Adequate exposure can be achieved with a midline incision that extends enough ros- trocaudally to allow lateral retraction toward the affected side. This may be most appro- priate for completely intradural tumors or those with foraminal involvement without much extension into the extraforaminal space. The techniques involved for tumors that
can be approached with a midline incision in the thoracolumbar spine are very similar to those described earlier for the cervical spine. For tumors with large paraspinal com- ponents, the lateral extracavitary approach can be used. This is described in detail here.
After the patient is turned to the prone position onto an appropriate table, a smaller bolster is placed on the side opposite the operative approach. The table is initially rotated toward the operative side to establish a true prone position. This maximizes the operative table rotation of the patient into a three-quarter prone position when anterior compartment visualization is needed. A hockey stick inci- sion with the midline long limb centered over the abnormality and the short limb gently curving 8 to 10 cm toward the operative side is planned. The midline portion of the incision is continued down through the subcutaneous tissue to the spinous processes. A routine subperiosteal elevation of the paraspinal muscles is performed bilaterally if posterior spinal exposure is required for laminectomy and/or poste- rior instrumentation. Adequate caudal midline spinal exposure, particularly for the placement of spinal instrumentation, must be assured at this point, because further caudal exposure is limited once the lateral limb of the incision has been opened.
After the midline soft tissue dissection is complete, the lateral limb of the skin incision is opened. This incision is continued down to the longitudinally oriented paraspinal muscles (i.e., erector spinae). In the thoracic region, portions of the trapezius muscle, the rhomboids, and the upper latissimus dorsi muscles are trans- versely incised in line with the skin incision, whereas only the thin thoracodorsal fascia is incised in the lumbar region. The myocutaneous flap is elevated to expose the longitudinally oriented paraspinal muscle mass. The lateral margin of this muscle mass is identified and medially elevated to expose the ribs in the thoracic region and the quadratus lumborum and psoas muscles at lumbar levels. The paraspinal muscle dissection continues over the facet joints to join the midline dissection. This allows the surgeon to simultaneously or alternatively work on either side of the now completely mobilized paraspinal muscle mass.
The lateral extracavitary approach may be modified according to the exposure requirements and surgical objective. Paraspinal tumors, for example, do not require intraspinal or posterior spinal exposure. In patients with these tumors, no midline subperiosteal dissection is performed. Instead, the midline incision is continued down only to the tip of the spinous processes. The midline insertion of the scapular muscles or thoracodorsal fascia is detached and laterally continued to elevate the superficial flap. Detachment of the lateral margin of the paraspinal muscle insertion from the iliac crest and resection of a posterosuperior iliac crest segment facilitate ventral exposure at lower lumbar levels (i.e., below L4). The sequence of the lateral extracavitary approach may also be varied as necessary according to the surgical strategy or tumor characteristics. Decompression of the spinal canal, for example, may proceed before the paraspinal exposure, or vice versa.
Complete dissection of the lateral spinal elements (facet joints, pedicle, transverse process, and ribs) one level above and below the pathologic segment is required to achieve adequate ventral exposure. The ventral and lateral surfaces of the ribs are subperiosteally exposed with Cobb elevators down to their vertebral body articula- tions. The intertransverse process ligament, psoas muscle, and diaphragm attach- ments (at upper lumbar levels) are released from the transverse process to the lateral pedicle margin at lumbar levels. The transverse processes at these levels are removed, as is a 6- to 8-cm segment of rib at the pathologic thoracic level. Removal of a shorter proximal rib segment (i.e., 3 to 4 cm) at adjacent thoracic levels provides adequate ventral spinal exposure while diminishing postoperative flail chest deformity. The segmental nerves at and one level above the pathologic level are identified. The intercostal neurovascular bundle lies deep to the intercostal muscles and is imbedded within the endothoracic fascia. The nerve is dissected free from the neurovascular bundle and elevated off the underlying fascia. The thoracic nerve roots are divided, and the proximal nerve stumps are elevated medially toward the foramen. Lumbar nerves are more difficult to identify, because they directly course through the psoas muscle. These nerves hinder ventral exposure, because they cross the surgical field and, unlike thoracic nerves, cannot be sacrificed and dorsally displaced. Dissection
of lumbar nerves over several centimeters allows for adequate nerve mobilization and prevents a postoperative stretch palsy. Vessel loop retraction of these nerves may improve ventral visualization. Initial exposure of the anterior thoracic paraspinal region is achieved by blunt ventral displacement of the pleura, once the intercostal nerves have been freed from its surface. Depression of the psoas muscle and/or dia- phragm after lumbar nerve dissection provides anterior lumbar paraspinal exposure.
Continued anterior compartment dissection is dictated by tumor characteristics. Para- spinal tumor components will be encountered before identification of the pedicle or lateral vertebral margin. Most tumors are closely located to the spine, and their margins can easily be separated from the surrounding pleura and endothoracic fascia in the thoracic spine. Table-mounted blade retractors and packing sponges maintain lung retraction, and the dissection remains entirely extrapleural for this area of the spine. The operating table is rotated away from the surgeon to improve ventral visu- alization. The pleura, diaphragm, and/or psoas muscle are bluntly dissected ventrally off the lateral vertebral body with Cobb elevators. Sharp dissection may be required at the disk space. Any remaining rib head is removed from the vertebral body artic- ulation. Dorsal segmental and foraminal branches from the intercostal or lumbar vessels, including the ascending lumbar vein, are cauterized and divided. Proximal dissection of the segmental nerves identifies the foramen. The margins of the pedicle are sharply defined with curettes. The pedicle is resected with Kerrison rongeurs and/or a high-speed drill. Partial resection of the superior facet joint and contiguous lateral vertebral elements exposes the lateral dural margin, which facilitates dissec- tion and improves ventral canal visualization. Mobilization of the segmental nerves continues to the lateral dural margin. This requires a section of proximal dorsal and autonomic rami and an often bloody dissection of an extensive perineural venous plexus. Once the lateral dural margin has been exposed, resection of the vertebral body and ventral spinal canal decompression may commence. Incision and evacua- tion of the disk precedes vertebral body resection, which is performed with rongeurs, curettes, and/or a high-speed drill. Ventral canal tumor and/or retropulsed bone fragments are quickly delivered down into the corpectomy defect with reverse-angle curettes to minimize epidural bleeding. This bleeding is controlled with absorbable gelatin sponges and cautery. Spinal stabilization with an anterior interbody strut graft and/or posterior fixation, if necessary, can be performed once tumor resection and hemostasis have been achieved. Divided thoracic nerve roots are clipped proximal to the dorsal root ganglion to prevent a painful postoperative neuralgia.
En bloc marginal resection of paraspinal and/or unilateral vertebral element or rib head neoplasms requires circumferential exposure of the tumor margins, ideally with a cuff of surrounding normal tissue. Development and disarticulation of the remain- ing medial vertebral element tumor attachment is achieved with osteotomes and ron- geurs. Occasionally, a spinal nerve may need to be included in the resected specimen.
The pleura are inspected for tears or an air leak. Most small tears can be repaired and do not require use of a chest tube if no air leak or lung collapse is detected.
This is preferable if the CSF space has been entered, to prevent a CSF-pleural fistula.
The remainder of the wound is closed in layers.
DISCUSSION OF BEST EVIDENCE
Accumulated clinical evidence over the past several decades has identified intradu- ral spinal nerve sheath tumors as well-encapsulated benign tumors with a relatively slow, but individually variable, growth rate. Numerous uncontrolled retrospective trials5-7,13-16 have established surgical resection as a safe and highly effective pri- mary treatment that provides long-term control or cure with preservation or even improvement of neurologic function. It is currently the treatment of choice for the majority of patients who harbor this tumor.
Less evidence is available for radiosurgery. Current evidence suggests that long- term tumor control may be achieved in some patients, but this effect is specifically unpredictable.8-12 Currently, high-risk patients, patients with multiple tumors, or those with recurrent or residual tumors may represent the best candidates for radiosurgery.
Evidence in support of nonoperative management of asymptomatic intradural spinal nerve sheath tumors is largely anecdotal and based on observation of the often extremely slow growth rate of many of these tumors. Since surgery is associ- ated with some risk, it is reasonable to offer some period of nonoperative manage- ment or at least to discuss the option with the patient.
COMMENTARY
“Surgery can’t make a normal patient better” is an aphorism well known to most sur- geons. Spinal neurosurgeons also generally adhere to the adage to “treat the patient, not the imaging.” Although the wisdom contained in these phrases is self-evident, it is also true that they overly simplify the numerous relevant factors that must be considered when determining the appropriate management options for each patient.
Often, the consequences of specific management recommendations—whether sur- gery, radiotherapy, or nonoperative observation—cannot be predicted at the indi- vidual level. One simply cannot know, in any particular case, whether a patient will have an adverse response to surgery such as a nerve root or spinal cord deficit, wound infection, CSF leak, or chronic neck or back pain. Furthermore, whether a specific tumor will respond to focused radiotherapy or will show an adverse effect cannot be predicted. In the absence of serial imaging, the biology and growth rate of the tumor cannot be known, and without a tissue sample one cannot be certain of the tumor histologic type or grade. Under such circumstances, there may be more than one rational treatment option. It is important, therefore, that the surgeon discuss these issues in detail with the patient so that the patient can make an informed deci- sion. From a surgeon’s perspective, it is important to consider the consequences of a wrong decision and to imagine the worst in relation to each management option. For example, if nonoperative management is recommended, what if the tumor type is more aggressive than initially assumed? This is why early follow-up imaging is done within narrow time frames in these patients and why early surgery is recommended for even small midline cauda equina tumors or tumors producing significant mass effect. On the other hand, consider the patient described in the Case Presentation, who has a small nerve sheath tumor at the L3 level. If early surgery were under- taken, she does risk permanent lumbar motor root deficit of moderate morbidity.
In many instances serial follow-up imaging will show little or no growth for several years. Thus, not performing surgery allows such a patient to remain neurologically normal for many years before undertaking the risks of surgery. If substantial growth is identified early on serial MRI scans, then both the patient and the surgeon clearly know that surgery should be undertaken despite the fact that the tumor may still be asymptomatic. Both patient and surgeon are better able to accept the small but pos- sibly significant consequences of surgery under these circumstances.
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