Peripheral deafness can be classified as nerve deafness, where loss of hearing is a result of damage to the cochlea or cochlear nerve, and conductive, where the cochlea is normal but sound is prevented from reaching it by problems with the outer or middle ears. Possible causes of conductive deafness include buildup of excess ear wax (cerumen), which is a comon problem in a number of dog breeds, especially those with long ears, and filling of the middle ear with purulent discharge (pus) from a middle ear infection. In the latter case, the body may take weeks to months after the infection clears to absorb this material. When an inner ear infection has occurred, the result may be a combination of both nerve and conduction deafness. Neither of the above conditions is likely in very young puppies. Rarely, congenital fusion of the ossicles can produce diminished sound transmission, but this has not been documented in dogs or cats. Aimals born without a tympanic memberane are able to hear, but at diminished levels.
Normal hearing testing in animals is performed with the brainstem auditory evoked response (BAER), also known as the brainstem auditory evoked potential (BAEP) or auditory brainstem response (ABR). The auditory stimulus is an air-conducted click sound which travels down the auditory canal to the tympanic membrane causing it to vibrate. These vibrations are transmitted by the three bones of the middle ear (ossicles: incus, maleus, stapes) to a membrane-covered opening to the cochlea, the oval window. Vibrations transmitted by the foot plate of the stapes to the oval window produce vibrations in the fluids of the inner ear, which bend the cilia of the hair cells of the organ of Corti to produce electrical activity in the auditory nerve. The recording system displays the detected response to this stimulus once the responses to a large number of stimuli are averaged. However, when an animal has conductive deafness the response to this test is a flat line, indicating deafness.
When there is reason to suspect conductive deafness, the BAER test is repeated using a vibratory stimulus transducer known as a bone stimulator instead of the air-conducted click produced by the insert earphones. The same stimulus signal is sent to the bone stimulator as to the earphones by the electrodiagnositc instrument, producing a brief vibration (which can still be heard as a sound). The bone stimulator is firmly held against the bone of the skull at a location known as the mastoid process (although the mandible and zygomatic arch also work) and the stimuli are produced. The stimuli are conducted through the bone of the skull to the cochlea (which is embedded in the mastoid bone), by passing the outer and middle ears. If the cochlea is functioning, a response will be detected that is very similar to that produced in a normally hearing animal in response to air-conducted stimuli. Peak latencies with bone stimulation are shorter because of the shorter time necessary for sound to travel through bone compared to that for the tubing of the insert earphone and the outer and middle ear. Sample recordings can be seen here. The picture below shows a bone stimulation transducer (left) and a silent whistle.
This test is more difficult to perform because of the difficulty of establishing and maintaining a good mechanical contact between the transducer and bone, especially in heavily muscled dogs. Some dogs show discomfort from the necessary intense pressure against the head, so air-conducted testing of both ears should be completed before performing bone testing. Interpretation of the results from bone-conducted stimuli is more subjective than with air-conducted stimuli because of the greater difficulty in obtaining satisfactory recordings.
Many BAER testing facilities do not offer bone stimulation because of the cost of the transducer and inexperience in its use. Instrument manufacturers offer them as accessories at $300-500 US, even though the parts only cost about $75.
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