Category:Cranial Nerve Examination - Horse

• The cranial nerves are numbered 1-12 from the most rostral to the most caudal.
• A systematic examination of the cranial nerves can aid accurate identification of the site of a lesion.

Cranial Nerve I

• Cranial nerve I is the olfactory nerve.
  • Abnormalities are rarely detected in this nerve.

Neurological Eye Examination - Horse

Face Sensation

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.

Pharyngeal and Laryngeal Function

• The pharynx and larynx are innervated by:
  • CN IX - the glossopharyngeal nerve
  • CN X - the vagus nerve
  • CN XI - the accessory nerve
• Pharyngeal and laryngeal function is best studied by:
  1. Observing the horse eat and swallow
  2. Endoscopy

Laryngeal Dysfunction

• Left recurrent laryngeal hemiplegia is the most common dysfunction in horses.
  • The left recurrent laryngeal nerve is a branch of the vagus nerve.
  • Horses are known as "roarers".

Pharyngeal Dysfunction

• Pharyngeal paralysis in horses is commonly associated with peripheral disease.
  • Especially guttural pouch disease.
    • Endoscopy of the guttural pouches should be therefore be performed in horses that are dysphagic.

Tongue Movement

• The hypoglossal nerve, CN XII provides motor innervation to the tongue.
• Normally, a horse resists the tongue being retracted from the mouth.
  • The horse will quickly return the tongue after it is pulled out to the side.
• Horses with hypoglossal weakness appear differently.
  • The tongue may protrude from the mouth.
  • Chronic unilateral hypoglossal lesions may result in unilateral tongue atrophy.
  • Bilateral hypoglossal lesions may result in difficulty prehending food.
• Horses with cerebral dysfunction may have signs corresponding to tongue weakness.
  • However, tongue atrophy is not present in these animals.