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Anatomy of the Retrosigmoid Approach

The retrosigmoid approach to the meatus is directed through a cranial opening situated just behind the sigmoid sinus, and down the plane between the posterior face of the temporal bone and the anterior surface of the cerebellum. In removing the posterior meatal wall, it is often necessary to sacrifice the subarcuate artery because it passes through the dura on the posterior meatal wall to reach the subarcuate fossa. This artery usually has a sufficiently long stem that its obliteration does not risk damage to the anterior inferior cerebellar artery from which it arises. However, in a few cases, the subarcuate artery and the segment of the anterior inferior cerebellar artery from which it arises will be incorporated into the dura covering the subarcuate fossa. In that case, the dura and artery will have to be separated together from the posterior meatal lip in preparation for opening the meatus.

The posterior semicircular canal and its common crus with the superior canal, both of which are situated just lateral to the posterior meatal lip, should be preserved in exposing the meatal contents if there is the possibility of preserving hearing, because hearing will be lost if they are damaged. Care is also required to avoid injury to the vestibular aqueduct, which is situated inferolateral to the meatal lip, and the endolymphatic sac, which expands under the dura on the posterior surface of the temporal bone inferolateral to the posterior meatal lip. The endolymphatic sac may be entered in removing the dura from the posterior meatal lip. There is little danger of encountering the cochlear canaliculus, which has a more anterior course below the internal auditory canal. An anomaly that may block access to the posterior meatal lip is an unusually high projection of the jugular bulb. The mastoid air cells commonly encountered in the posterior meatal lip must be closed carefully to prevent a postoperative cerebrospinal fluid (CSF) leak.

After the posterior wall of the internal acoustic canal has been removed, the dura lining the canal is opened to expose its contents. The facial nerve is identified near the entrance of the facial canal at the anterior superior quadrant of the canal rather than in a more medial location, where the direction of displacement is variable. It is easy to expose the vestibule if the tumor extends into the vestibule by drilling along the posterior and superior semicircular canals.

 
 

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