What neuroimaging is typically performed for facial nerve palsy?

Neuroimaging for Facial Nerve Palsy

MRI is the primary modality for evaluating facial nerve palsy, with high-resolution thin-cut pre- and postcontrast sequences being the method of choice when imaging is indicated. 1

When Imaging is Indicated

• In general, patients with Bell's palsy (idiopathic facial nerve palsy) do not need imaging unless symptoms are:
  • Atypical presentation 1
  • Recurrent episodes 1
  • Persistent for 2-4 months without improvement 1

MRI Protocol Recommendations

• High-resolution thin-cut MRI with pre- and postcontrast sequences 1
• Imaging protocols should cover the entire course of CN VII and its innervated structures 1
• 3T and volumetric imaging provide improved visualization of the facial nerve and surrounding perineural vascular plexus 1
• 3D heavily T2-weighted sequences are particularly useful for evaluating the nerve course 1
• MRI can detect enhancement patterns in the canalicular, labyrinthine, geniculate, tympanic, and mastoid portions of the nerve 1
• Recent studies show that 89% of patients with acute Bell's palsy (within 7 days of symptom onset) demonstrate abnormal contrast enhancement of the facial nerve 2

Complementary CT Imaging

• High-resolution temporal bone CT provides complementary information to MRI by characterizing:
  • Temporal bone fractures 1
  • Osseous integrity of the facial nerve canal 1
  • Presurgical osseous anatomy 1, 3
  • Nerve involvement with inflammatory middle ear disease 1
  • Foraminal expansion and patterns of bone erosion 1

Special Clinical Scenarios

Hemifacial Spasm

• MRI with 3D heavily T2-weighted sequences plus MRA is recommended to characterize vascular loops potentially compressing the facial nerve 1, 4
• Both imaging modalities correlate well with surgical findings (sensitivity and accuracy >95%) 1, 4
• MRI findings are supportive rather than diagnostic when selecting candidates for microvascular decompression 1, 4

Perineural Tumor Spread

• High-resolution thin-cut contrast-enhanced MRI is especially useful for evaluating perineural spread of tumors affecting CN VII 1
• Sensitivities for MRI detection of perineural tumor spread range from 73% to 100% 1

Ramsay Hunt Syndrome

• MRI can show enhancement of the facial and vestibulocochlear nerves in the internal auditory canal and labyrinth 5
• CSF analysis has 85% sensitivity for Ramsay Hunt Syndrome and should be considered alongside imaging 6

Imaging Modalities Not Routinely Recommended

• Ultrasound is not routinely used in the evaluation of facial nerve palsy 1
• FDG-PET/CT is not routinely used in the initial evaluation of facial nerve palsy 1
• CTA head alone is not recommended for isolated facial nerve palsy evaluation 1

Clinical Correlation

• The degree of facial nerve dysfunction should guide imaging decisions 7
• Patients with incomplete palsy (paresis) have excellent prognosis regardless of imaging findings 7
• Patients with complete paralysis, especially those with >90% denervation on electroneuronography (ENoG), have worse outcomes and may benefit more from imaging to exclude secondary causes 7

Common Pitfalls

• Normal enhancement can occur in the geniculate, tympanic, and mastoid portions of the facial nerve, which should not be misinterpreted as pathological 1
• There is a lack of consensus regarding the prognostic value of MRI in Bell's palsy 1
• MRI is most useful for excluding other causes of facial nerve palsy rather than confirming Bell's palsy 1
• Delayed imaging (>2 weeks after symptom onset) may miss early inflammatory changes in infectious causes of facial palsy 6, 5