Variations in the labyrinthine segment of facial nerve canal revealed by high-resolution computed tomography.

Objective: To study variations in the labyrinthine segment of fallopian canal and the associated middle and inner ear malformations.

Method: The high-resolution computed tomography (HRCT) images of the temporal bone in 24 patients with congenital variations in the labyrinthine segment of fallopian canal were analyzed retrospectively. The length of labyrinthine segment of the facial nerve and angle of the first genu of 10 normal subjects were also measured. Based on the original axial images, multi-planar reformation (MPR) and curved-planar reformation (CPR) images of bilateral ossicular chains, inner ear structure and fallopian canal were reconstructed. HRCT features of congenital variations in the labyrinthine segment of the facial nerve were analyzed, including its beginning site, dehiscence, length, angle of the first genu and the associated middle and inner ear malformations.

Results: Among the normal subjects, the length of labyrinthine segment of the facial nerve was 3.56±0.41mm, and angle of the first genu was 71.87±8.09°. Five types of variations in the labyrinthine segment of the facial nerve were found in 45 ears of 24 cases, including dehiscence in geniculate fossa in 25 ears, anteromedial displacement at the beginning site in 27 ears (widening of Bill's bar in 7 cases), enlargement of the angle of the first genu in 30 ears with an average value of 107.2° (96.0-126.0°), increase of length in 30 ears with an average length of 6.8mm (5.2-8.3mm) and bifurcation in one ear. Associated middle ear malformation in 6 ears and inner ear malformation in 36 ears were also found.

Conclusion: A variety of congenital variations may occur in the labyrinthine segment of the facial nerve and they are often associated with middle or inner ear malformations, which can be clearly displayed by HRCT with MPR or CPR images.