Intracranial neuromuscular choristoma: a case report and literature review.

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.

Another hypothesis regarding the histogenesis of intracranial NMCs is that cranial nerves trap skeletal muscle during embryogenesis. (6) According to other theories, neuroectoderm differentiates toward mesenchyme. (19-22)

Regarding the neoplastic potential of NMCs, Mitchell et al reported that they had found proliferative index marker Ki-67 activity in an NMC lesion, and they suggested that it is a slow-growing neoplasm. (12) They preferred to call this lesion a rhabdomyoma of the adult type. Their theory was supported by the tact that several patients with NMC experienced multiple recurrences following resection. (4,8,10,12) On the other hand, Louhimo and Rapola reported 2 cases of NMCs that underwent spontaneous regression following biopsy and subtotal resection. (2) They did not report whether or not there was any recurrence of these lesions.

We found that the lesion in our patient exhibited low proliferative activity on staining for Ki-67, and there was no recurrence at the 2-year follow-up. These findings support the idea that NMCs may be slow-growing tumors rather than hamartomas.

Table. Summary of clinical features of the 18 reported cases of
intracranial neuromuscular choristoma

                          Age/
Report                    sex        Symptoms

Zwick et al, (6)          2/M        Absent corneal reflex,
  1989                               facial numbness, wasted
                                     muscles
Vandewalle et al, (11)    41/M       R peripheral facial
  1995                               paresis, hearing loss
                                     (3 yr)
Smith et al, (13)         19/F       Hearing loss (3 mo)
  1997
                          61/F       Hearing loss (5 yr)
Giannini et al, (14)      20/F       Vision loss
  2002

Oeppen et al, (15)        2/M        R facial swelling
  2003

Wu et al, (16)            33/M       Unknown
  2003
                          50/F       Dizziness, hearing loss
                                     (12 yr)

                          39/M       Tinnitus, hearing loss
                                     (30 yr)

                          39/M       Tinnitus, hearing loss
                                     (6 yr)

                          23/M       Unknown
                          46/M       Tinnitus, hyperacusis

                          34/M       Hearing loss
                                     (10 yr)

                          49/F       Tinnitus, hearing loss
                                     (9 mo)

                          27/F       Tinnitus, hearing loss
                                     (8 mo)
                          36/M       Hearing loss
                                     (6 mo)
                          43/M       Hearing loss
                                     (6 mo)
Owor et al, *             44/M       Tinnitus, hearing loss
  2004
                                                     Nerve
Report                    Location                   excised

Zwick et al, (6)          L trigeminal nerve         L trigeminal
  1989

Vandewalle et al, (11)    R facial nerve             R facial
  1995

Smith et al, (13)         R cochlear nerve           R cochlear
  1997
                          R facial nerve             R facial
Giannini et al, (14)      L optic nerve              L optic
  2002

Oeppen et al, (15)        Unknown                    Unknown
  2003

Wu et al, (16)            R cochlear nerve           R cochlear
  2003
                          R cochlear nerve           R cochlear and
                                                     vestibular

                          L cochlear and             L cochlear and
                          vestibular branch          vestibular

                          L cochlear and             L cochlear and
                          vestibular branch          vestibular

                          R cochlear nerve           R cochlear
                          R cochlear and             R cochlear and
                          vestibular branch          vestibular

                          L cochlear and             L vestibular and
                          vestibular branch          partial cochlear

                          R cochlear and             R vestibular and
                          vestibular branch          partial cochlear

                          L cochlear nerve           L cochlear

                          L cochlear nerve           L cochlear

                          L cochlear nerve           L cochlear

Owor et al, *             R facial nerve             Partial R facial
  2004

Report                    Follow-up

Zwick et al, (6)          Trismus, infratemporal
  1989                    fossa scar

Vandewalle et al, (11)    Unknown
  1995

Smith et al, (13)         Unknown
  1997
                          Unknown

Giannini et al, (14)      Unknown
  2002

Oeppen et al, (15)        Unknown
  2003

Wu et al, (16)            Unknown
  2003
                          6 yr: increased
                          dizziness, complete
                          hearing loss
                          4 yr: increased
                          tinnitus, complete
                          hearing loss
                          1 yr: increased
                          tinnitus and dizziness,
                          complete hearing loss
                          Unknown
                          1 yr: increased
                          tinnitus and dizziness,
                          complete hearing loss
                          3 yr: baseline hearing

                          1 yr: baseline hearing,
                          same tinnitus

                          None

                          None

                          None

Owor et al, *             2 yr: no recurrence
  2004

* Present report.

References

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From the Department of Pathology, Albany (N.Y.) Medical Center (Dr. Owor and Dr. Qian), the Indianapolis Neurosurgical Group (Dr. Payner), and AmeriPath Indiana, Indianapolis (Dr. Martin and Dr. Shah).

Reprint requests: Yuan Sham MD,AmeriPath Indiana, 2560 N. Shadeland Ave., Indianapolis, IN 46219. Phone: [phone]; fax: [phone]; e-mail: [e-mail]

Originally presented in part at the 78th annual meeting of the American Association of Neuropathologists; June 20-23, 2002; Denver.