Vestibulocochlear nerve (CN) VIII; Ear, Hearing, Balance
The ear is the organ of hearing and balance. It comprises external (auricle and external acoustic meatus), middle and inner parts.
Deformities of the auricle (figure 30), such as bat ears, are common and anomalies may be associated with syndromes such as Down’s.
Figure 30 External ear
2. Crura of antihelix
3. Triangular fossa
5. Auricular tubercle
6. Upper and 10. lower parts of concha
7. Crux of helix
8. Arrow leading to external acoustic meatus
12. Intertragic incisura
Infection of the external acoustic meatus (otitis externa) may produce pain and discharge; earache may also be referred from the teeth, temporomandibular joint, larygopharynx and cervical spine. Additional symptoms of middle ear disease are deafness tinnitus and vertigo, although these may also occur in diseases of the central nervous, skeletal and cardiovascular systems, and with some drugs. Infection of the middle ear (otitis media) may produce redness and bulging of the eardrum. Extension of this infection into the mastoid air cells or the paranasal air sinuses may produce tenderness within the ear, or over the cheeks and frontal region (figure 31a,b). Otitis media may be complicated by perforation of the tympanic membrane and damage to the ossicles.
The outer cartilaginous part of the external acoustic meatus can be straightened by gently pulling the auricle upwards and backwards (figure 32), this facilitates detection of abnormalities within the canal.
A detailed view of the drum is provided with an auroscope, ensure that the hand holding the instrument is also resting on the head so that any head movement by the subject carries no risk of the instrument being forced further into the canal (figure 33). Examination with an auroscope or endoscope are often diagnostic of middle as well as external ear disease (figure 34a,b).
Assessment of auditory function of the vestibulocochlear nerve is with a whisper or a watch ticking in each ear (figure 35), having checked that there is no wax interfering with air conduction.
Air conduction should be better than bone conduction. Therefore, if the base of a vibrating tuning fork is placed over the mastoid process and the subject is asked to say as soon as the vibration stops, turning the vibrating fork near the ear should be accompanied by return of sound; this is known as Rinne’s test (figure 36a,b). If the sound does not return, the implication is that bony conduction is better than air and may indicate damage to the tympanic membrane or disease of the middle ear.
Comparing the two sides is by Weber’s test (figure 37) in which the base of a vibrating tuning fork is placed over the middle of the forehead and the subject is asked whether it is heard more distinctly in one ear than the other. The fork is normally heard centrally, but may be lateralised, e.g. in the presence of middle ear disease.
A more precise assessment of hearing is with audiometry, applying different noise levels at different frequencies and recording the responses. The balance component of the nerve is assessed by running cold water into each ear in turn. A positive response produces nystagmus towards the stimulated side.