Localization of the Internal Auditory Canal in Anterolateral Transpetrosal Approaches—Cadaver and Radiological Analysis

Purpose: Localization of the internal auditory meatus is one of the main goals in the anterolateral transpetrosal approaches. The extensive drilling of the petrous bone during these lateral approaches (anterior transpetrosal Kawase's approach, middle and extended middle fossa approaches, combined approaches) provides appropriate access to pathologies in the Meckel's cave and the superior petroclival area including the internal auditory canal, without a significant retraction of the neuroaxis. To maintain the hearing function the labyrinth should be preserved.

Method: The topographical relationships of the internal acoustic canal (IAC), the petrous ridge, the arcuate eminence (AE), the superior semicircular canal, and the GSPN were measured in a group of 50 adult skull bases (100 temporal bones, 1:1 male-to-female ratio). The results obtained in the laboratory were compared with 50 random HRCT examinations of patients admitted to the Masaryk Hospital during the last 2 years (100 temporal bones, 1:1 male-to-female ratio). The authors discuss the anatomical varieties and the techniques of localization of the internal auditory canal (Fisch, Portmann and Garcia Ibanez, House, neuronavigational systems) in hearing preservation skull base surgery.

Result: The mean angle between the GSPN and the IAC was 50.2 degrees (33 to 80 degrees) and between the IAC and the AE it was 54.1 degrees (34 to 66 degrees). The mean difference between these angles in individual temporal bones was 12.3 degrees (1 to 39 degrees). The position of the EA was misinterpreted or not identifiable in 42% of cases. The dehiscence of the geniculate ganglion was found in 14% of cases. There was no statistical difference between the sides and the gender.

Conclusion: There was a relative high variability of metric relationships and high incidence of not clearly identifiable landmarks. The arcuate eminence situation is the most variable. Every method of IAC identification has its own limitations and risks. The frameless navigation is helpful for a quick and safe identification of the IAC. A precise knowledge of the anatomy is a mandatory premise.