited risk of recurrence following resection.2 Most are World Health Organization (WHO) grade I or II meningothelial and psammomatous types. Malignant examples, either de novo or transformation, account for only ~2% of reported cases. Calcification has been noted in approximately one in five spinal meningiomas, potentially complicating resection in that subset.1,3,4,8 Despite their homogeneous histopathology, spinal meningiomas do exhibit some degree of biological heterogeneity with respect to growth rate. This underscores the importance of close periodic surveillance for newly diagnosed incidental lesions.
Surgical Approaches
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 ( Table 39.1 ). Tumor vascularity and consistency can also influence the choice of operative exposure, particularly for ventrally located tumors, but these characteristics are difficult to determine preoperatively. Anterior or anterolateral tumors may require a more complicated anterior approach, which could be problematic in a medically compromised patient. Approaches described in the literature for the cervical spinal canal include posterior, posterolateral, anterolateral, and anterior corpectomy.9–11 Approaches described for the thoracic region are more varied and may carry a higher risk of morbidity 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 approach.12–16 Other approaches should be divided by upper thoracic and lower thoracic lesions. Upper thoracic lateral and anterior approaches include the trapdoor approach and the parascapular extrapleural approach.17–20 The lower thoracic spine is accessible through the lateral extra-cavitary approach and the retropleural approaches.19–23 The lumbar spine is reached predominantly through the posterior, posterolateral, and lateral extracavitary approaches.15,16,22,23
Posterior Approach with Laminectomy
The majority of spinal meningiomas can be adequately accessed and safely removed through a standard posterior approach with laminectomy. Minimally invasive exposures have also been described.13,24,25 Following routine endotracheal intubation and placement of the appropriate vascular access and monitoring lines, the patient is placed in a prone position. We generally don’t find awake fiberoptic intubation necessary for compressive cervical intradural pathology. Care is maintained to pad all bony prominences and subcutaneously coursing nerve trunks. A Mayfield head clamp is used for lesions of the upper thoracic (above T4) and cervical spine. The arms are tucked at the side for tumors above the T6 level. Perioperative intravenous antibiotics and steroids are administered. Fluoroscopy is utilized to localize the tumor level. Spinal level identification from fluoroscopy may need to be reconciled from the level noted on preoperative imaging, usually MRI, to ensure concordance. Specifically, issues related to number of lumbar vertebrae, transitional levels, and size and level of last rib should be addressed. A routine midline skin incision and subperiosteal dissection of the paraspinal muscles are performed. The amount of bone removed may be tailored to the individual situation. It is rare that spinal stability is compromised during posterior exposure of intradural pathology in the adult population. Meticulous hemostasis of the muscle, bone, and epidural space should be secured before dural opening because the intradural operative field is located in the most dependent part of the exposure. In general, we prefer to remove enough of the lamina so that the dural opening may extend beyond the rostral and caudal tumor poles. Unilateral laminectomy may be appropriate for laterally or ventrolaterally located tumors. Significant facet resection is usually not needed. A longitudinal mid-line dural opening is accomplished. The incision may be paramedian for eccentric tumors or for tumors with a midline posterior dural attachment. The dura is tented laterally with suture to the paraspinal muscles to maximized intradural visualization and tamponade the epidural venous plexus. Nearly all meningiomas arise from, and are attached to, a dural base. Unlike intracranial meningiomas, bony involvement or invasion almost never occurs because of a well-defined spinal epidural space. The arachnoid is opened over the surface of the tumor, and the rostral and caudal tumor poles are identified. A small cottonoid sponge can be placed at either pole. Depending on the size and location of the tumor, internal decompression may be performed with an ultrasonic aspirator, laser, or suction/cautery method. A surgical bone tool (Sonopet Ultrasonic Aspirator, Stryker, Kalamazoo, MI) can be extremely useful for highly mineralized or densely calcified tumors. In general these tumors have a very friable and moderately vascular surface. We will often leave the covering arachnoid layer intact on the tumor surface to facilitate much of the dissection. Dorsal and dorsolateral tumors typically present no significant problems with visualization and safe delivery off the spinal cord. Tumors located ventral to the dentate ligament, however, can be more challenging. Fortunately, in most cases, the tumor is either completely lateral to the spinal cord or eccentrically located in the ventral canal, producing some degree of lateral cord displacement and rotation that provides an adequate surgical corridor from the dorsal approach. This exposure can be further optimized by section and light suture retraction of the dentate ligaments ( Fig. 39.1 ). Additional care with this maneuver must be taken with upper cervical lesions to avoid injury to the spinal accessory nerve. This nerve arises from a series of branches from the first six cervical levels that combine to form a trunk of ~1 mm in diameter that ascends to the craniocervical junction on the dorsal surface of the dentate ligament. Injury to this nerve causes a cosmetic and functional shoulder dysfunction, potentially of considerable morbidity. At other levels, either dorsal or ventral nerve roots may be reflected onto the tumor surface. In most cases these fascicles can be preserved, but in some instances it may not be possible to salvage the root. This is particularly true with tumors that originate from the lateral dura at the root exit zone. In cases of ventral tumor extension, the tumor is first debulked through a central trough. Tumors that are particularly vascular may need early devascularization of their dural origin with bipolar cautery. Ideally, the medial tumor component is completely disconnected from the dural attachment. The medial tumor can then be safely delivered away from the spinal cord. It may not be possible to directly visualize the ventral tumor–spinal cord interface. Fortunately, however, a well-developed arachnoid plane allows safe and gentle development of this plane to be achieved by tactile feel alone.
Management of the dural tumor origin varies according to surgeon preference and tumor location. Excision of the dural attachment often facilitates removal of dorsal and dorsolateral tumors, and competent dural patch grafting can be readily accomplished. Such is not the case with ventral and lateral attachments. We prefer to scrape the inner dural layers at the tumor origin and leave the outer layers intact ( Fig. 39.2 ). There does not appear to be a difference in tumor recurrence rates related to whether the dural tumor origin is excised or preserved.
The dura is reapproximated with a running locked 4–0 silk or Prolene suture (Ethicon, Inc., Somerville, NJ). Valsalva to 35 mm Hg is performed to test the adequacy of the dural repair. A small muscle patch can be used for small dural defects. DuraGen (Integra LifeSciences Corp., Plainsboro, NJ) and/or Duraseal (Baxter Healthcare Corp., Deerfield, IL) may be used to augment the repair. A careful, layered closure of the muscle, fascia, and skin is performed. A subfascial drain can be placed. The patient is maintained flat in bed until postoperative day 2 and then progressively mobilized.
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