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Inferior vestibular nerve

The inferior vestibular nerve is one of the two divisions of the vestibular portion of the vestibulocochlear nerve (CN VIII). It arises within the internal auditory canal (IAC) and innervates the posterior semicircular canal and the saccule, mediating balance sensations related to head position and vertical acceleration.

This nerve travels in the inferior compartment of the IAC, separated from other neurovascular bundles by bony plates (Bill’s bar and the falciform crest). It plays a crucial role in vestibular function, and its involvement is common in vestibular neuritis, schwannomas, and labyrinthitis.

Synonyms

  • Inferior division of the vestibular nerve

  • Saccular and posterior ampullary nerve trunk

Origin, Course, and Branches

  • Origin: Arises from the vestibular portion of CN VIII in the cerebellopontine angle (CPA).

  • Course:

    • Enters the internal auditory canal along with facial and cochlear nerves

    • Occupies the inferior-posterior quadrant of the IAC

    • Passes toward the vestibule and posterior semicircular canal via small bony canals

  • Branches:

    • Posterior ampullary nerve: Innervates the posterior semicircular canal

    • Saccular nerve: Innervates the saccule

Relations

  • Superiorly: Superior vestibular nerve

  • Anteriorly: Cochlear nerve

  • Posteriorly: Posterior semicircular canal

  • Inferiorly: Inferior aspect of IAC fundus

  • Medially: Internal auditory canal cisternal opening

  • Laterally: Vestibule and saccule

Function

  • Provides sensory input from the saccule (vertical linear acceleration)

  • Provides sensory input from the posterior semicircular canal (angular head movement in coronal plane)

  • Essential for balance, spatial orientation, and vestibulo-ocular reflexes

  • Clinical dysfunction causes dizziness, imbalance, positional vertigo, and abnormal VEMP (vestibular evoked myogenic potentials)

Clinical Significance

  • Vestibular schwannomas: May arise from the inferior vestibular nerve; often detected in IAC fundus

  • Vestibular neuritis: Inflammation affecting inferior branch commonly produces abnormal oVEMP/cVEMP

  • Labyrinthitis: Can cause abnormal enhancement along nerve course

  • Trauma: Temporal bone fractures may disrupt inferior vestibular fibers

  • Imaging role: Differentiation of nerve bundles in IAC is essential for diagnosing CPA masses

MRI Appearance

T1-weighted images:

  • Normal nerve: Intermediate-to-low signal, thin linear structure in IAC

  • Surrounded by IAC fat: bright on T1, improving contrast

  • Pathology:

    • Schwannoma: Iso- to hypointense mass expanding IAC

    • Inflammation: subtle thickening or intermediate hyperintensity

T2-weighted images:

  • Normal nerve: Intermediate-to-low signal, distinct within bright CSF

  • Vestibule and semicircular canals: fluid-filled structures appear bright

  • Pathology:

    • Schwannoma: heterogeneous but generally bright T2 mass

    • Labyrinthitis: increased T2 signal in vestibule or canals

    • Nerve edema: mildly increased signal intensity

3D High-Resolution Sequences (3D T2 DRIVE / SPACE / FIESTA / CISS):

  • Provide excellent visualization of all IAC nerves

  • Inferior vestibular nerve: thin, sharply marginated structure in inferior-posterior IAC quadrant

  • Posterior ampullary and saccular branches also identifiable in high-resolution scans

  • Pathology:

    • Schwannomas: well-defined, filling inferior-posterior IAC, expanding the canal

    • Nerve displacement: superior vestibular or cochlear nerve draped by tumor

    • Labyrinthine fluid abnormality: hyperintensity within vestibule or PSC

T1 Fat-Sat Post-Contrast:

  • Normal inferior vestibular nerve: no significant enhancement

  • Pathologic enhancement:

    • Vestibular schwannoma: strong homogeneous enhancement

    • Neuritis/labyrinthitis: linear enhancement along nerve

    • Metastasis or infiltrative disease: patchy or nodular enhancement

    • Inflammatory enhancement also seen in post-viral vestibulopathy

CT Appearance (Temporal Bone CT)

Non-Contrast CT (High Resolution):

  • Inferior vestibular nerve itself not directly visualized

  • Bony landmarks demonstrate its course:

    • IAC inferior-posterior quadrant

    • Bony canals to saccule and posterior semicircular canal

MRI images

Inferior vestibular nerve MRI axial image 1

MRI images

Inferior vestibular nerve MRI axial image