may be secondary to destruction of the middle or inner ear. Other causes include trauma to the petrous
temporal bone, loud noises (eg, gunfire), demyelinating conditions, ototoxic drugs (eg, aminoglycoside antibiotics
[gentamicin, kanamycin, neomycin, streptomycin] or salicylates), neoplasms involving the ear or brain stem, and
degeneration of the cochlea in aged dogs. Unilateral deafness or partial hearing loss, or both, is possible in
some of these instances. Cochlear degeneration in aged dogs is the most common cause of acquired deafness.
Congenital deafness can be inherited or result from damage (toxic or viral) to the developing fetus. An
autosomal gene in cats causes white fur, blue eyes, and deafness; it is dominant with complete expression for
white fur and incomplete expression for blue eyes and deafness. Deafness in this instance is due to
cochleosaccular degenerative changes that are expressed in the first week of life. Merle and white coat colors
are associated with congenital deafness in dogs and other animals. Dog breeds commonly affected include the
Dalmatian, Australian Heeler, Catahoula, English Setter, Australian Shepherd, Boston Terrier, Old English
Sheepdog, Great Dane, West Highland White Terrier, and Boxer. The list of affected breeds (now >48)
continues to expand and may change due to breed popularity and elimination of the defect through selective
breeding. For example, Cocker Spaniels were known to have hereditary deafness, but the trait is no longer
common in the breed.
Diagnosis requires careful observation of the animal’s response to sound. It is helpful to consider the owner’s
description of behavior and to ask appropriate questions. The response to visual, tactile, and olfactory stimuli
must be differentiated from the response to sound. In young animals or in animals kept in groups, deafness may
be difficult to detect, because the suspect individual will follow the response of others in the group. If the animal
is observed as an individual after an age when responses to auditory stimuli are predictable (~3-4 wk for dogs
and cats), then the deafness may be detected. The primary sign of deafness is failure to respond to an auditory
stimulus, eg, failure of noise to awaken a sleeping dog or failure to alert to the source of a sound. Other signs
include unusual behavior such as excessive barking, unusual voice, hyperactivity, confusion when given vocal
commands, and lack of reflex-alerting and attention movements of the pinnae. An animal that has gradually
become deaf, as in old age, may become unresponsive to the surroundings and refuse to answer the owner’s
call. These signs should be differentiated from cognitive dysfunction (see also other canine behavioral problems,
Other Canine Behavioral Problems). Unilateral deafness is difficult to detect, except by astute observation or by
electrodiagnostic procedures. Otoscopic examination of the external ear, radiography of the tympanic bullae,
and neurologic examination may reveal the cause, especially in cases of acquired deafness. In congenital
deafness, these procedures usually reveal normal anatomic structures, but the animal shows no evidence of
hearing, except in cases of unilateral deafness. Brain stem auditory evoked responses (BAER, an
electrodiagnostic test) are used to determine the presence and level of an auditory defect from either or both
ears. This test is useful in assessing hearing in puppies of breeds prone to congenital deafness. Impedance
audiometry can evaluate the integrity of the middle ear and the conduction system.
Deafness due to occlusion of the external ear canal usually responds to appropriate surgical or medical
treatment. This deafness is consistent with conduction deafness based on BAER testing and is usually not a
complete deafness. Deafness due to bacterial infections of the middle and inner ear may respond to appropriate
antibiotic treatment, which should be based on culture and sensitivity results. Recovery from deafness due to
persistent intense noise, trauma, or viral infections may be complete, partial, or nil. Recovery from deafness
caused by ototoxic drugs is rare. Hereditary deafness may be eliminated from a breed by removal of identifiable
carriers from the breeding program. The mode of inheritance of the deafness trait may be determined by study
of pedigrees or by test mating. More recently, the BAER test has been used to identify both unilaterally and
bilaterally affected dogs, which can then be eliminated from the breeding program.
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