Transmastoid Decompression of Labyrinthine-Meatal Segment of the Facial Nerve: A Comparative Radio-Anatomic Study

Objective: To evaluate the routes for approaching to labyrinthine-meatal segment of the facial nerve (FN) via transmastoid approach by drilling the ampullar ends of the lateral semicircular canal (LSC). Materials and Methods: Ten temporal bones of adult cadavers underwent high-resolution computed tomography (HRCT). Among them in 6 temporal bones the FN was decompressed under operating microscope via transmastoid approach by drilling the ampullar ends of the LSC. Within the remaining four of temporal bones the ampullary ends of the LSC were preserved. All temporal bones were re-evaluated by HRCT scans following the dissection. Distances between labyrinthine-meatal segment and surrounding anatomical landmarks were measured through the HRCT scans. Results: The mean distances from labyrinthine-meatal segment to the midpoint of the basal turn of the cochlea, LSC and vestibule were 9.5±0.6 mm (range 8.1-10.4), 10.1±1.2 mm (range 8.5-11.9 mm), 2.6±0.4 mm (range 1.9-3.2 mm), respectively. The mean distance of remaining bony part of fallopian canal near the meatus was determined as 0.57±0.3 mm in the temporal bones drilled from the ampullary ends of the LSC and 2.07±0.5 mm in the temporal bones preserved from ampullary ends of the LSC. This difference was significant (p<0.05). Conclusions: The results showed that the labyrinthine-meatal segment of the FN could be adequately accessed via transmastoid approach by skletonizing the ampullary end of the LSC. This approach may be used as an alternative technique in the decompression of labyrinthine-meatal segment of the FN.