A neurilemmoma of the parotid gland: Report of a case.

Abstract

Benign primary tumors of the facial nerve are frequently misdiagnosed because of the variety of their clinical manifestations. Much attention has been paid to neurilemmomas in the recent otolaryngologic literature, but far less has been focused on intraparotid facial nerve neurilemmomas. In this article, we describe a new case of this truly rare tumor, and we review its diagnosis, pathology, and treatment.

Introduction

Both benign and malignant tumors of the parotid gland frequently present a significant challenge to the physician because of their nature or their extension. [1] Tumors of the facial nerve are rare, but they have stimulated the interest of many clinicians and pathologists.

The origin of these tumors is sometimes difficult to determine. Schwann's cells have often been identified as the cells of origin. [2-6] Tumors of the facial nerve arise from the neural crest (neuroectoderm) and can be either benign (e.g., neurilemmomas, neurofibromas, ganglioneuromas, and paragangliomas) or malignant (e.g., neurosarcomas, neuroblastomas, and malignant paragangliomas). Schwannomas are the most common benign tumors of the facial nerve, and they are classified into two categories: neurilemmomas and neurofibromas. [3,5] These two types of schwannoma are usually distinctive from each other both clinically and pathologically. [3,6,7] Some authors use the term schwannoma as a synonym for neurilemmoma. [3,5,6] In our review of the literature, we found that schwannomas are frequently seen in the intratemporal segment of the facial nerve. On the other hand, only a few reports of intraparotid neurilemmomas have been published. [8,9]

In this article, we describe an additional case of this rare and benign tumor. We discuss the difficulties encountered in patient evaluation, including the absence of specificity for topographic testing, as well as the peculiar behavior of the facial nerve prior to surgery. We also mention the difficulties encountered in intraoperative evaluation and management.

Case report

A 44-year-old woman was admitted to our clinic with a left parotid mass, which had appeared 2 months earlier and which had been growing ever since. She complained of no other symptoms, and her otolaryngologic and general physical examinations were completely normal. The mass was firm, moderately hard, not adherent to the skin, and slightly tender to palpation. It measured 2 x 3 cm and was localized behind the posterior margin of the mandibular ramus. The patient's facial nerve function was normal on clinical evaluation. Two fine-needle aspiration cytologies showed inflammatory changes compatible with chronic sialadenitis.

Magnetic resonance tomography clearly showed that the tumor was located deep in the left parotid gland (figure 1). The mass featured a lack of homogeneity that could possibly have been associated with cystic degeneration.

The patient underwent a left total parotidectomy with a complete excision of the tumor. During surgery, the tumor was noted to be circumscribed and of a firm consistency. Analysis of intraoperative frozen sections did not specify its exact pathology, but it did reveal that the tumor was not malignant. All efforts to identify the facial nerve trunk were very laborious, and finally we were unable to dissect the nerve. However, the mandibular branch of the facial nerve was identified along its course with the posterior facial vein. Postoperatively, the patient experienced a complete facial nerve paralysis. On gross study, the tumor was whitish-tan, oval, encapsulated, and not attached to any identifiable nerve. It had a solid appearance and was marked by partly myxoid areas. The final pathology report identified it as a facial nerve neurilemmoma made up almost exclusively of Antoni B areas and foci of degenerative changes (figure 2).

Six months postoperatively, the patient was readmitted for a hypoglossal-facial nerve anastomosis. At the 2-year followup, the patient exhibited sufficient recovery of facial nerve function.

Discussion

Neurilemmomas were first described in 1908 by Verocay, who later called them neurinomas. [10] In 1935, Stout proposed the term neurilemmoma, believing that this type of tumor contains no neural elements but arises from the nerve sheath or Schwann's cells. [11] According to Stout, only 194 adequately described neurilemmomas were reported in the literature during the 100 years prior to 1935. Since then, many authors have reported cases involving the head and neck. Between 25 and 45% of all reported neurilemmomas have occurred in the head and neck region, but intraparotid facial nerve neurilemmomas are infrequent and generally unsuspected. [8,9,12] The fact that these tumors often grow to a large size without causing any neurologic deficit is impressive. [8,9] This might explain the difficulty that physicians encounter in establishing a correct preoperative diagnosis. Facial nerve palsy, complete or partial, can often be associated with an intratemporal schwannoma of the facial nerve. [2,13,14]

Neurilemmomas arise from the neural sheath of the peripheral sensory, motor, sympathetic, and cranial nerves (with the exception of the optic and olfactory nerves). [12,14] These tumors are characteristically slow growing, well encapsulated, round or fusiform, and closely associated with the nerve of origin, and they are usually solitary and painless. [6,15] Unremoved, they can become exceedingly large and cause pressure symptoms. Depending on the adjacent anatomy, these symptoms can include vocal fold dysfunction, hoarseness, cough, Horner's syndrome, and paresthesia among others. [7,12,15]

Neurilemmomas are not associated with any known etiologic factor. They can arise at any age, although they do occur more often between the third and fourth decades of life. Women are thought to be affected more often than men in ratios of 3:2 to 2:1, [16] although a study reported by Conley and Janecka found an equal distribution between the sexes. [17]

In most cases, neurilemmomas do not undergo a malignant transformation. [3,6,18] However, some cases have been reported in the literature, mainly in association with radiation therapy and the presence of neurofibromatosis (von Recklinghausen's disease); the incidence of malignancy among cases of neurilemmoma has been reported to range from 6 to 30%. [2-5,19] Facial nerve neurilemmomas are rather rare. Of the 140 cases that have been reported, most have occurred in the intratemporal portion of the facial nerve. [14, 20] Intraparotid neurilemmomas of the facial nerve can mimic the more common parotid neoplasms. Up until 1975, only 20 cases of intraparotid neurilemmoma had been reported in the English-language literature. [21] Since then, 11 others have been reported. [9]

Facial nerve palsy or paralysis can occur in cases of intratemporal neurilemmoma. [2,13,14,20,22] These tumors are soft and either white, yellow, or pink. They occasionally feature areas of calcification and/or cystic degeneration. Their capsule is continuous with the epineurium, the most external nerve sheath. [3]

Histopathologically, the hallmark of a neurilemmoma is its pattern of alternating Antoni A and B areas. [6,7,14] The Antoni A area is characterized by the presence of elongated and spindle-shaped Schwann's cells, and their nuclei are aligned in a palisading pattern (Verocay bodies). The hypocellular Antoni B area has a varying degree of cell pleomorphism; irregular cell types are scattered in loose connective tissue, and there is no definable palisading of tumor cell nuclei. Both Antoni areas are usually found in the same tumor, but their respective proportions vary. Nerve fibers are not part of the tumor because the mass arises from a Schwann's sheath and pushes the nerve axons aside. [3,6] Unlike neurilemmomas, neurofibromas are characterized by a profound infiltration of local tissue, no recognizable capsule, and an increase in cellularity, pleomorphism, and mitoses (especially in the malignant form). [34,6,14,8] Neurilemmoma must be included in the differential diagnosis of all parotid tumors, and a froz en section should be obtained whenever an unusual intraoperative finding is encountered in the parotid region. [21] Even so, as our case illustrates, this is not always helpful.

The definitive treatment of a neurilemmoma is local excision. [15] Even during surgery, it can be difficult to determine whether the tumor originated in the parotid gland or in the facial nerve. [21] In a patient reported by Wade, neither the trunk of the facial nerve nor the main branches of the nerve could be discerned. [23] Likewise, in one of the two patients reported by Avery and Sprinkle, no facial nerve was found close to the stylomastoid foramen. [9] Koide et al were able to identify the peripheral branches of the facial nerve in their patient, but the main trunk could be found only at the stylomastoid foramen. [24] In these cases, as in our own, the nerve could not be readily identified because the nerve fibers were splayed over the capsule of the tumor and appeared as fine connective tissue filaments. Under such conditions, the nerve can be easily disrupted during dissection, especially if the surgeon believes he or she is dealing with a parotid tumor. [21]

If a tumor is recognized as originating in the facial nerve when it is first encountered surgically, the surgeon should attempt to dissect it and free it from the nerve fibers. When a neurilemmoma arises from a major peripheral nerve, Seddon recommends enucleation by incision of the epineurium and dissection between the tumor capsule and the nerve fasciculi. [25] However, it can be difficult if not impossible to excise a cranial nerve neurilemmoma without removing a segment of the nerve.

The prognosis after surgical excision of a solitary schwannoma is excellent. If a tumor has not been completely excised, it can recur. Fortunately, because of its slow rate of growth, a recurrent tumor can be re-excised without the need for extensive or complicated methods. [17,21]

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