Surgery of Intramedullary Tumors

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Chapter 30 Surgery of Intramedullary Tumors

INTRODUCTION

Intramedullary spinal cord tumors are rare, accounting for about 4–10% of all central nervous system tumors. Astrocytomas and ependymomas are the most commonly encountered spinal intramedullary tumors, and are found in up to 70% of all spinal intramedullary tumors. Most intramedullary cord tumors are benign gliomas. The determination of the optimum treatment of these tumors is controversial. In the past, there has been the traditional approach of biopsy, dural decompression, and radiation therapy, despite the recognition that after a relatively short remission, progression ensues, and the patient quickly becomes seriously disabled. This treatment was based on the assumption that astrocytomas are infiltrative tumors and that radical resection poses a high probability of inflicting neurological injury to the patient. These assumptions are debatable because most of these neoplasms are low-grade lesions. Recent advances in microsurgical technology, such as the ultrasonic aspirator, laser, intraoperative ultrasound, and intraoperative neurophysiological monitoring, permit a safer aggressive surgical resection. A radical surgical approach for intramedullary spinal cord tumor has been proposed by some surgeons. The radical resection without adjuvant treatment has been the rule for the intramedullary lesions.1 The postoperative functional performance is determined mainly by the preoperative deficits. The rate of aggravation is less than 20%. The most influencing prognostic factor for the postoperative result is the extent of tumor removal.2 The goal of surgery is maximal removal of the tumor mass without additional functional deficits.

SURGICAL TECHNIQUES OF INTRAMEDULLARY CORD TUMOR RESECTION

IDENTIFICATION OF MIDLINE

There are two routes to intramedullary tumors: through the posterior midline or through a posterolateral myelotomy through the root entry zone.4 The former follows the posterior median sulcus, and the spinal cord is split between the two posterior columns.

The midline of the spinal cord is anatomically identified with branches of the dorsal medullary vein penetrating the median sulcus. The thin membrane from the arachnoid attaches to the dorsal midline surface of the spinal cord (area posticum). Sometimes a spinal cord edema makes it difficult to identify the midline on the posterior surface of the spinal cord. The vessels usually are located off midline and do not constitute reliable markers. If the anatomical midline is not definite, the imaginary line is assumed from the bilateral dorsal root entry zone. The tumor-infiltrated spinal cord possesses a swollen appearance.5 In cases of a hypervascular mass or tumor hemorrhage, the bluish discoloration is seen through the surface of the spinal cord6 (Fig. 30-1).

The posterolateral approach offers a lateral route where the posterior roots enter the spinal cord. The spinal cord is opened through the dorsal root entry zone (DREZ) between two or more roots. This route offers an avenue for resecting intramedullary lesions that is off the midline and closer to the lateral surface of the spinal cord. This approach can be applied to the astrocytoma or cavernous hemangioma. A myelotomy should be chosen based on the patient’s symptoms and MRI findings.

MYELOTOMY

The myelotomy can be performed with a No. 11 blade, a No. 59 beaver-blade, a CO2 laser, or a neodymium:yttrium-aluminum garnet (Nd-YAG) contact laser.4 Some surgeons routinely use the latter during myelotomy. Compared with electrocauterization, this technique causes no artifact during electrophysiological monitoring.

Postoperatively, patients may complain of transitory dysesthesia or of diffuse, ill-defined sensory symptoms. The symptoms are caused by microvascular damage to the posterior columns during the myelotomy.

After midline identification, the small pial vessels crossing the midline are coagulated. The large vessels, running longitudinally along the pial surface, should be saved with dissection. Pial incision starts from the maximum enlargement area of the spinal cord, extending to the cephalocaudal margin of the tumor mass. The midline myelotomy is performed using a fine arachnoid knife. Careful dissection is imperative.

When the length of the myelotomy falls short, the resection margin cannot be identified. Discontinuous myelotomy is a viable technical option whenever the presence of large vessels on the median sulcus would make the standard midline myelotomy unsafe. After a small incision is made, the median sulcus is gently spread with the aid of microdissectors or microforceps to deepen the myelotomy until the tumor’s pole or cyst is exposed or opened both rostrally and caudally. The interface of the median sulcus usually can be identified by small vessels running over its surface, even if the midline has deviated to either side as a result of tumor compression (Fig. 30-2).

The mass is encountered about 2 cm deep from the surface. The whole length of the mass is exposed, then the incision margin of the pia is sutured with a fine 6-0 or 7-0 nylon to the reflected dura. The spinal cord tissue over the tumor mass is retracted to expose the tumor mass (Fig. 30-3).

A sufficiently long myelotomy should be made over the extension of the tumor mass to avoid traction injury. The middle of the tumor is first dissected from each of its lateral margins, and a dissection plane between the tumor and normal tissue is confirmed on the basis of intraoperative microscopic observation of color and tissue consistency. The dissection plane is then extended bilaterally toward the rostral and caudal ends. Further dissection is repeated longitudinally into deeper regions. A deeper dissection of the lateral margin of the tumor may injure the corticospinal and lateral spinothalamic tracts.

Once the rostral or caudal pole of the tumor is freed, the tumor is lifted with tumor forceps, and the ventral side of the tumor is gently dissected from the spinal cord. In this way, en bloc removal is attempted. The bleeding from the surface of the tumor-dissected plane usually stops by itself, or it can be stopped with Surgicel, avoiding bipolar coagulation.

If no plane is apparent between the tumor mass and the surrounding spinal cord, then it is likely that an infiltrative tumor is present. A biopsy specimen is first obtained to confirm a pathological diagnosis.