The endolymphatic sac drains posteriorly to the sigmoid sinus. The vestibular aqueduct is a canal within the petrous bone connecting the vestibule to the endolymphatic sac (a pouch within the two layers of the dura posterior to the IAC). The dura around the IAC has important anatomical features regarding surgical approaches to the cerebellopontine angle. have reported that the subarachnoid space continues up to the fundus of the IAC ( Lescanne et al., 2002 Lescanne et al., 2008). However, the observation of tumors without an arachnoid interface with the facial nerve cast doubts on this theory ( Neely, 1981 Luetje et al., 1983 Jääskeläinen et al., 1994). Most authors believe that the subarachnoid space ends before the fundus of the IAC and, therefore, believe that vestibular neuromas arising at the fundus of the canal press the arachnoid membrane medially and therefore have a duplicate arachnoid covering, which aids in safe resection of the tumor and protection of the facial nerve ( Tarlov, 1980 Yaşargil, 2002 Pellet and Roche, 2004). The superior vestibular area accommodates the superior vestibular nerve that gives rise to the utricular, lateral ampullary, and superior ampullary nerves ( Rhoton, 2000d Lescanne et al., 2008). The superior fundal compartment is divided by a vertical bony crest (Bill's bar) into a facial area anteriorly and a superior vestibular area posteriorly ( Fig. The inferior vestibular area allows passage of the inferior vestibular nerve that gives rise to the posterior Ampullar (aka singular nerve) and the saccular nerve. The inferior fundal compartment contains the inferior vestibular area posteriorly and the cochlear area anteriorly. The fundus of IAC is divided into superior and inferior compartments by the transverse crest ( Fig. It continues to the level of the facial canal and envelopes the geniculate ganglion and becomes the epineurium ( Lescanne et al., 2008). Moving toward the fundus, the dura mater becomes progressively thinner ( Lescanne et al., 2002). The dural covering of this canal is an invagination of the dural covering of the posterior aspect of the petrous bone and continues to reach the fundus of the canal, where it attaches to the bony prominences in the fundus. It is directed anterolaterally and contains the acoustic-facial complex, the labyrinthine artery, and sometimes a loop of the anterior inferior cerebellar artery ( Rhoton, 2000c). The internal auditory canal (IAC) is situated at the posterior aspect of the petrous part of the temporal bone. Arnau Benet, in Handbook of Clinical Neurology, 2020 Dural reflections at the region of the internal auditory canal The literature also describes rare cases of bilateral, duplicated IACs that were associated with labyrinthine anomalies. 55 After cochlear implantation, some patients with narrow IACs have experienced facial pain and twitching without useful auditory sensation. 143 A narrow IAC has been considered a relative contraindication to cochlear implantation, because it suggests that the eighth cranial nerve may be insufficiently developed to conduct an auditory signal. Interestingly, in patients with atresia of the IAC, the facial nerve may take an aberrant course to establish facial motor function. A narrow IAC may accompany inner ear malformations or may be the sole radiographically detectable anomaly in a deaf child. When a patient has normal facial function and an IAC less than 3 mm in diameter, it is likely that the bony canal transmits only the facial nerve ( Fig. Jackler, in Cummings Pediatric Otolaryngology (Second Edition), 2021 Narrow Internal Auditory CanalĪ narrow IAC may indicate a failure of eighth cranial nerve development.
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