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

Abstract 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.摘要与引言：贝尔麻痹（Bell's palsy）的确切病因至今仍不明确，目前唯一被证实的病理改变为面神经（facial nerve）炎症与水肿，进而导致面神经在面神经管（facial canal）内发生嵌顿。研究目的：明确贝尔麻痹严重程度与面神经管管径之间是否存在关联，并探究面神经管针对贝尔麻痹的潜在解剖易感性，包括此前尚未被研究过的面神经管区段。研究方法：本回顾性临床研究纳入了34例贝尔麻痹患者的病历资料与颞骨计算机断层扫描影像。对每位患者的双侧面神经管（患侧与健侧）的6个区段分别测量管径，包括迷路段、膝状神经节区、鼓室段、面神经第二膝、乳突段以及茎乳孔。从病历中提取每位患者就诊时及治疗后3个月的House-Brackmann（HB）面神经功能分级结果。采用配对样本t检验与Wilcoxon符号秩检验比较双侧面神经管的管径差异；采用Wilcoxon符号秩检验分析面神经管管径与贝尔麻痹严重程度的相关性。本研究以p=0.05作为检验水准（数据分析采用IBM SPSS Statistics for Windows, Version 21.0；纽约州阿蒙克市，国际商业机器公司）。研究结果：本研究共纳入34例贝尔麻痹患者，其中女性16例，男性18例；平均年龄为40.3±21.3岁（标准差）。根据HB面神经功能分级标准，8例为V级，6例为IV级，11例为III级，8例为II级，1例为I级。患侧颞骨面神经管迷路段的平均管径显著小于健侧（p=0.00）；而在膝状神经节区（p=0.87）、鼓室段（p=0.66）、面神经第二膝（p=0.62）、乳突段（p=0.67）以及茎乳孔（p=0.16），双侧面神经管管径均无显著差异。在HB分级与面神经管管径的相关性分析中，迷路段（p=0.41）、鼓室段（p=0.12）、乳突段（p=0.14）、膝状神经节区（p=0.13）以及茎乳孔（p=0.44）均未发现显著相关性，但面神经第二膝区段存在显著相关性（p=0.02）。研究结论：本研究证实面神经管迷路段管径是贝尔麻痹的解剖学危险因素。同时发现面神经第二膝区段的管径与HB分级存在显著相关性。未来需开展更多研究（如MRI-CT联合成像或三维建模）以进一步验证该相关性，尤其是面神经第二膝区段。由此，未来或可针对重度贝尔麻痹患者实施特定区段的选择性面神经减压术。