Intracranial neuromuscular choristoma: a case report and literature review

Ear, Nose & Throat Journal, August, 2004 by Geraldine Owor, Jiang Qian, Troy Payner, Anita Martin, Yuan Shan

Abstract

Neuromuscular choristoma (NMC) is an uncommon tumor that usually involves a large nerve trunk. Only 28 cases of NMC have been previously reported in the English-language literature, 17 of which involved cranial nerves. We report a new case of intracranial NMC that arose from a facial nerve at the cerebellopontine angle in a 44-year-old man. The patient was taken to surgery, where the lesion was found to involve the right facial nerve. The tumor was partially removed, and at the 2-year follow-up, the patient showed no sign of recurrence.

Introduction

Neuromuscular choristoma (NMC)--also called neuromuscular hamartoma/rhabdomyoma or benign triton tumor--is an uncommon tumor that usually involves a large nerve trunk. Until now, only 28 cases of NMC have been reported in the English-language literature. (1-18) The first reported case of NMC, published in 1895, involved the sciatic nerve. (1) Since then, 17 cases of cranial nerve NMC have been reported (table). (6,11,13-16) The first of these, published in 1989, involved the trigeminal nerve in a child. (6) The second, reported in 1995, involved the facial nerve. (11) Additional cases involving the facial, (13) cochlear, (13,16) and optic (14) nerves were subsequently reported. The largest series was reported in 2003 by Wu et al, whose case series included 11 patients seen over a 10-year period. (16)

In this article, we report a new case of intracranial NMC that arose from a facial nerve. We also review the earlier reports of intracranial NMCs.

Case report

A previously healthy 44-year-old man came to us with a history of recent-onset progressive hearing loss and tinnitus in the left ear, right epiphora, and mild headache. No hearing loss was present in the right ear, and he reported no facial numbness. On physical examination, he was alert and spoke fluently. The cranial nerves were intact, but he exhibited mild facial weakness of the lower motor neuron type on the right. Facial sensation was normal. Magnetic resonance imaging (MRI) detected an 8-mm enhancing nodule in the right acoustic meatus (figure 1).

[FIGURE 1 OMITTED]

The patient was taken to surgery. Intraoperatively, the nodule was found to intimately involve the right facial nerve, and it was partially removed. The excised specimen was a 0.5-cm irregular fragment of pink and tan tissue. It was fixed in 10% formalin and embedded in paraffin. Histologic sections were stained with hematoxylin and eosin (H&E). Other special stains included trichrome and Bielschowsky's silver. Immunostaining was performed with glial fibrillary acidic protein, neurofilament protein, S-100, and the proliferation index MIB-1 (Ki-67).

Histologic examination identified benign striated muscle fibers, most of which were grouped in small fascicles, oriented haphazardly, and separated by fibrous tissue that contained areas of adipose tissue and small nerve fascicles (figure 2, A through D). The muscle fibers were well developed and featured multiple peripherally located small nuclei, deeply eosinophilic cytoplasm, and easily discernible cross-striations. No degenerating or regenerating basophilic muscle fibers were seen, and no nuclear atypia or mitoses were noted. The muscle fibers were intermixed with myelinated nerve fascicles. A small focus of a neuropil-like structure with rare glial cells and corpora amylacea was located adjacent to the skeletal muscle (figure 2, C). The nature of the myelinated nerve fascicles was well demonstrated by trichrome (figure 2, E) and Bielschowsky's staining (figure 2, F), as well as by immunohistochemical staining for neurofilament protein (figure 2, G). No neurons or ganglion cells were identified. A few Ki-67-positive cells were also noted (figure 2, H). The positive nuclei were apparently located in the connective tissue and nerve fibers and were not associated with skeletal muscle.

[FIGURE 2 OMITTED]

At the 2-year follow-up, the patient showed no sign of recurrence, and his only complaint was occasional headache.

Discussion

A true triton tumor is a malignant peripheral nerve sheath tumor with rhabdomyoblastic differentiation, and it is more common than a "benign triton tumor." (17,18) In most of the earlier case reports, the NMC involved a peripheral nerve, often one of a major nerve trunk. (1-5,7-10,12) Only 3 of these cases occurred in adults, and all 3 adults had experienced neuromuscular symptoms dating back to childhood or adolescence. (10)

Intracranial NMCs occur primarily in the acoustic canal and cerebellopontine angle, and 15 of the 18 reported cases (including ours) involved the vestibulocochlear or facial nerves. Two exceptions were tumors of the trigeminal (6) and optic (14) nerves; in 1 case, (15) the exact location was unreported. While extracranial NMCs are more common in children, 16 of the 18 cases of intracranial NMCs occurred in adults.

Histologically, the reported cases of intracranial NMC were similar. These tumors were made up of an admixture of benign, mature elements of neural, muscular, vascular, and adipose tissue in various proportions. The exact histogenesis of NMC is unknown, but Markel and Enzinger postulated that it originates as a hamartomatous malformation secondary to a combination of different mature tissue elements. (3) In normal circumstances, no skeletal muscle or adipose tissue is present in the cranium. But the most distinctive histologic feature of intracranial NMCs is the well-formed, mature skeletal muscle with various amounts of connective tissue and nerve fibers.