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An anatomic and radiologic evaluation of access to the lateral internal auditory canal via the retrosigmoid approach and description of an internal labyrinthectomy.

OBJECTIVE: The retrosigmoid approach to the posterior petrous bone may be used as a hearing preservation operation for extirpation of posterior fossa and internal auditory canal (IAC) lesions. However, it is usually not possible to remove tumor from the most lateral portions of the IAC even after removing the retrolabyrinthine bone down to the posterior semicircular canal. Our goal was to examine the advantages and disadvantages of the retrosigmoid approach with respect to approaching the lateral IAC, to find what is gained by drilling down the retrolabyrinthine bone, and to describe an internal labyrinthectomy whereby the labyrinth is removed via a retrosigmoid approach.

MATERIALS AND METHODS: Using a 3.5-cm craniotomy, a retrosigmoid approach was performed on one side in each of five whole fresh cadaveric heads. The IAC was identified, and the length of the IAC and the depth of the acoustic porus from the center of the craniotomy were measured. The bone posterior to the labyrinth was removed, and the length of the portion of the IAC still inaccessible was measured. These measurements were compared with measurements using computed tomography (CT) scans of each cadaveric head. Additionally, we present the findings of 11 patients who had an internal labyrinthectomy performed by the senior author.

RESULTS: The average length of the IAC based on CT scanning was 11.2 mm (SD, 0.84 mm; range, 10-12 mm, 95% confidence interval [CI], 9.44-12.96). The potential gain predicted by the CT scans was 8.4 mm (SD, 0.9 mm; range, 7-9 mm). In the anatomic study, the average length of the IAC was 11.0 mm (SD, 2.3 mm; range, 8.0-13.0 mm; 95% CI, 9.26-12.78). The average length of the IAC that was inaccessible after removing the retrolabyrinthine bone was 6.7 mm (SD, 1.5 mm; range, 5.0-7.8 mm), or 61%. The average gain in access to the lateral IAC was 4.3 mm (SD, 1.0 mm; range, 2.7-5.2 mm). After performing an internal labyrinthectomy, the fundus was accessible, as was the labyrinthine segment of Cranial Nerve VII and the geniculate ganglion.

CONCLUSION: The retrosigmoid approach provides access to the posterior petrous bone, and removal of the bone posterior to the labyrinth provides some additional access to the lateral IAC. However, an internal labyrinthectomy is necessary to provide access to the fundus of the IAC via the retrosigmoid approach.

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