Internal Acoustic Meatus: Morphological Peculiarities and Clinical Implications

Background/Objectives: The variability of the internal acoustic meatus (IAM) and its morphological peculiarities are of high clinical significance in otologic interventions and neurosurgery. Considering the complexity of the neurovascular structures housed by the IAM, including passage of the facial nerve, the aim of the study was to highlight the morphological peculiarities of the IAM and their clinical significance. Methods: The age peculiarities of the IAM course were studied on 18 serially, transversely sectioned human embryos and fetuses, as well as on 20 newborn and 20 adult formalin-fixed hemiheads. The morphometric parameters of the IAM (width, height, length, and angles between the anterior and posterior walls of the IAM and the longitudinal axis of the petrous ridge (LAPR) were evaluated on 82 dry temporal bones. Results: In embryos and early fetal stages of development, the course of the IAM formed an acute, posteromedially open angle with the longitudinal axis of the petrous part of the temporal bone (LAP); in newborns, it was a right angle; in adults, it was an acute, anteromedially open angle. Various shapes of the IAO, such as oval (51.2%), semioval (23.2%), quadrangular (13.4%), heart-like (3.7%), and triangular (8.5%), were highlighted. The mean width of the internal acoustic opening (IAO) was 7.7 ± 2.11 mm (right/left-8.2 ± 2.22 mm/7.2 ± 1.89 mm), with a statistically significant difference (p = 0.031). The mean height of the IAO was 5.3 ± 1.43 mm (right/left-5.6 ± 1.58 mm/5.0 ± 1.22 mm), p = 0.082. The mean length of the IAM was 10.1 ± 2.83 (right/left-10.5 ± 2.96 mm/9.7 ± 2.67 mm), p = 0.214. A partial septum was revealed in 3.6% of cases, while total septation of the IAM was observed in 2.4%. The mean angle between the anterior wall of the IAM and the longitudinal axis of the petrous ridge (AWIAM/LAPR) was 21.4 ± 13.67° (right/left-22.2 ± 14.62°/20.5 ± 12.78°), p = 0.564. The mean angle between the posterior wall of the IAM and the longitudinal axis of the petrous ridge (PWIAM/LAPR) was 82.4 ± 8.02° (right/left-84.3 ± 7.93°/80.5 ± 7.76°); p = 0.032. Conclusions: In the initial stages of an individual's ontogenesis, the IAM forms an acute, posteromedially open angle with the LAP; in newborns-a right angle; in adults-an acute, anteromedially open angle. Both morphological characteristics and morphometric parameters of the IAO and IAM vary bilaterally. The oval, semioval, quadrangular, heart-like, and triangular shapes of the IAO were highlighted. Partial and total septation of the IAM were revealed. Only the TDAIO and PWIAM/LAPR angle were statistically significant, p < 0.05, while the bilateral correlation was negligible. The established morphological variants of the IAM could predispose to facial and vestibulocochlear nerve impairments.