Lab 15: Cranial Nerve Testing

Cranial Nerve Testing (the important cranial nerves are listed in bold)

1. Cranial nerves should be evaluated sequentially from the olfactory (I) through the hypoglossal (XII).
2. Olfactory nerve (I):  Sensory pathway for smell
   1. Observe the animal’s response to pungent or pleasant odors such as alcohol or food, respectively.
   2. Olfaction is rarely tested in clinical practice.
3. Optic nerve (II): Sensory pathway for both vision and the pupillary light reflexes
   1. Assess vision
      1. Observe the animal walking in an unfamiliar environment
      2. Menace response – closure of the eyelids or retraction of the globe in response to a threatening gesture in the visual field (such as a hand moving rapidly past the animal’s face)

• The response of blinking requires an intact facial nerve (CN VII).

1. Menace response is absent before 2-3 months of age.

1. Assess the pupillary light reflex
   1. Examine the pupils for symmetry; unequal pupil size is called anisocoria.
   2. Shine a light in one eye and examine the eye for evidence of pupillary constriction.  This is the direct response.

• The other pupil should simultaneously constrict, this being the indirect or consensual response.

1. The efferent arm of the reflex is mediated by the oculomotor nerve (CN III).

4. Oculomotor nerve (III): Causes pupillary constriction and provides motor innervation to several of the extraocular muscles
   1. Examine the pupils for symmetry; damage to this nerve or its associated nucleus causes ipsilateral pupillary dilatation.
   2. Assess the pupillary light reflex; with a lesion of CN III, the pupil will not constrict when either the ipsilateral or contralateral eye is stimulated.
   3. Assess eye position at rest; lesions to the nerve or associated nucleus leads to relative paralysis of the eye and ventrolateral deviation of the eye known as strabismus.
      1. Strabismus is more commonly the result of vestibular disease.
   4. Assess for size of the palpebral fissure; slight drooping or ptosis of the upper lid may be apparent.
5. Trochlear nerve (IV): Innervates the dorsal oblique muscle of the eye
   1. Lesions cause lateral rotation of the dorsal aspect of the eyeball
   2. This is very rarely recognized in small animals.
6. Trigeminal nerve (V): Provides sensation to the face, and innervates muscles of mastication
   1. Assess sensation to the face
      1. Stimulate ophthalmic branch by touching the medial canthus of the eye and assessing for a blink (palpebral reflex).
      2. Test maxillary branch by touching the upper lip level with the canine tooth or by brushing the whiskers and looking for an eye blink.

• Evaluate the mandibular branch by pinching the skin overlying the mandible by the lower canine, or by brushing the whiskers and looking for an eye blink.

1. Use a blunt probe (hemostats) to touch the nasal mucosa: the animal usually jerks its head away, even if depressed or stoic.

1. Assess muscle mass of the masseter and temporalis muscles and jaw tone.
   1. Unilateral lesions cause ipsilateral atrophy of these muscles.
   2. Bilateral lesions result in a dropped jaw and an inability to close the mouth.
2. Abducens nerve (VI): Innervates the lateral rectus and retractor bulbi muscles of the eye
   1. Lesions cause a medial strabismus and an inability to retract the eye.
   2. This is very uncommon in small animals.
3. Facial nerve (VII): Innervates the muscles of facial expression and of the middle ear; also provides sensory fibers for taste to the cranial two thirds of the tongue, and innervates the lacrimal gland and the sublingual and mandibular salivary glands
   1. Assess for facial symmetry; lesions result in drooping of the lip and ear on the affected side.
   2. Assess for the ability to blink with the palpebral reflex and the menace response.
   3. Decreased tear production can be demonstrated using the Schirmer tear test.
4. Vestibulocochlear nerve (VIII): Sensory pathways for vestibular inputs and for hearing
   1. Assess for evidence of vestibular dysfunction:
      1. Head tilt
      2. Ataxic gait with ipsilateral leaning, falling or rolling

• Strabismus

1. Nystagmus: involuntary oscillation of the eyes
   1. Typically has definite fast and slow phases
   2. Named according to the direction of the fast phase
   3. Further characterized by the direction of the oscillations (e.g. vertical, rotary, or horizontal)
   4. Can sometimes be induced by rolling animal on its back
2. Loss of the oculovestibular reflex:  A nystagmus that develops in normal animals when the head is moved from side to side, with the fast phase of the nystagmus in the direction of head movement.

1. Assess hearing
   1. Loss of hearing can be crudely evaluated by observing the animal’s response to loud noises
   2. Electrophysiologic testing (BAER) can be used to more accurately test hearing.
2. Glossopharyngeal nerve (IX): Innervates the parotid and zygomatic salivary glands, provides taste to the caudal one third of the tongue, and (together with the vagus nerve) innervates the muscles of the larynx and pharynx
   1. Assess for dysphagia and loss of the gag reflex by touching the caudal pharynx with a finger or tongue depressor.
   2. A history of coughing after drinking and/or eating can be the most sensitive indicator of pharyngeal dysfunction.
3. Vagus nerve (X): Innervates the muscles of the larynx and pharynx
   1. Assess for dysphagia, loss of the gag reflex and historical evidence of dysphagia as discussed above.
   2. Other signs that may be seen include regurgitation (due to esophageal dysfunction) and bradycardia.
4. Spinal accessory nerve (XI): Innervates the trapezius, sternocephalicus and brachiocephalicus muscles
   1. Dysfunction due to involvement of this nerve is very rarely seen in animals.
5. Hypoglossal nerve (XII): Innervates the muscles of the tongue
   1. Dysfunction may lead to difficulty in eating and prehending food
   2. Assess for ipsilateral atrophy and fasiculations of the tongue; the tongue usually hangs from the mouth on the side of involvement.

A selective screening of testing of the important cranial nerves is summarized below:

Assessment of Vision and the Pupillary Light Reflex

Localization Of Visual Deficits

1. The presence or absence of conscious vision (assessed with the menace response) and the pupillary light reflex can be used to localize a lesion to a portion of the visual pathway.
2. If an animal has both loss of vision and an abnormal pupillary light reflex, the lesion must be in a part of the visual pathway that is shared by these two functions.
   1. Retina
   2. Optic nerve
   3. Optic chiasm
   4. Optic tract – pupillary light reflex will be present, but incomplete. Due to partial decussation of pathway at both optic chiasm and pretectal nuclei.
3. If the animal has only visual deficits, the lesion must be in the visual pathway in the telencephalon or diencephalon, beyond where the pupillary light reflex fibers branch off in the mesencephalon to synapse on the pretectal nuclei.
   1. Lateral geniculate nucleus (thalamus)
   2. Internal capsule
   3. Occipital cortex
4. If there are pupillary light reflex deficits but vision is normal, the lesion must be in the portions of the pathway specific to the reflex.
   1. Pretectal nucleus
   2. Parasympathetic nucleus of CN II
   3. CN III
   4. Ciliary ganglion, ciliary nerves, constrictor muscle of pupil