Auditory and Vestibular Disorders

1. Describe 2 tests commonly used to evaluate auditory function.

Human hearing is usually within the range of 2Hz to 20kHz with 0dB being the threshold of human hearing. Audiograms evaluate the auditory perception of tone frequencies up to 8kHz and measure threshold for each frequency. A sharp drop off in hearing at a specific range of frequencies is indicative of noise damage. Bone conduction can be compared with air conduction to test if hearing loss is cochlear or conduction related. This is done by putting vibration on the skull to bypass the middle ear – if hearing is better, then the cochlear may be ok, but the conduction to sounds to the inner ear is impaired.

Auditory Brainstem Evoked Response
Computer evaluation of the timing of the auditory system using an average of 2,000 EEG-type recordings during repetitive presentation of a brief sound stimulus can test hearing in infants or situations where you cannot communicate with the patient regarding his or her hearing. Recordings can discriminated at least 6 separate peaks which can be correlated with activities in structures of the auditory pathway responding to sound stimulus.

2. Describe 3 tests commonly used to evaluate vestibular function.

Balance Test
Stand with eyes closed to take out visual stimuli, and (optionally) on foam to take out somatosensory stimuli, and move platform to test otolith organs. Modern platforms incorporate measures of the location of center of gravity.

Barany Test
Rotate patient in a “Barany chair” and monitor eye movements to test each pair of semicircular canals. Modern chairs are computer controlled.

Caloric Test
Tilt head back 60 degrees to put the horizontal semicircular canal in a vertical orientation. Then, put hot or cold water into the external ear canal to heat or cool the endolymph in the horizontal semicircular canal and establish convection currents. Eye movements are then monitored to test each horizontal semicircular canal unilaterally. Normally, the direction of nystagmus (saccade) is away from the ear into which water is put.

3. Define conductive hearing loss and describe 4 disorders of this type.

Conductive hearing loss is the blockage or impairment of conduction of sound to the choclea.

Excessive Wax in External Acoustic Meatus
Excessive ear wax can block air vibrations; treatment is to remove wax.

Perforated Tympanic Membrane
Perforation of tympanic membrane decreases sound transmission to the middle ear, resulting in hearing loss of 5-20 dB across entire frequency range. Perforations will self-heal if not too large; otherwise, transplanted material may be necessary.

Otitis Media
Fluid in the middle ear may result from inadequate function of the Eustachian tube. Middle ear bone movements are impaired because of the fluid accumulation, resulting in hearing loss of 20-40 dB. There is a danger of infection to spread to the mastoid bone or brain. Decongestants, antibiotics, and antihistamines can aid recovery; in severe cases, it may be necessary to insert a ventilating tube into the tympanic membrane to drain the middle ear.

Abnormal spongy bone formation around the oval window and stapes footplate results in impairment of stapes movement. This can lead to gradual progressive hearing loss at all frequencies and as much as 40-60 dB of intensity as spongy bone growth enlarges. Tinnitus may also result. Otosclerosis can be treated with stapedectomy and replacement with stapes prosthesis but the loss of the stapedius muscle can result in peculiar sensitivities to certain sounds.

4. Define sensorineural hearing loss and describe 7 disorders of this type.

Sensorineural hearing loss is due to impared cochlea or auditory nerve function, commonly accompanied by vestibular disorder.

Noise Damage
Very loud noise can lead to damage in the organ of Corti, resulting in hair cell destruction, especially at the basal turn of the cochlea. This results in hearing loss at high frequencies (around 4 kHz) and sometimes tinnitus. There is no good treatment so protect your ears.

Old-age hearing loss is related to degeneration of cochlear hair cells especially at more basal turns. Hearing loss occurs especially for high frequencies. There is no good treatment. Bummer.

Drug-induced damage to the inner ear can be due to streptomycin, neomycin, kanamycin, high dose asprin, quinine, and some anti-tumor drugs (cisplatin/carboplatin). Cochlear hair cell destruction typically occurs earliest at more basal turns and progresses toward the apex over time. This results in bilateral hearing loss, especially at higher frquencies. Tinnitus, vertigo, and disequilibrium may also occur. No good treatment available other than detecting ototoxicity early and changing drugs.

Labyrinthitis results from bacterial or viral infection of the labyrinth. Symptoms include vertigo and hearing loss.

Meniere’s Disease
Meniere’s disease is caused by increased endolymph pressure (endolymphatic hydrops) leading to vestibular and cochlear malfunction, generally in one ear. Patients may have feeling of increased pressure in ear. Vestibular symptoms include spells of vertigo, pathological nystagmus, nausea, and vomiting. Auditory symptoms include tinnitus, and fluctuating hearing loss, mostly at low frequencies. Treatments include drugs (antihistamines) to reduce endolymph pressure, shunting from endolymphatic to subarachnoid space, destruction of vestibular hair cells with ototoxic drugs, labyrinthectomy, or section of vestibular nerve to relieve symptoms.

Congential Malformation of Labyrinth
Genetic basis accounts for 40% of malformations of the labyrinth. Prenatal rubella (German measles) virus infection accounts for 13%. Congenital malformations of the labyrinth result in vestibular dysfunction and hearing loss. In cases of complete hearing loss, a cochlear prosthesis may be used to apply artificial electrical stimulation of the auditory nerve in the inner ear.

Eighth Nerve Tumors (Acoustic Neuromas)
Eighth nerve tumors usually occur at the cerebellopontine angle and are benign. Abnormal proliferation of cells of the vestibular nerve sheath is the most common basis, leading to compression of CN VIII and pontomedullary portion of the brain. Earliest symptoms are vestibular and auditory dysfunctinos and are typically unilateral, including hearing loss, tinnitus, disequilibrium, and vertigo. Later symptoms reflect compression of CN V and CN VII: weakness of ipsilateral facial muscles and loss of ipsilateral facial sensation. Treatment is usually to remove the tumor as soon as possible.

5. Explain the basis for both auditory and vestibular symptoms in disorders of the inner ear.

Central hearing disorders can result in hearing deficits generally more complicated than just hearing loss.

Central Tinnitus
Cause of central tinnitus (perception of monotonous sound not present in environment) is unknown but may results form changes in central auditory pathways after damage to cochlea. There is no known cure.

Disorders of Brain Blood Supply
Disorders in brain blood supply may leads to anoxia in central auditory centers such as the cochlear nucleus and vestibular nuclei, which are supplied by the anterior (AICA) and posterior inferior cerebellar (PICA) arteries.

Brain Tumors
May lead to compression of central auditory centers, resulting in malformation.

Chemical Imbalances
Poorly understood at present.

6. Define vestibular compensation.

The loss of vestibular function in one side due to peripheral damage will result in loss of balance, etc. However, within 1-2 months of damage, the remaining side will compensate, allowing recovery of balance, demonstrating remarkable plasticity in the central vestibular pathways.