Removal of vestibular schwannoma and facial nerve preservation using small suboccipital retrosigmoid craniotomy

Transcription

1 274 Original article Removal of vestibular schwannoma and facial nerve preservation using small suboccipital retrosigmoid craniotomy CHEN Ling, CHEN Li-hua, LING Feng, LIU Yun-sheng, Madjid Samii and Amir Samii Keywords: vestibular schwannoma; internal auditory meatus; retrosigmoid transmeatal approach; facial nerve; monitoring A Background Vestibular schwannoma, the commonest form of intracranial schwannoma, arises from the Schwann cells investing the vestibular nerve. At present, the surgery for vestibular schwannoma remains one of the most complicated operations demanding for surgical skills in neurosurgery. And the trend of minimal invasion should also be the major influence on the management of patients with vestibular schwannomas. We summarized the microsurgical removal experience in a recent series of vestibular schwannomas and presented the operative technique and cranial nerve preservation in order to improve the rates of total tumor removal and facial nerve preservation. Methods A retrospective analysis was performed in 145 patients over a 7-year period who suffered from vestibular schwannomas that had been microsurgically removed by suboccipital retrosigmoid transmeatus approach with small craniotomy. CT thinner scans revealed the tumor size in the internal auditory meatus and the relationship of the posterior wall of the internal acoustic meatus to the bone labyrinths preoperatively. Brain stem evoked potential was monitored intraoperatively. The posterior wall of the internal acoustic meatus was designedly drilled off. Patient records and operative reports, including data from the electrophysiological monitoring, follow-up audiometric examinations, and neuroradiological findings were analyzed. Results Total tumor resection was achieved in 140 cases (96.6%) and subtotal resection in 5 cases. The anatomical integrity of the facial nerve was preserved in 91.0% (132/145) of the cases. Intracranial end-to-end anastomosis of the facial nerve was performed in 7 cases. Functional preservation of the facial nerve was achieved in 115 patients (Grade I and Grade II, 79.3%). No patient died in this series. Preservation of nerves and vessels were as important as tumor removal during the operation. CT thinner scan could show the relationship between the posterior wall of the internal acoustic meatus and bone labyrinths, that is helpful for a safe drilling of the posterior wall of the internal acoustic meatus. Conclusions The goal of every surgery should be the preservation of function of all cranial nerves. Using the retrosigmoid approach with small craniotomy is possible even for large schwannomas. Knowing the microanatomy of the cerebellopontine angle and internal auditory meatus, intraoperating neurophysiological monitoring of the facial nerve function, and the microsurgical techniques of the surgeons are all important factors for improving total tumor removal and preserving facial nerve function. ttempts at the surgical removal of vestibular schwannomas were made by Cushing and later by Dandy, two pioneers and legendary neurosurgeons who understood the natural history of the disease and set the landmarks of the current surgical approach to the cerebellopontine angle. In this century of medicine the goal of every surgery should shift from a life-threatening affection to the actual standard of cranial nerve functional preservation of all cranial nerves and conservation of quality of life. 1,2 To remove the tumor thoroughly and preserve facial nerve function, even of the vestibulocochlear nerve, is the optimal result of the operation on vestibular schwannoma. We excised vestibular schwannomas microsurgically from January 2001 to December 2007 by the suboccipital retrosigmoid transmeatus approach with small craniotomy; here we retrospectively analyzed and summarized 145 patients with complete data. METHODS Patients The mean age of patients at the time of surgery was (42.3±11.2) years (range, years), with 77 men and 68 women, and the course of disease varied from 4 months to 16 years. The location of the lesion on the right DOI: /cma.j.issn Department of Neurosurgery, Xuanwu Hospital, Capital Medical University, China International Neuroscience Institute, No. 45 Chuangchun Street, Beijing , China (Chen L, Chen LH and Ling F) Department of Neurosurgery, Xiangya Hospital, Central South University, Changsha, Hunan , China (Liu YS) International Neuroscience Institute, Alexis-Carrel-Strasse 4, Hannover 30625, Germany (Samii M and Samii A) Correspondence to: Dr. CHEN Li-hua, Department of Neurosurgery, Xuanwu Hospital, Capital Medical University, China International Neuroscience Institute, Beijing , China (Tel: Fax:

2 Chinese Medical Journal 2010;123(3): or left did not matter. Five patients with increased tumors after gamma knife treatment were included. Symptoms and signs The most typical and popular symptom was progressive nerve deafness (sensorineural deafness) on the lesion side. Initial symptoms mainly showed cochlear nerve irritation symptoms, including 93 cases of high-tone tinnitus or dysaudia. The main symptoms and physical signs were as follows. (1) Symptoms of irritation or destruction of vestibulocochlear nerve: 93 cases had tinnitus, 139 cases had hearing loss or deprivation, and 56 cases had vertigo. (2) Symptoms of involved adjacent nerves: the most common symptoms were caused by trigeminal nerve involvement including 54 patients with facial numbness, and 21 with hoarseness and facial incomplete paralysis. (3) Symptoms caused by brainstem or cerebellum involvement: 115 cases had disorder of coordinate movement and 12 had weak muscle strength. (4) Symptoms of intracranial hypertension: 17 cases had headache and papilledema. Examination All the patients were checked by MRI, and 140 of them were checked by CT, including transmeatus CT, at the same time. CT showed rounded or lobulated masses with relatively clear boundaries. MRI showed that tumors grew towards the cerebellopontine angle area centered by the orifice of the internal acoustic meatus (IAM). The bony internal acoustic meatus usually expanded with a funnel shape. Based on Samii s classification, and according to the maximal diameter of the tumor at the cerebellopontine angle, tumors were divided into large tumors, if the size was more than 3 cm in diameter, in 119 cases, and middle or small tumors in 26 cases. Surgical technique Monitoring and exposure of the tumor All the tumors were removed by a suboccipital retrosigmoid transmeatus approach which has not changed over the years. 3-5 Eighty-one of 145 cases were settled in the lateral position and the other 64 cases in the semi-sitting position. A 2.5 cm 3.0 cm surgical bone window was made with exposing the connection of transverse sinus and sigmoid sinus (Figure 1). We investigated the usefulness of a microscope-based navigation system (Zeiss, NC-4, Germany) in conjunction with a neuronavigational system (BrainLab, Germany) based on three-dimensional volumetric imaging for the removal of the posterior wall of the IAM. Intraoperative monitoring included ECG, Brainstem auditory evoked potentials (BAEPs), continuous measurement of arterial blood pressure and superior vena cava pressure. EMG recordings of the orbicularis oris and oculi muscles were used to monitor facial nerve function. A bipolar stimulus with an intensity of 1 ma and duration of 0.1 millisecond was used to assess facial nerve response. Facial nerve and BAEP monitoring was used in all cases. After dissecting the dura by scissors and suspending it, an operative microscope was settled. First, the cisterna magna and the pericisterna of the bulbus venae jugularis were opened and we discharged the cerebrospinal fluid, resulting in cerebellum rebound. Then we adjusted an ecarteur to pull the cerebellar hemisphere backwards. At this time two layers of arachnoid covering the tumor s surface could be seen. Careful attention was paid so that the function of the brain spatula was to protect the cerebellum but not drag it. Tumor isolation A large vestibular schwannoma compressed the brain stem and cerebellum resulting in disfiguration and displacement. There was a layer of arachnoid that remained between them and tumor all the time. Even if the tumor grew into brain stem or cerebellum it was still located outside of the intrinsic arachnoid of the brain tissue. It was the anatomic base of total operative removal of the vestibular schwannoma. First we separated the lower pole of the tumor between the two layers of arachnoid, then cut the arachnoid which covered the IX, X, XI cranial nerves. We found the anterior inferior cerebellar artery, posterior inferior cerebellar artery and their branches, then isolated them from the tumor wall. The petrosal vein and the trigeminal nerve were protected. After separating a part of the tumor wall, the capsule was dissected dorsally. The mass of the tumors were excised piece by piece using Cavitron Ultrasonic Surgical Aspirator (CUSA). This directly exposed the internal acoustic meatus of the middle or small vestibular schwannoma (Figure 2). We drilled out the posterior wall of the internal acoustic meatus first, found the facial nerve, then isolated the tumor and its wall. We did not attempt to see the whole tumor at the early exposure. Tumor removal We made sure by monitoring facial nerve during the operation that the facial nerve was not in the tumor capsule which was prepared to be cut open. The capsule was cut after fulgerize. For tumors smaller than 3 cm in diameter, the internal auditory canal was drilled open first. After removing part of the tumor and separating the facial nerve and vestibulocochlear nerve in the internal auditory canal, the intracranial part of the tumor was then removed. For tumors larger than 3 cm in diameter we excised the intracranial part of the tumor in partitions within the capsule first. After most of the tumor was decompressed the internal auditory canal was drilled open to identify the facial nerve. The residual tumor was removed and the facial nerve was dissected and preserved. CUSA was useful for the excision inside the tumor. We then separated the tumor wall and the surrounding adhesive tissues carefully along the arachnoid on the tumor surface. It was most important to protect the integrity of the envelope lest the unidentified nerves and vessels were damaged. We covered the tumor with cotton shields to protect the brain stem, cerebellum, blood vessels and cranial nerves around the tumor. The small artery branches that penetrated the tumor could be fulgurated

3 276 Figure 1. Suboccipital retrosigmoid transmeatus approach with small craniotomy. Figure 2. Drill and open the posterior wall of internal acoustic meatus. Figure 3. Separate the tumor in internal acoustic meatus and find the facial nerve. Figure 4. To distinguish the courser direction of facial nerve. Figure 5. Preoperative MRI image in transverse section. Figure 6. Postoperative MRI image in transverse section. Figure 7. Preoperative MRI image in coronary section. Figure 8. Postoperative MRI image in coronary section. and cut off, or be separated and preserved. Tearing open the tumor envelope can damage the facial nerve. On the upper pole of the tumor the trigeminal nerve and petrosal vein must be protected. A nerve usually forms a layer of vitreous membrane on the lateral tumor wall. It s an advisable decision not to attempt to separate the nerve at the broad and thin site. Delay further separation temporarily and concentrate your energy on a site that is easier to be separated. As the facial nerve was obviously stretched, it was generally not hard to do that. Managements of the tumor in the internal acoustic meatus We decided whether to decompress the inside of the tumor or to drill the internal acoustic meatus first based upon tumor size and adhesion to the brain stem and cranial nerves. If it was a middle or a small size tumor, or the tumor adhered closely to the brain stem, or it was difficult to identify or isolate the cranial nerves, the posterior wall should be drilled open first (Figure 2). Intraoperatively, the dura from the posterior IAM was removed, and the bone here was drilled away with a high-speed diamond burr under continuous saline solution irrigation until the intrameatal tumor extension was exposed. We separated the tumor in the internal acoustic meatus, found the facial nerve (Figure 3), identified the direction in which the facial nerve lays (Figure 4), then excised and separated the tumor exposing the cerebellopontine angle (CPA) area. The tumor was then separated from the cistern segment of the facial nerve and finally we cleaned up the remaining tumor on the facial nerve. If the tumor was too large to let the internal acoustic meatus be revealed, it should first be excised in partitions from the inside, then we opened the internal acoustic meatus. After identifying the direction in which the facial nerve lay, we separated and excised the tumor in the cerebellopontine angle cistern. When drilling off the bone the depth to which the posterior wall of the internal acoustic meatus was drilled should be proper. If it was not drilled deep enough the tumor in the internal acoustic meatus would be insufficiently revealed. If it was drilled excessively, it was possible to damage the lateral semicircular duct and even the jugular bulb and the danger of a cerebrospinal rhinorrhea occurring was increased. The depth we drilled to was 4 7 mm. There was a need to avoid the secondary damage to facial and vestibulocochlear nerves caused by excessive electric coagulations and by blindly clamping. It must be remembered to keep any cotton out when using a high-speed electric drill. A little cotton involved would result in irreversible injury. Facial nerve preservation Early identification of the facial nerve medially facilitated continued medial-to-lateral dissection of the tumor from the nerve and other neural structures. Facial nerve stimulation-monitoring was performed whenever further visualization of the nerve at different angles was necessary. Once the tumor had been completely removed, the facial nerve was routinely stimulated at its root, most proximal to the brainstem. If no response was obtained the stimulus intensity was increased by 0.1-mA increments until a response was obtained. If the intensity was increased to 0.4 ma and no response was obtained, the facial nerve was tested again throughout its course to the lateral IAM to determine the site of injury. The facial

4 Chinese Medical Journal 2010;123(3): nerve could be involved with the tumor, either by passing through it or being infiltrated by it. Intraoperative monitoring was performed on the patient who was anesthetized with or without a mild relaxant medication. After opening the IAM the intrameatal tumor extension was inspected and the position of the seventh and eighth nerves was determined by their electrophysiological response. Continuous electromyographic recording provided the feedback for identifying all the segments of the facial nerve. Nerve continuity could be confirmed by light touching or by electrical stimulation of the nerve. Follow-up Contrast-enhanced MRI must be performed within 3 days of the operation. Audiometry and CT with a bony window were performed 1 week after surgery. Estimation of the outcome was based primarily on the surgeon s perspective. All patients were evaluated with MRI/CT follow-up from 3 months to 1 year after operation. For evaluation we checked for facial symmetry at rest and in motion, separate movements of all three segments (forehead, eyes and mouth), and incidence of conjunctivitis and synkinesias, then graded the results by the H-B scale. If the patient could not come to the hospital a questionnaire was mailed to him/her 1 year postoperatively, questioning his/her perception of the functional recover, as compared with the clinician s observation. RESULTS General conditions after operation Excellent: 115 patients had no neural dysfunctions (not including the functions of facial and vestibulocochlear nerves). Good: the 15 remaining cases had different degrees of neural dysfunctions such as abduction dysfunction, defective coordination of the extremities, dysarthria or hoarseness. There were no severe neural dysfunctions or operative deaths. Degree of the tumor removal Complete resection was accomplished in 140 cases, accounting for 96.6% (Figures 5 8). Subtotal resection was done in 5 cases, where tumors closely adhered to nerves or the brain stem. Changes in heart rate and blood pressure were frequently seen during isolation of tumor near the brain stem, or tumor adhering too closely to facial and vestibulocochlear nerves that had to be separated. If separated by force the facial nerve function would be impaired. So subtotal resection had to be performed. Facial nerve anatomy and functional preservation The facial nerve was anatomically preserved in 132 cases accounting for 87.8%. The facial nerve could not be preserved in 13 cases because it was encapsuled by the tumor or was adhered much too closely with the tumor; Seven of the 13 cases were performed with end-to-end anastomosis of the facial nerve. One hundred and fifteen cases had functional preservation of the facial nerve (H-B grades I and II), accounting for 79.3%. Postoperative complications Subcutaneous effusion occurred in 3 cases, edema was seen after a cerebellar contusion in 5 cases, and aggravation of facial nerve dysfunction was seen in 12 cases. Temporary dysfunction of the posterior group of cranial nerves was seen in 3 cases and intracranial air occurred in 2 cases (semi-sitting position operation). DISCUSSION Anatomy of internal acoustic meatus The internal acoustic meatus is 4 mm in diameter and its overall length is about (9.9±0.9) mm in Chinese patients. Being restricted from the operation angle, the mean largest drilling range is 7.09 mm and the mean longest posterior wall of the internal acoustic meatus being drilled open is near 2/3 of the total length. 6,7 But the lengths of the posterior wall all vary over a large range. The variable range of the distance between the common crus and internal acoustic meatus is also large. The common crus is easily damaged when the posterior wall of the internal acoustic meatus is drilled open, which can lead to hearing loss. The distance between the jugular bulb and internal acoustic meatus varies a lot. We met a patient with a high jugular bulb. The jugular bulb intruded on the petrosal bone and reached up to the level of internal acoustic meatus. Because of the big variation of the petrosal bone and the close relationship between the posterior wall of the internal acoustic and bony canals of the ear, the vestibulum and the mastoid air cell may intrude on the posterior wall of the internal acoustic meatus and the jugular bulb may also reach the level of the internal acoustic meatus. Thus, thin layer CT scan (include a bony window image) on the petrosal bone, measuring the angle and length of internal acoustic meatus, the gasfication of the petrosal bone, the positions of cochlea, semicircular canals and jugular bulbs should be done regularly before the operation. The drilling range of the internal acoustic meatus needs to be decided according to the relationship between the internal acoustic meatus and the surrounding structures to direct the safe drilling of the posterior wall of the internal acoustic meatus. This has a great value in protecting the functions of the facial nerve in the internal acoustic meatus. 8 Some researchers consider that solid tumors easily cause internal acoustic meatus changes. And a case that has internal acoustic meatus changes will result in reduced function of facial and vestibulocochlear nerves after the operation. 9 The length of the drilled posterior wall of the internal acoustic meatus should be proper. If the IAM is drilled insufficiently, the tumor in it will not be revealed. If the IAM is drilled excessively, it is possible to damage the lateral semicircular canal and the danger of cerebrospinal rhinorrhea increases. The range we drilled is generally 6 7 mm. We used biogel or muscles to block the internal acoustic meatus to avoid the

5 278 leakage of cerebrospinal fluid. But if the wadding is too excessive it can easily form scar tissue in the internal acoustic meatus. This can lead to nerves and blood vessels compressed and delayed neural disfunction after the operation. The arachnoid of the internal acoustic meatus is the continuation of the arachnoid of the cerebellopontine angle cisterna. It reaches the bottom of the internal acoustic meatus. There is no arachnoid to separate the tumor from nerves. 10 The nerves in the internal acoustic meatus are enwrapped by the arachnoid. When vestibular schwannoma occurres, the entrance of internal acoustic meatus is usually enlarged due to long time compression by the tumor. When drilling opens the posterior wall of internal acoustic meatus, the drilling range is limited to revealing the extrinsic pole of the tumor and it s not necessary to drill to the bottom of internal acoustic meatus. When vestibular schwannoma occurs, the facial nerve near the internal acoustic pore can be flattened, elongated and squeezed in different directions. Sometimes it is difficult to differentiate it from the tumor wall. Removal of the tumor in internal acoustic meatus The tumor adheres to the entrance of internal acoustic meatus. The tumor in the internal acoustic meatus usually is excised last, lest the facial nerve is impaired by the traction due to the gravitational effect of the tumor. 11 The excision of the tumor in the internal acoustic meatus depends on the adequate stripping of the posterior wall of the internal acoustic meatus. One case with high jugular bulbs in this group did not have the internal acoustic meatus drilled out sufficiently due to hemorrhage during the stripping. Examination after the operation showed that tumor remained in the internal acoustic meatus. We need to recognize the posterior border of the entrance of the internal acoustic meatus first. Before drilling to open the internal acoustic meatus, the operative field should be checked. No cotton shield should be left in the field. The drilled width and depth of the posterior wall of the internal acoustic meatus should be large enough to adequately expose the tumor in the internal acoustic meatus. The facial nerve is usually justled and forms an angle on the anterior border of the entrance of the internal acoustic meatus. Sometimes it is difficult to differentiate it from the tumor wall. The cutting direction is parallel to the longitudinal axis of the internal acoustic meatus. The tumor in the internal acoustic meatus was separated carefully. The benmost border of the tumor was found and separated from the facial nerve. At this time the remaining tumor tissues was along the interface between the facial nerve and tumor capsule (Figure 4). In order to excise the tumor in the internal acoustic meatus and protect the facial vestibulocochlear nerve during the operation, we should stay as flush as possible when drilling off the posterior lip of the internal acoustic meatus lest the heat conduction impairs the facial nerve. After drilling open the posterior wall of the internal acoustic meatus, we should longitudinally cut open the cerebral dura mater in the internal acoustic meatus. Thus the tumor in the internal acoustic meatus can be excised in an open field. The segment of the facial nerve in the internal acoustic meatus is not closely adhered to the tumor and the tumor can be separated from the facial nerve surface gently with a detacher. The internal auditory artery should be preserved during the operation. We should not use bipolar electrocoagulation when dealing with the tumor in the internal acoustic meatus especially when the facial nerve adheres to the tumor closely. This is advantageous to keep its blood supply. It is of the same importance to preserve the internal auditory artery as the cochlear nerve. They are the preconditions for keeping auditory acuity. Recognize and protect the facial nerve The operation was performed by small suboccipital retrosigmoid craniotomy for all patients. This approach for small vestibular schwannomas (VS) yields a high rate of facial function preservation 12 and combined with auditory monitoring for a small vestibular schwannoma, can accomplish total tumor removal with an excellent hearing preservation rate. 13 The principle of the vestibular schwannoma operation is to separate the tumor from the neural tissue, not to dissect the neural tissue from the tumor. The anatomic preservation of the facial nerve is the foundation to preserve the neural functions of the facial nerve. It is most important to recognize in time the exact pathologicoanatomic relationship between the facial nerve and the tumor during the operation. There are two layers of arachnoid on the tumor surface. The outer layer is the arachnoid of the cerebellopontine angle cistern and the inner layer is the arachnoid on the vestibulocochlear nerve surface. The tumor lies outside the arachnoid and the facial and vestibulocochlear nerve lie under the arachnoid. This is the anatomic foundation to preserve the anatomic structure of the facial nerve: keeping the arachnoid integrated as much as you can and separating between the two layers of the arachnoid. It is profitable to separate the adhesions between the tumor and surrounding structures, preserve the facial nerve and decrease hemorrhage. Our experience is to excise the main body of the tumor intratumorally at first but keep the capsule wall integrated, then excise the tumor on the upper pole, lower pole and in medial, in the internal acoustic meatus. When separating the tumor capsule, we should first find the brain stem end of the facial nerve which is not adhesive to the tumor, sharp separate it gradually along the lying direction after confirmation, recognize the internal auditory meatus segment of the facial nerve which does not adhere to the tumor in the internal acoustic meatus, turn over the tumor capsule gradually and excise it in partition. It is an effective method to acquire the anatomic preservation of the facial nerve. Whether to separate it with a blunt or sharp knife when the tumor is closely adhesive to the facial nerve, different authors have different standpoints. We can

6 Chinese Medical Journal 2010;123(3): prevent more impairment of the facial nerve when separating it in the neural axial direction from the normal area to the abnormal area using a blunt-sharp combined method. It can prevent the facial nerve from further damage. Under normal conditions the facial nerve and vestibulocochlear nerve accompany each other. Facial nerve lies forward and upward and vestibulocochlear nerve lies backward and downward. Large vestibular schwannomas change the direction of the facial nerve and prolong it even to 3 5 cm and push it forward and downward or forward and upward of the cerebellopontine angle. 14,15 The facial and vestibulocochlear nerves are often compressed to a thin film. The arachnoid adheres to the tumor closely. It is hard to find the facial nerve even use a microscope. It is very difficult to keep the arachnoid integrated at this time. It requires the assistance of neural monitoring to identified the tumor wall and facial nerve at this point, then separate them. Facial nerve electrostimulation can help find the direction of the facial nerve. We should first accurately locate the direction of the facial nerve, after confirming there is no facial nerve involved, separate the arachnoid on the tumor wall towards the upper and lower poles and keep the arachnoid integrated. Samii et al 16 reported an anatomical nerve preservation rate of 98.5% overall, and 100% in smalland middle-sized vestibular schwannomas. Sasaki et al 17 analyzed the tumor capsule and the tumor-nerve interface in vestibular schwannomas through histological observations, and found the so-called tumor capsule in vestibular schwannomas is the residual vestibular nerve tissue itself, consisting of the perineurium and underlying nerve fibers. Complete tumor resection can be achieved by removal of both tumor parenchyma and tumor capsule when a clear border between the tumor capsule and facial or cochlear nerve fibers can be identified intraoperatively. Conversely, when a severe adhesion between the tumor and facial or cochlear nerve fibers is observed dissection of the vestibular nerve-tumor interface is recommended for preservation of the functions of these cranial nerves. Kanno et al 18 paid attention to the important anatomical areas so called last 10 mm: depends on the type of pathological change that the intraarachnoidal portion of the nerve has undergone. In fact, the tumor is not completely covered by arachnoid. That is particularly obvious between the tumor and seventh and vestibulocochlear nerves near to the internal acoustic meatus, where the arachnoid is absent. So the last 10 mm is very important for the preservation of the function of facial nerve and cohlear nerve. Finally, the majority of vestibular schwannomas smaller than 3 cm may be considered to be treated by radiosurgery or a combination of both according to the opinions of some authors, but we still do not have enough long term results from stereotactic radiosurgery Given the use of microsurgery and facial EMG monitoring, the basic goal of contemporary vestibular schwannoma therapy should be the complete tumor removal along with preservation of facial nerve function. The goal of vestibular schwannoma treatment should be taken according to tumor size. But even in large vestibular schwannomas, preservation of facial nerve function (H-B grades I or II) should be prioritized over total resection. For tumors larger than 3 cm, the goal of low morbidity and maintenance of normal facial nerve function can be attained with the retrosigmoid transmeatal approach, refined microsurgical technique, and intraoperative facial nerve monitoring. REFERENCES 1. Pellet W. History of vestibular schwannoma surgery. Prog Neurol Surg 2008; 21: Samii M, Gerganov V, Samii A. Hearing preservation after complete microsurgical removal in vestibular schwannomas. Prog Neurol Surg 2008; 21: Samii M, Matthies C. Management of 1000 vestibular schwannomas (acoustic neuromas): the facial nerve preservation and restitution of function. Neurosurgery 1997; 40: Gharabaghi A, Samii A, Koerbel A, Rosahl SK, Tatagiba M, Samii M. Preservation of function in vestibular schwannoma surgery. Neurosurgery 2007; 60 (2 Suppl 1): Mistra B. Modern management protocol and outcome in acoustic neuroma. Asian J Neurosurg 2007; 1: Jiang T, Wang CC, Yu CJ, Liu JX, Chen F, Wang FM. Drill open the posterior wall of internal acoustic meatus with suboccipital retrosigmoid approach and protect bony labyrinths. Chin Neurosurg (Chin) J 1999; 15: Jia W, Yu CJ, Wang FM, Chen F. Research on suboccipital retrosigmoid transmeatus approach microanatomy. J Cap Med Univ (Chin) 2004; 25: Yokoyama T, Uemura K, Ryu H, Hinokuma K, Nishizawa S, Yamamoto S, et al. Surgical approach to the internal auditory meatus in acoustic neuroma surgery: significance of preoperative high-resolution computed tomography. Neurosurgery 1996; 39: Yamakami I, Uchino Y, Kobayashi E, Saeki N, Yamaura A. Prognostic significance of changes in the internal acoustic meatus caused by vestibular schwannoma. Neurol Med Chir 2002; 42: Lian YZ, Liu EZ, Li JS. Dissect and observe the arachnoid in internal acoustic meatus and observe its relationship with the acoustic neuroma. Chin Microinv Neurosurg J (Chin) 2004; 9: Yu CJ, Wang CC, Guan SS, Jiang T. Discussion on the excision of acoustic neuroma and facial nerve reservation. Chin J Neurosurg (Chin) 2001; 17: Yang J, Grayeli AB, Barylyak R, Elgarem H. Functional outcome of retrosigmoid approach in vestibular schwannoma surgery. Acta Otolaryngol 2008; 128: Yamakami I, Yoshinori H, Saeki N, Wada M, Oka N. Hearing preservation and intraoperative auditory brainstem response and cochlear nerve compound action potential monitoring in the removal of small acoustic neurinoma via the retrosigmoid

7 280 approach. J Neurol Neurosurg Psychiatry 2009; 80: Koos WT, Day JD, Matula C, Levy DI. Neurotopographic considerations in the microsurgical treatment of small acoustic neurinomas. J Neurosurg 1998; 88: Chen LH, Liu YS, Chen L, Liu ZX, Yang ZQ, Xu LX, et al. Operation and pathoanatomy of large acoustic neuroma. Chin Microinv Neurosurg J (Chin) 2004; 9: Samii M, Gerganov V, Samii A. Improved preservation of hearing and facial nerve function in vestibular schwannoma surgery via the retrosigmoid approach in a series of 200 patients. J Neurosurg 2006; 105: Sasaki T, Shono T, Hashiguchi K, Yoshida F, Suzuki SO. Histological considerations of the cleavage plane for preservation of facial and cochlear nerve functions in vestibular schwannoma surgery. J Neurosurg 2009; 110: Kanno T, Karagiozov KL. Preservation of facial and cochlear nerve in cerebellopontine angle (CPA) neurinoma surgery- our current surgical technical considerations (technical note). Asian J Neurosurg 2007; 1: Lunsford LD, Niranjan A, Flickingsr JL, Maitz A, Kondziolka D. Radiosurgery of vestibular schwannoma: summary of experience in 829 cases. J Neurosurg 2005; 102 (Suuppl): Rowe J, Grainger A, Walton L, Silcocks P, Radatz M, Kemeny A. Risk of malignancy after Gamma knife stereotactic radiosurgery. Neurosurgery 2007; 60: (Received August 8, 2009) Edited by WANG De