Anatomic Assessment of the Limits of an Endoscopically Assisted Retrolabyrinthine Approach to the Internal Auditory Canal

Objective: The retrolabyrinthine approach is a hearing preservation approach that can be applied to internal auditory canal (IAC) pathology with endoscopic assistance. Information regarding the surgical advantages and anatomic constraints of this approach is lacking. This study aims to define the minimum amount of retrosigmoid dural decompression necessary for exposure of the entire IAC and the degree of surgical freedom afforded by this approach.

Methods: Presigmoid retrolabyrinthine approaches were performed on fresh cadaveric heads. The IAC was exposed under endoscopic guidance. The retrosigmoid posterior fossa dura was decompressed in a step-wise fashion until the fundus of the IAC was exposed and successive measurements were made. Degree of surgical freedom at the fundus of the IAC was calculated for each specimen after both the retrolabyrinthine approach and translabyrinthine approach.

Results: The IAC was entirely exposed in 9 specimens with a median length of 12 mm (range: 10–13 mm). Complete IAC exposure could be achieved with 1 cm of retrosigmoid dural decompression in eight of nine mastoids. One mastoid required 2 cm of dural decompression to expose the entire IAC. For the retrolabyrinthine approach, the median anterior-posterior surgical freedom was 13° (range: 6–23°) compared with 46° (range: 36–53°) for the translabyrinthine approach (p = 0.014). For the retrolabyrinthine approach, the median superior–inferior surgical freedom was 40° (range: 33–46°) compared 47° (range: 42–51°) for the translabyrinthine approach (p = 0.022).

Conclusions: Using endoscopic assistance, the retrolabyrinthine approach can expose the entire length of the IAC. We recommend that at least 1.5 cm of posterior fossa dura is exposed for this approach. Compared with the translabyrinthine approach, this strategy provides significantly less instrument freedom in both the horizontal and vertical axes. This approach may be appropriate for carefully-selected patients with intact hearing and small- to medium-sized tumors involving the IAC.

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No conflict of interest has been declared by the author(s).