Applied Anatomy of cranial nerves
3. The effect of lesions to each cranial nerve (I to XII) in relation to their distribution I.e. in terms of:
(i) General sensory
(ii) Special sensory
(iii) Motor (Somatic or Branchial)
(iv) Parasympathetic (Visceral Motor)
CN I (Olfactory):
 Special sensory
 Not tested routinely, but can test with familiar smells of vanilla, coffee, peppermint
 Causes of lesion: upper respiratory tract infection, smoking and increasing age, basal skull or frontal
fracture, and following meningitis
 Effect of lesion: partial or full loss of sense of smell, which will be accompanied by significant loss of
flavour sensation in the mouth.
CN II (Optic):
 Special sensory
 Test visual acuity: use a hand-held Snellen’s chart. If cannot see top line of chart, count fingers, if not then
hand movement, failing this light perception.
 Causes of lesion: uni/bilateral occipital lobe infarction, uni/bilateral optic nerve damage (e.g. methanol
poisoning), diabetic retinopathy, nerve compression
 Effect of lesion: uni/bilateral blindness with rapid or gradual onset. If unilateral and nerve damage is to the
lateral nerve, blindness will be in the medial direction, if damage is to the post-chiasma medial nerve,
blindness will be in the lateral direction, on the opposite side.
CN III (Oculomotor), CN IV (Trochlear) and CN VI (Abducens):
 Motor
 Test Pupils: size, shape, equality and regularity tested by Light reflex with a torch.
 Test eye movements: ask patient to follow movement of finger around four regions.
 Causes of lesion: trauma, nerve compression,
 Effect of lesion: Failure of eye movements:
o CN III: adduction (medial rectus), depression (inferior rectus), elevation (superior rectus) and
external rotation (inferior oblique), raise upper eyelid (levator palpebrae superioris)
o CN IV: inversion (superior oblique) – cannot look downwards & outwards together
o CN VI: abduction (lateral rectus) – cannot look outwards
The effect is that the eye is paralysed in the region of the effected muscle, and will not respond to a moving
object in moving in that vicinity.
Double vision (diplopia) and nystagmus: eyes are normally maintained at rest in the middle line by the
balance of tone between opposing ocular muscles. Disturbances of this tone (which depends on pulses from
the retina) allows the eyes to drift off in one direction. This movement is corrected by a quick movement
(saccadic) back to the original position.
CN V (Trigeminal):
 Motor & sensory
 Test motor: palpation of tension in mastication muscles
 Test sensory: pain/light touch sensation to three facial regions (correspond to three nerve branches)
 Causes of lesion: trauma, nerve compression,
 Effect of lesion: Muscle weakness of the trigeminal nerve may lead to the patient’s complaining of difficulty
chewing (eating) or talking. Total loss of sensation in all three divisions of the nerve suggests that the level
of the lesion is at the ganglion or the sensory root. If there is total sensory loss in one division only this
suggests a postganglionic lesion. Loss of pain or light touch sensation from either of the regions suggests
damage to that nerve branch.
CN VII (facial):
 Motor & sensory
 Test motor: facial expressions
 Test sensory: a taste examination using a small amount of sugar on the anterior two thirds of the tongue
 Causes of lesion: vascular lesions or tumours (lesions of the frontal lobes may cause weakness of the
emotional movements of the face alone), MS, meningitis, otitis media, Lyme disease, Guillain-Barre
syndrome.
 Effect of lesion: Patient may have noticed the onset of difficulty forming facial expressions, speaking and
keeping liquids in the mouth or have facial asymmetry. They may have dry eyes/mouth, and loss of some
taste perception. Difficult muscle movements include wrinkling the forehead, shutting eyes tightly (Bell’s
phenomenon: upward movement of the eyeball and incomplete closure of the eyelid).
CN VIII (Vestibulocochlear):
 Special sensory
 Test: nerve or conduction deafness: Rinne’s vibration test, stand on one leg (balance)
 Causes of lesion: family Hx, exposure to loud noise, trauma, ear infections, tumour, toxicity. Unilateral loss
more likely to be nerve lesion. Conductive deafness due to: wax, otitis media, otosclerosis or Paget’s disease
 Effect of lesion: Hearing loss, complaints from patient/relatives, injury from falling, difficulty walking.
CN IX (Glossopharyngeal):
 Motor & sensory
 Tests: gag reflex, ask the patient to say “Ah” and check for deviation of the uvula to the normal side.
 Causes of lesion: vascular lesions (e.g. lateral medullary infarction), tumours, motor neurone diseases
 Effect of lesion: No definite symptoms. Patient may experience difficulty swallowing dry foods.
Glossopharyngeal neuralgia is a sudden shooting pain radiating from one side of the throat to the ear.
CN X (Vagus):
 Motor & Sensory
 Test sensory: Parasympathetic Afferent: Taste (root of tongue) and taste buds on the epiglottis.
 Test motor: test swallowing using intrinsic laryngeal muscles and nominal extrinsic tongue muscle.
 Causes of lesion:
 Effect of lesion: Unilateral vagus nerve paralysis may cause difficulty swallowing solids and liquids and
hoarseness. Lesions may be associated with vagal effects such as bradycardia, projectile vomiting,
hypotension and slow respiratory rate. Parasympathetic efferent lesion cause irregularities in normal
functioning of thoracic and abdominal viscera.
CN XI (Accessory):
 Motor
 Test: shrug shoulders – feel bulk of trapezius and attempt to push shoulders down.
 Causes of lesion: trauma involving neck or base of skull, basilar invagination, tumours near jugular foramen,
motor neurone disease, Guillian-Barre syndrome.
 Effect of lesion: inability to control head and upper back movements.
CN XII (Hypoglossal):
 Motor
 Test: difficulty in swallowing and Ask the patient to poke out the tongue which may deviate towards the
weaker side if there is a unilateral lower motor neurone lesion.
 Causes of lesion: vascular lesions, motor neurone disease or tumours. Peripheral causes: posterior fossa,
aneurysms of tumours, chronic meningitis and trauma.
 Effect of lesion: tongue wasting and fasciculations (which indicate a lower motor neurone lesion).