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

• Nerve fibres giving sensation to the face are carried in cranial nerve V - the trigeminal nerve.
  • The trigeminal nerve provides sensory innervation to:
    • The skin of the face
    • The ears
    • The nasal mucosa
    • The cornea
  • There are three divisions of the trigeminal nerve
    • The mandibular branch
    • The maxillary branch
    • The opthalmic branch
• Facial sensation is tested by observation for an avoidance response or reflex movement following stimulation.
  • All the divisions of the trigeminal nerve should be tested. This is achieved by:
    • Stimulation within the nostrils
    • Stimulation within the ears
    • Stimulation between the mandibles
    • Testing the corneal reflex.
      1. Pressure is exerted on the corneal surface through the eyelid.
      2. The opthalmic branch of the trigeminal nerve provides sensory (afferent) input.
      3. Efferent signals are sent via the abducens nerve (CN VI)
      4. The retractor oculi muscle retracts the eye

Masticatory muscles

• The mandibular branch of the trigeminal nerve carries motor fibres.
  • It is the only division of the trigeminal nerve that does so.
• The muscles of mastication are innervated by these fibres.
  • The masseter and temporalis muscles are particulaly influenced by the mandibular brach of the trigeminal nerve.
    • These muscles of mastication are also the easiest to assess.
    • These muscles should be closely examined for signs of atrophy.
      • Atrophy of the temporalis and masseter muscle indicates damage to the mandibular branch of CN V.

Facial Expression

• The muscles of facial expression are innervated by branches of cranial nerve VII - the facial nerve.
• The facial nerve also carries the parasympathetic supply to the lacrimal glands.
  • Horses with facial nerve dysfunction are therefore prone to corneal ulceration, due to :
    • An inability to blink
    • Poor or absent tear production

Facial Nerve Dysfunction

• Facial nerve dysfunction is common in the horse.
• It is readily identified by one or more of the following:
  • A lip droop
  • Muzzle deviation
    • Deviation is towards the normal side.
  • An ear droop
• Early or mild dysfunction may be reflected by:
  • Slight changes to nostril size
  • Nostril flare
  • Reduced ear movements in response to audible stimuli
• The branch of CN VII supplying the muzzle and nostrils crosses the vertical mandibular ramus and the surface of the masseter muscle.
  • A lesion to that side of the face, for example a kick, may cause signs confined to the nose.
  • Alternatively, a more central lesion will give both ear and nostril signs.

The Palpebral Reflex

• The palpebral reflex examines the function of both CN V (afferent) and CN VII (efferent).
• The reflex is elicited by lightly touching the eyelids and watching for reflex closure.

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.