How to identify facial nerve injury on a computed tomography (CT) scan?

Identifying Facial Nerve Injury on CT

High-resolution temporal bone CT with thin sections is the preferred CT technique for identifying facial nerve injury, focusing on osseous abnormalities of the facial nerve canal rather than direct nerve visualization, as CT cannot directly image the nerve itself. 1

Key Principle: CT Shows Bone, Not Nerve

CT does not directly visualize the facial nerve—it identifies injury by demonstrating osseous abnormalities along the facial nerve canal (Fallopian canal) through the temporal bone. 1, 2 MRI is superior for direct nerve visualization, but CT excels at characterizing bony pathology that suggests nerve injury. 1, 3, 2

CT Findings Indicating Facial Nerve Injury

Temporal Bone Fractures

• Fracture lines crossing the facial nerve canal are the primary CT indicator of potential facial nerve injury. 4, 5, 6
• The perigeniculate area (region of the geniculate ganglion) is the most commonly injured site in temporal bone trauma, involved in 46% of cases. 5
• A characteristic fracture pattern extends from the petrotympanic fissure at the glenoid fossa to the anterior inferior aspect of the medial bony external auditory canal, resuming at the scutum and extending laterally along the external canal wall—when projected medially, this crosses the horizontal portion of the facial nerve. 4
• Mixed fractures (68% of cases) are more common than pure longitudinal or transverse patterns in patients with facial nerve injury. 4

Osseous Canal Abnormalities

• Facial canal foraminal expansion suggests mass effect or chronic nerve pathology. 1
• Patterns of bone erosion along the facial nerve canal indicate tumor involvement or chronic inflammatory disease. 1
• Bony dehiscence of the facial canal may be congenital or acquired. 1

Associated Findings

• Middle ear inflammatory disease with involvement of the facial nerve canal. 1
• Intrinsic bone tumor matrices affecting the temporal bone and facial canal. 1
• Ossicular chain disruption often accompanies facial nerve injury in trauma (23 cases in one series). 6

Technical Requirements for Optimal CT

Acquisition Parameters

• High-resolution thin-cut temporal bone CT with thin sections (1 mm or less) using a bone window algorithm is mandatory—routine head CT is inadequate. 1, 4, 5
• Multislice spiral CT with multiplanar reconstructions in all three spatial planes facilitates exact evaluation of the facial nerve canal. 3, 2
• Axial and coronal planes are both necessary: axial CT detects most fractures, but coronal imaging is essential for precise topographic analysis of fracture course and facial canal involvement. 6

Contrast Administration

• Contrast may be useful when infection or tumor is suspected but is not routinely required for trauma evaluation. 1
• There is no evidence supporting combined pre- and postcontrast imaging. 1

Anatomical Segments to Evaluate Systematically

Trace the facial nerve canal through its entire intratemporal course: 1, 7

• Meatal segment (internal auditory canal)
• Labyrinthine segment (narrowest portion, most vulnerable to compression)
• Geniculate ganglion (most common injury site in trauma) 5
• Tympanic (horizontal) segment
• Mastoid (vertical) segment
• Stylomastoid foramen (exit point)

Diagnostic Accuracy and Limitations

• Overall sensitivity and specificity of high-resolution CT for detecting facial nerve canal fractures are 77.5% and 77.7% respectively, but vary by anatomical location. 5
• CT results should be interpreted with caution in certain areas where sensitivity is lower—the diagnostic value differs according to fracture location. 5
• In 22 of 27 patients (81%) with facial nerve palsy, CT successfully demonstrated facial canal lesions, with the geniculate ganglion region being the most common injury site. 6

Clinical Context for CT Utilization

• Routine CT head alone is not appropriate for evaluating facial nerve palsy—dedicated temporal bone CT is required. 1
• CT is most valuable in acute trauma settings to rapidly assess for temporal bone fractures and surgical planning. 1, 4
• For non-traumatic facial nerve palsy, MRI is the primary modality, with CT serving as a complementary study to characterize osseous anatomy. 1, 8

Common Pitfalls to Avoid

• Do not rely on routine head CT—it lacks the spatial resolution to adequately evaluate the facial nerve canal and will miss subtle fractures. 1, 4
• Do not assume CT can directly visualize nerve injury—it only shows bony abnormalities that suggest nerve involvement. 1, 2
• Do not skip coronal imaging—axial views alone may miss the precise extent of facial canal involvement. 6
• Do not forget that absence of visible fracture on CT does not exclude nerve injury—sensitivity is only 77.5%, and nerve injury can occur without visible bony disruption. 5