Chapter 29 Surgery of Extramedullary Tumors
Extramedullary tumors are composed of intradural and extradural tumors. Intradural tumors account for 10–20% of primary central nervous system neoplasms in adults. About two-thirds are extramedullary, well-circumscribed, and histologically benign. The tumors considered here include schwannoma, meningioma, and filum terminale ependymoma. Extradural lesions are meningeal cyst, synovial cyst, and pure extradural schwannoma. From a surgical exposure perspective, the extramedullary lesions can be divided into extraspinal (the vertebral column and the paraspinal region) and the intraspinal compartments. The surgical approach is different depending on the location to the spinal cord (ventral or dorsal) and the extraspinal extension. Intraspinal lesions can be intradural or extradural. In most cases, intraspinal lesions can be removed with a posterior midline approach. However, when the extraspinal portion is large, the extent of bony resection is wide and a posterolateral approach should be considered. In cases of large extraforaminal lesions, an anterior approach must be considered. Large extraforaminal lesions can invade the vertebral body and erode it.
Although most tumors of the thoracic and lumbar spine are readily accessible via standard spinal exposures, there is a group of tumors that cannot be removed with a simple posterior approach. They are large dumbbell tumors with significant intraspinal and paraspinal involvement (Fig. 29-1), paraspinal tumors located in the upper thoracic region (T1–3) or imbedded within the psoas muscle adjacent to the lumbar spine, and extensive unilateral anterior and posterior paraspinal tumors with significant spinal canal and vertebral column involvement.2 Posterior midline approaches provide bilateral exposure of the entire posterior elements (lamina and facets, the dorsal epidural space, the entire intradural space, and the posterior paraspinal region) and lateral spinal elements (pedicle and lateral epidural space); however, access to the anterior element is limited. For tumors with a large extraforaminal portion, the posterolateral (e.g., lateral extracavitary approach) route should be added.
At the cauda equina level, the posterior element is totally removed for exposure of the dural sac and foraminal nerve root. Disarticulation is performed above and below the facet joint. The unilateral complex of superior articular process, transverse process, and inferior articular process is removed from the vertebral body at the point of the pedicle. A part of the spinal canal and unilateral foramen is opened. An underlying mass is seen to originate from the nerve root. The tumor mass–laden nerve root is dissected from the underlying vertebral body. A cottonoid is put under the nerve root. A dural incision is made on the lateral margin of the spinal cord and extended over the tumor mass (Fig. 29-2). The intradural mass is dissected from the surrounding cauda equina, and the origin nerve is cut at the surface of the tumor mass. The distal end of the tumor mass is cut intracapsularly or extracapsularly.
A patient presents with a schwannoma originating from the L3 nerve root that formed a large extraforaminal paraspinal mass at the paraspinal area (Fig. 29-3). The extraforaminal mass has enlarged to expand the left psoas muscle (Fig. 29-4). For the removal of the large extraforaminal portion, extensive paraspinal exposure is required. The lateral extracavitary approach is useful in this situation. This single-staged approach provides excellent exposure of intradural structures as well as extensive access to the anterior and posterior paraspinal region, ventral spinal canal, and vertebral body (Fig. 29-5).
Fig. 29-5 The posterolateral route provides an excellent view of intradural structures as well as extensive access to the anterior and posterior paraspinal region, ventral spinal canal, and vertebral body.
For one-stage removal of the tumor mass, the approach should be performed from both the midline and paramedian route. A hockey-stick incision with the midline long limb centered over the abnormality and the short limb gently curving 8–10 cm toward the tumor side is made. With the midline approach, the intraspinal mass is removed. Once the midline soft tissue dissection is complete, the lateral limb of the skin incision is opened. The thoracodorsal fascia is incised. The lateral margin of the paraspinal muscle is identified and medially elevated to expose the quadratus lumborum and psoas muscles. 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 (i.e., below L4) 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 L3 and L4 transverse process should be resected to expose the cephalocaudal extent of the tumor mass. The intertransverse ligament and psoas muscle are released from the transverse process to the lateral pedicle margin, at which time the posterior tumor capsule is seen. After the tumor capsule incision is complete, internal debulking is attempted. After the interior of the tumor mass is decompressed, the capsule shrinks. As the tumor capsule is tractioned, the interface between the tumor capsule and psoas muscle is dissected (Fig. 29-6). During dissection, feeding vessels are encountered and should be ligated and cut down. Internal debulking and capsule dissection are repeated. The L2 and L4 nerve roots can 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. Proximal dissection of the segmental nerves identifies the foramen. Vessel loop retraction of these nerves may improve ventral visualization. Depression of the psoas muscle after lumbar nerve dissection provides anterior lumbar paraspinal exposure. Most paraspinal or dumbbell tumors are closely applied to the spine. The psoas muscle is bluntly dissected ventrally off the lateral vertebral body with Cobb elevators. Sharp dissection may be required at the disc space. Dorsal segmental and foraminal branches from the lumbar vessels are cauterized and divided. The margins of the pedicle are sharply defined with curettes. Posterior element (facet joint, pedicle, and transverse process) removal exposes the lateral dural margin, which facilitates dissection and improves ventral canal visualization. Mobilization of the segmental nerves continues to the lateral dural margin. If a two-stage operation is planned, the extraforaminal paraspinal mass is removed with a retroperitoneal anterolateral approach.
Fig. 29-6 With a paramedian approach, the tumor capsule is exposed and incised. The internal debulking is performed, after which the lateral side of the capsule is dissected from the muscle tissue. During the lateral wall dissection, the bleeding from small perforating vessels should be controlled.
Although paraspinal tumors at the lumbar spine are more effectively exposed through the retroperitoneal approach, tumors that are imbedded deep within the psoas muscle immediately adjacent to the lumbar spine can complicate the anterolateral approach because the lumbar spinal nerves, plexus, and femoral nerve can be injured during dissection of the psoas muscle. Large tumors stretch the longitudinally oriented psoas muscle fibers. It can be difficult to identify these nerves during mobilization of this muscle. The posterolateral approach, however, provides proximal exposure of the lumbar nerves and early tumor identification without extensive psoas muscle dissection. These features facilitate tumor removal and minimize the risk of lumbar nerve injury.
A 40-year-old man had suffered from three episodes of severe back pain with left leg pain in the previous 5 years. There was no family history of neurofibromatosis. At examination, he showed no constant back pain and no neurological deficits.