Since the ear canal does not open until approximately 5 days in cats and 14 days in dogs, and deaf puppies and kittens cue off of the responses of litter mates, it is not uncommon for deafness to go unrecognized for many weeks. In some breeds, bilaterally deaf puppies will display more aggressive play with litter mates because they do not hear cries of pain, and both puppies and kittens after weaning will not waken at feeding time unless jostled. Bilateral deafness can usually be detected by behavioral testing with sound stimuli presented outside of the visual field or with the animal blindfolded, taking care to avoid visual or vibratory cues. The minimum desired response is a Preyer's reflex, or twitch of the ears in response to the sound; many animals will also orient to the sound source. However, hearing animals, especially the young, quickly adapt and stop responding, resulting in equivocal results. Further, unilateral deafness cannot be detected by these measures; at best such animals may demonstrate difficulty in localizing the origin of a sound.
Objective assessment of the presence of auditory function requires a test known variously as the brainstem auditory evoked response (BAER), brainstem auditory evoked potential (BAEP), or auditory brainstem response (ABR). In this test, a computer-based system detects electrical activity in the cochlea and auditory pathways in the brain in much the same way that an antenna detects radio or TV signals or an EKG detects electrical activity of the heart (Sims & Moore, 1984a; Sims, 1988; Strain, 1992a). The response waveform consists of a series of peaks identified by Roman numerals: peak I is produced by the cochlea and cochlear nerve, and later peaks are produced within the brain (Figure 3A). The response is collected with a special computer through small subdermal electrodes. The most common electrode montage consists of one in front of each ear, one at the top of the head, and one between and just caudal to the eyes. It is uncommon for an animal to show any evidence of pain from the placement of the electrodes, but it may object to the restraint and the irritation of wires hanging in front of its face. Recordings from cats often require a cat bag or other form of restraint. The stimulus click produced by the computer is directed into the ear with a foam insert earphone or headphones. Because of the microvolt amplitude of the response, a computer must average responses to a large number of stimuli (typically 1000) to unmask them from the scalp EEG and EMG activity within which the waveform is buried. Each ear is tested individually, and the test is usually complete in 10-15 minutes. When dogs or white cats are screened for congenital inherited deafness, a single sound intensity is generally used by examiners since the deafness, when present in an ear, is total; 95 dB nHL (normal hearing level) is used by the author, but no consensus standard has been developed. Assessment of partial hearing loss is usually evaluated at several intensities to gauge the extent of loss. Sedation or anesthesia are generally unnecessary unless the animal becomes extremely agitated, which can usually be avoided with patient and gentle handling. However, the response is unaffected by chemical restraint, so some practitioners routinely employ sedatives. With complete peripheral deafness, peak I of the BAER is totally absent, as are the subsequent peaks (Figure 3B). With partial hearing loss, as is seen with presbycusis and some cases of ototoxicity, the time to occurrence of peak I is increased and the amplitude of the peaks is diminished (Figure 3C). The BAER changes during postnatal development of the auditory system, so appropriate reference values must be used when evaluating young animals (Strain et al., 1991; Buchwald & Shipley, 1986). BAER testing of puppies is not usually performed before five weeks of age, when the strial and hair cell degeneration of congenital deafness is complete. Many breeders of at-risk dog breeds routinely test entire litters before placing the dogs, and bilaterally deaf puppies are frequently euthanized.
Animals that test as deaf with the BAER, but in whom conductive deafness is suspected, can be further evaluated by BAER using a bone stimulator instead of air-conducted clicks (Strain et al., 1993). A vibratory stimulus transducer is firmly held against the skull, preferably over the mastoid process, and BAER recordings are obtained in the usual manner. The auditory stimulus travels through bone to the cochlea, bypassing the outer and middle ears. When the deafness is conductive, a normal-appearing response is recorded (Figure 3D). Because there are no known forms of inherited conductive deafness in dogs or cats, animals with this form of deafness are not precluded from breeding considerations.
Other diagnostic tests of auditory function are also available, but may be more difficult to employ, require additional high-cost equipment, or require anesthesia. Impedance audiometry (Penrod & Coulter, 1980; Sims, 1988) permits assessment of middle ear function. The middle latency auditory evoked potential (Sims & Moore, 1984b), similar to the BAER, tests auditory pathways up through the auditory cortex. Most recently, it has been shown that the cochlea generates very low intensity otoacoustic emissions in response to auditory stimuli, a response thought to reflect the active processes of outer hair cells. Transiently evoked otoacoustic emissions can be used to assess cochlear function (Sims et al., 1994)
Fig. 3. Brain-stem auditory evoked responses recorded from dogs. (a) Air-conducted response recorded from a normal adult. (b) Air-conducted response recorded from a deaf Dalmatian puppy. (c) Air-conducted response recorded from a 15-year-old Boston Terrier with presbycusis. Note the delayed peak latencies and decreased peak amplitudes compared to (a). (d) Bone-conducted response from a normal adult. Latencies are shorter than in (a) because bone conduction transmission time is shorter than the air transmission time in the insert earphone tubing.