What is the management of dehiscence of the facial nerve canal?

Management of Facial Nerve Canal Dehiscence

Middle fossa decompression surgery is the recommended approach for managing facial nerve canal dehiscence in patients with severe facial nerve dysfunction, particularly when there is >90% reduction in amplitude on electrodiagnostic testing and no voluntary EMG activity.

Definition and Prevalence

• Facial canal dehiscence (FCD) refers to the absence of bony covering over the facial nerve canal, most commonly found in the tympanic segment 1, 2
• The prevalence of FCD ranges from 11.4% to 51.2% in various studies, making it a relatively common anatomical finding 3, 4
• In healthy populations without middle ear disease, FCD has been reported in up to 51.2% of temporal bones examined by high-resolution CT 4

Diagnostic Evaluation

• High-resolution temporal bone CT is the preferred imaging modality to evaluate the facial nerve canal and identify dehiscence 5
• Ultra-high-resolution CT with a scoring system can help identify FCD with high sensitivity (82%) and specificity (93%) 1
• MRI provides complementary information, particularly for evaluating the nerve itself and surrounding soft tissues 5
• Electrodiagnostic testing (ENoG and EMG) is crucial for determining the severity of nerve dysfunction and guiding treatment decisions 5

Risk Factors

• Cholesteatoma, chronic otitis media, and mastoiditis can cause or exacerbate facial canal dehiscence 6
• Anatomical factors such as a wider angle at the second genu of the facial nerve (mean 117.8° vs. 114°) are associated with higher rates of dehiscence 6
• Hereditary factors may play a role, with a 29% chance of contralateral dehiscence if one ear shows the condition 3

Management Algorithm

For Asymptomatic Incidental FCD:

• Observation is recommended as most cases do not require intervention 5
• Document the dehiscence for future reference, especially if otologic surgery is planned 2

For Symptomatic FCD with Facial Nerve Dysfunction:

1. Initial Assessment:

   • Determine severity using House-Brackmann facial nerve grading 5
   • Perform electrodiagnostic testing (ENoG and EMG) 5

2. Medical Management:

   • First-line treatment with corticosteroids, especially in Bell's palsy cases 5
   • Consider antiviral medications in suspected viral etiology 5

3. Surgical Intervention Criteria:

   • Complete facial paralysis with >90% reduction in amplitude on ENoG testing 5
   • Absent volitional nerve activity on EMG 5
   • Intervention ideally within 3-14 days of symptom onset 5

4. Surgical Approach Selection:

   • Middle fossa decompression is preferred for labyrinthine segment dehiscence 5
   • Transmastoid approach has shown less benefit and may risk conductive hearing loss due to incus disarticulation 5

Evidence for Middle Fossa Decompression

• Gantz et al. demonstrated that 91% of patients achieved House-Brackmann I/II outcomes with middle fossa decompression compared to only 42% with steroids alone (p=0.0002) 5
• Fisch reported better return of function in surgically decompressed patients (79-93%) compared to non-decompressed controls (64%) 5
• Kim et al. showed 11 of 13 patients achieved House-Brackmann I/II scores after middle fossa decompression 5

Surgical Considerations

• Middle fossa approach requires craniotomy but allows for removal of bone surrounding the facial nerve at the meatal foramen without affecting hearing structures 5
• Timing is critical - best outcomes are seen when surgery is performed within 14 days of symptom onset 5
• The labyrinthine segment starting at the meatal foramen is the most common site of constriction in Bell's palsy and should be the target of decompression 5

Potential Complications and Caveats

• Surgical risks include hearing loss, CSF leak, infection, and vascular complications 5
• Patient preferences play a significant role in decision-making due to the psychological impact of facial paralysis 5
• Preoperative CT evaluation is essential to identify other potential bone defects that may be present alongside FCD (e.g., lateral semicircular canal fistula in 11.7% of cases) 4
• False negative and positive rates on CT imaging should be considered when planning surgical intervention 2

Special Considerations

• For elderly patients (>80 years), conservative surveillance or stereotactic radiosurgery may be preferred over microsurgical resection 5
• In cases where FCD is associated with tumors like vestibular schwannomas, management should follow specific guidelines for those conditions 5