Chapter 6 - Auditory & Vestibular Function

Questions

Define the following terms:

conductive hearing loss, sensorineural hearing loss, tinnitus, vertigo, nystagmus.
Conductive hearing loss is the loss of the ability to transmit sound waves to the inner ear. Obstruction of the external ear, problems with the tympanic membrane or problems with the middle ear (or the ossicular chain) are the cause.
Sensorineural hearing loss is hearing loss due to damage to the hair cells of the organ of Corti or to the auditory part of the vestibulocochlear nerve.
Tinnitus is ringing or buzzing in the ears.
Vertigo is the illusion of movement.
Nystagmus is a to-and-fro movement of the eyes. It can be pendular (even swings in both directions, often due to congenital vision problems) or can be "jerk." In jerk nystagmus, there is a fast and a slow component, typically due to vestibular problems (the direction is named by the fast component).

6-1. What effect do lesions of the CNS have on hearing in one ear?

Answer 6-1. It is nearly impossible to cause loss of hearing in one ear by damage to the brain beyond the point where the vestibulocochlear nerve enters the brain stem. This is because the distribution of hearing is bilateral at all levels of the central nervous system auditory pathway. Localization of sound may be slightly affected by auditory cortex lesions. In fact it is nearly impossible for central nervous system lesions to cause clinically detectable hearing loss and if these is hearing loss you must look at the conductive system, the inner ear and the vestibulocochlear nerve.

6-2. What kind of hearing loss will be produced by damage to the inner ear?

Answer 6-2. Inner ear and CN VIII lesions will produce senorineural deafness.

6-3. What do you call problems in which the sound wave can not reach the inner ear?

Answer 6-3. Blockage of the outer ear or damage to the tympanic membrane or middle ear is called conductive deafness.

6-4. What would it mean if Weber's test lateralized to the left?

Answer 6-4. Weber's test lateralized toward the side of conductive deafness and away from sensorineural.

6-5. What is the most common symptom of damage to the vestibular system?

Answer 6-5. Vertigo is common with vestibular damage.

6-6. What does it mean when someone says that nystagmus was in a particular direction?

Answer 6-6. Nystagmus is named according to the direction of fast movement.

6-7. How can you distinguish vertigo from inner ear damage from that caused by damage to the central nervous system?

Answer 6-7. With peripheral causes of vertigo (the illusion of movement), nystagmus is proportional to the amount of vertigo and nystagmus is always in the same direction regardless of the direction that the patient looks. Also, peripheral nystagmus is almost never in a vertical (up or down) direction. With central vertigo (cerebellum, vestibular nuclei and brain stem) nystagmus is usually greater than vertigo and may shift direction depending on gaze direction.

6-8. What is the only way to examine the integrity of the vestibular system on one side?

Answer 6-8. Caloric testing is the only way to check each inner ear vestibular function independently.

6-9. What would you anticipate finding during cold-water caloric testing in the intact patient who is awake?

Answer 6-9. When the patient is awake, the eyes will drift slowly toward the side of cold water with rapid correction to the opposite side (the opposite for warm water). There is tonic balance in vestibular input from each ear; cold water caloric testing decreases tonic input from the inner ear (warm water increases it).

6-10. What would you expect to find in the comatose patient whose brain stem was still working if ice-water was infused into the right ear?

Answer 6-10. The eyes would drift toward the side of the ice-water infusion and remain there for several minutes. There would be no nystagmus (which is a response generated by the conscious cerebral cortex when the visual image slips across the retina).

6-11. Damage to the inner ear produces responses that look like (cold/warm) water caloric testing (choose one)?

Answer 6-11. Damage to the inner ear or vestibular nerve decreases vestibular input from that ear (acts like cold water caloric).