Relevance of the Cochlear Aqueduct in Presigmoid Approaches: A Radio-Anatomical Study

Introduction: The cochlear aqueduct (CA) is a bony canal that drains perilymph from the basal turn of the cochlea into pars nervosa of the jugular foramen. When visualized in presigmoid surgery, the CA is inferior to the internal auditory canal (IAC) and superior to the jugular bulb (JB). In this study we aim to describe whether the CA serves as a surgically relevant reference for JB height in presigmoid approaches, especially when the JB is not obviously high or low in preoperative imaging.

Methods: On eleven sides of six formalin-fixed, latex-injected cadaveric heads, retrolabyrinthine, translabyrinthine, and transcochlear approaches were performed. After performing the surgical approach, the CA was classified as supra-jugular or unseen/infra-jugular. The vertical distances from the JB to these structures were measured (mm): inferior border of posterior semicircular canal (PSCC), tegmen dura, porus of CN IX, inferior IAC, and superior IAC. Horizontal distance of CA to the petrous internal carotid artery (ICA) was measured. Visualization of the following structures in the prepontine cistern was assessed: basilar artery, CN VI, and Dorello’s canal. Student’s T-tests assessed for significant differences in measurements of interest between specimens with a supra-jugular CA and specimens with an unseen/infra-jugular CA.

One hundred sides of fifty CT head scans of patients were studied for presence of the CA. The CA’s position to the JB was described as either superior or inferior.

Results: The CA was supra-jugular in seven sides (64%), with a mean distance of 3.1 mm (range: 2–4 mm) to the JB. With a supra-jugular CA, the mean distances between the JB and the structures of interest were always longer than in specimens with an unseen/infra-jugular CA. Specifically, the distances from the JB to tegmen dura (24.8 vs. 14 mm, p < 0.01), inferior border of PSCC (7.1 vs. 0 mm, p < 0.01), inferior border of IAC (11.6 vs. 3.25 mm, p < 0.01), and superior border of IAC (16.2 vs. 8.3 mm, p < 0.01); CN IX was closer to JB (0.6 vs. -5.7 mm, p = 0.04). The mean distance from the CA to the petrous segment of the ICA was 9.3 mm (range: 4-16 mm). In specimens with a supra-jugular CA, CN VI, the basilar artery, and Dorello’s canal were consistently visualized during retrolabyrinthine exposure, but in specimens with an unseen/infra-jugular CA, only the proximal cisternal portion of CN VI was reliably seen.

With CT head scans, the CA could be identified on 90 sides (90%). With respect to the JB, it was superior on 70 sides (78%) and inferior on 20 sides (22%).

Conclusion: The presence of a supra-jugular CA predicts a significantly larger presigmoid window and better access to prepontine structures as the JB is significantly lower in these cases and more infralabyrinthine drilling is possible. Further, presence of the CA and its relationship to the JB is discernible on 80% of preoperative CT scans. Consequently, the CA may help surgeons decide whether a presigmoid approach should be utilized—especially in cases when the JB is not obviously high or low.

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Artikel online veröffentlicht:
05. Februar 2024