Cranial Nerve VIII

Auditory Nerve

There are two components: auditory and vestibular.

AUDITORY

WHAT TO DO

Test the hearing

Test one ear at a time. Block the opposite ear; either cover it with your hand or produce a blocking white noise, e.g. crumpling paper.

Hold your watch by the patient's ear. Discover how far away from the ear it is still heard. Alternative sounds are whispering or rubbing your fingers together. Increase in volume to normal speech or loud speech until your patient hears.

If the hearing in one ear is reduced, perform Rinne's and Weber's tests.

Rinne's test

• Hold a 256 or 512 Hz tuning fork on the mastoid process (bone conduction (BC)) and then in the front of the ear (air conduction (AC)).

• Ask the patient in which position the sound is louder.

Weber's test

• Hold the 256 or 512 Hz tuning fork on the vertex of the head.

• Ask in which ear the sound is louder: the good ear or the deaf ear.

WHAT YOU FIND

	Rinne test in deaf ear	Weber test
Conductive deafness	BC > AC	Deaf ear
Sensorineural deafness	AC > BC	Good ear

N.B. With complete sensorineural deafness in one ear, bone conduction from the other ear will be better than air conduction.

WHAT IT MEANS

• Conductive deafness. Common causes: middle ear disease, external auditory meatus obstruction, e.g. wax.

• Sensorineural deafness:

– Lesion of the cochlea (common): otosclerosis, Ménière's disease, drug- or noise-induced damage.

– Lesions in the nerve (uncommon): meningitis, cerebellopontine angle tumours, trauma.

– Lesions in the nucleus in the pons (very rare): vascular or demyelinating lesions.

VESTIBULAR

BACKGROUND

The vestibular system is not easy to examine at the bedside because it is difficult to test one part of the system, or even one side, in isolation. In some respects this is fortunate, as it is this ability of the vestibular system that allows patients to make good recoveries even after severe unilateral vestibular lesions by learning to operate on only one functioning vestibular system.

The vestibular system can be examined indirectly by checking gait, looking for nystagmus and carrying out more specific tests (see below).

Gait

See Chapter 4. Always test heel–toe walking. Gait is unsteady, veering to the side of the lesion.

Nystagmus

See Chapter 10. Vestibular nystagmus is associated with vertigo, horizontal and unidirectional. It may be positional.

Head impulse test

See Chapter 25. This is a dynamic test of vestibular function.

Caloric test

This is normally performed in a test laboratory.

The patient lies down with his head on a pillow at 30 degrees so the lateral semicircular canal is vertical.

Cool water (usually about 250 ml at 30°C) is instilled into one ear over 40 seconds. The patient is asked to look straight ahead and the eyes are watched. This is repeated in the other ear, and then in each ear with warm water (44°C).