Revisiting the Intratemporal Course of the Facial Nerve

Abstract

Introduction The facial nerve displays a lot of variations and anomalies in its course. Having sound knowledge about the surgical anatomy along with its intricacies is essential in mastoid surgeries. In this study, we have documented the angle of deviation of the mastoid segment of the facial nerve during its intratemporal course and the importance of this angle during mastoid surgeries. Aim and objectives The objectives of this study were to document and assess the orientation of the intratemporal course of the mastoid segment of the facial nerve and to measure the angle of deviation of the mastoid segment of the facial nerve as it traverses from the second genu to the stylomastoid foramen. Methodology A prospective observational study was conducted between December 2023 and June 2024 on 20 wet temporal bone specimens. Following canal wall down mastoidectomy, the facial nerve was explored and completely traced from the second genu to the stylomastoid foramen. High-quality images were captured and uploaded to the GIMP 2.10.36 software. At the second genu, a tangential line was drawn in the sagittal plane. Another line was drawn along the course of the facial nerve until its exit at the stylomastoid foramen. The angle between these two lines was estimated, and the angle of deviation was documented using this software and statistically analyzed. Results The mean angle of deviation at the second genu was found to be 15 (12-18) degrees in all 20 specimens. It was found that as the facial nerve traverses towards the stylomastoid foramen, from its anteromedial course it becomes more lateral and superficial. The anteroposterior angulation between males and females had a p-value of 0.02591, showing that it was significant. Similarly, the medial to lateral angulation between males and females had a p-value of 0.0458, which showed significance. However, the anteroposterior and medial to lateral angles did not show any significance when compared to the standard angle of 15 degrees as per literature, showing a p-value of 0.456 and 0.275 respectively. This helped us understand and document the highly variable course of the mastoid segment of the facial nerve, which can be emphasized during further research. Conclusion This angle of deviation of the mastoid segment of the facial nerve, around 15 degrees, will help surgeons be more careful during mastoid surgeries as it helps in assessing the depth and highly variable course of the facial nerve during its intratemporal course. This further acts as a guiding tool for budding surgeons in preventing inadvertent facial nerve injuries.

Keywords: angulation; facial nerve; facial nerve course variation; intratemporal course; mastoid segment; mastoid surgeries; stylomastoid foramen.

Figure 1. Left temporal bone showing cortical mastoidectomy. (a) Black arrow: non-ampullary end of superior semicircular canal. (b) Red arrow: ampullary end of lateral semicircular canal. (c) Blue arrow: sigmoid sinus.

Figure 2. Facial canal traced in its complete length: (a) Tympanic segment (pink dotted line) and Mastoid segment (blue dotted line). (b) Purple arrow: second genu. (c) Black arrow: opening of stylomastoid foramen. (d) Red arrow: sigmoid sinus bulge.

Figure 3. Exposure of the facial nerve after removing the bony canal and nerve sheath.

Figure 4. Measurement of the antero-posterior angle of the facial nerve along its exit toward the stylomastoid foramen: (a) Black arrow: horizontal line; (b) Red arrow: tangential line; (c) Green arrow: antero-posterior angle formed; (d) Blue arrows: direction of the stylomastoid foramen.

Figure 5. Measurement of medial to lateral angulation of the facial nerve as it exits the stylomastoid foramen: (a) Black arrow: first horizontal line parallel to the mastoid segment; (b) Yellow arrow: second horizontal line at the exit of the mastoid segment through the stylomastoid foramen; (c) Red arrow: third line tangential to the second line; (d) Green arrow: medial to lateral angle.