PBL 5 - Schwannoma of the Vestibulocochlear Nerve

Question 1

Describe the course of the facial nerve (CN7).

Answer 1

1. Origin: lateral side of pons/medulla
2. Passes across posterior fossa of cranium as 2 branches:
   a. Large motor root
   b. Smaller sensory root (the intermediate nerve)
3. Roots pass through the internal acoustic meatus
4. Roots pass through the facial canal of the temporal bone
5. Roots fuse to form the facial nerve, which enlarges to form the geniculate ganglion
6. Branches of the geniculate ganglion:
   a. Greater petrosal nerve
   b. Nerve to the stapedius
   c. Chorda tympani nerve
7. Exits skull through the stylomastoid foramen

Question 2

Describe the course of the vestibulocochlear nerve (CN 8).

Answer 2

1. Origin: lateral surface of pons/medulla
2. Crosses the posterior cranial fossa with the facial nerve roots
3. Leaves the cranial cavity via the internal acoustic meatus
4. Splits into the vesticular and cochlear divisions in the petrous part of the temporal bone
5. Vestibular part sends fibres to the ampulla of each of the 3 semicircular canals
6. Cochlear part sends fibres to their hair cells in the cochlea

Question 3

Define tinnitus.

Answer 3

Sensation of sounds in the ear in the absence of external sounds

Question 4

Define vertigo.

Answer 4

State in which the patient feels that either they or their surroundings are in a constant state of movement

(Most commonly a spinning sensation - but may also be a tilting feeling)

Question 5

Define Meniere’s disease.

Answer 5

A disease of the inner ear characterised by episodes of deafness, tinnitus and vertigo, usually preceded by a sense of fullness in the ear; caused by build-up of fluid in the inner ear

Question 6

Describe the pathophysiology of Meniere’s disease.

Answer 6

1. Increased production of endolymph
   a. This causes increased pressure
   b. This causes rupture of Reissner’s membrane (that separates the scala vestibula from the scala media)
   c. This causes influx of potassium-rich endolymph into the perilymphatic space of the scala media
   d. This causes damage to sensorineural parts of the ear
2. Decreased production of perilymph
   a. This leads to a compensatory increase in endolymph production
3. Decreased absorption of endolymph
   a. Caused by: malfunction of endolymphatic sac/blockage of endolymphatic pathways

Question 7

Describe the signs and symptoms of Meniere’s disease.

Answer 7

Common triad:

1. Hearing loss (fluctuating at first, then gradually becomes permanent)
2. Tinnitus
3. Vertigo

Other symptoms:

1. Pallor
2. Sweating
3. Nausea
4. Vomiting

Question 8

Define “schwannoma of the vestibulocochlear nerve”.

Answer 8

A slow-growing benign tumour arising on one of the vestibulocochlear nerves, it arises from the Schwann cells surrounding the nerve

Question 9

Describe causes of schwannoma.

Answer 9

1. Tumour suppressoor gene mutations on chromosome 22 (protein: merlin/schwannomin)
2. Neurofibromatosis 2 (results in bilateral schwannomas)

Question 10

Describe the signs and symptoms of schwannoma of the vestibulocochlear nerve.

Answer 10

1. Assymetrical hearing los
2. Facial numbness
3. Progressive episodes of dizziness
   a. Intermittent
   b. May be associated with nystagmus
   c. Caused by: cerebellar compression, vestibular dysfunction
4. Other hearing related symptoms:
   a. Tinnitus
   b. Difficulty localising sounds
5. Rare symptoms associated with facial nerve involvement:
   a. Facial weakness
   b. Increased/decreased tears
   c. Metallic/reduced taste
   d. Slow blink
   e. Difficulty swallowing

Question 11

Describe the pathophysiology of vestibulocochlear schwannoma.

Answer 11

1. Overproduction of Schwann cells causes a tumour
   a. Usually very slow growing (2-3mm/year)
2. Common tumour locations:
   a. Nerve entry point in the medulla
   b. Inside the internal acoustic meatus
   c. Cerebellopontine angle
   d. Vestibular division of the vestibulocochlear nerve
3. Small tumours causes compression of the vestibulocochlear nerve
4. Larger tumours can also cause compression of:
   a. Trigeminal nerve (facial numbness)
   b. Lower cranial nerves and brainstem (swallowing difficulties, cerebellar symptoms, nystagmus, gait changes)
   c. Facial nerve
   d. 4th ventricle (hydrocephalus)

Question 12

List the 3 surgical approaches which may be used to operate on vestibular schwannoma.

Answer 12

1. Middle fossa approach
2. Suboccipital retrosigmoid approach
3. Translabyrinthine approach

Question 13

Describe the process of the suboccipital retrosigmoid approach.

Answer 13

1. Craniotomy
   a. Exposes posterior part of sigmoid sinus and inferior part of transverse sinus
2. Removal of tumour
   a. Debulking
   b. Opening of the body part of the internal auditory canal
3. Reconstruction of the internal acoustic meatus
   a. Dura mater flap is placed over the hole
   b. Muscle/fat graft soaked with fibrin glue is placed on top
4. Closing the craniotomy
   a. Biological glue to prevent CSF leakage
   b. Bone flap is replaced and secured with titanium screws

Question 14

List the complications which may be caused by the suboccipital retrosigmoid approach.

Answer 14

1. Facial weakness/palsy
2. Eye problems (e.g. unable to close it properly)
3. Hearing loss
4. Balance problems
5. Chronic headache (due to severed occipital nerves)

Question 15

List the features of facial nerve palsy.

Answer 15

1. Dry eye (paralysis of orbicularis oculi m.)
   a. Corneal drying
   b. Corneal ulceration
   c. Infection of the eye
   d. Blindness
2. Pain
3. Paralysis of muscles of facial expression
   a. Difficulty speaking
   b. Difficulty eating
4. Loss of sensation in face
5. Altered taste
6. Excess/reduced salivation/tearing
7. Loss of hearing