Tumor Anatomy, Pathology, and Pathogenesis

FNSs arise sporadically from the Schwann cell sheaths. They may involve any area of the facial nerve, but have a predilection for the perigeniculate area, the tympanic segment, and the mastoid segment ( Fig. 14.1 ).6,7 Skip lesions, or multiple segments of involvement, are not infrequent. Despite having the appearance of multiple discrete tumors on a nerve, histopathological studies have noted discrete intraneural connections between separate portions of the lesion.8 Their gross appearance is that of a diffuse bulging of the facial nerve, sometimes over several segments. The tumors are relatively homogeneous tan to gray in color, with larger tumors exhibiting areas of cystic formation. Microscopically, intact nerve fibers can permeate a tumor and thus biopsy of these lesions may precipitate a facial paralysis.8 Typically, the tumors are well encapsulated and consist of two distinct histological patterns. Antoni A areas show broad interlacing ribbons of spindle cells with elongated nuclei, generally arranged in wave or whirl patterns. Verocay bodies, which are zones where the cells are arranged in palisades at either end of a bundle of parallel fibers, can be seen in the Antoni A regions. Antoni B areas are composed of a loose tissue arrangement that is often described as stellate and pleomorphic cells embedded in a myxoid ground substance with occasional multinucleated giant cells.

Clinical Presentation

As with other schwannomas, FNSs typically grow slowly over time. FNSs usually do not cause symptoms until they are fairly large and may manifest initially with hearing symptoms. Symptoms include slowly progressive facial paresis or paralysis and hearing loss, though tinnitus, pain, vestibular symptoms, or an ear canal mass may be present.9 In the presence of a middle ear or ear canal mass, biopsy must not be performed because of the likelihood of resulting facial paralysis. Lipkin and colleagues, in their review of 238 cases, found patient age ranged from 4 to 81 years, with a mean age of 39 years.9 Gender and sidedness were equally distributed. The most common symptoms were facial paralysis (73%), hearing loss (50%), tinnitus (13%), and ear canal mass (13%). Patients also reported pain (11%), vestibular symptoms (11%), and otorrhea (6%).

Facial nerve symptoms in FNSs have been classically described as having a slowly progressive pattern. Jackson and colleagues noted that facial paresis that progresses to complete paralysis over 6 to 12 weeks is not Bell palsy.10 May and Hardin argued that progression of a facial paralysis beyond 3 weeks was suggestive of a neoplasm.11 Facial twitching followed by progressive paresis is very characteristic of these tumors and should trigger suspicion for a neoplasm. Furthermore, a diagnosis of Bell palsy should not be accepted if there are no signs of recovery at 6 months.

However, FNSs can masquerade as Bell palsy presenting with a sudden complete paralysis with the occurrence ranging from 14–21%.9,12,13 In addition, the presentation of recurrent facial palsy should trigger a search for neoplastic involvement of the facial nerve. One should not be lulled into the diagnosis of “recurrent Bell palsy.” FNSs that arrive in the internal auditory canal and cerebellopontine angle may manifest with progressive sensorineural hearing loss similar to that caused by a vestibular schwannoma (VS). The diagnosis of FNSs in these cases may not be established until they are identified surgically. In a patient with a suspected VS, the presence of coexisting facial nerve symptoms should raise the surgeon′s suspicion for an FNS. The solicitation of hyperkinesis, slowly progressive facial paralysis, recurrent ipsilateral paralysis, or asymmetric hearing loss on history or the presence of a mass in the ear canal, middle ear, or parotid should prompt radiological investigations.

Diagnostic Studies

Audiological testing should be obtained in all patients with facial paralysis. It should be mentioned that Bell palsy does not cause hearing impairment and, thus, any abnormality on the pure tone audiogram should mandate further investigation. Opinions differ on the utility of electroneurography (ENoG) and electromyography (EMG) testing in the diagnosis stage. Problems with standardization and potential sources of error as well as intertest and interdevice variability have been noted. However, the utility of these studies is optimized for those who use them frequently and are familiar with the limitations of their own test procedures. ENoG showing increased latency and/or greater amplitude reduction than expected for incomplete facial paralysis and an EMG showing fibrillations and polyphasic potentials simultaneously can be clues for pushing one to pursue radiological evaluation. The latter is consistent with compression neuropathy.14 These studies may help in instances where the diagnosis of an FNS is in question, with the alternative being a VS, and may help the surgeon to better prepare and counsel the patient accordingly.

Radiological imaging, meaning computed tomography (CT) and magnetic resonance imaging (MRI), are easily obtained in the modern era of medicine. Findings on CT provide a detailed look at the bony anatomy and may show dilation of the fallopian canal or intralabyrinthine segment ( Fig. 14.2a ), bone erosion around the geniculate ganglion or otic capsule, and possibly a middle ear mass. MRI with gadolinium, however, is the initial imaging study of choice and is superior in demonstrating disease in the cerebellopontine angle and extratemporally in the parotid as well as demonstrating tumor extension along the facial nerve. FNSs are isointense or slightly hypointense compared with brain on T1-weighted MRI, exhibit homogeneous enhancement with gadolinium, and demonstrate increased signal on T2-weighted images with characteristics similar to cerebrospinal fluid. Proximal tumors may imitate acoustic neuromas ( Fig. 14.2b ).

Tumor Anatomy, Pathology, and Pathogenesis

FNHs are probably not true neoplasms but are more likely to be vascular malformations, specifically venous vascular malformations.16 A classification for vascular lesions based on clinical, histopathological, and cytological features was introduced by Mulliken and Glowacki in 1982 and has been subsequently refined ( Table 14.1 ).17,18 The refined classification system argues that the term “hemangioma” should be reserved for benign vascular tumors that arise by cellular hyperplasia, whereas the term “malformation” should be used for errors of vascular morphogenesis that develop in utero and persist postnatally. Benoit and colleagues argue that what have been previously called FNHs exhibit clinical, histopathological, and immunohistochemical characteristics consistent with venous vascular malformations.16

On gross inspection, hemangiomas are rubbery red or purple masses that have the appearance of a sponge on sectioning. Histologically, FNHs appear as spaces lined by single layer of endothelial cells and filled with red blood cells. More specifically, Benoit et al provide a modern histologic description noting the presence of irregularly shaped dilated vessels with scant mural smooth muscle without an internal elastic lamina consistent with venous or lymphatic malformation.16 Benoit and colleagues note that, in all specimens examined, lesional endothelial cells were mitotically inactive. Unlike traditional hemangiomas, facial nerve or geniculate ganglion hemangiomas do not stain positively for glucose transporter protein isoform 1 or Lewis Y antigen, and demonstrate immunoreactivity for podoplanin, a marker for lymphatic endothelial differentiation.16

Traditionally, it is thought that hemangiomas of the facial nerve originate from the vascular plexuses that surround the nerve. Vascularity is most intense around the geniculate ganglion and, to a lesser extent, around the internal auditory canal and midmastoid portion.19 As a result of their origin from the vascular plexus, these tumors are typically extraneural, although infiltration of the nerve does occur.20 Thus, FNHs tend to occur around the geniculate ganglion. Although extension to the labyrinthine or tympanic segment can occur, extension to the internal auditory canal or beyond the cochleariform process is uncommon.21

Clinical Presentation

Despite being characteristically slow growing, FNHs tend to produce significant facial dysfunction at early stages out of proportion to the actual tumor size.21 Some have suggested that this is due to a “vascular steal” phenomenon. In a review of the experience with geniculate ganglion hemangiomas at the House Clinic, which represents the largest single institution published experience in the literature, the most common presenting symptoms and signs were progressive facial paresis or paralysis and facial twitching followed by hearing loss due to either disruption of the ossicular chain or from erosion into the cochlea ( Fig. 14.3a, b ).

Table 14.2 depicts the distribution of presenting signs and symptoms. The House-Brackmann (HB) grading at presentation was I/II, III/IV, and VI in seven (38%), six (28%), and six patients (34%), respectively ( Table 14.3 ).

Diagnostic Studies

While audiometry and acoustic reflex testing are important, they are nonspecific in patients with FNHs. However, the underlying hearing status is important for planning of surgical management and approach. Due to their small size and lack of unique angiographic appearance from other vascular lesions, angiography plays little role in the evaluation.

On ENoG, some lesions may demonstrate significant decrease in the response amplitude, and EMG may show a characteristic pattern of mixed regeneration and degeneration with concomitant presence of fibrillation potentials and polyphasic action potentials. This resulting dyssynchrony may explain the discrepancy seen in some patients between the severity of amplitude reduction noted on ENoG and the relatively mild facial nerve dysfunction.

High-resolution, fine-section CT and MRI are essential for the evaluation of a patient when a facial nerve hemangioma is suspected. CT may demonstrate enlargement of the geniculate ganglion and possible enlargement of distal labyrinthine or proximal tympanic segments of the facial nerve (see Fig. 14.3a, b ). Typically, the lesion is centered on the geniculate ganglion and may exhibit adjacent nonsharp bony erosion with irregular borders, intralesional stippled calcifications, and spicules of bone. On MRI, FNHs tend to be slightly hypo- or isointense with brain on T1-weighted imaging and enhance intensely with gadolinium ( Fig. 14.3c ).

On T2-weighted imaging, FNHs tend to be hyperintense to brain, although they can be less intense than FNSs. Sometimes, intratumoral calcifications are present, and these lesions may exhibit a salt-and- pepper appearance on MRI. Lo and colleagues have shown that contrasted MRI was more sensitive in detecting hemangiomas of the internal auditory canals but missed four of six geniculate ganglion hemangiomas.22 Thus, in patients with progressive facial palsy, a gadolinium-enhanced MRI of the internal auditory canal is the initial study of choice. If MRI is negative, then a high-resolution CT scan of the temporal bone is recommended to evaluate for geniculate ganglion hemangioma.