Cranial nerve examination

Cranial nerve examination is an important, but often daunting, part of the neurological exam due to negative memories of neuroanatomy class. It’s fantastic if you know which cranial nerves are being tested for each test described below, as well as the origin and projection of each cranial nerve. Fortunately, the cranial nerves are conveniently numbered from rostral to caudal and, from a practical standpoint, you just need to remember the following to help you localize the lesion.

  • Cranial nerves I and II are associated with the forebrain
  • Cranial nerves III and IV originate in the rostral brainstem (midbrain)
  • All other cranial nerves are in the caudal brainstem (pons, medulla oblongata)

Complete cranial nerve chart

Olfaction

Sensory: Olfactory n. (CN I)
Technique: Watch for a behavioral response to something pleasant smelling placed in front of the patient. Avoid using a cotton ball soaked in alcohol because this can irritate trigeminal nerve endings in the nose leading to a false positive result.
NOTE: This author does not routinely perform this test. Olfaction is tested when there is a client complaint regarding decreased smell.

Menace response

Sensory: Visual pathway (ipsilateral retina, ipsilateral optic nerve, optic chiasm, contralateral optic tract, contralateral lateral geniculate nucleus/thalamus, contralateral optic radiations, contralateral occipital lobe)
Motor: Cranial nerve 7 (facial n.)
Technique: Cover one eye and make a “threatening” gesture in front of the other eye. The normal response is closure of the eyelids. Be careful not to create a wind current or inadvertently touch the whiskers as this will give a false positive result.

NOTES

  • This is a learned response and NOT a reflex.
  • Most dogs and cats have a positive menace response by 12 weeks of age.
  • Some normal patients will not menace due to a stress response. Tap the head gently over the frontal sinus to get their attention.
  • The contralateral forebrain must be functioning normally to interpret the “threatening” gesture.
  • The ipsilateral cerebellum also plays a role in the menace response, but the exact pathway is uncertain.

Visual testing

Sensory: Ipsilateral retina, ipsilateral optic nerve; optic chiasm; contralateral optic tract, lateral geniculate nucleus/thalamus, optic radiations, occipital lobe
Technique: Drop cotton balls through patient’s field of vision (ideally standing behind patient), having patient navigate an obstacle course, see if patient will track a laser pointer (especially cats)
Normal response: Behavioral response – turning to look

Pupillary light response (PLR)

Sensory: Ipsilateral retina, optic nerve, optic chiasm and first part of contralateral optic tract
Motor: Cranial nerve 3 (Oculomotor n.)
Technique: Shine a bright light source (transilluminator or halogen penlight) into one eye and look for pupil constriction in both eyes. Test in both ambient light and dark room. Ideally, do not use a disposable pen light since these do not have a strong light source.
Normal response: Constriction of the ipislateral pupil (direct response) and partial constriction of contralateral pupil (indirect / consensual response)

PLR pathway

Fundic exam

While technically not part of the cranial nerve exam, it’s an important part of the neurological exam. This is the only test in which one can directly observe a part of the nervous system, the optic disc, which is the convergence of ganglion cell axons leaving the globe forming the optic nerve. Abnormalities of the retina or optic nerve can occasionally provide clues to systemic disease and the cause of neurological signs. For example, enlarged or tortuous retinal blood vessels or retinal hemorrhages can be a sign of systemic hypertension which could lead to infarction as a cause of neurological signs.

Technique: Examine the retina with an indirect lens (20 diopter) and transilluminator or with an ophthalmoscope. For indirect fundic exam, hold the light source near your eye and shine into the eye and look for a tapetal reflection. With your other hand, place the lens in front of the eye at finger distance. Move the lens and/or your head to visualize the entire retina.

TIP: There is nothing wrong with direct ophthalmoscopes, but I prefer not to have my face so close to the mouth of an animal that I don’t know well. I would rather have my fingers bitten while performing an indirect exam with a lens/transilluminator than my face bitten while using an ophthalmoscope.

Indirect ophthalmoscopy

Indirect ophthalmoscopy using a 20-diopter lens and transilluminator.

Hypertensive retinopathy

Multiple retinal hemorrhages in a dog with systemic hypertension.

Horner Syndrome

Horner Syndrome is a constellation of abnormalities that occur following sympathetic denervation to the eye.

Clinical signs:

Possible lesion locations: (pathway)

  • Brainstem
  • Spinal cord (C1-T3)
  • Vagosympathetic trunk
  • Cranial cervical ganglion / tympanic bulla
  • Intracranial
  • Retrobulbar

Horner Syndrome OS in a cat with left-sided otitis media-interna.

Corneal sensation & globe retraction

Sensory: Trigeminal n. (CN V)
Motor: Facial n. (CN VII) to close eyelids and abducens n. (CN VI) to retract globe
Technique: Gently touch the cornea with a soft cotton swab or a piece of cotton.
Normal response: Closure of the eyelids and retraction of the globe into the orbit.

Jaw tone & Muscles of mastication

Technique: Open mouth and assess jaw tone. Observe and palpate muscles of mastication.
Abnormalities:

  • Dropped jaw – bilat mandibular branch of trigeminal n. (CN V)
    • Trigeminal neuritis most common
    • IMPORTANT: Check rabies vaccines status – rabies can cause dropped jaw
  • Muscle atrophy
    • Unilateral – disease of ipsilateral trigeminal n. (CN V) – Trigeminal nerve sheath tumor most common
    • Bilateral – Masticatory Muscle Myositis most common

Dropped jaw in a Chesapeake Bay Retriever with Idiopathic Trigeminal Neuritis.

Facial sensation & lip retraction

Sensory: Trigeminal n. (CN V), Facial n. (CN VII) for inner pinnae
Motor: Facial n. (CN VII)
Technique: Touch or gently pinch the lips and look for retraction of the lips. Also, touch the medial nasal mucosa with a hemostat and look for retraction of the head.
NOTE: If response to nasal stimulation is decreased or absent, but there is a response to pinching the lips, consider a CONTRALATERAL forebrain disorder…the contralateral forebrain has to recognize the nasal stimulation as annoying for the patient to retract the head.

Muscles of facial expression

Motor: Facial n. (CN VII)
Technique: Observe the muscles of facial expression
Abnormal: Facial droop, deviation of the nose laterally (usually chronic facial n. dysfunction), or ear droop
Localization: Ipsilateral brainstem (medulla) or peripheral facial n. dysfunction

Left idiopathic facial nerve paralysis in an English Setter.

Palpebral reflex (blink reflex)

Sensory: Trigeminal n. (CN V)
Motor: Facial n. (CN VII)
Technique: Gently touch the medial & lateral canthus and look for closure of the eyelids.
Localization: Incomplete or absent closure of the eyelids is most often due to a facial nerve disorder. This would be confirmed by a lack of menace response if the patient is visual or if there is a lip or ear droop on the same side. A trigeminal nerve sensory disorder should be suspected if there is an absent palpebral reflex in the face of a normal menace response (indicating normal motor response) and/or absent lip retraction.

Physiological nystagmus (“Doll’s eye” reflex)

Sensory: Vestibular receptors in inner ears
Motor: Oculomotor (III), trochlear (IV), and abducen (VI) nerves
Technique: Move the head slowly laterally in both directions and up and down while observing the eyes for nystagmus. This is a normal, inducible nystagmus. The fast phase of physiological nystagmus is in the direction of head movement.
Localization: Delayed or absent physiological nystagmus is most often due to vestibular dysfunction. However, dysfunction of one of the nerves innervating the extraocular muscles should be suspected if strabismus is present and/or there are no other signs of a vestibular disorder (e.g., head tilt, vestibular ataxia, pathological nystagmus)

Pathological nystagmus

Technique: Examine the patient for evidence of spontaneous nystagmus in all body positions. If the patient will tolerate it, evaluate the patient in dorsal recumbency with the head/neck extended.

Characterization:

  • Horizontal, rotary, or vertical?
  • Direction of fast phase – Right or left if horizontal or rotary?
  • Changing direction of fast phase – Does fast phase direction change in different body positions? (e.g., patient has a resting right horizontal nystagmus, but then has a left horizontal nystagmus when placed on its back)

Localization: Horizontal and vertical nystagmus are fairly self-explanatory. Rotary nystagmus is present when the eyeball is rotating. The direction of the fast phase is usually away from the side of the lesion. Horizontal nystagmus (points toward the ears) is almost always peripheral and vertical nystagmus (points toward the brain) is almost always central. Rotary nystagmus can be seen with peripheral or central vestibular dysfunction. Be careful interpreting nystagmus during the acute stage of severe vestibular dysfunction as patients may look like they have vertical nystagmus when it is actually rotary with only a small rotational movement. Change in direction of fast phase (e.g., right to left) or change to vertical nystagmus in any body position usually indicates central vestibular dysfunction. Some have written that development of a positional nystagmus in patients that do not have a resting nystagmus is suggestive of central vestibular dysfunction. However, resting nystagmus often improves or resolves before positional nystagmus so patients with peripheral vestibular dysfunction might be incorrectly localized.

Gag reflex

Motor: Glossopharyngeal (CN IX) and vagus (CN X) nerves
Technique: Touch the oropharynx with the tip of your finger or a cotton tip applicator and watch for contraction of the pharyngeal muscles.
TIP: Never do this in any cat or an aggressive dog. Instead, you can palpate the larynx externally and look for swallowing.

Tongue

Motor: Hypoglossal n. (CN XII)
Technique: Look at the tongue for any atrophy, deviation, or asymmetry. With a unilateral hypoglossal nerve lesion, the tongue will deviate toward the side of the lesion and there will be atrophy of the ipsilateral side of the tongue. The patient may have difficulty with prehension of food. Tongue fasciculations may be present due to hypoglossal nerve dysfunction or irritation.

Left-sided tongue deviation and atrophy in a dog with left hypoglossal dysfunction.

Left-sided tongue deviation and atrophy in a dog with left hypoglossal dysfunction.