The internal auditory canal (IAC) is a bony tunnel in the posterior petrous temporal bone transmitting the facial nerve (CN VII, anterior superior quadrant), the cochlear nerve (CN VIII cochlear, anterior inferior), the superior vestibular nerve (posterior superior), and the inferior vestibular nerve (posterior inferior), separated by the falciform crest (horizontal) and the vertical crest (Bill's bar). The IAC is 8-12 mm long and 4-8 mm in diameter, with the porus acusticus (opening) on the posterior petrous face.
Vestibular schwannoma (acoustic neuroma) is the most common tumour of the IAC, arising from the vestibular nerve Schwann cells within the canal and expanding into the CPA cistern. MRI with gadolinium demonstrates the enhancing tumour filling the IAC. Small intra-canalicular tumours are managed by observation, stereotactic radiosurgery (Gamma Knife), or middle fossa approach surgical resection preserving hearing. Facial nerve decompression for Bell's palsy requires identification of the IAC fundus through the middle fossa approach where the facial nerve meatal segment is most compressed.
Small vestibular schwannoma confined to the IAC produces unilateral sensorineural hearing loss, tinnitus, and imbalance; MRI with gadolinium demonstrates an enhancing mass filling the IAC fundus and extending to the porus acusticus; middle fossa surgical approach allows tumour removal with hearing preservation in small tumours with serviceable residual hearing.
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