Coexisting facial nerve schwannoma and monomorphic adenoma of the parotid gland

Ear, Nose & Throat Journal, March, 2004 by Jeremy D. Richmon, Carter E. Wahl, Stanley Chia

Abstract

We describe the case of a patient with a parotid mass that was found to consist of both a facial nerve schwannoma and a monomorphic adenoma. To our knowledge, this is the first report of these two lesions presenting as a single tumor. We also discuss the incidence, diagnosis, and treatment of parotid facial nerve schwannomas.

Introduction

A schwannoma can arise along any peripheral nerve. Approximately 25% of all schwannomas occur in the head and neck, and the vast majority of these occur along the vestibular portion of cranial nerve VIII. (1,2) Schwannomas occasionally originate in the facial nerve; in such cases, the intratemporal portion is most likely to be affected. (3) Schwannomas of the peripheral facial nerve are quite rare.

In this article, we describe an unusual case of a schwannoma that arose from the peripheral facial nerve and that was associated with a monomorphic adenoma of the parotid. To our knowledge, this is the first description of the simultaneous appearance of a peripheral facial nerve schwannoma and a monomorphic adenoma of the parotid.

Case report

A 54-year-old Filipino man presented with a slowly enlarging left preauricular mass of 2 years' duration. He was otherwise healthy and had no history of facial or neck masses or skin malignancy. On examination, the 2 x 2-cm mass was located anterior to the left tragus. It was firm, nontender, and mobile. No other masses were noted on the head and neck, and the facial nerve was intact throughout all its branches.

Fine-needle aspiration cytology revealed that the mass contained hypocellular material that featured a few clusters of bland-appearing cells with vesicular and round-to-elongated nuclei, ill-defined cell borders, and moderate-to-abundant cytoplasm on a background of blood. These features suggested, hut were not entirely diagnostic of, a pleomorphic adenoma. Computed tomography (CT) revealed that the well-circumscribed, nonenhancing mass in the left parotid gland was consistent with a pleomorphic adenoma (figure 1).

[FIGURE 1 OMITTED]

The patient underwent a left parotidectomy with facial nerve dissection. Dissection of the inferior division was uneventful. Upon dissection of the superior division, a lobulated cystic lesion with a mottled purple color was noted. The mass was densely adherent to the facial nerve and extended to the deep lobe of the parotid gland. With difficulty, the mass was separated from the epineurium of the superior division of the facial nerve. During the course of the dissection, the frontal branch of the nerve was transected, and a graft from the ipsilateral great auricular nerve was used to repair it. The remainder of the mass was dissected free from the deep lobe of the parotid.

At the 2-week follow-up, the patient exhibited paralysis of the frontal branch of the left facial nerve, but all other branches were intact.

During the resection, a biopsy specimen obtained from the left parotid gland was sent to pathology for an intraoperative frozen-section diagnosis. The material was not diagnostic, and a second sample was sent. Histologically, the second sample appeared to consist of cellular mesenchymal tissue with a focal area of cartilaginous differentiation. A diagnosis of pleomorphic adenoma was again made. Immunohistochemical staining for actin and keratin was ordered on the unfrozen tissue.

Subsequently, the left superficial parotid gland specimen was obtained in fragments for permanent sections. The largest fragment measured 5.0 x 4.0 x 2.0 cm in aggregate, and a suture marked the area that had been biopsied. Grossly, the parotid gland was made up of yellow-tan fibrofatty tissue with no identifiable lesions or nodules. Histologically, a tumor consisting of compact spindle cells arranged in intersecting fascicles was identified (figure 2, A). The tumor could be seen budding from a branch of the facial nerve (figure 2, B). A diagnosis of benign schwannoma was made. Almost the entire tumor exhibited the Antoni A pattern, with a few focal areas showing nuclear palisading and Verocay's bodies. An S-100 immunohistochemical stain was diffusely positive, supporting the diagnosis of a schwannoma (figure 2, C). The stains for actin and keratin were negative, ruling out a pleomorphic adenoma. Additionally, a separate and clinically unapparent encapsulated epithelial proliferation consistent with a monomorphic adenoma was discovered (figure 3). The adenoma contained bland-appearing cells with darkly staining nuclei arranged in a glandular pattern.

[FIGURES 2-3 OMITTED]

At the 7-month follow-up, the patient continued to experience left frontal nerve paralysis.

Discussion

Some 82% of all parotid tumors are benign; of these, 57% are pleomorphic adenomas. (4) Neuromas are uncommon benign parotid tumors; they include schwannomas (neurilemmomas) and neurofibromas, although both originate in Schwann's cells. A schwannoma is a solitary encapsulated tumor; a neurofibroma is not capsulated and contains nerve fibers within the lesion. Therefore, it is theoretically possible to dissect a schwannoma free from its associated nerve, whereas such is not possible with a neurofibroma. Additionally, although malignant transformation is rare with both tumors, it is more likely to occur with a neurofibroma.