Schwannoma of the intermediate nerve

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1 J Neurosurg 109: , 2008 Schwannoma of the intermediate nerve Case report CHRISTIAN SCHELLER, M.D., 1 JENS RACHINGER, M.D., 1 JULIAN PRELL, M.D., 1 MALTE KORNHUBER, M.D., 2 AND CHRISTIAN STRAUSS, M.D. 1 Departments of 1 Neurosurgery and 2 Neurology, Martin-Luther University of Halle-Wittenberg, Halle, Germany The intermediate nerve is seldom identified as the site of tumor origin in cerebellopontine angle schwannomas. A 29-year-old man presented with a 6-month history of slowly progressive hearing loss and dizziness; facial nerve weakness was not observed clinically. Magnetic resonance imaging revealed a tumor in the left cerebellopontine angle region extending up to the geniculate ganglion and along the course of the superficial petrosal nerve. A CT scan showed enlargement of the facial nerve canal. Microsurgery was performed via an extended retrosigmoid approach. Intraoperative and electrophysiological findings identified the intermediate nerve as the site of tumor origin. (DOI: /JNS/2008/109/7/0144) KEY WORDS cerebellopontine angle facial nerve intermediate nerve neurophysiological monitoring schwannoma vestibular nerve S CHWANNOMAS not arising from the vestibulocochlear nerve bundle are rare in CPA surgery. Authors of reports of large surgical series have noted that between 1.9 and 2.7% of tumors originate from the facial nerve. 21 Facial nerve paresis and hearing disturbances are the common symptoms of facial nerve schwannomas. 9,10,17 Preoperatively, it can be difficult to distinguish facial nerve schwannomas of the CPA from vestibular schwannomas, unless pronounced facial weakness or typical imaging signs are present. 4,6 8 Intermediate nerve schwannomas have not been studied in surgical series, and so far only 2 case reports have been published. 7,17 Case Report Abbreviations used in this paper: CPA = cerebellopontine angle; EMG = electromyography. History and Examination. This 29-year-old man presented to our institution with a 6-month history of progressive leftsided hearing loss and dizziness. Pure-tone audiometry and speech discrimination testing revealed a high-frequency hearing loss pattern and affected word discrimination classified as Hearing Class B. 1 On examination, facial nerve function was normal, but EMG revealed fibrillation potentials in the orbicularis oris and nasalis muscles distinct evidence of chronic damage. The Schirmer tear test and blink reflex test revealed normal findings, and the patient s senses of smell and taste were unimpaired. Contrast-enhanced T1-weighted MR imaging demonstrated a homogeneous, contrast-enhancing tumor in the left CPA region extending up to the geniculate ganglion and along the course of the superficial petrosal nerve (Fig. 1). A bone window CT scan showed pathological enlargement of the facial nerve canal up to the second genu of the facial nerve and an enlarged internal auditory meatus. Because the patient s symptoms had intensified and the tumor showed evidence of progression during the last 4 months preoperatively, surgery was undertaken. Operation. Microsurgery was performed via a retrosigmoid approach with the patient placed supine. Electrophysiological monitoring was performed using brainstem auditory evoked potentials and facial nerve EMG with 4 pairs of electrodes in each of the orbicularis oris, orbicularis oculi, and nasalis muscles. After opening the arachnoid membrane of the lateral cerebellopontine cistern, and after minimal retraction of the cerebellum, the tumor was exposed. The tumor capsule was mapped, initially using 0.5-mA constant-current bipolar stimulation (Neurosign 100, Inomed). On the posterior cranial capsule beside the superior 144 J. Neurosurg. / Volume 109 / July 2008

The tumor progressed along the course of the petrosus major nerve (D). J. Neurosurg.

2 Schwannoma of the intermediate nerve FIG. 1. Axial (A C) and sagittal (D) contrast-enhanced T1-weighted MR images showing a lesion in the CPA region extending into the base of the middle cranial fossa. An enlarged facial canal is visible (B). The tumor progressed along the course of the petrosus major nerve (D). J. Neurosurg. / Volume 109 / July 2008 vestibular nerve, EMG signals could be elicited from the orbicularis oris muscle with as little as 0.05 ma (Figs. 2 and 3). After tumor debulking and dissection of the superior vestibular nerve, a small nerve running parallel and anterior to the superior vestibular nerve was clearly detected as the site of tumor origin (Fig. 4). The facial nerve itself was localized in the area of the anterior cranial tumor capsule. Its electrical stimulation revealed EMG responses in all 3 branches (orbicularis oculi, nasalis, and orbicularis oris muscles) (Fig. 5). The facial nerve within the CPA had no contact with the tumor. It became attached to the capsule at the premeatal segment. The posterior meatal lip and the petrous bone between meatus and temporal base were drilled. After intracanalicular tumor resection, facial nerve function deteriorated, manifesting in the increasing proximal/distal ratio of EMG amplitudes. A trains were not observed. 12,14 Subsequently, capsule remnants were left at the geniculate ganglion to preserve the anatomical course of the facial nerve. The cochlear nerve was preserved, but brainstem auditory evoked potentials showed a slowly progressive deterioration of amplitudes. Histopathological Examination and Postoperative Course. Histopathological examination of the tumor revealed a schwannoma (World Health Organization Grade I). Postoperative vasoactive intravenous treatment consisting of hydrohyethyl starch and nimodipine was administered for 10 days. 3,15,16,20 Facial nerve function deteriorated to House Brackmann Grade IV. 5 Postoperative audiometry revealed FIG. 2. Intraoperative photograph demonstrating the area of positive electrical response (dotted line) exclusively in the orbicularis (orb) oris muscle. The facial nerve is seen anteriorly and superiorly. M = muscle; VII = facial nerve. 145

C. Scheller et al. FIG. 3. Traces (4 electrodes per branch) of the results of electrical stimulation (0.05 ma) of the intermediate nerve with motor response in the orbicularis (Orb.) oris muscle.

3 C. Scheller et al. FIG. 3. Traces (4 electrodes per branch) of the results of electrical stimulation (0.05 ma) of the intermediate nerve with motor response in the orbicularis (Orb.) oris muscle. Nasal = nasalis. Hearing Class C. 1 At the 4-month follow-up the patient s hearing remained Hearing Class C, and facial nerve function improved to House Brackmann Grade III with clear EMG evidence of recovery in all branches. The patient s chief complaint was a metallic taste sensation. Discussion The intermediate nerve is a relatively small nerve with a variable anatomical course and is usually not addressed in the pertinent literature. Recently, authors of anatomical studies on the intermediate nerve have described different sites of origin and variable numbers of rootlets. 11 The intermediate nerve can be divided into the following 3 parts: a proximal segment from the brainstem to the vestibulocochlear nerve, an intermediate segment coursing through the CPA together with the superior vestibular nerve, and a distal segment joining the facial nerve within the internal auditory canal. 7,13 Similarly, a split facial nerve course was recently observed in vestibular schwannomas. 19 The minor portion was described as running parallel to the superior vestibular nerve on the cranial tumor capsule, rejoining the major portion at the level of the porus. We identified the tumor origin in the present case based on the absence of facial nerve paresis preoperatively, the imaging exclusion of a diagnosis of vestibular schwannoma, and electrophysiological test results. Preoperative EMG revealed fibrillation potentials in the orbicularis oris and nasalis muscle, an indication of partial denervation. Direct intermediate nerve stimulation has resulted in EMG activity in the orbicularis oris muscle. 2 Modified 3-channel facial EMG may be a tool to reliably identify the intermediate nerve during surgery. In the present case, intraoperative electrical stimulation revealed that the suspected intermediate nerve carried fibers exclusively to the orbicularis oris muscle, whereas the major portion of the facial nerve FIG. 4. Intraoperative photograph of the dissected superior vestibular nerve (N. vest. sup.) showing the intermediate nerve (N. intermedius) clearly as the site of tumor origin. 146 J. Neurosurg. / Volume 109 / July 2008

Schwannoma of the intermediate nerve FIG. 5. Traces (4 electrodes per branch) of the results of electrical stimulation (0.05 ma) of the facial nerve with motor response in all 3 branches.

4 Schwannoma of the intermediate nerve FIG. 5. Traces (4 electrodes per branch) of the results of electrical stimulation (0.05 ma) of the facial nerve with motor response in all 3 branches. supplied all 3 branches of the facial nerve. 18,19 The study by Ashram et al., 2 the intraoperative observations of Strauss and colleagues, 18,19 and the intraoperative direct nerve stimulation in the present case revealed EMG activity exclusively in the orbicularis oris muscle. In conclusion, we can infer from the studies by Ashram et al. and Strauss et al. that it is possible to detect the intermediate nerve during surgery using selective direct nerve stimulation and multichannel recordings. Consequently, some facial nerve schwannomas might actually arise from the intermediate nerve. 2,19 References 1. Anonymous: Committee on Hearing and Equilibrium guidelines for the evaluation of hearing preservation in acoustic neuroma (vestibular schwannoma). American Academy of Otolaryngology-Head and Neck Surgery Foundation, Inc. Otolaryngol Head Neck Surg 113: , Ashram YA, Jackler RK, Pitts LH, Yingling CD: Intraoperative electrophysiological identification of the nervus intermedius. Otol Neurotol 26: , Bischoff B, Romstöck J, Fahlbusch R, Buchfelder M, Strauss C: Intraoperative brainstem evoked potential pattern and perioperative vasoactive treatment for hearing preservation in vestibular schwannoma surgery. J Neurol Neurosurg Psychiatry: , 2007 J. Neurosurg. / Volume 109 / July Fagan PA, Misra SN, Doust B: Facial neuroma of the cerebellopontine angle and the internal auditory canal. Laryngoscope 103: , House JW, Brackmann DE: Facial nerve grading system. Otolaryngol Head Neck Surg 93: , King TT, Morrison AW: Primary facial nerve tumors within the skull. J Neurosurg 72:1 8, Kudo A, Suzuki M, Kubo N, Kuroda K, Ogawa A, Iwasaki Y: Schwannoma arising from the intermediate nerve and manifesting as hemifacial spasm. J Neurosurg 84: , Lee KS, Britton BH, Kelly DL Jr: Schwannoma of the facial nerve in the cerebellopontine angle presenting with hearing loss. Surg Neurol 32: , Lipkin AF, Coker NJ, Jenkins HA, Alford BR: Intracranial and intratemporal facial neuroma. Otolaryngol Head Neck Surg 96: 71 79, O Donoghue GM, Brackmann DE, House JW, Jackler RW: Neuromas of the facial nerve. Am J Otol 10:49 54, Oh CS, Chung IH, Lee KS, Tanaka S: Morphological study on the rootlets comprising the root of the intermediate nerve. Anat Sci Int 78: , Prell J, Rampp S, Romstöck J, Fahlbusch R, Strauss C: Train time as a quantitative electromyographic parameter for facial nerve function in patients undergoing surgery for vestibular schwannoma. J Neurosurg 106: , Rhoton AL Jr, Kobayashi S, Hollinshead WH: Nervus intermedius. J Neurosurg 29: , Romstöck J, Strauss C, Fahlbusch R: Continuous electromyo- 147

5 C. Scheller et al. graphic monitoring of motor cranial nerves during cerebellopontine angle surgery. J Neurosurg 93: , Scheller C, Richter HP, Engelhardt M, König R, Antoniadis G: The influence of prophylactic vasoactive treatment on cochlear and facial nerve functions after vestibular schwannoma surgery: a prospective and open-label randomized pilot study. Neurosurgery 61:92 98, Scheller C, Strauss C, Fahlbusch R, Romstöck J: Delayed facial nerve paresis following acoustic neuroma resection and postoperative vasoactive treatment. Zentralbl Neurochir 65: , Sherman JD, Dagnew E, Pensak ML, van Loveren HR, Tew JM Jr: Facial nerve neuromas: report of 10 cases and review of the literature. Neurosurgery 50: , Strauss C: The facial nerve in medial vestibular schwannomas. J Neurosurg 97: , Strauss C, Prell J, Rampp S, Romstöck J: Split facial nerve course in vestibular schwannomas. J Neurosurg 105: , Strauss C, Romstöck J, Fahlbusch R, Rampp S, Scheller C: Preservation of facial nerve function after postoperative vasoactive treatment in vestibular schwannoma surgery. Neurosurgery 59: , Symon L, Cheesman AD, Kawauchi M, Bordi L: Neuromas of the facial nerve: a report of 12 cases. Br J Neurosurg 7:13 22, 1993 Manuscript submitted August 4, Accepted October 8, Address correspondence to: Christian Scheller, M.D., Department of Neurosurgery, Martin-Luther University of Halle-Wittenberg, Ernst-Grube-Strasse 40, Halle, Germany. christian. scheller@medizin.uni-halle.de. 148 J. Neurosurg. / Volume 109 / July 2008

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