Vestibulocochlear nerve

Special sensory function

Afferent system for hearing (cochlear nerve) and balance (vestibular nerve)

Anatomy

Cochlear nerve

Carries axons from the bipolar neurons of the spiral ganglion within the cochlea and modiolus. Passes from internal auditory canal (IAC) fundus to porus acusticus within the anteroinferior quadrant of IAC. Near porus acusticus it joins with the superior and inferior vestibular nerves to form the vestibulocochlear nerve.

The vestibulocochlear nerve then crosses the cerebellopontine angle cistern posterior to the facial nerve. It enters the brainstem laterally, at the pontomedullary junction.

The cochlear nerve reaches the ventral and dorsal cochlear nuclei in the dorsolateral caudal pons/rostral medulla oblongata.

Vestibular nerve

Carries the axons from bipolar neurons located in the vestibular ganglion (Scarpa’s ganglion) in the internal auditory meatus, where afferents from the utricle, saccule and semicircular canals synapse. The fibers form the superior and inferior vestibular nerves. Near porus acusticus the two nerves join with the cochlear nerve to form the vestibular nerve.

In the brainstem the afferent fibers terminate in the vestibular nuclear complex which is composed of four nuclei (lateral, superior, medial, inferior) located in the rhomboid fossa in the floor of the 4th ventricle in the caudal pons.

Clinical implications

• 95% of all lesions causing sensorineural hearing loss are vestibulocochlear schwannomas.
• In unilateral sensorineural hearing loss the lesion is almost always peripheral or at the root entry zone of the vestibulocochlear nerve or cochlear nucleus level. The auditory pathways decussate at multiple levels throughout their course in the brainstem making unilateral hearing loss very unlikely at lesion levels above the cochlear nuclei.