Eighth Cranial Nerve Lesions

Anatomy

Course of the Vestibulocochlear Nerve

• Central nuclei:
  • Vestibular nuclei → pons and rostral medulla
  • Cochlear nuclei → pons and rostral medulla
• Cochlear fibres → medial geniculate bodies → superior temporal gyrus
• Vestibular fibres → widely project throughout brainstem and cerebellum
• Emerges lateral to the Facial nerve in the pontomedullary junction
• Travels through the internal acoustic meatus with CN VII and labyrinthine artery
• Terminates in the labyrinth of the inner ear (petrous temporal bone)

Vestibulocochlear Nerve Innervations

Component	Innervations
Cochlear nerve	Special sense of hearing (organ of Corti)
Vestibular nerve	Sensory input from semicircular canals, utricle, saccule → essential for balance

Pathology

Classification of Hearing Loss

Type	Pathology
Conductive	Abnormality of external or middle ear
Sensorineural	Abnormality of inner ear, cochlear nerve, or brainstem
Mixed	Combination of conductive and sensorineural

Causes of Cochlear Nerve Dysfunction

Sensorineural Causes

Acute

Cause	Notes
Idiopathic Sudden Sensorineural Hearing Loss (ISSHL)	Most cases fall here
Noise-induced	Prolonged noise exposure
Meniere’s syndrome	Sensorineural loss + episodic vertigo + tinnitus
Ototoxic drugs	Aminoglycosides, quinine, aspirin, frusemide
Labyrinthitis	Viral/bacterial; associated vertigo and hearing loss
Acoustic neuroma	Progressive hearing loss
Small vessel disease	Hyperviscosity, autoimmune, microvascular
Brainstem lesions	Rare

Chronic

Cause
Presbyacusis (age-related loss)
Congenital infections (rubella, syphilis)

Conductive Causes

Acute

Cause
Wax (cerumen) impaction
Otitis media (acute/chronic/secretory)
Barotrauma
Temporal bone fracture
Tympanic membrane trauma
Ossicular dislocation
Perilymphatic fistula

Chronic

Cause
Otosclerosis
Paget’s disease

Clinical Assessment

History

Key questions:

1. Nature of hearing loss
   • Acute / gradual
   • Partial / complete
   • Unilateral / bilateral
2. Pain (infection, malignancy)
3. Trauma (including ear cleaning)
4. Noise exposure
5. Middle ear symptoms
6. Associated symptoms
   • Tinnitus: Often accompanies nerve or conductive deafness
   • Vertigo: Suggests vestibular involvement
7. Medications (esp. ototoxic drugs or overdose)

Examination

Step	Findings
Inspect external auditory meatus	Cerumen, foreign body
Inspect tympanic membrane	Infection, inflammation, fluid
Inspect for vesicles	Ramsay Hunt syndrome
Hearing tests	See below

Hearing Tests

Rinne’s Test

Step	Interpretation
512 Hz tuning fork on mastoid → move to external meatus	AC > BC = Rinne positive (normal or sensorineural loss); BC > AC = Rinne negative (conductive loss)

Note: Rinne alone cannot confirm sensorineural loss → requires Weber test.

Weber’s Test

Step	Interpretation
512 Hz tuning fork on mid-forehead	Localises to good ear = sensorineural loss; localises to bad ear = conductive loss

Figure	Interpretation
1	Normal results: Rinne test: positive on both sides (Air conduction>Bone Conduction) Weber test: normal referred equally to each ear, indicating symmetrical hearing in both ears with normal middle/outer ear function
2	Sensorineural deafness in the RIGHT ear: Rinne test: positive on both sides Weber test: referred to the left ear.
3	Conductive deafness in the RIGHT ear: Rinne test: negative on the right (patients right) (Bone Conduction>Ait Conduction) Rinne test: positive on the left Weber test is referred to the right ear

Audiometry

• Formal audiology assessment
• Definition of acute hearing loss:
  Sensorineural loss ≥ 30 dB across ≥ 3 contiguous frequencies within 3 days

Investigations

Blood Tests

1. FBC
2. U&Es / glucose
3. CRP
4. ESR
5. Others as indicated

CT Scan / CT Angiogram

• Screening for mass lesions
• CT angiogram → suspected aneurysm

MRI

• Imaging of choice
• Detects:
  • Middle ear pathology
  • Posterior fossa lesions
  • Brainstem lesions
  • Vestibulocochlear nerve pathology (e.g. acoustic neuroma)

Management

• Directed at underlying cause
• Sudden sensorineural hearing loss = otologic emergency → ENT referral prior to ED discharge
• Treat underlying causes:
  • Infections → antibiotics/antivirals
  • Vestibular disorders → ENT management ± vestibular rehab
  • Tumours → neurosurgical or oncologic referral
  • Trauma → ENT or neurosurgical management

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References

Publications

• Brazis PW, Masdeu JC, Biller J. Localization in Clinical Neurology. 8e 2021
• Fuller G. Neurological Examination Made Easy. 6e 2019
• O’Brien M. Aids to the Examination of the Peripheral Nervous System. 6e 2023

FOAMed

• Cadogan M. Tuning Fork Tests (Weber and Rinne). LITFL
• Coni R. Neuro 101: Cranial Nerves. LITFL
• Nickson C. The Brainstem Rules of Four. LITFL
• Ercleve T. The rule of 4 of the brainstem. LITFL
• Nickson C. Cranial nerve lesions DDx. LITFL