Diagnostic Challenges and Imaging Considerations for Intraparotid Facial Nerve Schwannoma: A Case Report and Literature Review

Introduction

A rare site of origin for schwannomas is the parotid gland. Schwannomas arise from the Schwann cells of myelinated cranial or peripheric nerves. They are benign encapsulated tumors originating from the neuroectoderm [1,2]. Usually, head and neck schwannomas arise in the vagus nerve, sympathetic trunk, or vestibular nerve. Facial nerve schwannomas (FNS) are rare and uncommon neoplasms of the seventh cranial nerve. Most FNS arise in the skull base. Extratemporal growth is described in only 9% of cases [3,4]. There is a strong association between FNS and neurofibromatosis types 1 and 2 [2].

The management of FNS is exceptionally complicated because the diagnosis is usually only made intraoperatively. In addition, resection of the tumor could lead to facial nerve palsy, with significant esthetic consequences. On the other hand, if surgical treatment is delayed, tumor progression can lead to severe but rare complications, such as the development of hydrocephalus [5]. Therefore, the decision as to when therapy should be initiated and in what form it should be carried out should be carefully considered [6].

Case Reports

The first case was a 28-year-old male patient with a mass in the right parotid gland incidentally diagnosed during epilepsy imaging. The patient presented to our clinic for a more detailed evaluation. The facial nerve function was normal. However, magnetic resonance imaging (MRI) showed a cystic mass along the course of the facial nerve in the mastoid segment and right parotid gland, with minimal contrast affinity (Figures 1, 2), which was not suspicious for malignancy. We immediately examined the right parotid gland, which revealed a large, sharply circumscribed, lobulated, partially septated mass with dorsal acoustic enhancement without internal perfusion (Figure 3). The subsequent ultrasound and MRI examinations showed a constant size and aspect. Electromyographic measurement of the facial nerve showed unremarkable findings (House-Brackmann grade I).

The second case involved a 34-year-old female patient who noticed a progressive mass on the right mandibular angle for several weeks. Neurological deficits and other symptoms were also denied. To further evaluate the patient’s concerns, we performed ultrasonography of the head and neck region (Figure 4), an MRI scan of the head (Figures 5, 6), and an electromyographic measurement of the facial nerve. Surprisingly, the findings were similar to those of the first patient.

The various therapeutic options were carefully discussed with the patients. Without clinical symptoms and size constancy, both patients opted for a wait-and-scan approach.

Discussion

Schwannomas (also called neurinomas) are benign tumors that originate from myelin-forming cells and are characterized by generally slow growth. The symptoms depend on the growth behavior of the location and size of the tumor and on the function of the corresponding nerve. Schwannomas can occur anywhere in the body where peripheral nerves are located, but they are most common in the head, face, and extremities. Vestibular schwannoma is the most common, accounting for over 90% of cases. In contrast, non-vestibular cranial nerve schwannomas are rare lesions and account for only 5% to 10% of cases. These include, in descending order, trigeminal nerve schwannomas, lower cervical nerve schwannomas, FNS, and, in very few cases, oculomotor nerve schwannomas. FNS can arise anywhere along the course of the facial nerve and have been classified according to their sites of origin: the cerebellopontine angle and the labyrinthine, geniculate, tym-panic, mastoid, and parotid segment. In particular, intraparotid facial nerve schwannomas (IPFNS) are extremely scarce and account for approximately 14.6% of FNS [6,7]. Due to their extracranial course, they are often mistaken for more conventional tumors, such as pleomorphic adenomas [8,9]. Since there are often no clinical symptoms, like in our patients, preoperative diagnosis is complex, and there are no standard diagnostic criteria. Facial nerve paresis could occur in both benign and malignant parotid tumors. According to Marchioni et al, 72 of 79 patients (91.1%) with a diagnosis of IPFNS had normal facial nerve function. The probability of facial nerve palsy in IPFNS is approximately 19% [2]. This is due to the absence of nerve compression in a bony canal in a solely extratemporal tumor location.

Ultrasonography is a standard imaging modality in diagnosing parotid gland tumors. However, as described in case reports, IPFNS present as hypoechogenic, with sharply marked lesions with dorsal acoustic enhancement, and therefore, can be mis-diagnosed as neoplasms of salivary gland tissue. The presented cases illustrate that extended imaging should be considered, especially in space-occupying lesions in broad contact with the mastoid process. The criterion standard of imaging modality is an MRI scan [2]. In concordance with our case report, imaging features suggestive of facial nerve schwannoma include well-circumscribed margins, displacement of adjacent structures, cystic formation, and heterogeneous relatively strong signal intensity, especially on T2-weighted MR images [10]. Due to the rare occurrence of IPFNS and clinically and radiologically uncharacteristic features, preoperative diagnosis is challenging. Marchioni et al could not identify defining radiological imaging features after reviewing 79 cases of patients with IPFNS [9]. In the association of the tumor to the fallopian canal, the diagnosis of IPFNS by MRI is more likely. A computed tomography scan can strengthen this finding to detect bony remodeling in the fallopian canal as an indication of growth tendency [11,12]. It is challenging to distinguish facial nerve sheath tumors from other neoplasms, such as vestibular nerve schwannomas or meningiomas located in the cerebellopontine angle or petrous bone. A hint that can indicate IPFNS is the involvement of the geniculate ganglion or the various segments of the facial nerve, such as its labyrinthine, tympanic, or mastoid course [12].

Another diagnostic method is fine needle aspiration cytology (FNAC). FNAC performed preoperatively often leads to incorrect diagnoses and inconclusive results [8,9]. Other studies showed that IPFNS could be detected preoperatively by FNAC in only 15 of 67 patients (22%). A recent case report showed that in only 31.6% of cases, FNS could be detected by FNA [10]. According to many authors, there are better methods than FNAC [9,10]. A more reliable way to diagnose IPFNS is an intraoperative diagnosis through surgical excision and histopathologic evaluation [9,10]. Adequate treatment of IPFNS is challenging for surgeons. Surgical diagnosis confirmation carries many risks for the patient regarding aesthetics and quality of life. Patient age, facial nerve function, growth progression, and tumor location are important factors that should be considered in the decision-making process [11]. Numerous authors consider a House-Brackmann grade of IV or worse as an indication for surgery [11,13]. Other important surgical indications are complications such as hearing loss, development of hydrocephalus, or brain stem compression in cases of intratemporal manifestation [14]. In cases with a House-Brackmann grade of III or better and uncritical tumor location, in which no other symptoms are present, and there is a strong suspicion of IPFNS radiologically, we agree with other authors that conservative techniques such as wait and scan is the method of choice [1,13,14]. When considering observation as a method of choice, it is essential to know that facial schwannomas have a reported annual growth rate of 2.0 mm per year. Most schwannomas remain constant in size for years without progression. Doshi et al showed that of 14 patients with a House-Brackmann grade III or better, 12 remained without clinical or radiographic progression after 13 years of observation [11]. A study by McMonagle et al strengthens this conclusion. The authors demonstrated that 20 of 53 patients with facial nerve schwannoma were followed up for 8 years and clinically showed good facial nerve function [11]. Thus, as in our case report, a wait-and-scan approach could be a good option for selected patients.

Stereotactic radiosurgery is another established treatment option in selected patients [14–16]. Multiple studies have shown that stereotactic radiosurgery provides reasonable short-term tumor control rates. In a meta-analysis of 45 patients, tumor control was achieved in 93% of cases after 2 years. Sheehan et al showed, after reviewing 42 cases, that 90% of the patients were in progression-free survival after 5 years. The mean marginal dose of radiation was 12.5 Gray. These findings agreed with the study of Hasegawa et al, in which after 5 years, the progression-free survival rate was 92% [11,16]. The probability of post-radiogenic, new-onset worsening of facial nerve function after doses less than 14 Gray was less than 5% [11]. However, it is essential to mention that radiosurgery has 2 significant drawbacks. First, the possibility of complete tumor removal eliminates full histologic evaluation. On the other hand, surgical therapy offers the possibility of patient-specific facial nerve rehabilitation. Second, surgical therapy following radiotherapy is associated with increased complications. Numerous case reports elucidate IPFNS, delineating diverse methodologies and offering intriguing deductions. For example, De Ceulaer et al documented a 68-year-old patient with pre-existing impairment of the facial nerve’s frontal branch. A diagnosis of IPFNS was established based on medical history, symptoms, and MRI examination. Intraoperative frozen section examination confirmed the diagnosis, leading to surgical resection with nerve reconstruction, using a nerve graft. Postoperatively, facial nerve function partially recovered, reaching House-Brackmann grade II [17]. The 2018 case report of Simone et al presented 2 instances of IPFNS. In the first case, a parotidectomy was performed, involving tumor stripping from the nerve. The second case underwent nerve resection with complex reconstruction using hypoglossal-facial neurorraphy. Similar to our findings, the authors emphasized the challenge of preoperative diagnosis but stressed its crucial nature. Postoperative paralysis was expected, and FNAC often lacked diagnostic accuracy. A 24-month follow-up revealed House-Brackmann grade II in temporal-ocular and grade III in facial-cervical branches for the first case, while the second case showed House-Brackmann grade III [18]. Marchioni et al reported on a 74-year-old patient presenting with facial nerve palsy (House-Brackmann grade II). During parotidectomy, the intraoperative sacrifice of the facial nerve occurred and was subsequently reconstructed using a great auricular nerve graft. Immediate postoperative assessment revealed complete facial nerve palsy. A 6-month followup indicated House-Brackmann grade V. The authors concluded that tumor extent and involvement of different facial nerve branches are crucial for prognosis and treatment planning [19].

In our case presentation of IPFNS, we emphasize unique diagnostic challenges and the pivotal role of trans-cervical ultrasound imaging. These 2 cases highlight the complexity of IPFNS diagnosis, particularly in the absence of standard diagnostic criteria, and the potential for misdiagnosis as more conventional salivary gland tumors. Our findings underscore the necessity of extended imaging for accurate diagnosis and management of IPFNS, especially when lesions are in close contact with the mastoid process. This approach is crucial for distinguishing IPFNS from other neoplasms, a challenge accentuated by their rare occurrence and non-characteristic clinical and radiological features.

Conclusions

Diagnosis and therapy for IPFNS remain challenging. When deciding on therapy, it must be considered that significant consequences for the patient and his quality of life can result. Therefore, whether and when surgery is indicated must be weighed. A vital aspect highlighted is the emphasis on ultrasound as the preferred imaging modality for masses in the parotid gland. The differential diagnosis of IPFNS is particularly crucial. Examiners should be familiar with the sonographic findings of these pathologies to initiate further diagnostic imaging when IPFNS is suspected. Our report underscores this point by demonstrating that trans-cervical ultrasound is critical for initial assessment and subsequent diagnostic procedures. The treatment decision is based on facial nerve function, patient age, tumor location and size, and last but not least, the patient’s preferences. An observational wait-and-scan approach is an option for patients with small tumors and normal facial function. On the other hand, surgical approaches or stereo-tactic radiosurgery can be vital for patients with already limited facial nerve function.