Vestibular System Examination

The Vestibulocochlear Nerve

• Cranial nerve VIII is the vestibulocochlear nerve.
• The vestibulocochlear nerve carries
  • Auditory, or cochlear, signals.
  • Balance, or vestibular, signals.
• Signals from CN VIII are relayed to the vestibular nuclei in the brainstem.
  • The nuclei in turn relay information to
    • The eyes
    • The body and limbs
    • Higher centres.
• Many efferent signals are controlled in part by cerebellar input.

Unilateral Vestibular Disease

• Horses with unilateral vestibular lesions often have a head tilt towards the side of the lesion.
• If nystagmus is absent, determining whether a horse with a head tilt is also weak is helpful in deciding whether vestibular disease is central or peripheral.
  • In peripheral diease, the horse may be ataxic but weakness is not normally seen.

Acute

• In the acute stages of vestibular disease, spontaneous nystagmus may be present.
  • The appearance of nystagmus is different depending on the type of vestibular disease present.
    • Central vestibular disease
      • Nystagmus often is variable; i.e. rotary, horizontal and vertical.
    • Peripheral vestibular disease
      • The fast phase of the nystagmus is away from the side of the lesion.

Chronic

• Nystagmus may resolve in more chronic lesions.
  • However, it may return with changes in head position.
    • I.e. positional nystagmus.
• Visual accomodation improves ataxic movements.
  • However, these may return dramatically on blindfolding.

Bilateral Vestibular Disease

• An observable head tilt may not be present.
• Movements are likely to be markedly ataxic.
  • This is probably due in part to involvement of ascending proprioceptive and descending motor pathways that run through the brainstem.