Neurology II: Cranial Nerves and Their Disorders

• What do the cranial nerves form a part of?

  The peripheral nervous system (PNS)
• What are the 12 cranial nerves?

  I - (Olfactory)

  II - (Optic)

  III - (Oculomotor)

  IV - (Trochlear)

  V - (Trigeminal)

  VI - (Abducens)

  VII - (Facial)

  VIII - (Auditory or Vestibulocochlear)

  IX - (Glossopharyngeal)

  X - (Vagus)

  XI - (Spinal Accessory)

  XII - (Hypoglossal)
• Describe the structure of the olfactory nerve.

  -Shortest cranial nerve

  -Unmyelinated

  -Sensory component only

  
  

  Specialised epithelium at the top of the nasal cavity contains olfactory nerve fibres

  
• What is the function of the olfactory nerve?

  Sense of smell
• What can damage to the olfactory nerve result in and how can damage be caused?


  Damage to the olfactory nerve can cause altered sense of smell, or complete loss of sense of smell (anosmia).

  Causes include head injuries, tumours and neurodegenerative disorders(Temporary changes to smell can be caused by infection)
• How would you test to see if the patient's olfactory nerve is functioning well?

  Ask about any recent changes to sense of smell

  
  

  Then, with the patient's eyes closed, ask them to occlude one nostril and identify the smell (e.g. coffee, vanilla)

  Repeat on the other side
• The optic nerve only has a sensory component, true or false?


  True
• What is the function of the optic nerve?

  Transmission of sensory information from the retina to the primary visual cortex of the brain

  
• What are some potential causes of optic nerve defects?

  Trauma

  Tumour (e.g. pituitary adenoma)

  Multiple sclerosis (optic neuritis-inflammation damages optic nerve)

  Stroke
• What do you have to test in order to examine the optic nerve?

  Visual acuityVisual fieldsPupillary reflexesFundoscopy

  Fundoscopy is performed with anophthalmoscope& it allows visualisation of the retina and optic disc (optic nerve head) but this is rarely carried out in dental setting
• How can visual acuity be formally assessed?

  Snellen chart (letters with diff sizes) with the patient sat 6 metres away.

  Colour vision is also assessed using Ishihara plates (numbers in different colours)

  Dental setting:Ask the patient to read from a printed page (glasses, contact lenses should be worn)

  Test one eye at a time
• How can visual fields be formally assessed?

  Tested through confrontation: sit facing each other with a distance of ~an arm's length

  Have the patient looking directly at your eye or nose and test each quadrant in the patient's visual field by having them count the number of fingers that you are showingThis relies on the examiner having normal visual fields & blind spots
• What are some common visual field defects?

  Monocular blindness (e.g. trauma)

  
  

  Bitemporal hemianopia (classically seen in acromegaly)

  
  

  Homonymous hemianopia (e.g. stroke)

  
• How is the pupillary reflex tested?

  
• If you had a left optic nerve lesion, what would happen when testing the pupillary reflex?


  Shine a pen torch into the left eye:

  Left direct reflex lost(the left pupil will not constrict)Left consensual reflex maintained (left pupil will constrict when light shone in the right eye)

  Right direct pupillary response is maintained

  Rightconsensualreflex is lost(right pupil will not constrict when light is shone into left eye)
• If you had a left oculomotor nerve lesion, what would happen when testing the pupillary reflex?


  Shine a pen torch into the left eye:

  Left direct reflex lostLeft consensual reflex lost

  Right direct reflex maintainedRight consensual reflex maintained
• Which cranial nerves are usually assessed together and why?

  III (oculomotor), IV(trochlear), & VI (abducens)

  
  

  All 3 have a motor function and supply the extra-ocular muscles

  
• What does the oculomotor nerve supply?


  Supplies medial, superior & inferiorrectus, inferior oblique, levator palpebrae superioris

  Also supplies parasympathetic fibres involved in pupillary constriction

  
• What does the trochlear nerve supply?

  Superior oblique

  
• What does the abducens nerve supply?

  Lateral rectus

  
• What will palsies of cranial nerves III, IV, & VI result in?

  Diplopia - double vision
• What are potential causes of palsy in the oculomotor nerve and how does it manifest?

  Causes: diabetes, increased intra-cranial pressure

  Manifestation: eye is fixed down and out, unless looking towards the affected side

  Additional signs: ptosis (drooping), dilated pupil (mydriasis)
• What is a potential cause of palsy in the

  trochlear nerveand how does it manifest?Cause: trauma

  Manifestation: eye cannot move down and in (vertical diplopia when looking inferiorly)
• What are potential causes of palsy in the

  abducens nerveand how does it manifest?Causes: stroke, multiple sclerosis

  Manifestation: cannot look to the affected side
• How do you test for these palsies?

  Usually performed alongside cranial nerve II, after visual fields

  
  

  Ask the patient to follow a pen as you draw out the letter H

  
  

  Ensure they keep the head still and only move the eyes
• Does the trigeminal nerve have both sensory and motor function?

  Yes
• What are some potential causes of

  trigeminal nerve palsy?Upper motor neurone lesions (i.e. within the brain cortex) include tumours, multiple sclerosis

  
  

  Lower motor neurone lesions (affecting the peripheral nerve fibres) include cavernous sinus lesions, iatrogenic damage to IAN/lingual nerve
• How do you test the sensory component of the trigeminal nerve?

  Sensory component divided intoophthalmic(V1),maxillary(V2) andmandibular(V3) divisions

  Tested through light touch and pin prick (use a Neurotip)Corneal reflex (V1) not routinely tested

  
• How do you test the motor component of the trigeminal nerve?

  Motor component supplies muscles of mastication

  
  

  Inspect temporalis/masseter for wasting

  
  

  Palpate temporalis/masseter muscle bulk with patient clenching

  
  

  Ask the patient to open the jaw against resistance from your hand

  
  

  (Jaw jerk reflex- not routinely performed)
• What functions does the facial nerve have?

  Sensoryfor taste to the anterior 2/3rds viachorda tympani

  Motorto the muscles of facial expression, nerve to stapedius

  Secretomotorto lacrimal, submandibular & sublingual salivary glands
• What are potential causes of facial nerve palsy?

  Upper motor neurone lesions:stroke

  Lower motor neurone lesions:Bell’s palsy, parotid tumour

  Remember that inlowermotor neurone lesions, the entire half of the face is affected

  Inuppermotor neurone lesions, the forehead is spared (as there is bilateral innervation from the cortex)
• How do you test the facial nerve?

  Ask about changes to sense of taste

  
  

  Ask about hearing (hyperacusis)

  
  

  Facial movements (tested against resistance)- raise eyebrows, close eyelids, puff out cheeks, purse the lips, show your teeth
• What is the function of the vestibulocochlear nerve?

  It has asensoryfunction

  Is involved in hearing and balance
• What are some common causes of vestibulocochlear nerve defects?

  Acoustic neuroma (tumour that develops on vestibular nerve)

  
  

  Paget's disease (bones grow larger+weaker)
• How do you test the vestibulocochlear nerve?

  Ask about changes to hearing

  
  

  Whisper into the ear (choose a bi-digit number or two syllable word) and ask the patient to repeat

  
  

  Rinne and Weber test are performed using a tuning fork to assess for sensorineural vs conductive deafness
• What are the functions of the glossopharyngeal nerve?


  Sensoryandmotorfunctions

  -Principal role issensoryto tonsillar fossa and pharynx

  -Taste to the posterior 1/3rd of the tongue

  -Parasympathetic innervation to the parotid glands
• What are some potential causes of glossopharyngeal nerve palsy?


  Trauma, tumour, diphtheria

  
  

  (IX nerve problems usually occur alongside CN X problems)
• What will patients with a glossopharyngeal nerve palsy have?


  Impaired gag reflex

  
  

  (Testing gag reflex is not routinely performed)
• What is the function of the vagus nerve?

  Motorfunction supplying the pharynx, larynx and soft palate
• What are some causes of vagus nerve palsy?

  Trauma, brainstem lesion