Image-guided, microsurgical topographic anatomy of the endolymphatic sac and vestibular aqueduct via a suboccipital retrosigmoid approach

The endolymphatic sac (ES) and the vestibular aqueduct (VA) are often in the surgical field when posterior fossa lesions are targeted using retrosigmoid approaches. The purpose of this work is to validate neuronavigator accuracy in predicting VA location as well as to give guidelines to preserve the ES and VA. A retrosigmoid approach was performed bilaterally in six specimens in the semisitting position. Preoperatively, we registered in the CT scans the position of the VA genu (virtual genu). After the approach execution, ES and VA genu topographic relationships with evident posterolateral cranial base structures were measured using neuronavigation. Next, we exposed the VA genu: its position coincided with the virtual VA genu in all the specimens. On the average, the ES was 17.93 mm posterosuperolateral to the XI nerve in the jugular foramen, 12.26 mm posterolateral to the internal acoustic meatus, 20.13 mm anteromedial to the petro-sigmoid intersection at a point 13.30 mm inferior to the petrous ridge. The VA genu was located 7.23 mm posterolateral to the internal acoustic meatus, 18.11 mm superolateral to the XI nerve in the jugular foramen, 10.27 mm inferior to the petrous ridge, and 6.28 mm anterolateral to the endolymphatic ledge at a depth of 3.46 mm from the posterior pyramidal wall. Our study demonstrates that is possible to use neuronavigation to reliably predict the location of the VA genu. In addition, neuronavigation may be effectively used to create a topographical framework that may help maintaining the integrity of the ES/VA during retrosigmoid approaches.