Approach to CN5, CN7, CN8 Palsy

Question 1

CN 7 - facial nerve functions

Answer 1

1. Motor function
   - Facial muscles (expression)
   - Orbicularis oculi (eyelid closure)
2. Sensory
   - Anterior 2/3 of tongue
   - Subjective sensation of the face (objective normal)
3. Hearing - stapedius muscle - loudness control
4. Parasympathetic
   - Lacrimal gland
   - Salivary gland

Question 2

CN 7 - facial nerve anatomical course

Answer 2

Origin: pons - motor root and sensory root
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Enters cerebellopontine angle – together with CN8
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Travels through internal auditory meatus and enters facial canal (motor and sensory root merges) to form facial nerve
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Branches:
- Greater petrosal nerve (salivary gland)
- Stapedius nerve (stapedius muscle)
- Chorda tympani (anterior 2/3 tongue)
- Posterior auricular nerve (motor muscles around ear)
- Digastric and stylohyoid muscle
- Main trunk (motor root)
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Motor root continues into parotid gland
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Terminal motor branches:
- Temporal branch
- Zygomatic branch
- Buccal branch
- Marginal mandibular branch
- Cervical branch

Question 3

CN7 - facial nerve testing (mixed nerve)

Answer 3

1. Forehead asymmetry (frontalis weakness in LMNL)
   - Look up, wrinkle forehead
2. Orbicularis oculi weakness
   - Squeeze eyes shut, bury eyelashes
   - Test power of 1 eye at a time with both index fingers
3. Facial asymmetry
   - Observe for drooping
   - Puff cheeks
   - Smile, show your lower teeth - platysmal asymmetry
4. Hyperacusis on affected side - stapedius nerve involvement
   (Conversely deafness or tinnitus signify acoustic neuroma or large brainstem lesion)
5. Look for cerebellar signs (cerebellopontine angle lesion)
6. Parotid tumours or surgery - peripheral facial nerve palsy

Wishlist
1. Otoscopy - Ramsay-Hunt syndrome
2. Taste function testing
3. Hypo or hyperlacrimation

Question 4

How do you differentiate CN7 UMNL vs LMNL?

Answer 4

CN7 UMNL - frontalis muscle sparing (from bilateral innervation)

CN7 LMNL - affects both upper and lower parts of the face

Caveats:
1. Dense stroke (UMNL) may have whole face weakness mimick Bell’s palsy
2. Limited lower facial fibre Bell’s palsy affecting only lower parts of the face

Question 5

What are the causes of unilateral CN7 UMN palsy?

Answer 5

1. Contralateral stroke
2. Tumour

Question 6

What are the causes of unilateral CN7 LMN palsy?

Answer 6

A. Pontine level - CN5/6/7/8, contralat pyramidal weakness, ataxia
1. Pontine infarct (brainstem syndrome)
2. Demyelination
3. Tumour deposits

B. Cerebellopontine angle - with CN8 defect
4. CP angle tumour - meningioma, acoustic neuroma

C. Facial canal - unilateral CN7 LMNL palsy
5. Middle ear infection
6. Bell’s palsy (idiopathic) - 95%
7. Tumour deposits
8. Fracture of skull base
9. Carcinomatous basal meningitis

D. Geniculate ganglion - vesicle in auditory canal
10. Ramsay Hunt syndrome

E. Peripheral branches - parotid scar/swelling/pain
11. Parotid gland infection, tumour, surgery

F. Mononeuritis multiplex - DM, PAN, CS, WG, RA, SLE, Sjogren, sarcocidosis, lymphoma, Lyme, leprosy

Question 7

What are the causes of bilateral CN7 LMNL palsy?

Answer 7

1. Guillain-Barre syndrome (GBS)
2. Myasthenia gravis (NMJ)
3. Myopathies
   - Dystrophia myotonica
   - Facio-scapulo-humeral dystrophy
4. Infections
   - Lyme disease
   - Polio
5. Sarcoidosis
6. Carcinomatous basal meningitis

Question 8

What are the investigations to evaluate for CN7 palsy?

Answer 8

Usually self limiting, however to offer investigations:

1. Blood glucose and HbA1c - Bells palsy a/w DM
2. MRI brain suprageniculate hyperintensity on T2, as well as TRO intracranial tumours, stroke
3. CRP, ESR, infective serologies
4. Anti-ganglioside antibodies (GBS)
5. Nerve conduction studies
6. Lumbar puncture in multi-CN palsy, mononeuritis multiplex, meningeal infiltration

Question 9

How would you manage CN7 LMN palsy?

Answer 9

1. High dose prednisolone
2. Oral acyclovir
3. Corneal protection - eye lubricants and eye shield

Question 10

What are the complications of Bell’s palsy?

Answer 10

1. Persistent facial weakness
2. Corneal abrasion
3. Subjective pain and sensory disturbance over facial nerve
4. Aberrant re-innervation with crocodile tears (tearing while talking or eating)
5. Jaw-eyelid synkinesia
6. Hemifacial spasm

Question 11

Prognosis of Bell’s palsy

Answer 11

• 85% patients improve over 3-4 weeks of onset
• 70% achieve complete recovery
• 15% persistent lesion (aberrant reinnervation)