The role of facial canal diameter in the pathogenesis and grade of Bell's palsy: a study by high resolution computed tomography.

Introduction: The exact etiology of Bell's palsy still remains obscure. The only authenticated finding is inflammation and edema of the facial nerve leading to entrapment inside the facial canal.

Objective: To identify if there is any relationship between the grade of Bell's palsy and diameter of the facial canal, and also to study any possible anatomic predisposition of facial canal for Bell's palsy including parts which have not been studied before.

Methods: Medical records and temporal computed tomography scans of 34 patients with Bell's palsy were utilized in this retrospective clinical study. Diameters of both facial canals (affected and unaffected) of each patient were measured at labyrinthine segment, geniculate ganglion, tympanic segment, second genu, mastoid segment and stylomastoid foramen. The House-Brackmann (HB) scale of each patient at presentation and 3 months after the treatment was evaluated from their medical records. The paired samples t-test and Wilcoxon signed-rank test were used for comparison of width between the affected side and unaffected side. The Wilcoxon signed-rank test was also used for evaluation of relationship between the diameter of facial canal and the grade of the Bell's palsy. Significant differences were established at a level of p=0.05 (IBM SPSS Statistics for Windows, Version 21.0.; Armonk, NY, IBM Corp).

Results: Thirty-four patients - 16 females, 18 males; mean age±Standard Deviation, 40.3±21.3 - with Bell's palsy were included in the study. According to the HB facial nerve grading system; 8 patients were grade V, 6 were grade IV, 11 were grade III, 8 were grade II and 1 patient was grade I. The mean width at the labyrinthine segment of the facial canal in the affected temporal bone was significantly smaller than the equivalent in the unaffected temporal bone (p=0.00). There was no significant difference between the affected and unaffected temporal bones at the geniculate ganglion (p=0.87), tympanic segment (p=0.66), second genu (p=0.62), mastoid segment (p=0.67) and stylomastoid foramen (p=0.16). We did not find any relationship between the HB grade and the facial canal diameter at the level of labyrinthine segment (p=0.41), tympanic segment (p=0.12), mastoid segment (p=0.14), geniculate ganglion (p=0.13) and stylomastoid foramen (p=0.44), while we found significant relationship at the level of second genu (p=0.02).

Conclusion: We found the diameter of labyrinthine segment of facial canal as an anatomic risk factor for Bell's palsy. We also found significant relationship between the HB grade and FC diameter at the level of second genu. Future studies (MRI-CT combined or 3D modeling) are needed to promote this possible relevance especially at second genu. Thus, in the future it may be possible to selectively decompress particular segments in high grade BP patients.