Due to long route of the facial nerve various pathology can affect it at any level of its course and both computed tomography and magnetic resonance imaging represents the main modalities for revealing that benign and malignant pathological conditions.
While CT imaging provides higher spatial resolution for inspecting the bony structures,
the facial canal and any calcifications within lesions,
MRI is superior at evaluation of the facial nerve itself and most of its pathology.
Neoplasms of the facial nerve have to do primary or secondary with it.
Schwannomas are the most common primary neoplasms of the facial nerve,
best seen on MRI and characterized by uniform,
avid enhancement in T1 weighted postcontrast images.
In T1 images can be seen iso-to hypointense and hyperintense in T2 images.
Very small facial schwannomas may be distinguished from vestibular in CE high resolution MR imaging,
primarily based on their origin while when large and limited to the cerebellopontine angle and the internal auditory canal are very difficult to differentiate from vestibular.
High resolution CT imaging is required to access the internal auditory canal for erosion.
(Figures 9,10)
Fig. 9: MR T1W image after IV CM shows a neuroma at the right VII - VIII cranial nerves and cerebellopontine angle.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Fig. 10: MR T1W image after IV CM shows a facial neuroma at the left middle ear.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Cholosteatomas at MRI can be seen with reduced diffusion while CT can reveal an expansile soft-tissue mass with erosion of the lateral wall and various opacification of the middle ear and mastoid air cells.
In advance cases there is also bony destruction including facial canal.
(figure11)
Fig. 11: CT image shows a cholosteatoma that extends to the left internal auditory canal.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Meningiomas limited to auditory canal can mimic both facial and vestibular schwannomas and can expanded across the bony canal.
They also demonstrate enhancement on both CT and MRI and while MRI can show the presence of a "dural tail" CT can help further to differentiate meningiomas from schwannomas revealing the small petechial calcifications and the permeative sclerotic process,
characteristic of meningiomas.
(figures 12,
13,
14)
Fig. 12: MR T1W image after IV CM. Leptomeningeal metastases at the cerebellum and bilateral CN VII - VIII.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Fig. 13: MR T1W image after IV CM. Meningeal carcinomatosis at the right CN VII - VIII in a patient with breast cancer.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Fig. 14: MR T1W image after IV CM. Malignant meningioma compressing the left facial nerve.
References: Department of Radiology, KAT General Hospital, Athens, Greece
CNS lymphoma,
hematogenous metastatic lesions and perineural spread of tumors in the cerebellopontine angle represent relative uncommon other malignancies affecting the facial nerve and can be revealed by CE MRI as markedly enhancement and moderate thickening of the nerve and should be considered to any patient with eccentric enhancement of it and history of cancer.
(figures 15,
16)
Fig. 15: CT image shows a meningeal metastasis in the right cerebellopontine angle.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Fig. 16: MR T1W image after IV CM. Meningeal metastasis at the right facial nerve in a patient with NHL.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Glomus tumor of the facial nerve are rare and can be evaluated on CE MR images as markedly enhancing expansile lesion with smooth margins,
hemorrhage or calcification and salt-and-pepper appearance in larger lesions due to flow voids.
(figure 17)
Fig. 17: MR T1W image after IV CM shows a very small tumor - glomus tympanicum at the left facial nerve.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Infectious conditions
- Reactivation of herpes simplex virus infection along the facial nerve consists of the majority of Bell's palsy and represents an acute and rapid paralysis of the nerve to a previously asymptomatic patient.
MRI can show the swelling of the facial nerve and the degree of increased signal correlates directly with worse prognosis.
CT has limited or no role in the evaluation of Bell's palsy however can safely excluded from differential diagnosis if CT reveals expansion of the facial canal.
- Ramsay - Hunt syndrome (varicella zoster virus infection) can be seen in axial FS T1 - weighted CE MRI as avid enhancement of distal canalicular and labyrinthine segments of the facial nerve.
(figure 18)
- Lyme disease, when untreated can develop neurologic symptomatology involving facial nerve and should be considered in cases of patients in endemic areas with facial neuritis.
In that cases MRI can show small,
nodular enhancement of the nerve.
- Sarcoidosis may be another infectious condition that involves facial nerve and can be demontsrated - although nonspecific - in CE MR coronal T1-weighted images as bilateral thickening and irregular enhancement of the facial nerve.
- Fungal infections can be evaluated with CT whereas irregular soft - tissue expansion to mastoid segment of facial nerve can be seen.
- Multiple sclerosis can also affect the facial nerve and in that cases CE MRI may show edema,
reduced diffusion of ongoing inflammation and/or abnormal enhancement.
Fig. 18: Right facial nerve neuritis. Coronal CE T1 weighted image showing the enhancement of the facial nerve.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Vascular abnormalities
- Hemangiomas in the auditory canal can be seen on CE MR and CT images as avid enhancement,
expansile masses.
High resolution CT can reveal the characteristic pattern of contrast enhancement of hemangiomas (specifically for the facial nerve: markedly enhance of the anterior section and moderate enhance of the posterior portion) and trabecular calcifications differentiating them from other conditions such as schwannomas.
MRI shows iso or slightly hypo intense in T1W images,
hyper intense in T2W images and intense enhancement in CE T1W.
- Aneurysms,
tortuous and elongated vertebrobasilar artery or posterior inferior cerebellar artery can also compress the facial nerve leading to ipsilateral neurological symptomatology.
(figure 19)
- Brainstem ischemia after a stroke affects the facial nerve due to edema causing compression over the facial nerve.
(figure 20)
Fig. 19: MR T1W image after IV CM shows basal artery aneurysm at the left cerebellopontine angle.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Fig. 20: Brainstem ischemia causing supranuclear facial palsy. MR PD image showing the increased signal intensity.
References: Department of Radiology, KAT General Hospital, Athens, Greece
Congenital issues may affect the facial nerve due to anatomic variations or disorders.
- External auditory canal atresia can be seen in high resolution CT as atretic bony plate with displacement of mastoid segment of facial nerve.
CT can also reveal oval window atresia relates with inferomedially displacement of the tympanic and descending segments of the facial nerve.
- Facial canal meningocele is rare condition and refers to a defect that allows spontaneous herniation of meninges into it.
T2W MRI is preferred to confirm the signal of CSF into the canal.
In addition CT can show a widened labyrinthine canal and lobulated enlargement of the geniculate fossa.
Traumatic or iatrogenic causes
- Temporal bone fractures or malpositioned prosthetic elements in patients with cochlear implants can be evaluated with CT which can show the fracture line involving the tympanic segment.
- Radiation neuritis in patients treated with radiotherapy could be imagined at MRI as nonspecific enhancement of the facial nerve.