Atypical Tumors of the Facial Nerve

Case Series and Review of the Literature

Lindsey Ross, M.D; Doniel Drazin, M.D; Paula Eboli, M.D; Gregory P. Lekovic, M.D., Ph.D.

Disclosures

Neurosurg Focus. 2013;34(3):e2 

In This Article

Discussion

We present a series of rare facial nerve tumors with features atypical from those seen in the more common facial nerve schwannoma, which in autopsy series may have an incidence as high as 0.8%.[36] Facial nerve schwannomas typically involve the geniculate ganglion, IAC, and CPA (Fig. 5). Facial nerve schwannomas, such as the case included here, entirely confined to the temporal bone are exceedingly rare.

Figure 5.

Atypical facial nerve tumor illustration. Diagram by Lindsey Ross and Brian Nicholas. Printed with permission from Lindsey Ross, M.D.

Facial nerve hemangiomas were first described by Politzer in 1901.[25] Again, this lesion has a predilection for the geniculate ganglion. Once thought to occur here as result of pertinent anastomosis, it has recently been found to arise as a consequence of its dense capillary but anatomically distinct network. When compared with the neighboring segments of the geniculate ganlgion (that is, the tympanic and labyrinthine segments), there are upward of 46 cases reported in the literature of facial nerve hemangioma involving the geniculate ganglion.[2] Hemangiomas are also commonly known as benign vascular tumors. There is evidence to prove that facial nerve hemangiomas should be correctly categorized as venous malformations, given the lack of internal elastic laminae noted on histological examination.[1,4] In fact, Benoit et al.[4] attempted to reclassify facial nerve hemangiomas using histological and immunohistochemical markers in the context of commonly accepted vascular lesion nomenclature. The distinction is understood when the true definition is studied. Benign vascular tumors arise directly from cellular hyperplasia, whereas malformations arise from errors in vascular morphogenesis.[4]

Facial nerve hemangiomas usually present in midlife, specifically between the 3rd and 6th decades. There appears to be an equal distribution of males and females. Approximately 97% of patients with atypical facial nerve tumors present with some degree of facial nerve deficit.[14] Quite often, there is a spectrum of motor deficit that varies from hemifacial spasms to facial paresis. Hearing loss is also common and may be characterized as conductive if the horizontal segment of the facial nerve is involved or sensorineural if the tumor affects the labyrinthine segment or geniculate ganglion. Again, ipsilateral hearing loss is the most prevalent presenting symptom. Nonetheless, the deficit often is slow in progression or renders a recurrent episodic course, which allows us to make the distinction between other facial nerve tumors. Patients are often misdiagnosed and treated for idiopathic facial nerve weakness (Bell palsy), a much more common etiology of facial nerve paresis. The temporal characteristic of facial paralysis is of utmost importance in diagnoses. With Bell palsy, 85% of patients experience recovery of facial paresis to House-Brackmann Grade I or II in approximately 8–12 weeks, whereas patients with atypical facial nerve tumors, such as facial nerve hemangiomas, often experience a more indolent course without recovery of facial motor function. Uniquely, facial nerve hemangioma size is not directly correlated to the extent of deficit. Small tumors (< 10 mm) may lend themselves to grave deficits. There is debate in the literature with respect to the etiology of the neurological deficit. Previous reports in the literature have claimed that facial nerve deficits arise as a consequence of direct compressive forces; newer schools of thought believe that there is an element of a vascular steal phenomenon wherein blood flow to the highly vascular facial nerve is detoured toward the tumor, resulting in ischemic insult.[4,35] For this reason, facial nerve schwannomas of similar size may lead to less severe cranial nerve dysfunction.[26]

The relationship between facial nerve hemangioma and cavernous malformations isolated to the seventh cranial nerve is controversial. Deshmukh et al.[8] described 2 patients who presented with acute hearing loss and facial nerve paresis; MRI revealed hyperintense lesions, without contrast enhancement, which were found to have small cavernous malformations. Importantly, extraaxial cavernous malformations may also enhance following the administration of Gd, so the presence of enhancement is not sufficient to rule out the presence of a cavernous malformation. The incidence of these lesions is too low to determine whether these lesions are truly distinct from facial nerve hemangioma, or merely histological variations of the same clinicopathological entity.

Although meningiomas are the second most common tumor of the CPA, geniculate meningiomas are exceedingly rare.[27] Although the etiology is unclear, noted associations with progesterone, breast cancer, and radiation therapy have been described.[17] Meningiomas arise from arachnoid villi, which are invaginations of the arachnoid mater along the walls of the dural and venous sinuses. They are also located along the neural foramina of the cranial nerves. Facial nerve meningiomas most likely arise from the arachnoid villi along the porus acousticus (opening between the CPA cistern and IAC) and gasserian envelope. This can be explained embryonically. The seventh and eighth cranial nerves arise from a common primordium. At 5 weeks of gestation, the fibers of the facial nerve exit the neural tube along with a sheath of arachnoid and dura. Although the dura terminates at the IAC, the arachnoid may continue toward the geniculate ganglion and beyond as it gradually fuses with endoneurium.[19,31] Extracranial extension of a neural foramen meningioma is quite rare. More commonly, the tumor would originate intracranially and extend extracranially.

Glomus tumors are also known as paragangliomas or chemodectomas that arise from chemoreceptor cell derivatives of neural crest cells. They originate from paraganglionic tissue typically at the carotid bifurcation (carotid body tumors), jugular foramen (glomus jugulare), vagus nerve (glomus vagale), and tympanic plexus (glomus tympanicum).[21] They may occur sporadically or as a part of hereditary syndromes such as multiple endocrine neoplasia Type II (MEN II), von Hippel-Lindau syndrome, and neurofibromatosis Type 1.[21] In 80% of hereditary cases and 20% of sporadic cases, the patient presents with multiple lesions.

A landmark histological study by Guild in 1941 described 73 temporal bone paragangliomas.[32] The thought is that the glomus tumor may arise from the Arnold nerve, an auricular branch of the vagus nerve that traverses through the mastoid canaliculi from the jugular bulb, superior to the fallopian canal at the stylomastoid foramen, where an ascending branch merges with the facial nerve. This is the same nerve that gives rise to glomus jugulare and glomus tympanicum.[9]

Preoperative Evaluation

Any patient with a suspected tumor of the facial nerve should be evaluated with high-resolution CT scanning of the temporal bone, MRI with and without contrast, and direct otoscopic examination. Large tumors involving the tympanic segment of the facial nerve may be visible on otoscopic examination. Gross features such as vascularity may aid in diagnosis, and transcanal biopsy may be performed in select cases. Electroneurography may also be of assistance in select cases.

Careful review of preoperative radiographs may help to establish the diagnosis of facial nerve schwannoma, especially in the presence of geniculate ganglion enhancement, anterior position of the tumor in the IAC, or linear enhancement of the facial nerve in the mastoid temporal bone.[38] Schwannomas will typically create a smooth but enlarged course along the fallopian canal and may be differentiated from hemangiomas, which lack distinct margins and often contain bony spicules on thin-cut CT. They will also enhance on postcontrast T1-weighted MRI.

Facial nerve hemangiomas are best visualized via high-resolution CT scanning of the temporal bone, namely enlargement of the fallopian canal with a lesion exhibiting irregular margins, amorphous shape, and possibly intratumoral bony spicules. The typical honeycomb or sunburst radiographic appearance is indicative of an ossifying hemangioma. This occurs as a result of osteoclastic remodeling resulting in intralesional lamellar bony trabeculae.[4,29] The honeycomb sign is pathognomonic for hemangioma and helps to differentiate between schwannoma and meningioma. Nevertheless, this is present only 50% of the time.[11] This is also known as an ossifying hemangioma.[29] Magnetic resonance imaging of the brain, with thin-cut sequences through the temporal bone at the IAC, is used if the lesion is not visualized. One would expect to see iso-, hyper-, or variable intensity on T1-weighted images and hyperintensity on T2-weighted images as well as enhancement with Gd contrast. Of note, one would expect normal variations of mild to moderate enhancement of certain aforementioned highly vascularized segments of the facial nerve, namely the geniculate ganglion and tympanic and labyrinthine segments.

Circumferential expansion of the facial nerve canal with well-preserved margins and smooth architecture are typical findings in cases of glomus facialis and are findings comparable to characteristics of schwannoma on CT scanning. Magnetic resonance imaging of the temporal bone, when performed, may reveal a pathognomonic salt-and-pepper pattern as described by Olsen et al.[33] in paragangliomas larger than 2 cm. Otherwise, the lesion has been noted to show hypointensity to muscle on T1-weighted imaging and heterogeneous enhancement with Gd contrast injection. On T2-weighted imaging, there is isointensity to muscle.[18] Angiographic findings include hypervascularity, an enlarged feeding artery, and possible draining vein. This often assists in diagnosis, as rapid arteriovenous shunting is not seen in hemangiomas.

Management

Due to their rarity, there is little guidance in the literature for the appropriate management of facial nerve tumors. Recommendations may be reasonably extrapolated from the approach to the more common facial nerve schwannoma. Many authors, including Shirazi et al.,[37] have advocated conservative treatment of facial neuromas when patients present without facial motor and hearing deficits.[20,28] Treatment strategies include radiological observation, drainage of any cystic component of the tumor for histological diagnosis, and/or bony decompression of the tumor.[37] Even decompression may lend itself to a delayed management approach in attempts at preserving residual nerve function. Nevertheless, if the lesion is symptomatic, resection with attempts at anatomical continuity is the gold standard. Surgical approaches include the most commonly used translabyrinthine approach, as well as the retrosigmoid approach, transmastoid or middle fossa approach, combined middle fossa–transmastoid approach, and transmastoid–transparotid approach. This is often performed with a cable nerve graft interposition of either the sural or greater auricular nerve. Falcioni et al.[12] stated that the chances of satisfactory facial nerve recovery decreases significantly postoperatively if resection is not performed within the 1st year of initial clinical facial nerve dysfunction.

Hemangiomas of the facial nerve at the geniculate ganglion are best approached through the middle fossa. If the vertical segment of the facial nerve is involved and hearing is preserved, a transmastoid approach appears to obtain the best visualization. Often a combined approach is appropriate. Upon dissection a soft, dark red, or even blue, easily dissectible mass will be encountered. As a consequence, the facial nerve is often salvageable. Surgery should be offered within the 1st year of diagnosis, prior to perineural fibrosis, or actual neural infiltration develops, making resection more difficult.[1] Facial nerve decompression and removal of any compressive bony spicules may prolong facial function and minimize the chances of postoperative worsening of facial paresis. Aggressive resection will likely exacerbate preexisting facial paresis and is warranted only in cases of high-grade facial nerve dysfunction (House-Brackmann Grade V or VI).

If resection is appropriate, cable grafting such as with a greater auricular nerve graft may be used. This intervention may preserve facial nerve function in the range of 50%–75%.[10]

Role of Radiosurgery

Recently, our group has published on the changing paradigm for treatment of facial nerve schwannoma, with an increased reliance on radiosurgery for the treatment of these tumors.[39] This is often performed on the basis of MRI and CT imaging alone (that is, without tissue diagnosis). While radiosurgery has had generally favorable results for the treatment of facial nerve schwannoma, the role of radiosurgery in these atypical tumors, especially facial nerve hemangiomas, is less clear.

Other rare causes of facial nerve paresis from tumor involvement of the intratemporal facial nerve include epidermoid (cholesteatoma), metastasis, and direct invasion from skull base carcinoma. The facial nerve perineurium serves as a gateway to the temporal bone for neoplasms such as parotid mucoepidermoid carcinoma, benign pleomorphic adenoma of the parotid gland, and squamous cell carcinoma.[16] Tumors extrinsic to the intratemporal segment including primary temporal bone tumors, pontine gliomas, and parotid tumors may also affect facial nerve function.

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