vestibulocochlear nerve 8

VESTIBULOCOCHLEAR NERVE

- NITHIN NAIR

INTRODUCTIONStato-acoustic or Auditory nerve8th cranial nerve2 components: a) cochlear (hearing) b) vestibular

(equilibrium)

OBJECTIVES 1) Anatomy and functions.2) Purpose of test3) How to test 4) Interpretations

ANATOMY AND FUNCTIONSCochlear Nerve:Dorsal and ventral cochlear nuclei -- situated---

inferior cerebellar peduncle.The auditory receptors -- hair cells of the organ of

corti Innervated – peripheral processes of bipolar neurons

of spiral ganglionTheir central processes form cochlear nerve which

ends in dorsal and ventral cochlear nuclei

AUDITORY PATHWAYThe 1st sensory neuron lies in the spiral ganglion of

bipolar cell. The peripheral processes are distributed to the organ of corti.The central processes form cochlear nerve –ends in dorsal and ventral cochlear nuclei.

The 2nd neuron fibres start from the dorsal and ventral cochlear nuclei; their axons form trapezoid body and ends in dorsal nucleus of trapezoid body of the same and opp. side

AUDITORY PATHWAYThe 3rd neuron fibres from dorsal nuclei of trapezoid

body ascend as lat. Lemniscus on both sides. Some of the fibres of lat. Lemniscus end in inf. Colliculus. Rest of the lat. Lemniscus relays in med. Geniculate body.

The 4th neuron fibres from med. Geniculate body – auditory radiations – pass thru sublentiform part of internal capsule – projected to auditory cortex of temporal lobe.

Vestibular Nerve: The 4 vestibular nuclei are situated partly in medulla

and partly in pons in relation to the floor of 4th ventrical.

Receptors – maculae of saccule and utricle (static balance) and cristae of the ampullae of semicircular ducts (kinetic balance)

Innervated – peripheral processes of bipolar neuron of vestibular ganglion (acoustic meatus)

Their central processes –form vestibular nerve – ends in vestibular nuclei.

Vestibular pathway:

Innervates the maculae of the urticle and succule (sensitive to static changes in equilibrium)

The ampullae of semicircular canals(sensitive to dynamic changes in equilibrium)

Impulses pass: vestibular nerve vestibular nuclei (pontomedullary junction)

Fibres from vestibular nuclei pass via inferior cerebellar peduncle cerebellum

Clinical symptoms:

Damage to cochlear nerve produces tinnitus and sensorineural deafness.

Damage to vestibular nerve results in vertigo or dizziness and nystagmus.

Types of nystagmus: 1) Peripheral vestibular nystagmus2) Central nervous system nystagmus3) Optokinetic (railroad or freeway)

nystagmus

Purpose of test:To detect whether any deafness is bilateral or

unilateral, and whether sensorineural or conduction.

To determine whether disturbance of vestibular function originates in the labyrinth, the vestibular nerve, or the brain stem.

HOW TO TEST

Method of testing:

A) For cochlear component• 1) Rinne’s test• 2) Weber’s test• 3) Absolute bone conduction (ABC)

test• 4) Schwabach test• 5) Audiometric test• 6) Evoked response

B) For vestibular component

• 1) Rotational test.• 2) Caloric test.• 3) Dix Hallpike maneuver for

nystagmus.• 4) Electronystagmography

Rinne’s test

Weber’s test

Audiometric test

Evoked response

Rotational test

Caloric test

Dix hallpike maneuver

Electronystagmography (ENG)

INTERPRETATIONS

InterpretationsWhether hearing loss is senorineural or conductive

TEST NORMAL CONDUCTIVE DEAFNESS

SENSORINEURAL DEAFNESS

Rinne AC > BC BC > AC AC > BC

Weber Not lateralized Lateralized to diseased ear

Lateralized to better ear

ABC Equal Lengthened Shortened

Schwabach Equal Lengthened Shortened

InterpretationsWhether nystagmus is peripheral or central

FINDING PERIPHERAL CENTRALLATENCY Yes 3 – 10 sec NoFATIGABILITY Yes No

NYSTAGMUS DIRECTION

Fixed, typically mixed rotational

Changing, variable and pure vertical or pure horizontal

SUPPRESSION BY VISUAL FIXATION

Yes No

SEVERITY Markedly severe Mild to moderateCONSISTENCY Less consistent More consistentPAST POINTING In direction of slow

phaseIn direction of fast phase

References...BD CHAURASIA’S HUMAN ANATOMYNEUROLOGICAL EXAMINATION BY

NAVNEET KUMARBICKERSTAFF’S NEUROLOGICAL

EXAMINATIONNEUROLOGICAL EXAMINATION MADE

EASY – GERAINT FULLERPJ MEHTA’S PRACTICAL MEDICINEPRACTICAL PHYSIOLOGY – VD JOSHIWWW.GOOGLE.COM