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by ROBERT
G. OJEMANN, M.D. © Congress of Neurological Surgeons Honored
Guest Presentation Disclaimer: The information and reference materials contained herein are intended solely to provide background information. They were written for an audience of physicians. They are in no way intended to constitute medical advise. For medical advise a physician must, of course, be consulted. Contents
Overview The microsurgical removal of an acoustic neuroma can be done by a suboccipital, translabrynthine, or middle fossa approach. Good results from all three approaches have been reported by experienced groups of neurosurgeons (43). For most patients we have preferred the suboccipital (posterior fossa) approach because of the wide visualization it allows, the ability to save hearing in appropriate cases, and the good results we (35-44) and others (1, 8, 10, 12, 18, 19, 47, 53) have reported. The technical aspects of the operation have been reported and illustrated in detail in previous publications (39, 42, 43). In a few patients with no useful hearing and intracanalicular tumors or with tumors extending a few millimeters into the posterior fossa, we have used a translabrynthine approach. Perioperative Medical Therapy Steroids are usually started 48 hours prior to operation and a higher dose (methyprednisolone, 80 mg, intravenously) is given just before the operation. The blood sugar is carefully monitored. The high steroid dose is continued every 6 hours during the operation and then is gradually tapered off over 5-10 days depending on the size of the tumor and facial nerve function. An antibiotic, usually a cephalosporin, is given intravenously starting just before surgery. This is continued for 24 hours after the operation. After anesthesia is induced, an indwelling Foley catheter is inserted and 10-20 mg of furosemide are given intravenously. During the preparation and exposure of the dura a 20% solution of mannitol is given intravenously, in a dosage of 1-1.5 g/kg, over 20-30 minutes. Monitoring Continuous electrophysiological monitoring of facial nerve function during the operation has become an established procedure (43).
The benefits of this monitoring have been documented (16). A continuous drip of a muscle relaxant is carefully administered so facial nerve function can be assessed. The dosage is monitored by following the twitches elicited with ulnar nerve stimulation. Facial nerve function is monitored by continuous recording of electromyographic activity with two recording electrodes, one in the orbicularis oculi and the other in the orbicularis oculus muscles (Fig. 24. 1). The muscle contractions, which could occur from stimulation of the facial nerve during coagulation when the electrodes are inactive, are recorded from a motion sensor placed on the cheek. Monopolar stimulation is used to locate the seventh nerve. Fifth nerve function is monitored with electrodes placed in the masseter and temporalis muscles. When auditory evoked responses are monitored during an attempt to save hearing, we use a system developed by Dr. Robert Levine (see Cochlear Nerve Function) (21-25, 42). A transtympanic electrode is placed for electrocochleography, scalp electrodes are inserted for brainstem auditory evoked potential monitoring, and a microphone system is placed in the external ear canal to provide the sound stimulus (Fig. 24. 1). Position
The operating table is turned so the surgeon can sit behind the patient's head and place the feet under the table. The patient lies supine with the shoulder that is ipsilateral to the tumor slightly elevated (Fig. 24.2). The head is turned parallel to the floor, elevated, and held with a three-point skeletal-fixation headrest. During the operation the line of sight to the brainstem may be altered by rotating the table from side to side. An arm rest is placed for the surgeon's arm nearest the vertex. The other arm rests on the patient. In patients with previous neck pathology, it may be necessary to elevate the shoulder more or use a lateral position. Incision and Exposure A vertical incision is centered 2 cm medial to the mastoid process (Fig. 24.2). A graft of peticrartial tissue, about 4 cm in diameter, is taken from the occipital region by extending the superior aspect of the incision as needed. This graft is used in closing the cerebral convexity dura at the end of the operation. The suboccipital muscles and fascia are incised in line with the incision and carefully separated from their attachments to the bone using subperiosteal dissection and electrocautery. Special care is taken to occlude the arterial vessels as they are encountered in the muscle. An ernissary vein is usually exposed in the region of the medial mastoid area.
The bone over the lateral two thirds of the cerebellar hemisphere is exposed. It is usually not necessary to visualize the midline bone or lim of the foramen magnum. A burr hole is placed, the dura carefully separated from the overlying bone, and a bone flap cut. This opening exposes the dura over the lateral two thirds of the cerebellar hemisphere and exposes the transverse sinus. Further bone is removed as needed to expose the turn from the transverse sinus to the sigmoid sinus and the edge of the petrous bone laterally. This allows the edge of the sinus to be retracted, with the sutures placed to hold the dural flaps, and yields a direct line of sight down the posterior surface of the petrous bone. The mastoid air cells are usually entered and are occluded with bone wax. The dura is opened vertically, keeping an area of medial dura intact to protect the retracted cerebellum. Stellate dural incisions provide superior, lateral, and inferior flaps of dura, which are held back with sutures (Fig. 24.3). The cerebellum is then gently elevated, the arachnoid is opened, and cerebrospinal fluid is allowed to drain (Fig. 24.3). This usually relieves any bulging of the cerebellum and allows exposure of the cerebellopontine angle with minimal retraction. The arachnoid should be opened enough to allow cerebrospinal fluid to continue to drain during the operation. In most tumors of >2 cm the tip of a small catheter (number 10 Bardic) is placed in the cistern and sutured to the inferior medial comer of the dural opening to drain cerebrospinal fluid continuously during the operation. In some patients with large tumors a small portion of the lateral cerebellar hemisphere is removed to facilitate the exposure. Following placement of the self-retaining Greenberg refractors, the operating microscope is positioned. Removal of Small Tumors and Hearing Preservation
Under the microscope the arachnoid over the tumor is opened and the petrosal vein is coagulated and divided. The refractors are repositioned. With small tumors the eight nerve complex is seen coming into the inferior medial side of the tumor. The next step is usually exposure of the tumor in the internal auditory canal. Dura is removed over the region of the internal auditory canal and bone is carefully removed using an air drill, with constant suction-irrigation for cooling. When hearing preservation is a consideration, the bone removal extends for a distance of no more than 10 mm laterally. A more lateral exposure risks entering the labyrinth. Dissection then depends on an assessment of the relationship of the tumor to the vestibular and cochlear nerves. In some patients the vestibular nerve fibers entering the medial edge of the tumor are divided, the cochlear and facial nerves are identified, and the dissection proceeds from the medial to lateral sections. In other patients it may be difficult to define the cochlear nerve medially. The tumor is then carefully rotated near the lateral end of the canal, with a search for the seventh nerve anteriorly superiorly and the cochlear nerve anteriorly inferiorly (Fig. 24.4). It is important to avoid stretching or putting tension on these nerves. The position of the seventh nerve is confirmed with stimulation. An internal decompression of the tumor may be done using sharp dissection to facilitate the exposure. Dissection along the facial and cochlear nerves is done with fine straight or curved microdissectors or canal knives and sharp dissection is done with microscissors. Dissection is alternated from different directions depending on what seems to give the best exposure, the easiest plane of dissection, and the least traction on the nerves. When the cochlear and facial nerves have been clearly defined, the vestibular nerves coming into the tumor are divided on both the medial and lateral aspects of the tumor. In some patients the lateral end of the tumor may not be exposed because of the limitation in bone removal. In these patients the tumor is transacted near the end of the canal and the lateral extent of the tumor is removed with a small ring curette. During the dissection there may be intermittent bleeding along the nerves. A fine suction keeps the field clean and does not damage the nerves. Most of the bleeding stops spontaneously. When trying to save hearing, an attempt is made to preserve any significant arterial vessel entering the internal auditory meatus. Removal of Medium and Large Size Tumors
The arachnoid over the posterior capsule of the tumor is opened. A separate cystic collection of cerebrospinal fluid may occasionally be loculated in relation to the tumor capsule, with thickened arachnoid and xanthochromic fluid. The petrosal vein, which usually comes off the cerebellum or middle cerebellar peduncle to the petrosal sinus just above the tumor, is coagulated and divided. In order to complete the initial exposure of the posterior capsule it is usually necessary to shrink the cerebellar tissue next to the tumor with bipolar coagulation or to remove a small amount of cerebellar tissue which may obscure the inferior medial pole. In some large tumors up to 1 cm of lateral cerebellum may be resected. The posterior capsule is stimulated to locate the facial nerve. In almost all patients the facial nerve is on the anterior surface of the tumor and there is no response on this first stimulation. The most common direction of displacement is directly anteriorly (Fig. 24.5). The next most common location is a anterior medial displacement along the brainstem and over the anterior superior aspect of the tumor. In this circumstance the facial nerve may be displaced against the fifth nerve. On rare occasions the nerve is inferior or across the posterior surface. Only one patient in my series of 410 cases had the facial nerve displaced posteriorly. The ninth, 10th, and llth cranial nerves are identified and arachnoid adjacent to the cerebellum is carefully dissected to aid exposure of the inferior medial capsule. With larger tumors the ninth and 10th nerves are carefully reflected off the tumor capsule and a small rubber dam is placed over them for protection during the rest of the operation.
The next step is internal decompression of the tumor, which is done intermittently as needed. This allows all the pressure to be placed on the tumor capsule while it is separated from the cranial nerves and brainstem. The ultrasonic aspirator, bipolar coagulation, and sharp dissection are used for internal decompression. Dissection begins inferiorly and medially. In medium size tumors, the eight nerve complex can usually be defined with moderate dissection (Fig. 24.6). In larger tumors these nerves are usually not seen initially. After carefully reflecting the capsule laterally and superiorly into the area of decompression, one looks for the eight nerve complex along the inferior medial capsule. Being a right-handed surgeon, I prefer having fine suction in my left hand to retract the tumor and keep the area of dissection clean. The vestibular and cochlear nerve fibers entering the tumor are divided using bipolar coagulation and sharp dissection (Fig. 24.7). Arterial vessels adjacent to the tumor are preserved, dividing only branches entering the tumor.
As the dissection of the capsule progresses, not only is further internal decompression done as indicated but sections of the tumor capsule are removed to allow room to reflect the capsule laterally. It may also be advantageous to alternate dissection of the inferior medial capsule with dissection superiorly and medially to defme the fifth nerve and brainstem attachments (Fig. 24.8). There are often vascular attachments and significant compressionin the region of the fifth nerve root entry zone. Small rubber darns may be placed on the brainstem for protection as the dissection progresses.
If the seventh nerve has not yet been localized, intermittent stimulation is used. In some patients spontaneous electromyographic activity indicates when one is on or near that nerve. This nerve is usually located by reflecting the inferior and medial tumor capsule further laterally and superiorly. The nerve is seen anterior to the divided eighth nerve complex (Fig. 24.9). It often can be recognized by its slight grayish color, which is different from that of the adjacent brainstem. In some patients the facial nerve may be displaced against the brainstem and take a superior course before turning and heading laterally. When displaced superiorly it may lie against the fifth nerve. Usually the facial nerve forms a solid band on the tumor capsule but it may be spread out over a wide area and occasionally is surrounded by the tumor.
Dissection extends along the seventh nerve toward the internal auditory meatus. We use fine straight or curved microdissectors, canal knives, and sharp dissection with microscissors as needed (Fig. 24.10). When the point is reached where the bone over the internal auditory canal is impeding further dissection or the dissection is difficult, attention is directed to the tumor in the internal auditory canal. The exposure is the same as described for small tumors. In some patients the tumor extends quite far laterally in the canal and it is advisable to make the usual exposure, remove a portion of the tumor, and then do further drilling within the lateral end of the canal as needed. When drilling along the inferior margin one must remember that, occasionally, the jugular bulb may come over this area.
After separation of the tumor from the facial nerve in the internal auditory canal, the attachments along the edge of the internal auditory meatus are divided. The facial nerve starts to turn anteriorly as the posterior fossa is entered. The surgeon can then decide how best to proceed. The dissection may be continued medially along the brainstem and cere bellapeducle, by dividing arachnoid and vascular attachments as they are encountered and gradually freeing the facial nerve. On occasion a large branch of the anterior inferior cerebellar artery is embedded in the tumor capsule but it can usually be dissected free by dividing the small branches directly supplying the tumor. In large tumors the trochlear nerve and superior cerebellar artery may be adherent superiorly, the sixth nerve adherent anteriorly, and the ninth and 10th nerves adherent inferiorly. The objective is to reduce the bulk and attachments of the tumor so that finally the surgeon is dealing only with dissection from the facial nerve. The facial nerve is usually most adherent to the tumor capsule in the posterior fossa near the internal auditory meatus, where the nerve may be splayed over the anteiior-supeiior capsule or occasionally surrounded by tumor. The dissection in this area is often complicated by vasctflar and fibrous attachments. The surgeon must adapt to the characteristics of the tumor and it may be necessary to work alternately from vaidous angles. In some patients with large tumors the capsule is so intimately adherent to the brainstem and cranial nerves that a plane cannot be developed. Closure Once the tumor is removed, hemostasis is checked. The area of bone removal over the internal auditory meatus is carefully waxed to occlude air cells. In addition, an adipose tissue graft taken from a superficial incision on the lower abdomen is carefully placed in the area where bone has been removed. Surgicel is used to hold this graft in place and is also used to cover the area of resected or retracted cerebellum. I like to close the dura in a water-tight fashion using the graft of pericranial tissue taken at the beginning of the operation. The dura is covered with Gelfoa.m. The bone flap is replaced and held with number 28 wire, which does not interfere with any future imaging, and a dural tenting suture. The wound is thoroughly irrigated with Bacitracin solution prior to closure. Management of Postoperative Complications
During the initial exposure particular attention is focused on occluding the arterial vessels in the muscles as they are encountered. Then during the closure the muscles are again carefully checked for bleeding. A significant epidural hematoma can result with hemorrhage from these blood vessels. Prior to closure of the dura, the systemic blood pressure is elevated to approximately 140-150 mm Hg. On more than one occasion this maneuver has provoked hemorrhage, which is controlled with bipolar coagulation. Postoperatively the blood pressure is controlled, beginning in the operating room with an intravenous antihypertensive medication, and is monitored continuously for as long as necessary. If the cerebellum is unusually full at the end of the operation and there has been good cerebrospinal fluid drainage, cerebellar infarction or hematoma should be considered. In this situation a resection of the lateral 1-2 cm ofcerebellum may need to be done. If the patient does not recover promptly fron, anesthesia or there is an unexpected significant neurological deficit or delayed neurological deterioration, a CT scan is done immediately to look for cerebellar hematoma or infarction. Prompt removal of a significant hematoma or area of infarction can lead to a dramatic recovery. Cerebrospinal Fluid Fistula If a cerebrospinal fluid leak develops, a lumbar drain is placed for 72 hours; this often resolves the problem. When the leak persists, a transmastoid repair using an adipose tissue graft is done. Hydrocephalus In the Postoperative period the patient my develop neurological syrnptoms that suggest that hydrocephalus or a tense subgaleal fluid collection may be present. Ventricular size is followed by CT scan. Fortunately, persistent hydrocephalus is a rare complication. Most patients recover spontaneously, a few require a temporary lumbar drain, and only occasionally is a ventriculoperitoneal shunt needed. Meningitis When there is Postoperative fever with headache or stiffness in the neck, the possibility of either aseptic or bacterial meningitis must be considered. CT with contrast is done to look for an area that might suggest a local infection. A lumbar puncture is then done and administration of broad-spectrum antibiotics is started. Subsequent treatment is guided by the results of the cerebrospinal fluid examination and cultures. If the findings suggest aseptic meningitis, steroids are used. Wound Infection When the infection is superficial and the organism is sensitive to antibiotics, it may not be necessary to remove the bone flap. ff the infection is extensive, debridement of the wotmd and removal of the bone flap is necessary. Neurological Complications If there is any significant Postoperative disability the patient is seen by physical and occupational therapists. Difficulty swallowing due to impaired function in the ninth and tenth nerves should be Carefully evaluated with a modified barium swallow and followed by a specialist in swallowing disorders. Often the patient can be given instructions that facilitate their swallowing and prevent aspiration. On rare occasion a gastrostomy is needed. Headache After suboccipital operation a small
percentage of patients complain of persistent headache. The magnetic
resonance imaging (MRI) scan rarely shows a structural abnormality
such as hydrocephalus. In most patients it seems to be a myofascial
problem, in a few a neuroma of the occipital nerve, and in some
cervical spine degenerative disease. Many patients are benefited
by a program of physical therapy and local blocks as indicated.
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