internal auditory meatus

**Anatomy** - Location: petrous part of the temporal bone. The opening (porus acusticus internus) faces the posterior cranial fossa; the lateral end (fundus) is closed by a bony plate separating it from the inner ear (vestibule and cochlea). - Length is about 1 cm. - Contents: facial nerve (CN VII), including the nervus intermedius; vestibulocochlear nerve (CN VIII), with its cochlear and superior and inferior vestibular divisions; and the labyrinthine (internal auditory) artery. The labyrinthine artery usually arises from the anterior inferior cerebellar artery (AICA), less often from the basilar artery. - The fundus is divided by a transverse (falciform) crest into upper and lower compartments. A vertical crest (Bill's bar) further separates the facial nerve from the superior vestibular nerve in the upper compartment. - Upper anterior compartment: facial nerve. - Upper posterior compartment: superior vestibular nerve. - Lower anterior compartment: cochlear nerve. - Lower posterior compartment: inferior vestibular nerve. - Memory aid: "Seven Up, Coke Down." CN VII is up and the cochlear nerve is down. - The facial nerve exits the canal into the facial canal, then leaves the skull via the stylomastoid foramen. The vestibulocochlear nerve does not exit the skull. It ends in the inner ear. - The internal auditory meatus is distinct from the external auditory meatus, which is part of the outer ear (tympanic membrane, ceruminous glands).

**Clinical significance** - **Vestibular schwannoma (acoustic neuroma):** - A benign tumor of Schwann cells, typically arising from the superior or inferior vestibular nerve at the Schwann–glial junction (Obersteiner-Redlich zone) within the canal. - Presents with unilateral sensorineural hearing loss, tinnitus, and imbalance. Larger tumors in the cerebellopontine angle cause facial numbness (CN V) and facial weakness (CN VII), and can compress the brainstem or cause hydrocephalus. - Bilateral vestibular schwannomas are pathognomonic for **neurofibromatosis type 2** (NF2 gene mutation, chromosome 22, merlin/schwannomin). - Diagnosis: MRI with gadolinium, which shows an enhancing mass widening the internal auditory canal ("ice cream cone" appearance). Treatment is observation, stereotactic radiosurgery, or surgical resection. - **Temporal bone fractures:** Transverse fractures are more likely to cross the internal auditory canal and otic capsule. They cause sensorineural hearing loss and facial nerve palsy more often than longitudinal fractures do. - **Facial nerve palsy:** A lesion in the internal auditory canal causes LMN facial paralysis. It may also cause loss of taste to the anterior two-thirds of the tongue, decreased lacrimation and salivation, hyperacusis (stapedius), and hearing loss or vertigo from CN VIII involvement. - **Vascular:** An AICA infarct (lateral pontomedullary or lower pontine syndrome) can cause ipsilateral facial paralysis, deafness, and vertigo, because the labyrinthine artery is usually an AICA branch. Labyrinthine artery occlusion causes sudden sensorineural hearing loss and vertigo. - **Other lesions at the cerebellopontine angle or canal:** Meningioma, epidermoid cyst, and Ramsay Hunt syndrome (VZV reactivation of the geniculate ganglion, with vesicles in the ear).

**High-yield pearls** - Imaging of the internal auditory canal with MRI is the test of choice for unilateral sensorineural hearing loss. - The most common cerebellopontine angle tumor is vestibular schwannoma (~80–90%), followed by meningioma. - Early vestibular schwannoma often affects hearing before facial function. The facial nerve is relatively resistant to compression.