The ratio of facial nerve to facial canal as an indicator of entrapment in Bell's palsy: A study by CT and MRI.

OBJECTIVES: To find out if the ratio of facial nerve to facial canal diameter plays any role in the etiopathogenesis and grade of Bell's palsy. PATIENTS AND METHODS: Twenty-nine consecutive patients (16 females, 13 males) diagnosed with unilateral Bell's palsy were enrolled into the study. At admission, 5 patients were grade V, 11 were grade IV, 11 were grade III and 2 were grade II. The grade of Bell's palsy was documented by House-Brackmann facial nerve grading system at admission, 15th day, 1st month and 3th month. Temporal MRI and CT scans were obtained at the time of diagnosis. Diameter of facial nerve and facial canal at the middle of five different segments were measured equidistantly at the same workstation. Both sides of every patient (affected and unaffected) were measured by one radiologist who was not informed about the side of the paralysis. MRI and CT values of each segment were compared between affected and unaffected sides. In addition, FN/FC ratio of affected and non-affected sides was calculated and compared for each segment. We also analyzed if a relationship exists between above-mentioned measurements and the House-Brackmann grade. RESULTS: Considering MRI values; statistically significant thickening of facial nerve between affected and unaffected sides was found at labyrinthine (p = 0.012), tympanic (p = 0.023) and geniculate parts (p = 0.04). Considering CT values; statistically significant difference between affected and unaffected sides was not found at any segment. Comparison of FN/FC ratio of both sides revealed statistically significant increment of the affected side at labyrinthine segment (p = 0.015) and geniculate ganglion (p = 0.032). We determined positive correlation between diameter of FN and HB grade at labyrinthine segment (p = 0.03, R = 0.531). On the other hand, we determined negative correlation between diameter of FC and HB grade at labyrinthine segment (p < 0.001, R = -318). A positive correlation between HB grade and FN/FC ratio was found only at the labyrinthine segment (p = 0.003, R = 0.673). CONCLUSION: FN/FC ratio of labyrinthine segment and geniculate ganglion was found to be increased in patients with Bell's palsy. In addition, a positive correlation was determined between this ratio and grade of Bell's palsy particularly at labyrinthine segment. Basing the current study, if FN decompression is recommended in Bell's palsy patients with objective findings of bad prognosis, conservative surgery targeting the labyrinthine segment and geniculate ganglion alone might be safer.