Suboccipital- Transmeatal Approach

A number of operative approaches to the cerebellopontine angle have been described, including a suboccipital-transmeatal, a trans­labyrinthine, a middle fossa, a translabyrinthine-transtentorial and a subtemporal-transtentorial approach. The authors prefer the suboccipital-transmeatal approach.

Operative Technique

Following the induction of general anaesthesia, the patient is placed on the operating room table in a lateral position. Some neurosurgeons insert lumbar spinal drain and the distal end of the tubing is brought out underneath the operating room table for the anaesthesiologist, who controls the CSF drainage during surgery. (In general the drain remains closed until the dura is exposed and the surgeon is ready to open the dura.) The patient is then moved into the final lateral position with a roll under the lower axilla. The shoulders and hips are taped to the table to allow manipulation of the table in all planes without risk of the patient slipping. It is important to place gentle traction on the shoulder of the upper arm parallel to the body to pull the shoulder out of the operative field. The head is then fixed with a Mayfield clamp with a moderate degree of flexion and slight rotation to bring the mastoid tip to the top of the operative field.

Table rotation is used to the advantage of the surgeon during the operation as the line of sight into the cerebellopontine angle is maximized by moving the patient rather than the surgeon, who usually sits in one position during the operation. The surgeon's chair is on casters and has adjustable arm rests adaptable for any surgeon and any patient. The combination of moving the chair and the operating room table brings almost all cerebellopontine angle lesions into a comfortable line of sight for the surgeon.

The skin preparation and draping are routine. Furosemide and mannitol are given prior to making the skin incision. In the unusual patient who presents with clinically manifested hydrocephalus, we perform a ventriculo­peritoneal shunt 7 to 10 days before the tumor surgery. The time delay allows the wound to heal and the risk of infection is then minimal. It is wise to consider placing the shunt on the side opposite the tumor so the tubing does not encroach on the suboccipital surgical site.

A two-limbed incision that begins 2 to 3 cm below and 1 cm medial to the mastoid tip and which extends vertically to the level of the top of the pinna and then curves medially toward the external occipital protuberance. The medial limb can be shortened or lengthened depending on the degree of exposure needed. This incision gives adequate exposure of the occipital bone and at the time of closure, provides sufficient galea to cover most of the upper transverse aspect of the wound. It also eliminates the need to transect the suboccipital musculature in a nonanatomic plane. The scalp flap can be easily dissected off the occipital bone with monopolar electrocautery. Caution must be used when opening the inferior portion of the vertical limb of the incision. medial to the mastoid tip. Rarely an anomalous vertebral artery can be found coursing up between the muscles and the bone. A variable number of emissary veins, connecting the external venous system of the scalp with the underlying sinuses, can be encountered during the dissection. Although they may bleed profusely they are easily controlled with coagulation and bone wax.

The craniectomy is performed using multiple burr holes followed by bone removal with a rongeur or drill. The craniectomy should be large enough to expose the sigmoid sinus laterally and the transverse sinus superiorly. It is not uncommon to encounter large venous channels during the bony dissection, especially as the sigmoid sinus is approached. Bleeding from the bone is controlled with bone wax, whereas bleeding from the sinus can be controlled with a small piece of Gelfoam . Mastoid air cells overlying the sigmoid sinus are also frequently encountered during the craniectomy. These can serve as a guide as one approaches the sinus and should be waxed thoroughly before the dura is opened. As the craniectomy is completed, the spinal drain is opened, if it was inserted.

The dural opening is started in the center of the craniectomy and is opened first in the direction of the junction of the transverse and sigmoid sinuses. A second incision is made toward the inferior aspect of the craniectomy, completing a triangle with the sigmoid sinus as its base. The resulting flap of dura is then reflected back and tacked to the cervical musculature with nylon suture. The dural opening is completed by making two more radial cuts starting from the apex of the dural triangle. The first cut extends toward the transverse sinus and the second cut extends toward the inferior medial portion of the craniectomy. The resulting smaller leaves of dura are tacked back. The object, however, is to completely open the dura to provide adequate exposure regardless, of the number of dural incisions.

At the time of closure, lyodura is used if the dural leaves do not approximate easily. By the time the dura is opened, the combination of spinal drainage and gravity has pulled the cerebellum away from the petrous bone. The patient is then rolled toward the surgeon far enough to position the petrous bone vertically. This manoeuvre, plus slight retraction with a 1-cm blade of a Greenberg retractor, exposes the arachnoid of the cerebellopontine angle. The superior petrosal vein, extending from the cerebellum to the junction of the tentorium and the petrous bone is coagulated as soon as it is visualized. Traction on this vein. which can be made up of multiple smaller vessels, can lead to troublesome, although not dangerous bleeding. An effective way to manage a disrupted vein is to cover it with a small piece of Gelfoam and a cottonoid and after the bleeding has stopped, remove the pack and coagulate the vessel. Bleeding from these veins looks quite serious in the small confines of the cerebellopontine angle but is always low-pressure bleeding and can always be managed with conservative measures.

With slight retraction of the cerebellum at its junction with the tumor, the surgeon can pull the arachnoid tight and divide it between the surface vessels of the tumor. As the surface vessels are identified, they are coagulated and dissected carefully along the arachnoid. This simple manoeuvre of opening the arachnoid establishes the critical tissue planes between the tumor and the side of the pons, as well as the lower cranial nerves and the AICA. The latter is often found buried in the arachnoid at the junction of the cerebellum and the dome of the tumor. Once a clear view of the tumor is achieved, stimulation of the exposed surface in an attempt to locate the seventh nerve is performed because the relationship of the nerve and the tumor may be variable, especially with meningiomas. With acoustic tumors the course of the nerve can also be variable, but is usually located anterior to the tumor. After the seventh nerve leaves the pontomedullary junction, it may course directly toward the internal auditory canal under the lower pole of the tumor and at other times it may course superiorly along the side of the pons up toward the root entry zone of the trigeminal nerve and follow the course of this nerve back to the petrous bone . In large tumors, the nerve is often very thin and difficult to identify, but with stimulation it is possible.

There are several possible techniques applicable to the ultimate removal of the tumor. These include the laser, the ultrasonic aspirator and bipolar coagulation with concomitant suction. Most neurosurgeons use primarily a bipolar coagulation technique with continuous irrigation and suction. This allows for a bloodless field. In large tumors, the removal begins in the center of the surface facing the surgeon. The slow and meticulous removal of the tumor internally and gradually outward allows the capsule to fall inward. This decompression of the tumor in essence changes a large tumor into a small one and allows the eventual visualization of the cranial nerves and vessels and permits the surgeon to define their relationship to the tumor. After the center of the tumor has been decompressed, the dissection is best carried out rostrally in the region of the fifth nerve. This nerve is easily identified and tolerate dissection better than the lower cranial nerves.

By following this nerve medially to identify the entrance into the brain stem, this is then followed by exposure of cranial nerves IX, X, and XI, located at the lower pole of the tumor. As the interface between the pons and the tumor becomes apparent, small pieces of Gelfoam are inserted between the two to give gentle retraction and at the same time protect the side of the pons and the related vessels. Attempts to pull on the tumor or to manipulate it without adequate tumor decompression will, under most circumstances, result in serious problems.

Special care must be exercised in the management of the ninth and tenth cranial nerves. These nerves are extremely sensitive to traction or trauma and must be protected very carefully. As soon as possible, we free them from the underlying arachnoid and tumor surface with sharp dissection and cover them with cottonoids.

After the tumor has been reduced in volume to the point at which it is anatomically free of the fifth, ninth, tenth and eleventh cranial nerves and the lateral aspect of the pons, it becomes possible to identify the seventh nerve as it exits from the brain stem beneath the choroid plexus protruding from the foramen of Luschka. This location is ventral and slightly above the root entry zone of the vestibular nerves. The facial nerve has a distinct. silvery, shiny appearance. In contrast, the vestibular nerves are dull in colour and are somewhat tan.

The removal of smaller cerebellopontine angle lesions is easier because the relationship of the lesion to most of the important structures can be defined readily. In the removal of small acoustic tumors and in large acoustic tumors after they have been reduced in size, the internal auditory canal must then be unroofed. By drilling off the roof of the canal back to the fundus, it is possible to assure a complete removal of the tumor and of equal importance, to expose a portion of the seventh nerve that is free of tumor. This serves as an excellent starting point for developing the plane between the seventh nerve and the tumor.

Unroofing of the canal is started by coagulating the dura adherent to the petrous bone. The bone can be removed with any high-speed drill. The drilling must go far enough to expose the fundus of the canal and particularly the transverse crest (which is oriented vertically in the lateral position). When viewed in the lateral position, the transverse crest separates the superior and inferior vestibular nerves superficially in the canal and the facial and cochlear nerves deep in the canal. In general, the unroofing of the canal is started with a cutting burr: the surgeon switches to a diamond drill as the outline of the canal becomes apparent. Prior to drilling, the relationship between the jugular bulb and the canal should be ascertained from the CT scan because they may be in close proximity. During the unroofing process, air cells in the posterior wall of the internal auditory canal may be opened. It is essential that these be sealed prior to closure. either with bone wax, Gelfoam, muscle, or fibrin glue, for they are a potential source of postoperative CSF leakage. Following the bony dissection of the canal. the dura of the canal is opened with microscissors. starting at the medial end.

Commonly, there is a 1- to 2-mm area in the fundus that is free of tumor and, by gently dissecting in this area, the nerves may be exposed and identified. The intracanalicular portion of the tumor generally has a loose attachment to the seventh nerve, usually at the origin: subsequent sharp and blunt dissection allows easy separation along their anatomic plane. Once the tumor has been freed from its attachments within the canal it can be dissected back to the lip of the canal. Care must be exercised by the surgeon at this stage of the operation because the seventh nerve becomes broad and thin and sometimes takes on the appearance of thickened arachnoid, especially as it goes over the edge of the lip of the canal. This thinned out portion is the point where the seventh nerve is most vulnerable.

After removal of the last remnant of tumor, air cells that have been exposed during drilling must be sealed. The sealing process can be with bone wax applied with a Penfield dissector and tamped into place with a cottonoid, If a number of air cells are opened or are difficult to seal with bone wax. a piece of Gelfoam covered with muscle and held in place with fibrin glue provides an effective seal. Extra time spent during this phase of the operation is worthwhile because it minimizes the risk of postoperative CSF leakage.

Prior to dural closure, multiple Valsalva manoeuvres are performed to confirm venous haemostasis. The subarachnoid space is irrigated until clear. An attempt is always made to close the dura primarily: however closure with lyodura or thick periosteum harvested nearby, may be necessary.

In cases where a large tumor has been removed and a potential for cerebellar swelling exists, the dura is left patulous. It is  preferable to monitor postoperative intracranial pressure by way of a subdural catheter. This catheter is tunnelled out from the incision though a separate stab wound above the horizontal limb of the opening. This catheter may also serve as a CSF drain.If a small bone flap has been turned during the opening, this can be replaced. Alternatively, if the patient is concerned about cosmesis, a cranioplasty can be performed.

The scalp wound is then closed in a two-layered fashion with 2-0 Vicryl for the galea and 3-0 nylon interrupted mattress or subcuticular sutures for the skin. Care must be taken to close the muscle and fascial layers of the inferior portion of the vertical limb of the incision because this region may leak CSF if not adequately closed.

Results

Mortality rate is ranging between 1.6 - 5  percent depending upon the patients categories. Anatomical preservation of the facial nerve is ranging between 60-90 percent depending upon various factors, such as tumor size. Preservation of sound sensation  also ranging between 20-66 percent.