DURAL INCISION

A midline dural incision is made. Great care should be taken not to injure the arachnoid membrane. Under the microscope, the arachnoid membrane is sharply incised. The arachnoid membrane is tacked up by suturing it to the dura. Most tumors are totally intramedullary and are not apparent on surface inspection. An intraoperative ultrasound may be used to localize and determine the rostrocaudal tumor extent.³

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).

Fig. 30-1 The surface of the spinal cord after dural opening in an intramedullary hemangioma.

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).

Fig. 30-2 At the interface of the median sulcus, the small vessels are penetrating into the pial surface.

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).

Fig. 30-3 After myelotomy, pial suture is performed and the tissue is retracted to expose the tumor.

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.

DISSECTION OF THE TUMOR MASS

Ependymoma

The gross appearance is soft, friable, red to gray, and not encapsulated. Usually ependymomas have a smooth, glistening tumor surface. However, they are well-demarcated from the surrounding cord tissue, being dissected with blunt manipulation. The rostral end of the ependymoma accompanies the cyst, but the caudal part of the mass sometimes is connected to the central canal with a tough, fibrous band (Fig. 30-4). Large tumors may require internal decompression with an ultrasonic aspirator. The dissection for the ventral surface of the tumor mass is difficult because the feeding artery from the anterior spinal artery is usually found. The feeding arteries should be cauterized and cut on the side of the tumor mass with the tumor mass tractioned posteriorly.

Fig. 30-4 After myelotomy, the soft tumor mass is exposed with the proximal syrinx.

It is important to preserve the pia of the ventral surface (subpial tumor removal) to prevent damaging the anterior spinal artery and its branches.