Hearing Sparing Retrolabyrinthine Presigmoid Approach to the Anterior Cerebellopontine Region: Expanding the Limits of Trautmann's Triangle

Background: The presigmoid retrolabyrinthine approach has several advantages, such as lower morbidity and preservation of hearing. However, the anatomical area exposed through the exposure of Trautmann’s Triangle may not provide a sufficient working area. Attention has been placed in the relationship of the Jugular Bulb (JB) to the IAC but there is little known on the need to skeletonize the Sigmoid Sinus (SS) based on its relationship to the Posterior Semicircular Canal (PSC). In this study we hypothesized that an expansion of Trautmann’s Triangle provides a larger exposure of lesions located at cranial nerve entry zones and lateral brain stem. The aim of this study was to measure the additional exposure and visibility provided by skeletonizing the SS and to determine if there are a subset of patients that would not benefit from this based on the depth of the operative field provided by the relationship between the PSC and SS.

Methods: Human cadaver heads were subjected to a thin temporal bone CT scan for a total of 10 sides. The estimated preoperative surgical working area was calculated based on the relationship of the SS to the PSC, Superior Petrosal Sinus (SPS) and Jugular Bulb (JB) on imaging. The heads were then subjected to a mastoidectomy to calculate the following distances in the presigmoid retrolabyrinthine space: external auditory canal (EAC) to SS, PSC to JB, PSC to SS, and lateral semicircular canal (LSC) to SS after the PSC was drilled. The SS was then skeletonized and gentle retraction was applied. Measurements to the SS were then repeated. Samples were photographed.

Results: The distance between the EAC and SS measured intraoperatively was significantly correlated to the preoperatively measured distances by placing a laterally drawn line on the SS and measuring the shortest perpendicular distance to the PSC (r = 0.82; p = 0.02) and LSC (r = 0.859; p = 0.01) on axial CT imaging. Skeletonization of the SS significantly increased the distance between the PSC to SS by 4.9 mm (13.96 ± 3.13 mm vs. 9.05 ± 3.49 mm; p < 0.001), and LSC to SS by 4.2 mm (16.97 ± 3.50 vs. 12.76 ± 3.83; p = 0.01). On post mastoidectomy images, skeletonizing the sigmoid sinus helped improved anterior visibility on the majority of our samples that had a SS that was lateral to the PSC on axial imaging. In samples in which the SS is medial to the PSC or only has minimal lateral displacement, skeletonizing the SS did not markedly improve visibility of the retrolabyrinthine space.

Conclusion: Working area and visibility improves as the PSC and SS approach the same lateral plane on axial imaging. Preoperative evaluation of the laterality of the SS to the PSC may assist a surgeon in determining the need for skeletonizing the SS and avoiding possible vascular injuries.