MR imaging of facial nerve in a case of Bell's palsy

Clinical History

A 28-year-old lady presented with an acute history of sagging of the angle of the mouth on the left side with difficulty in eating food. The patient was diagnosed with lower motor neuron type facial nerve palsy on the left side at clinical examination. No antecedent cause for the same could be delineated.

Imaging Findings

Cerebral parenchyma was unremarkable with no altered signal areas on all conventional sequences.
Focal enhancement of the meatal segment and diffuse enhancement of the canalicular part (labyrinthine, ganglionic, tympanic and mastoid ) with mild thickening of the left facial nerve was noted.

Discussion

Idiopathic facial nerve palsy, popularly referred to as Bell’s palsy, is believed to result from inflammatory oedema of the facial nerve, with resultant compression of the vascular supply within the tight fallopian canal. The labyrinthine segment resides in the narrowest portion of the canal and hence is most susceptible to ischaemia. [1]
Mild enhancement of the labyrinthine segment, geniculate ganglion, and proximal tympanic segments can be normal on post-contrast MR, presumably secondary to the presence of a circumneural venous plexus in these segments. [1]
All the routine haematological and conventional imaging tests were normal in this patient. No other clinical complaint was present. The patient responded well to prednisolone therapy. No antiviral agents were used. No evidence of relapse was noted during the past year. A diagnosis of exclusion was made in conjunction with neuritis findings seen on MRI with otherwise unremarkable brain.
Mechanisms of gadolinium enhancement of the facial nerve in acute palsy are thought to be the disruption of the blood nerve barrier, which is formed by tight junctions of capillary endothelium, and hypervascularity or venous congestion at perineural structures. [2]
Signal intensity increase at IAC and labyrinthine segments seemed to have statistically significant correlation with the initial severity of facial nerve inflammation. [2]
Take Home Message:
Facial Nerve MR Imaging can play a central role in Bell's palsy, which is otherwise a diagnosis of exclusion.

Differential Diagnosis List

Final Diagnosis

Idiopathic facial nerve neuritis

URL:	https://www.eurorad.org/case/13892
DOI:	10.1594/EURORAD/CASE.13892
ISSN:	1563-4086