Plexiform neurofibroma of the facial nerve

Clinical History

A young male patient presented with a painless swelling of the face on the right side. On examination, a non-tender mass was palpable in the region of the parotid. The patient also had multiple hyperpigmented macules on the chest and a few small subcutaneous swellings in the lower limbs.

Imaging Findings

High-frequency ultrasound showed a large, heterogeneous, predominantly hypoechoic mass in the superficial lobe of the parotid gland. The deep lobe was not visualized on ultrasound. No ductal dilatation was noted. To further characterize the nature and extent of the lesion, an MRI was performed. A large well-defined lobulated mass was seen involving both the lobes of the right parotid gland. The mass appeared isointense to muscle on T1WI and markedly hyperintense on T2WI. On contrast administration, intense heterogeneous enhancement of the mass was noted. Intraparotid facial nerve could not be separately visualized from the mass. However, the facial nerve appeared normal in size and signal intensity at the point of its exit from the stylomastoid foramen. There was no invasion of the mass into the other adjacent structures. Based on the clinical and imaging findings, a diagnosis of plexiform neurofibroma of the facial nerve was made and confirmed on histopathological examination.

Discussion

Neurogenic tumours involving the facial nerve are rare and the majority of them occur in its intratemporal course. Intraparotid location of facial nerve tumours is very rare. Most facial nerve neurofibromas (especially the plexiform variant) are associated with neurofibromatosis type 1 [1].

On computed tomography, neurofibromas appear as soft tissue density masses. On MR images, they are isointense to muscle on T1WI and hyperintense on T2WI. Despite being hypovascular, they enhance intensely on postcontrast imaging. A fusiform shape, nerve root encasement and homogenous enhancement pattern favours the diagnosis of neurofibroma. Further, a “target”-like appearance on T2W images (central dark signal and peripheral bright signal) is considered characteristic of neurofibromas [2]. In contrast, schwannomas are usually round in shape, demonstrate inhomogeneous enhancement with intralesional necrosis and cystic changes. Unlike neurofibromas, schwannomas tend to displace the nerve roots. Even with these characteristics, it may be impossible to distinguish neurofibromas from schwannomas on cross-sectional imaging. Rather, the major role of cross-sectional imaging is to define the extent of the neurogenic lesion, which aids in surgical planning.

The term plexiform neurofibroma is used when there is diffuse nodular enlargement of a nerve and its branches. Multiple fascicles of a nerve can be involved and these masses can also infiltrate into multiple tissue layers [3]. They are pathognomonic of NF-1. They are most commonly diagnosed during early childhood with growth spurts occurring during puberty and pregnancy. They can grow to a large size, which often leads to significant morbidity, including disfigurement and functional impairment. It is also important to identify these tumours early because of their risk of malignant transformation, which is seen in up to 15% cases [4].

Facial nerve function plays a central role in the management of these tumours. If the facial nerve function is compromised, the mass is usually resected. In patients with intact facial nerve function, some authors advocate surgery while others suggest follow-up with serial electroneurography and CT.

Differential Diagnosis List

Final Diagnosis

Plexiform neurofibroma of the facial nerve

URL:	https://www.eurorad.org/case/12525
DOI:	10.1594/EURORAD/CASE.12525
ISSN:	1563-4086