Keywords
facial nerve - traumatic neuroma - facial nerve schwannoma - geniculate - hemangioma
Introduction
Traumatic neuroma (TN) is a nonneoplastic proliferation of a peripheral nerve.[1] The size of the TN is inversely proportional to the extent of healthy neural regeneration
during re-establishment of nerve continuity.[2] TN are composed of disorganized nerve fascicles, Schwann cells, and endoneurial
and perineurial fibroblastic cells in a dense collagenous matrix.[2] Facial nerve TN is rare and can develop following extensive injury or more subtle
trauma that does not result in nerve discontinuity. It may also develop in association
with chronic middle ear inflammation.[3] These lesions may occur in any segment of the facial nerve, but most commonly involve
the labyrinthine, geniculate, and tympanic segments.[4]
[5] In this report, a case of facial nerve TN following blunt trauma is presented, along
with imaging, histopathology, and management presented in a surgical video.
Case Report
A 30-year-old man presented to the authors' institution with complete right-sided
facial paralysis following a downhill skiing injury. More remotely, he was diagnosed
with Bell's palsy 3 years earlier and was treated with corticosteroids with incomplete
recovery. Imaging at that time did not demonstrate any abnormality of the facial nerve.
The skiing accident occurred 9 months prior to presentation to our institution. During
evaluation, he endorsed mild imbalance and denied hearing loss. Examination demonstrated
normal cranial nerve function with the exception of House–Brackmann (HB) grade VI
facial function. Facial electromyography demonstrated active right facial neuropathy,
and an audiogram demonstrated normal hearing thresholds bilaterally.
Contrast enhanced magnetic resonance imaging (MRI) performed approximately 3 years
prior did not reveal any evidence of a facial nerve lesion. However, recent imaging
revealed avid enhancement of the right geniculate ganglion with widening of the labyrinthine
segment and the geniculate ganglion ([Fig. 1]). Computed tomography (CT) demonstrated an expansile lytic process with honeycomb
stippling involving the right facial nerve centered on the labyrinthine segment and
involving the geniculate ganglion ([Fig. 2]). A cochlear fistula was also evident. Radiographic findings were most compatible
with a facial nerve hemangioma.
Fig. 1 (A) Postcontrast T1-weighted MRI 2.8 years before presenting to our center demonstrating
no evidence of lesion. (B) Recent postcontrast T1-weighted (C) T2-weighted fast spin echo, and (D) thin slice heavily T2-weighted MRI demonstrated an expansile mass involving the
right meatal, labyrinthine, geniculate, and proximal tympanic facial nerve.
Fig. 2 (A) Coronal and (B) axial temporal bone computed tomography demonstrating expansion of the labyrinthine
and geniculate segments of the fallopian canal with honeycomb stippling (intralesional
spindle-like calcifications) and a cochlear fistula.
Based on imaging characteristics and poor preoperative facial nerve function, our
team offered an option of microsurgical resection with interpositional nerve graft.
To access the nerve from the internal auditory canal (IAC) to the mastoid, a combined
mastoid-middle fossa craniotomy approach was utilized.
Procedure
A cortical mastoidectomy was performed. The facial nerve was decompressed in the mastoid
segment and traced proximally toward the geniculate ganglion. A temporal craniotomy
centered over the external auditory canal was used to expose the middle fossa floor.
The facial nerve was exposed from the IAC to the geniculate ganglion. A lesion involving
the meatal, labyrinthine, geniculate, and tympanic segments was identified and excised.
Frozen section analysis revealed atypical cells. Upon removal of the geniculate segment,
the cochlear fistula was identified and great care was taken to avoid suctioning of
perilymph. A small blood patch was placed over the fistula along with fascia and fibrin
glue. A segment of great auricular nerve was obtained through a high cervical incision
and was interposed between the meatal and mastoid segments. A neurorrhaphy tube and
fibrin glue were used to secure the graft in place.
Histological sections obtained from formalin-fixed paraffin-embedded tissue demonstrated
disorganized nerve fascicles, involving peripheral ganglia. No associated cellular
proliferation or abnormal blood vessels were identified ([Fig. 3]). Immunohistochemical staining with neurofilament (2F11 clone; Dako, Santa Clara,
California, United States) highlighted disorganization of the nerve fascicles. In
addition, this also demonstrated foci with relative aggregation of axons, suggestive
of axonal clusters. There was no proliferation of Schwann cells demonstrated by S100
(polyclonal; Dako) or of perineurial cells revealed by Epithelial membrane antigen
(EMA) (E29 clone; Dako). No macrophage aggregates were detected by CD68 (KP1 clone;
Dako), and no increase in collagen IV staining (CIV 22 clone; Dako) was seen in association
with the disorganized nerve fascicles. Overall, the appearance of disorganized nerve
fascicles, demonstration of axonal aggregates reminiscent of axonal sprouting/regeneration,
and absence of any cellular proliferation or vascular malformation were considered
to favor a TN. At 3 months postoperatively, MRI demonstrated no evidence of recurrent
lesion. Postoperatively, the patient developed ipsilateral profound sensorineural
hearing loss.
Fig. 3 (A) Histological sections demonstrate disorganized nerve fascicles, which also involved
peripheral ganglia (hematoxylin and eosin; ×100 magnification). (B) Longitudinal and cross-sections of the disorganized nerve fascicles, suggesting
variably density of the axons (hematoxylin and eosin; ×100 magnification). (C) Neurofilament highlights the axonal disorganization in the longitudinal section
and demonstrates axonal aggregates in the cross-sections, suggestive of axonal clustering
(×100 magnification).
Discussion
Traumatic facial neuromas are rare entities that comprise a very small fraction of
lesions affecting the facial nerve. Prevalence estimates are based on cadaveric temporal
bone studies and range between 0.002 and 0.8%.[1]
[6]
[7] As TN of the facial nerve are seldom encountered, more common lesions of the facial
nerve must be considered in a patient with facial paralysis and imaging demonstrating
an abnormality of the facial nerve along its course. The two most common tumors of
the intratemporal or intracranial facial nerve are schwannoma and hemangioma. Neurofibromas
of the facial nerve, more common in neurofibromatosis type 1, frequently affect distal
branches in the parotid gland but can extend intratemporally.[8] While malignant peripheral nerve sheath tumors of the facial nerve have been reported,
parotid tumors with overt or occult perineural spread account for the most common
cause of tumor infiltration causing facial paralysis.[9] Histopathologically, facial nerve TN demonstrates disorganized nerve fascicles.
Demonstration of axonal sprouting with an associated Schwann cell can be best seen
by electron microscopic examination of glutaraldehyde-fixed plastic-embedded sections.
In this case, given the clinical suspicion of a neoplastic process, only formalin-fixed
paraffin-embedded sections were available for evaluation. Even so, the presence of
axonal clusters in the disorganized nerve fascicles, highlighted by a neurofilament
immunostain, is suggestive of axonal sprouting as part of regeneration. Notably, the
absence of a cellular proliferation excludes a neoplastic process.
There have been 14 previously reported cases of TN involving the facial nerve. While
blunt trauma without disruption of neural continuity accounts for the most common
etiology of facial TN, several cases have been identified following mastoid surgery,
thought to be related to middle ear inflammation. Two cases were considered idiopathic.[3]
[10]
[11] In the setting of chronic middle ear or mastoid inflammation, facial nerve function
was typically normal and the neuroma was discovered incidentally or on postmortem
evaluation.[3] In contrast, traumatic facial neuromas are commonly associated with some degree
of facial paralysis.[3]
[10]
[11]
Based on location, history, and imaging characteristics, the leading differential
diagnosis in this case was geniculate ganglion hemangioma. These “tumors” were originally
thought to develop from the rich capillary plexus surrounding the geniculate ganglion.
Based on histology and immunohistochemistry, these lesions are now favored to be a
venous malformation rather than a true neoplasm. Less commonly, facial nerve hemangioma
can occur in the IAC, at the second genu, and near the takeoff of the chorda tympani
nerve.[12] In contrast to facial nerve schwannoma, geniculate ganglion hemangioma is classically
associated with more advanced nerve symptoms (including spasm, recurrent palsy with
intervals of recovery, or progressive facial paralysis) out of proportion to tumor
size.[13] Vertigo and pulsatile tinnitus are rare but may result from invasion of the labyrinth.[14] Facial nerve hemangiomas are typically T1 iso/hypointense, T2 hyperintense, and
heterogeneously enhanced on MRI.[14] CT imaging demonstrates ill-defined margins with intralesional bone spicules characterized
as a “honeycomb” pattern as seen in the index case.
Given its rarity and overlapping appearance with geniculate ganglion hemangioma and
facial nerve schwannoma, definitive preoperative diagnosis is challenging. However,
a recent report by Chhabra et al suggest the use of MR neurography as a means of distinguishing
TN from neoplasia using features such as perilesional scarring, lack of a split fat
or target sign, and absence of abnormal enhancement.[15] In situations with a convincing prior history of trauma, MR neurography may be considered
as an adjunct to high-resolution cross-sectional imaging with MR and CT.
While there is no established treatment algorithm for TN, one may adapt treatment
strategies for facial nerve schwannoma or geniculate ganglion hemangioma to these
rare cases.[2] The best facial nerve functional outcome yielded by interpositional nerve grafting
is HB grade III. A majority of patients enjoy good resting tone, but a portion still
require adjunct procedures to aid in eyelid closure. Therefore, if the facial paresis
is severe (i.e., HB grades IV–VI), it is reasonable to opt for nerve resection with
interposition nerve graft placement or primary reanastomosis. To prevent muscle atrophy
and degeneration of motor end plates, it is ideal to proceed with nerve grafting within
12 to 18 months in situations with complete facial paralysis. Unlike in facial nerve
schwannoma or geniculate ganglion hemangioma, however, neural preservation strategies
(i.e., subtotal resection with observation of residual tumor) are not feasible in
TN.
Conclusion
Facial nerve TN is a rare consequence of trauma. Given the overlapping clinical presentation
and imaging characteristics of facial TN and other more common primary facial nerve
lesions, such as geniculate ganglioma hemangioma and facial nerve schwannoma, preoperative
diagnosis remains challenging. In general, management of facial paralysis associated
with TN parallels that of geniculate ganglion hemangioma and schwannoma—interposition
nerve graft or donor nerve procedures should be considered in cases with HB grade
IV or poorer function and no evidence of improvement after 6 to 12 months of observation.
