Nuclei and fibre types
CN VIII carries special somatic afferent (SSA) fibres in two functional parts, each with its own ganglion and brainstem nuclei.
| Part | Receptor | Ganglion | Nuclei |
|---|---|---|---|
| Cochlear (hearing) | Organ of Corti in the cochlea | Spiral ganglion, in the modiolus (central bony core) of the cochlea | Dorsal and ventral cochlear nuclei, at the pontomedullary junction on the inferior cerebellar peduncle |
| Vestibular (balance) | Cristae of the semicircular ducts; maculae of the utricle and saccule | Vestibular ganglion, in the internal acoustic meatus | Superior, inferior, medial and lateral vestibular nuclei, in the floor of the fourth ventricle |
The ganglion cells are bipolar. The nerve also contains a small number of efferent fibres, the olivocochlear bundle, which adjust the sensitivity of the cochlear hair cells.
Emergence and skull foramen
CN VIII enters the brainstem at the cerebellopontine angle, at the pontomedullary junction, immediately lateral to the facial nerve. It runs in the internal acoustic meatus of the petrous temporal bone and does not leave the skull; its receptors lie in the inner ear within the same bone.
At the fundus (deep end) of the meatus, a horizontal transverse crest and, above it, a small vertical crest divide the space into four quadrants. The facial nerve lies anterosuperior, the cochlear nerve anteroinferior, the superior vestibular nerve posterosuperior and the inferior vestibular nerve posteroinferior.
Course and branches
CN VIII is short: it runs from the inner ear, across the internal acoustic meatus and the cerebellopontine angle cistern, to the brainstem, travelling with the facial nerve and the labyrinthine artery.
- Cochlear nerve: formed from spiral ganglion axons leaving the modiolus.
- Superior vestibular nerve: from the utricle, the anterior and lateral semicircular ducts and part of the saccule.
- Inferior vestibular nerve: from most of the saccule and the posterior semicircular duct.
Central connections
The auditory pathway runs from the cochlear nuclei to the superior olivary complex, then up the lateral lemniscus to the inferior colliculus, the medial geniculate body of the thalamus and the primary auditory cortex in the transverse temporal gyri (Heschl's gyri). Fibres cross at several levels, so each ear is represented in both hemispheres, and a unilateral cortical lesion does not cause deafness in one ear.
The vestibular nuclei connect to the cerebellum, to the spinal cord through the vestibulospinal tracts for posture, and to the nuclei of CN III, IV and VI through the medial longitudinal fasciculus. The last connection drives the vestibulo-ocular reflex, which keeps the eyes fixed on a target when the head moves.
The labyrinthine artery supplies the inner ear, usually as a branch of the anterior inferior cerebellar artery. It is an end artery, so its occlusion causes sudden deafness and vertigo.
Clinical testing
CN VIII is tested by bedside hearing tests with a 512 Hz tuning fork and by examining eye movements and balance.
Hearing
- Whispered voice test: a rough screen, masking the other ear by rubbing the tragus.
- Rinne test: compares air conduction with bone conduction on the mastoid. Normally air conduction is louder (Rinne positive).
- Weber test: the fork on the vertex or forehead; the sound should be heard centrally.
| Finding | Conductive loss (affected ear) | Sensorineural loss (affected ear) |
|---|---|---|
| Rinne | Negative: bone louder than air | Positive: air louder than bone, both reduced |
| Weber | Lateralises to the affected ear | Lateralises to the better ear |
Balance
Look for nystagmus, perform the head impulse test for vestibular function, use the Dix–Hallpike manoeuvre to provoke benign paroxysmal positional vertigo, and assess gait and Romberg's test. Formal audiometry and caloric testing confirm bedside findings.
Lesions and palsies
A CN VIII lesion causes some combination of sensorineural hearing loss, tinnitus, vertigo and imbalance on the affected side.
- Vestibular schwannoma (acoustic neuroma): a benign tumour of Schwann cells of the vestibular part of the nerve, arising in the internal acoustic meatus and growing into the cerebellopontine angle. It usually presents with unilateral hearing loss and tinnitus rather than vertigo. Larger tumours compress CN V (reduced corneal reflex), CN VII and the cerebellum. Bilateral tumours suggest neurofibromatosis type 2.
- Vestibular neuritis: acute severe vertigo with nausea and nystagmus but normal hearing, usually after a viral illness. Labyrinthitis adds hearing loss.
- Ménière's disease: episodic vertigo with fluctuating hearing loss, tinnitus and aural fullness.
- Ototoxicity: aminoglycosides and some chemotherapy drugs damage hair cells.
- Temporal bone fracture: fractures crossing the otic capsule cause sensorineural loss; those through the middle ear cause conductive loss.
- Age and noise: presbycusis and noise damage cause bilateral high-frequency sensorineural loss.
Unilateral sensorineural hearing loss is investigated with MRI of the internal acoustic meatus to exclude a vestibular schwannoma.