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Auditory and vestibular disorders
Korean Journal of Audiology 2011;15(3):147-151.
A Case of Facial Nerve Schwannoma Presenting as an External Auditory Canal Mass.
Da Jung Jung, Jun Ho Seok, Kyu Yup Lee, Sang Heun Lee
Department of Otorhinolaryngology-Head and Neck Surgery, Kyungpook National University Hospital, School of Medicine, Kyungpook National University, Daegu, Korea. leeshu@knu.ac.kr
Abstract
Facial nerve schwannoma is a rare benign tumor that arises from the Schwann cell sheath of facial nerve. Although the tumor can occur anywhere along the course of the peripheral nerve, it is frequently present as an internal auditory canal mass in the head and neck region. We experienced a rare case of facial nerve schwannoma on the mastoid segment, which was presented as an external auditory canal mass in a 24-years-old man. The lesion was removed via the transmastoid approach and the facial nerve was grafted using the greater auricular nerve. The patient's facial nerve function was preserved postoperatively as House-Brackmann grade III.
Keywords: Nerve sheath neoplasms;Neurilemmoma;Facial nerve

Address for correspondence : Sang Heun Lee, MD, Department of Otorhinolaryngology-Head and Neck Surgery, Kyungpook National University Hospital, School of Medicine, Kyungpook National University, 130 Dongdeok-ro, Jung-gu, Daegu 700-712, Korea
Tel : +82-53-420-5785, Fax ; +82-53-423-4524, E-mail : leeshu@knu.ac.kr

Introduction


Benign tumors, which originate from the peripheral nerve sheath that surrounds the axons of peripheral nerves, can be divided into two major groups: schwannoma and neurofibroma.1) Schwannoma is a tumor that originates from Schwann cells in the peripheral neurilemma, and approximately 25 percent of these tumors are known to occur in the head and neck region.2,3) The prevalence rate of facial nerve schwannomas is 1/23,000,4) and facial nerve schwannomas on the external temporal bone have been reported in 15% of all cases.5) The occurrence of facial nerve schwannomas in the middle ear and mastoid cavity is relatively rare. We report here on a case of schwannoma that originated from the mastoid segment of the facial nerve, which was presented as external auditory canal mass.

Case Report

A 24-year-old male patient presented to us a symptom in the right ear fullness, which had been occurring for a year. The ear fullness was felt only in the morning at first, but the pain had become severe in the last two to three months ago, and lasted all day. When he visited our hospital, there was a House- Brackmann Grade II facial nerve paralysis, and polyp-formed mass with a smooth surface that had filled the right external auditory canal. The eardrum could not be found due to the mass (Fig. 1). In pure-tone audiometry of 500 Hz, 1,000 Hz and 2,000 Hz, the right ear had an air conduction threshold of 35 dB HL and bone conduction threshold of 5 dB. In addition, the air-bone gap was 30 dB. In the computed tomography (CT) scan of the temporal bone, a shade of soft tissue that filled some of external auditory canal and mastoid was connected to the stylo-mastoid foramen. There were defects on the posterior wall of the external auditory canal due to the mass, but there were no specific ossicles and tympanic segment found on the facial nerve (Fig. 2). In the magnetic resonance image (MRI) T1 weighted image, the same relative signal strength and contrast enhancement with the brain parenchyma was observed and the mass, which was 1.3×1.4 cm, was connected to the part of the mastoid and stylo-mastoid foramen, through the external auditory canal (Fig. 3). It was diagnosed as schwannoma in preoperative incisional biopsy results, which were positive for S-100 protein staining. The dia-gnosis was facial nerve neurilemmoma, which caused the conductive hearing loss and invaded the entire external auditory canal and the middle ear. Surgery was performed using the transmastoid approach. The mass in the external auditory canal was connected to the part of the epitympanum and the mastoid cavity, along the posterior wall of external auditory canal. The mass started at the second genu of the facial nerve, and was connected to the stylomastoid foramen across the entire mastoid segments. However, It had a distinct border with the tympanic segment, so it was relatively easy to dissect. After resection of the mass, neurorraphy with the great auricular nerve was performed using a 9-0 nylon suture. After Tissue glue (Tisseel kit
® 2 mL, Baxter) was applied to the connected nerves, we covered the nerves with tragal cartilage, tragal perichondrium and temporalis muscle fascia, and obliterated the opening with an inferior based musculocutaneous flap (Fig. 4). Keratin had gathered began to form a cholesteatoma in the external auditory canal that was inside of the mass. In addition, part of the tympanic membrane was lost due to the mass but the ossicles were intact. In the histopathologic report, Antoni type A, which are the spindle cells, and the surrounding connective tissues were well-arranged, and Antoni type B, which has relatively loose phlegmatic temparament, were scattered. Thus, this case was diagnosed as a typical schwannoma, which was positive for S-100 protein staining (Fig. 5). He had no symptoms such as dizziness after the surgery. It has been 3 months since the surgery and the facial nerve palsy of patients is House-Brackmann Grade III (H-B Gr III) and the patient has been visiting the hospital regularly for check-ups (Fig. 6).

Discussion

Schwannoma is a benign tumor that originates from schwann cells. It can occur in any nerve containing a Schwann cell, but facial nerve schwannomas are relatively rare. About 300 cases have been reported so far.6) Since the first reported by Schmidt in 1930, approximately 0.15 to 0.8% of facial nerve schwannomas have been reported by autopsy of temporal bone. However, extra temporal lesions are relatively rare and almost no cases are found as a mass in the external auditory canal.7) A mass in middle ear and external auditory cananl can be considered cholesteatoma, hemangioma, adenoma, osteoma, Exostosis or schwannomas. To diagnosis schwannoma, comprehensive analysis of clinical features, imaging scans, and histopathologic examination are required to discriminate from other tumors.9) In the case of facial nerve schwannomas, symptoms of progressive facial palsy, conductive hearing loss, and tinnitus occur. But a variety of clinical manifestations depending on the site of the schwannomas are possible; therefore, it is difficult to diagnose preoperatively.8) In this case, there were no specific features except for complaints of earfullness and the mass in the external auditory canal. Schwannoma has two pathological features, the Antoni type A and type B.10) In features of Antoni type A, spindle cells are tightly together, wavy, and twisted nuclei are palisading and form a verocay body. In Antoni type B, the spindle-shaped or oval cells are interspersed between loose interstitial tissues. In addition, hyalinization and hemorrhagic necrosis of interstitial tissues can be found.10) Furthermore, these cases are positive for S-100 protein immunohistochemical staining. The S-100 protein is a specific marker of the neural crest and, can be informative in distinguishing from other tumors such as cholesteatoma, hemangioma, adenoma or osteoma, which can occur in the external auditory canal.11) The therapeutical approach used for schwannoma is surgery since it is resistant to radiation therapy.10) In the case of facial nerve schwannoma, the goal is to remove the mass completely, while conserving facial nerves and hearing functions. However, the outcome can depend on the location, size of tumor and the timing of the surgery. Therefore, there has been much controversy about the timing and surgical approach.9,12) Some reports have insisted that the surgical treatment should be postponed as long as possible when the function of the facial nerve seems normal or mildly alterd, due to the deterioration of the facial nerve after surgery.9) According to Lee, et al.,9) among 11 patients with facial nerve paralysis who received surgical treatment, 7 of them deteriorated and were shown to have H-B Gr III disorder. In studies by Chung, et al.,13) all 8 cases of tumor extraction, nerve graft or end-to-end nerve anastomosis on the facial nerve Schwannoma of different parts had the H-B Gr III disorder after the surgeries. In addition, repetitive denervation and regeneration can cause collagenous in distal regions, therefore, the function of facial nerve may decrease after surgery. Approximately 18-29% of the facial nerve schwannomas includes the involvement of the inner ear, and this disorder can deteriorate the function of the inner ear; thus, some studies have suggested earlier surgical treatment.12,14,15) In this case, the schwannoma obstructed the external auditory canal and invaded the middle ear. Thus, because of the formation of cholesteatoma, the surgery was used in this case.
Therapeutic plans and surgical approaches to treat facial nerve schwannoma can vary, and the decision of removing a facial nerve along with extraction of the tumor is especially important.9) In theory, the facial nerve schwannomas are covered with a capsule. Therefore, the function of the nerve can be preserved by removing the mass including the facial nerve capsule, but this procedure can be performed only if the tumor is very small and does not interfere with the facial nerve fiber.16) In addition, Hajjaj and Linthicum17) found that facial nerve fibers were widely distributed inside the tumor in 8 cases out of 23 facial nerve schwannoma cases when the samples were immunohistochemistry stained with an antibody targeting the nerve filament. These findings demonstrated that enucleation of tumor has large risk of recurrence and that this method can cause degradation of the facial nerve function.14) On the other hand, when the tumor is large and interferes with the facial nerve, part of the facial nerve has to be removed along with mass.9) Various opinions are available for reconstructing the facial nerve.18) End-to-end anastomosis may be the best method for facial nerve reconstruction. However, when there is tension, a free nerve graft is performed using the great auricular nerve or sural nerve.9) In this case, the surgery was performed using the transmastoid approach due to conductive hearing loss and facial nerve schwannoma invasion of the middle ear and the external auditory canal. Boundaries were determined based on the preoperative CT scan, MRI and gross findings during the surgery and the extraction of tumors was complete. Free nerve graft was performed using the great auricular nerve in order to avoid tension. Some studies have reported that facial nerve paralysis can occur even if there was no facial nerve paralysis before the surgery. Once the facial nerve is amputated, H-B grade III can occur even after facial nerve reconstruction.9) When amputation and reconstruction of the facial nerve is performed, the deterioration of the facial nerve function after surgery due to degeneration of the nerves can be gradually improved to the H-B grade III after end-to-end anastomosis.5) In this case, after extraction of a large tumor from the mastoid cavity, external auditory canal, and some parts of the middle ear, neurorrhaphy was performed using the great auricular nerve, which was long enough to avoid tension. When a mass is found in the external auditory canal, it could be cholesteatoma, hemangioma, adenoma, osteoma, exostosis or schwannomas, which is very rare. However, it is difficult in many cases to make a differential diagnosis based on only clinical manifestations. Only a partial density enhancement is observed in the contrast-enhanced computed tomography of schwannomas. In addition, only a moderate signal T1 and a high signal in T2 gadolinium can be used to enhance MRI images. Furthermore, it is difficult to make an accurate differential diagnosis even in radiographic images.19) For an accurate diagnosis of mass in the external auditory canal, a preoperative biopsy is often required in addition to radiological images. Care must be taken in resection and biopsy when considering the possibility of schwannomas, even this is rare.


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