Internal Auditory Canal Variability: Anatomic Variation Affects Cisternal Facial Nerve Visualization.

BACKGROUND: The internal auditory canal (IAC) is an important landmark during surgery for lesions of the cerebellopontine angle. There is significant variability in the position and orientation of the IAC radiographically, and the authors have noted differences in surgical exposure depending on the individual anatomy of the IAC. OBJECTIVE: To test the hypothesis that IAC position and orientation affects the surgical exposure of the IAC and facial nerve, especially when performing the translabyrinthine approach. METHODS: The authors retrospectively reviewed magnetic resonance imaging studies of 50 randomly selected patients with pathologically confirmed vestibular schwannomas. Measurements, including the anterior (APD) and posterior (PPD) petrous distances, the anterior (APA) and posterior (PPA) petro-auditory angles, and the internal auditory angle (IAA), were obtained to quantify the position and orientation of the IAC within the petrous temporal bone. RESULTS: The results quantitatively demonstrate tremendous variability of the position and orientation of the IAC in the petrous temporal bone. The measurement ranges were APD 10.2 to 26.1 mm, PPD 15.1 to 37.2 mm, APA 104 to 157°, PPA 30 to 96°, and IAA -5 to 40°. CONCLUSION: IAC variability can have a substantial effect on the surgical exposure of the IAC and facial and vestibulocochlear nerves. Specifically, a horizontally oriented IAC with a small IAA may have significant impact on visualization of the facial nerve within its cisternal segment with the translabyrinthine approach. The retrosigmoid approach is less affected with IAC variability in position and angle.