The internal acoustic meatus (internal auditory canal, IAC) is a short channel approximately 1 cm long within the petrous temporal bone, opening onto the posterior face of the petrous bone in the posterior cranial fossa. It transmits the facial nerve (CN VII), the vestibulocochlear nerve (CN VIII) divided into cochlear, superior vestibular, and inferior vestibular branches, and the labyrinthine artery. The canal is divided by a transverse crest (crista transversalis) into a superior compartment (facial nerve and superior vestibular nerve) and inferior compartment (cochlear and inferior vestibular nerves).
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| Ossification | Ossifies in the cartilaginous otic capsule of the petrous temporal bone during fetal life. |
The IAC is the primary location where vestibular schwannomas (acoustic neuromas) arise, typically from the vestibular nerve within the canal before extending medially into the cerebellopontine angle cistern. MRI with gadolinium demonstrates contrast enhancement of the IAC contents and any expanding tumour. Small intracanalicular tumours may be observed or treated with stereotactic radiosurgery, and surgical approaches include translabyrinthine (sacrifices hearing), retrosigmoid, and middle fossa (best hearing preservation potential for small tumours). The facial nerve in the IAC is at risk in all acoustic neuroma surgeries due to its intimate relationship with the tumour. The middle fossa approach to the IAC fundus is also used for facial nerve decompression, as the meatal foramen at the start of the labyrinthine segment is the narrowest part of the facial canal.
Acoustic neuromas arise in the IAC from Schwann cells of the vestibular nerve, producing progressive unilateral sensorineural hearing loss and tinnitus, expanding medially into the cerebellopontine angle to compress the brainstem and adjacent cranial nerves, managed by observation, stereotactic radiosurgery, or microsurgical removal.
The facial nerve in the IAC is draped over the surface of vestibular schwannomas in a variable and often displaced position, requiring intraoperative facial nerve monitoring and careful dissection to preserve function; facial nerve outcome correlates with tumour size and adherence to the nerve.
