Diagnosis and surgical outcomes of intraparotid facial nerve schwannoma showing normal facial nerve function

Abstract

The findings of intraparotid facial nerve schwannoma (FNS) using preoperative diagnostic tools, including ultrasonography (US)-guided fine needle aspiration biopsy, computed tomography (CT) scan, and magnetic resonance imaging (MRI), were analyzed to determine if there are any useful findings that might suggest the presence of a lesion. Treatment guidelines are suggested. The medical records of 15 patients who were diagnosed with an intraparotid FNS were retrospectively analyzed. US and CT scans provide clinicians with only limited information; gadolinium enhanced T1-weighted images from MRI provide more specific findings. Tumors could be removed successfully with surgical exploration, preserving facial nerve function at the same time. Gadolinium-enhanced T1-weighted MRI showed more characteristic findings for the diagnosis of intraparotid FNS. Intraparotid FNS without facial palsy can be diagnosed with MRI preoperatively, and surgical exploration is a suitable treatment modality which can remove the tumor and preserve facial nerve function.

A schwannoma is an encapsulated benign tumor originating from Schwann cells. Facial nerve schwannoma (FNS) can arise anywhere along the course of the nerve, whether intratemporal or extratemporal. Most FNS arise on the intratemporal segment, and only 9% occur on the intraparotid segment. Most patients with intraparotid FNS present as outpatients without facial palsy, and most are diagnosed during surgery, since they are often mistaken for other benign tumors due to the unspecific findings shown by fine needle aspiration (FNA) and imaging studies. Due to the possibility of facial nerve injury, management is usually conservative. However, since intraparotid FNS are slow growing, many patients visit with several years of disease duration, and cases of huge mass formation are occasionally encountered because there are no anatomical barriers around the lesion. Surgical removal is required in these cases, but surgeons may hesitate due to the possibility of facial nerve palsy.

In this study, a retrospective analysis of the medical records of patients who were diagnosed with intraparotid FNS was performed, and the characteristics shown on ultrasonography (US)-guided FNA, computed tomography (CT), and magnetic resonance imaging (MRI) were examined. Treatment guidelines are proposed.

Materials and methods

The medical records of 15 patients diagnosed with intraparotid FNS between 2005 and 2011 were analyzed retrospectively. Six of the patients were male and nine were female, and their mean age was 40 years (range 28–69 years). The average tumor size was 3.7 cm (range 1.4–8.0 cm). All patients visited with the complaint of a painless parotid mass, and facial palsy was not observed in any case. Patient information is presented in Table 1 . For diagnosis, FNA and imaging studies including US, CT, and/or MRI were performed.

Table 1
Summary of patients with intraparotid facial nerve schwannoma.
No. Sex/age (years) Presenting symptom Size (cm) FNA Preop. facial nerve function (HB) Surgery Postop. facial nerve function (HB)
1 M/29 Facial mass 3 Inconclusive I Sleeve I
2 M/45 Facial mass 3.4 Inconclusive I Debulking I
3 F/39 Facial mass 3.6 Inconclusive I Sleeve I
4 F/38 Facial mass 5.7 Schwannoma I Sleeve II
5 M/69 Facial mass 2.5 Pleomorphic adenoma I Sleeve I
6 M/28 Facial mass 2.5 Schwannoma I Sleeve I
7 F/53 Facial mass 3.5 Schwannoma I Observation I
8 F/34 Facial mass 1.6 Schwannoma I Sleeve I
9 F/35 Facial mass 5.3 Inconclusive I Sleeve I
10 F/36 Facial mass 4.7 Inconclusive I Sleeve I
11 M/37 Facial mass 3.8 Pleomorphic adenoma I Sleeve I
12 F/38 Facial mass 4 Inconclusive I Sleeve I
13 M/39 Facial mass 8 Inconclusive I Sleeve I
14 F/40 Facial mass 2 Inconclusive I Debulking I
15 F/41 Facial mass 3.5 Schwannoma I Sleeve I

FNA, fine needle aspiration; HB, House–Brackmann; Preop., preoperative; Postop., postoperative; M, male; F, female.

Surgery for tumor removal was performed in all 15 patients. The operative procedure is outlined below. After performing a superficial parotidectomy, the facial nerve was identified and the FNS arising from the facial nerve was confirmed. Then, the nerve fiber that runs above the capsule of the FNS was identified by microscope and an incision was made with a sickle knife at the portion of the tumor capsule where the nerve fiber was not observed. The tumor capsule was dissected from the incision and tumor removal was performed ( Fig. 1 ). In cases where dissection was not feasible, debulking of the inner portion of the tumor was done with brain forceps after capsule incision, and the pathology was confirmed. In cases where the border between the nerve fiber and tumor was not definite, the operation was terminated.

Fig. 1
(A) After superficial parotidectomy, a dumbbell-shaped tumor (white arrow) arising from the facial nerve trunk (star) was identified. The tumor was connected to the facial nerve with a small stalk, and the tumor was dissected from the capsule from the incision that was made upon the capsule where the nerve did not come across (case 4). (B) The facial nerve (star) is observed and the tumor, which was connected with a small stalk, was dissected after the incision was made on the capsule (case 5).

Results

Fine needle aspiration biopsy

US-guided FNA was performed in all patients. Findings in eight cases were reported as inconclusive, whereas in five cases, a schwannoma was suspected due to observed Verocay bodies. Two cases tended to indicate pleomorphic adenoma, but the pathologic report of the surgical specimen was reported as schwannoma ( Table 1 ).

Radiologic features ( Table 2 )

On US, all lesions were well defined. Hypoechoic lesions were observed in 11 subjects, whereas hyperechoic lesions were observed in four patients. Among the 11 hypoechoic lesions, a multilobulated cystic area was observed in two cases ( Fig. 2 ). On CT, well-defined low-attenuated lesions were observed in all cases, and widening of the stylomastoid foramen was observed in four cases ( Fig. 3 ). MRI was performed for six patients. On MRI, all lesions showed isointense signal intensity relative to muscle on T1-weighted images, and hyperintense signal intensity relative to muscle on T2-weighted images. The target sign, which refers to increased peripheral signal intensity and decreased central signal intensity on T2-weighted images, was found in only two cases, and all six subjects showed the target sign on gadolinium (Gd)-enhanced T1-weighted images ( Figs. 4 and 5 ). Multiple lesions were observed in two cases ( Fig. 6 ). MRI was not performed for nine subjects.

Table 2
Radiologic features of US, CT, and MRI.
No. US CT T1 T2 Gd enhanced Widening of ST foramen Target sign on T2 Target sign on Gd T1
1 Hypoechoic Low density Isointense Hyperintense Enhanced +
2 Hypoechoic Low density Isointense Hyperintense Enhanced +
3 Hyperechoic Low density Isointense Hyperintense Enhanced + +
4 Hypoechoic multilobulated Low density Isointense Hyperintense Enhanced + +
5 Hyperechoic Low density NE NE NE NE NE
6 Hypoechoic Low density NE NE NE NE NE
7 Hypoechoic lobulated Low density Isointense Hyperintense Enhanced + +
8 Hyperechoic Low density NE NE NE NE NE
9 Hypoechoic Low density NE NE NE NE NE
10 Hypoechoic Low density NE NE NE NE NE
11 Hyperechoic Low density NE NE NE NE NE
12 Hypoechoic Low density NE NE NE + NE NE
13 Hypoechoic lobulated Low density NE NE NE + NE NE
14 Hypoechoic Low density NE NE NE NE NE
15 Hypoechoic multilobulated Low density Isointense Hyperintense Enhanced + +
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Jan 24, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Diagnosis and surgical outcomes of intraparotid facial nerve schwannoma showing normal facial nerve function
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