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Canine Deafness
George M. Strain, PhD
INTRODUCTION
Auditory function is important to animals because it is a means by which much of the
interaction with its environment occurs; reduction or loss of this function can have
a mild or an extreme impact. Deaf animals can survive, but deafness or diminished
hearing precludes usefulness in working dogs, diminishes communication in pet-
family relationships, impedes communication with conspecifics, and can put affected
animals in jeopardy in settings where motor vehicles or predator animals can inflict
damage or even death.
1
Deafness (partial or complete inability to hear) may result from a wide variety of
causes. Remedies may exist for some types, but for other types there is no recourse.
Knowing an animal’s type of deafness is informative in understanding the impact it has
on the animal’s life, the prognosis for the condition, and any breeding implications.
Funding: N/A.
The author has nothing to disclose.
Department of Comparative Biomedical Sciences, School of Veterinary Medicine, Louisiana
State University, Baton Rouge, LA 70803, USA
E-mail address: strain@lsu.edu
KEYWORDS
Sensorineural deafness Conductive deafness Pigment-associated deafness
Hereditary deafness Presbycusis Age-related hearing loss
Noise-induced hearing loss Ototoxicity
KEY POINTS
Deafness is common in dogs and has a variety of causes; the most frequent is congenital
hereditary sensorineural deafness associated with white pigmentation.
Conductive deafness often may be resolved, whereas sensorineural deafness is, at
present, permanent.
In addition to hereditary causes of sensorineural deafness, hearing loss can also result
from aging (presbycusis), ototoxicity, noise trauma, otitis interna, anesthesia, and several
less common causes.
Because pigment-associated congenital deafness is hereditary, affected animals should
not be bred.
Definitive diagnosis of deafness requires brainstem auditory evoked response testing,
because behavioral testing has limited reliability.
Vet Clin Small Anim 42 (2012) 1209–1224
http://dx.doi.org/10.1016/j.cvsm.2012.08.010 vetsmall.theclinics.com
0195-5616/12/$ – see front matter Ó2012 Elsevier Inc. All rights reserved.
Author's personal copy
This article presents and discusses the types of deafness that may be encountered in
clinical practice.
Diagnosis of deafness by behavioral means is often unreliable, because it is difficult
to identify unilateral deafness or partial hearing loss by behavioral testing. Dogs able to
hear may not respond because of situational anxiety, or may discontinue responding
when the stimulus loses interest for them. Deaf dogs may respond because they
detect the stimulus through other senses, from visual cues, vibration, or even air
movement. Objective diagnosis requires electrodiagnostic testing, usually the brain-
stem auditory evoked response (BAER). This is covered in detail elsewhere in this
issue and in other references.
2–6
NORMAL AUDITORY FUNCTION
The frequency range of hearing for dogs is often reported to be 67 Hz to 45 kHz,
whereas that of humans, from similar sources, is 64 Hz to 23 kHz.
1
Although compar-
isons must be made with caution because dissimilar methods of determination have
been used, it is clear that dogs detect sounds at much higher frequencies than
humans, but differ little on the low frequency end. There is little difference between
dogs and humans in detection thresholds for sounds within their optimum frequency
range of hearing, in both cases being approximately 0 dB SPL (sound pressure level)
over the range of approximately 1 to 10 kHz.
1
Dogs of different sizes from different
breeds show little variation in hearing, based on measurements of frequency thresh-
olds and ranges for a single Chihuahua, Dachshund, poodle, and Saint Bernard, where
the audiograms were essentially identical.
7
However, the absence of any interbreed
differences has not been confirmed through systematic studies.
Successful detection of sound requires patency of the outer and middle ears, and
proper function of the middle and internal ears. Identification of the source of a sound
requires bilateral hearing, so unilateral deafness impairs this ability. Coverage of normal
auditory function is presented in detail elsewhere in this issue and in other sources.
1,8
Behavioral changes in deaf dogs can be subtle or grossly apparent. Bilaterally deaf
puppies often go undetected because they cue off of the behavior of their littermates;
they usually are very visually attentive and as a result may seem to have above average
intelligence. Unilaterally deaf dogs show difficulty localizing the source of a sound, but
often adapt. Hearing loss that is progressive (eg, age-related or from noise trauma)
often goes undetected by the owner until a significant level of loss is reached. In
hunting or field trial dogs, the distance at which a dog responds to a signal sound
decreases. Totally deaf dogs may be more susceptible to being startled, and so
should be approached with caution; these dogs should also be protected from unde-
tected dangers, such as motor vehicles. Bilaterally deaf puppies are more challenging
to train than hearing puppies, and may be more of a challenge than some owners are
willing to accept. Deaf dogs can learn to respond to hand signals and other cues, and
function better in households with other dogs whose behavior they can follow.
TYPES OF DEAFNESS
Deafness is not always bilateral, is not always total for an ear, and is not always hered-
itary. To describe deafness, it is useful to apply pairs of discriminant terms. The
discussion next follows that of
9
in explaining the possible presentations of deafness.
Unilateral Versus Bilateral Deafness
Deafness can affect one or both ears. Humans can report unilateral losses, but
animals with hearing loss in just one ear are frequently not identified as such unless
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and until both ears become affected, if the loss is progressive; some are never
detected. Unilaterally deaf puppies often exhibit difficulties determining the origin of
sounds, which requires bilateral hearing, but often adapt to the condition and no
longer show this deficit. Hereditary deafness and hearing loss associated with otitis
can be unilateral, whereas ototoxicity (from systemic drug administration), noise-
induced hearing loss (NIHL), and age-related hearing loss (ARHL; presbycusis) are
typically bilateral.
Partial Versus Total Deafness
If both ears are totally deaf, the animal is obvious in failing to respond to sounds. In
litters of puppies this may not be recognized because deaf dogs cue their behavior
based on littermates. If hearing loss is partial, the animal may not exhibit any deficit
unless it is progressive and until it reaches a level where the dog can no longer
cope. This seems acute in onset to the owner when the disease may have been pro-
gressing for months or years. Hereditary deafness is usually total in the affected ear,
whereas most other types are expressed as partial deafness, at least initially, including
NIHL, presbycusis, and ototoxicity.
Syndromic Versus Nonsyndromic Deafness
In many types of deafness in humans, hearing loss is accompanied by disease in other
organ systems or abnormalities in phenotype. These include Usher syndrome, where
deafness is accompanied by retinitis pigmentosa; Alport syndrome, where nephritis
accompanies deafness; and Waardenburg syndrome, where deafness is accompa-
nied by a variety of other disorders, including pigmentation abnormalities. There are
no known syndromic types of deafness in dogs. Canine hereditary deafness is usually
associated with white skin and hair, but the pigment patterns are not considered to be
abnormal and so the deafness is not syndromic.
Peripheral Versus Central Deafness
Deafness can result from retrocochlear (central) disease (cochlear nerve, brainstem,
auditory cortex) or from disorders of the peripheral auditory organ and support struc-
tures (external ear, middle ear, cochlea). The retrocochlear auditory pathway projects
centrally through several brain structures on the way to the thalamus and auditory
cortex; at most points in this pathway some auditory fibers ascend ipsilaterally,
whereas others decussate and ascend contralaterally. As a result it is difficult for
central deafness to result without significant concurrent brain damage that would
be reflected by extensive neurologic abnormalities.
TYPES OF PERIPHERAL DEAFNESS
Most cases of deafness in dogs are peripheral, and the rest of this discussion covers
types of peripheral deafness. Discriminant pairs of terms are also useful in describing
the possible types of peripheral deafness. Peripheral deafness can be sensorineural or
conductive, inherited or acquired, or congenital or later onset.
From these pairs of terms, eight possible combinations are theoretically possible.
Deafness may be sensorineural, inherited, and congenital, or it may be conductive,
acquired, and later onset, and so on.
Conductive deafness results when sound is reduced or blocked from reaching
the internal ear because of outer or middle ear pathologies. Conductive deafness
can often be corrected.
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Sensorineural deafness occurs when there is damage to the hair cells or afferent
neurons of the cochlea. Sensorineural deafness cannot be corrected at present
in mammals, although regeneration of hair cells can occur in avian species.
Inherited deafness has a genetic basis, whereas acquired deafness does not. At
present there are no known hereditary forms of deafness in dogs except those
that are congenital, with one possible exception being a condition in Cavalier
King Charles spaniels (see later).
Acquired deafness includes all nonhereditary types of deafness.
Congenital (literally “with birth”) deafness can be hereditary, but is not neces-
sarily so; several other conditions can produce deafness in the perinatal period.
Hereditary congenital sensorineural deafness actually does not develop until 3 to
4 weeks of age.
Deafness that occurs following the perinatal period is later onset.
The types of peripheral deafness that are clinically observed in dogs are summa-
rized in Table 1.
9
Conductive Deafness
Conductive deafness is nearly always acquired and later onset. Ear canal atresia is
a rare congenital cause; the ear canal may be delayed in opening or may be perma-
nently sealed. Surgery may be an option. Causes for later-onset conductive deafness
include otitis externa and media, middle ear effusion, cerumen impaction, ear canal
inflammation from awns or other foreign bodies, middle ear polyps, and possibly
otosclerosis, although the last has not been reported in dogs. Some cases of severe
and chronic otitis externa require total ear canal ablation and bulla osteotomy; in most
cases auditory function is present after the ablation if it was present before surgery.
10
The animal will have conductive deafness from muffling of sounds by the skin over the
former ear canal opening, but loud sounds will still be detected.
Table 1
Classifications and examples of different types of peripheral deafness in dogs
Sensorineural Conductive
Congenital Later Onset Congenital Later Onset
Hereditary Pigment-associated
Non–pigment-
associated
(Doberman, Puli)
None known None known Primary secretory
otitis media
(Cavalier King
Charles spaniel)
a
Acquired Perinatal anoxia
Dystocia
Intrauterine
ototoxin exposure
Ototoxin exposure
Otitis interna
Presbycusis
Noise trauma
Physical trauma
Anesthesia-
associated
Undetermined
Ear canal atresia Otitis externa
Otitis media
Cerumen impaction
Ear canal
inflammation
(awns)
Ear canal foreign
bodies
Middle ear polyps
Otosclerosis
a
Hereditary basis suspected but not confirmed.
From Strain GM. Forms and mechanisms of deafness. In: Deafness in dogs and cats. Wallingford
(UK): CAB International; 2011. p. 45; with permission.
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A condition called primary secretory otitis media (or glue ear, or otitis media with
effusion) is a later-onset form of conductive deafness discussed in more detail else-
where in this issue. Because the condition occurs primarily in the Cavalier King
Charles spaniel breed (although it has been reported once each in a Dachshund,
boxer, and Shih Tzu
ˆ),
11
there may be a hereditary basis for the disorder. It has also
been suggested that the condition may be the result of pharyngeal conformation,
resulting from a thick soft palate and reduced nasopharyngeal aperture.
12
Sensorineural Deafness
The sound-detecting structures of the internal ear, the cochlea and its organ of Corti,
contain two targets through which pathology may produce sensorineural deafness:
the hair cells and the stria vascularis (Fig. 1). The organ of Corti has one row of inner
hair cells, which are the primary transduction cells for audition, and three rows of outer
hair cells, which actively amplify sound. Loss of cochlear hair cells produces deafness.
The stria vascularis, a modified vascular structure on the outer wall of the cochlear
duct, is responsible for maintaining high potassium levels in the endolymph fluid
that surrounds the stereocilia of the hair cells. Damage to the stria vascularis, or the
absence of functioning melanocytes in the stria, results in secondary loss of hair cells
and deafness. Melanocytes in the stria play a critical role in potassium level mainte-
nance in the endolymph.
13
The most frequent clinical presentation of sensorineural
deafness is congenital hereditary deafness, followed by presbycusis, then ototoxicity
and noise trauma, then anesthesia-associated deafness.
Internal ear abnormalities, hereditary or from developmental anomalies, have been
classified into three types
14,15
: (1) morphogenetic, (2) neuroepithelial, and (3) cochleo-
saccular or Scheibe-type.Morphogenetic abnormalities include structural deformities
Fig. 1. Cross-section through the mammalian cochlea showing the three cochlear compart-
ments (the scala vestibuli, the scala tympani, and between them the cochlear duct or scala
media); the organ of Corti; the spiral ganglion; and the stria vascularis. Hair cells in the
organ of Corti and the stria vascularis are the targets of processes causing sensorineural
deafness. (Courtesy of Don W. Fawcett, MD.)
Canine Deafness 1213
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of either the bony or membranous labyrinths that result from mutations acting in early
development of these structures. Semicircular canals may be absent, and the
cochlear duct may be reduced or even absent. Because of the severity of the abnor-
malities the deafness is not easily classifiable as either sensorineural or conductive.
These are rare.
Neuroepithelial abnormalities appear at the end of cochlear development (3–4 weeks
after birth in dogs) and result from hair cell degeneration that occurs after the normal
pattern of development. The stria vascularis and Reissner membrane are normal until
late stages in the degenerative process. Vestibular dysfunction may be a component,
deafness is complete, and both ears seem to be symmetrically affected. This type
occurs in the Doberman pinscher and perhaps in the few other breeds with hereditary
congenital deafness that is not pigment-associated. The mechanisms for this type of
deafness are not known in dogs, but in humans and mouse models the causes are
frequently channelopathies: mutations in genes responsible for neuron membrane
ion channels, especially potassium ion channels.
16
Cochleo-saccular or Scheibe-type abnormalities also occur late in cochlear devel-
opment, but deafness results from initial degeneration of the stria vascularis. Strial
degeneration results in loss of the elevated potassium concentration in the endo-
lymph, and the hair cells degenerate, Reissner membrane collapses, and other
cochlear structures eventually collapse and degenerate, including spiral ganglion cells
whose axons form the cochlear branch of the eighth cranial nerve. In some species the
saccule of the vestibular system may also degenerate, but it is not clear how often this
occurs in affected dogs. Deafness may be unilateral or bilateral, and is total in an
affected ear, although in rare cases hearing for the very high frequencies may be
retained but at frequencies so high as to be of limited use in daily life. This retained
hearing may reflect the continuity of the fluids between the cochlear duct and the
vestibular organs, and the location in the cochlea for normal high-frequency hearing
at its base; the degeneration of the stria vascularis in the cochlea and loss of high
potassium levels around the hair cell stereocilia may be compensated for by vestibular
endolymph. Most occurrences of cochleo-saccular deafness are associated with the
genes responsible for white or lightened pigmentation in skin and hair (piebald or merle
in the dog) where the genes suppress melanocyte function to produce white. Suppres-
sion of melanocytes in the stria vascularis by these genes disrupts their regulation of
the endolymph composition, and deafness ensues as a secondary effect. Pigment-
associated deafness is the most common type of deafness in dogs and cats, and is
by far the most common form of congenital deafness.
Sensorineural deafness can also result from neuroepithelial and cochleo-saccular
cochlear degeneration that is a result of nongenetic causes.
Hereditary Deafness
Pigment-associated deafness
Most hereditary deafness in dogs is pigment-associated, of the cochleo-saccular
pathology type, and is specifically associated with either the recessive alleles of the
piebald gene (S) or the dominant allele of the merle (M) gene. Dog breeds with reported
congenital deafness number more than 90 (Box 1). Congenital deafness is not neces-
sarily hereditary, but there is good evidence for a genetic basis in many breeds, espe-
cially those with white or dilute pigmentation patterns. In recent years, significant effort
has gone into documenting the prevalence of deafness in breeds affected by pigment-
associated deafness
17–23
; rates vary by breed, ranging from a high of 30% (unilateral
and bilateral) in Dalmatians in the United States, to 1.3% in colored bull terriers.
18
Hearing testing of puppies can be performed beginning at about 5 weeks of age;
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breeders in breeds with significant deafness prevalence rates frequently have BAER
testing performed before placing puppies and may cull bilaterally deaf puppies.
The piebald and merle genes suppress melanocytes, producing white or dilution of
pigmentation in skin and hair, blue irises in some dogs, and disrupted function in the
stria vascularis, resulting in deafness. Significant associations between the absence of
iris pigment and deafness have been shown for several breeds with the piebald gene,
including Dalmatian, English setter, and English cocker spaniel.
18
Piebald
The Slocus has four alleles. The dominant allele S(for self) produces solid color. Three
recessive alleles express increasing white in the coat: Irish spotting (s
i
), piebald (s
p
),
and extreme white piebald (s
w
). Examples of breeds carrying these alleles are as
follows: Irish spotting, Basenji and Bernese mountain dog; piebald, English springer
spaniel, fox terrier, and beagle; and extreme white piebald, Dalmatian, bull terrier,
and Samoyed. Breeds may contain within their population two or even three of the
Box 1
Dog breeds with reported congenital deafness. Dogs of any breed can be affected from
a variety of causes, but breeds with white pigmentation are most often affected
Akita Dalmatian Old English Sheepdog
American bulldog Dappled Dachshund Papillon
American-Canadian shepherd Doberman pinscher Perro de Carea Leone
´s
American Eskimo Dogo Argentino Pit bull terrier
American hairless terrier English bulldog Pointer/English pointer
American Staffordshire terrier English cocker spaniel Presa Canario
Anatolian shepherd English setter Puli
Australian cattle dog Foxhound Rhodesian ridgeback
Australian shepherd Fox terrier Rat terrier
Beagle French bulldog Rottweiler
Belgian sheepdog/
Groenendael
German shepherd Saint Bernard
Belgian Tervuren German shorthaired pointer Samoyed
Bichon Frise Great Dane Schnauzer
Border collie Great Pyrenees Scottish terrier
Borzoi Greyhound Sealyham terrier
Boston terrier Havanese Shetland sheepdog
Boxer Ibizan hound Shih Tzu
ˆ
Brittney spaniel Icelandic sheepdog Shropshire terrier
Bulldog Italian greyhound Siberian husky
Bull terrier Jack/Parson Russell terrier Soft coated Wheaten terrier
Canaan dog Japanese Chin Springer spaniel
Cardigan Welsh Corgi Kuvasz Sussex spaniel
Catahoula leopard dog Labrador retriever Tibetan spaniel
Catalan shepherd Lo
¨wchen Tibetan terrier
Cavalier King Charles spaniel Maltese Toy fox terrier
Chihuahua Miniature pinscher Toy poodle
Chinese crested Miniature poodle Walker American foxhound
Chow chow Mongrel West Highland white terrier
Cocker spaniel Newfoundland Landseer Whippet
Collie Norwegian Dunkerhound Yorkshire terrier
Coton de Tulear Nova Scotia duck tolling
retriever
From Strain GM. Deafness in dogs and cats. Available at: http://www.lsu.edu/deafness/deaf.htm
Canine Deafness 1215
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different recessive alleles
24
; the identification of which alleles are present in a breed is
often uncertain because there are no genetic markers to distinguish them.
Merle
The Mlocus has two alleles: the recessive allele mproduces uniform pigmentation,
whereas the dominant allele M, known as merle or dapple, produces a random pattern
of diluted pigmentation overlying uniform pigmentation; it also increases the amount of
white spotting on the coat. Dogs homozygous for the dominant allele can be deaf and
frequently have ocular abnormalities; even heterozygous dogs can be deaf, but there
may be breed differences in the prevalence of deafness in merle carriers.
20
Breeds
carrying the merle allele include Shetland sheepdog, Australian shepherd, Cardigan
Welsh Corgi, and Dachshund. In general, the prevalence of deafness in dogs carrying
merle is similar to that of breeds with piebald. Some breeds carry merle and piebald,
and some breeds (eg, Great Danes) with merle also carry a modifier gene known as
harlequin.
25
Gene identification: merle
The merle gene has been sequenced, and has been identified to be a mutation in the
dominant allele of the SILV pigmentation gene, located on canine chromosome 10
(CFA10, Table 2).
26
The mutation is a 253 base pair short interspersed element located
just before exon 11 in the gene, and includes a multiple adenine repeat (poly-A) tail that
must be 90 to 100 adenine repeats in length to cause expression of the merle phenotype.
The identification of the gene has not yet provided an explanation of the basis for the
deafness associated with its presence in a dog. A locus for the harlequin gene, which
modifies expression of merle so that diluted areas become white, has been reported
on canine chromosome 9 (CFA9, see Table 2).
27
It is not known whether harlequin
has any association with deafness; Great Danes can carry merle, harlequin, and piebald.
Gene identification: piebald
The various alleles of Shave been reported to be mutations of the microphthalmia-
associated transcription factor (MITF) gene, located on canine chromosome 20
(CFA20, see Table 2).
28
Two mutations were identified in a 3.5-kb region upstream of
the M promoter region, that in various combinations were said to generate the different
alleles: a short interspersed element insertion present in piebald and extreme white
piebald breeds but missing in Irish spotting and solid breeds, and a polymorphism in
the promoter region of the gene present in solid dogs. A separate research group was
unable to duplicate all of the study’s reported findings
29
and a third ispursuing a relation-
ship between Irish spotting and the gene KITLG (c-Kit ligand) on canine chromosome 15
(CFA15, see Table 2).
30
KITLG plays a role in melanogenesis. As with the merle gene, no
studies of these genes have provided an explanation for the associated deafness.
Inheritance of pigment-associated deafness
The inheritance of pigment-associated deafness seems complex and does not parallel
the inheritance of the associated pigment genes merle (simple dominant) and piebald
(simple recessive). Numerous studies have attempted to identify the mechanism of
inheritance of deafness using either a statistical technique called complex segregation
analysis of pedigrees or microsatellite marker studies of DNA from affected dogs,
without a consensus
21,31–36
; most proposed some version of an autosomal-
recessive mechanism, but not one that is simple mendelian autosomal-recessive. A
recent study of deafness in Australian stumpy-tail cattle dogs used a whole genome
screen of microsatellite markers and identified a locus on canine chromosome 10
that was significantly linked to deafness.
21
Located within that locus is the gene
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Table 2
Genes implicated in pigment-associated hereditary sensorineural deafness in dogs
Common Name Chromosome Gene Name Gene Product/Function Mutation
Merle (M) CFA10 SILV Melanocyte protein Pmel17; plays
a role in pigmentation patterns
A 256 bp retrotransposon insertion
with a poly-A tail
Piebald (S) CFA20 MITF (microphthalmia-associated
transcription factor)
Regulates SILV and the gene
tyrosinase, involved in melanin
synthesis and melanocyte
development
Two mutations upstream of the M
promoter region of the gene;
different combinations result in
the different recessive alleles of S
Piebald – Irish spotting (s
i
) CFA15 KITLG
(c-Kit ligand)
Plays a role in melanogenesis;
possible origin of the Irish
spotting allele of S
Unknown
Harlequin CFA9 Unknown Modifies the effects of M; produces
complete hypopigmentation of
areas otherwise white in merles
Unknown
None CFA10 Possibly SOX10 Transcription factor that regulates
MITF
Possible deafness gene in Australian
stumpy-tailed cattle dogs
Unknown
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SOX10, a homeobox gene that is an activator of the gene MITF. However, sequencing
of SOX10 in several affected dogs showed no mutations, and the study has not been
repeated in dogs from other affected breeds, so it is not yet clear that a mutation of
SOX10 is causative.
At present, genes located on canine chromosomes 9, 10, 15, and 20 have been
investigated that play a role in pigmentation in the dog and that may play a role in deaf-
ness (see Table 2); many more genes have been identified to be involved in pigmen-
tation and linked to different forms of deafness in humans and in mouse mutations.
37
Much work remains to solve the genetic basis of this form of deafness. Until DNA-
based tests become available to identify dogs carrying gene mutations responsible
for hereditary deafness, affected dogs should not be bred, even those deaf in just
one ear.
Doberman pinscher
Bilateral congenital deafness accompanied by vestibular disease in Doberman
pinschers has a neuroepithelial cochlear pathology and has an autosomal-recessive
mode of inheritance based on pedigree analysis.
38
Vestibular signs of head tilt, ataxia,
and circling behavior develop between birth and 12 weeks of age, but the animals
adapt to them with time; the deafness is permanent. Similar presentations have
been reported in beagle, Akita, and possibly in Tibetan and Shropshire terriers.
25
No
gene linked to the disorder has been reported, and the mechanisms responsible for
the cochlear and vestibular hair cell degeneration are unknown.
Pointers
A champion field trial pointer bitch reputed to have experienced a nervous breakdown
was used as foundation breeding stock to develop a line of dogs with enhanced
anxious behavior to support research in human anxiety disorders.
39
Concurrent with
the breeding for increased anxious behavior was the appearance of deafness in
affected dogs. Most anxious dogs were bilaterally deaf by 3 months of age, but not
all were affected, suggesting incomplete penetrance; the cochlear degeneration is
of the neuroepithelial type.
40
Pedigree analysis suggested autosomal-recessive inher-
itance, but because of tight inbreeding, other mechanisms of inheritance could not be
ruled out.
39
Responsible genes and mechanisms have not been identified. Deafness
does not seem to develop in pointers unrelated to this special-bred kindred.
Presbycusis
Presbycusis, or ARHL, is primarily sensorineural but may also include conductive
hearing loss and central changes. It is progressive, usually bilaterally symmetric,
and generally affects high frequencies before low frequencies. In humans it may be
accompanied by tinnitus and has no known treatment or cure. The deficits can be
exacerbated by NIHL, hypertension, and diabetes. Human males are affected more
than females, but it is unclear if this holds true for dogs. The primary mechanism is
degeneration of the stria vascularis,
41
but concurrent degeneration of the organ of
Corti and ganglion cells is observed. Genetic factors are important in human ARHL,
but no studies have examined genetic effects in dogs.
A 7-year study documented tone burst-derived BAER thresholds in middle-sized
dogs as presbycusis developed.
42
Hearing thresholds began to increase at 8 to 10
years of age, and the mid- to high-range frequencies of 8 to 32 kHz were affected first.
The loss was progressive and expanded to cover the entire frequency range. Life
expectancies vary in dogs based on body size, with small breed dogs living longer
than large breed dogs, so the age of onset of ARHL likely differs between large and
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small breeds. The pattern of pathologies seen in geriatric dogs with ARHL included
reductions in the stria vascularis, numbers of hair cells, and numbers of spiral ganglion
cells,
43
mirroring human pathology.
Ototoxicity
Many drugs and chemicals have been identified that are toxic to the internal ear, both
the cochlea and the vestibular organs. More than 180 compounds and classes of
compounds have been identified as ototoxic.
44
Some produce effects that are revers-
ible if detected early, such as the salicylates, but most produce effects that are perma-
nent by the time of discovery. Effects can be auditory or vestibular or both, unilateral or
bilateral, and may result in partial or total functional loss. Toxicity may result from
parenteral or topical application, and can result from long-term and acute exposures.
Synergism may result from concurrent ototoxic drug exposure with presbycusis or
noise trauma. Mechanisms of toxicity may be through direct damage of hair cells
(neuroepithelial) or indirect effects through damage to the stria vascularis (cochleo-
saccular).
Ototoxic drugs and chemicals can be classified into broad groups: antibiotics
(aminoglycosides and others); loop diuretics; antiseptics; antineoplastic agents; and
miscellaneous agents. The drug most frequently associated with ototoxicity is the ami-
noglycoside gentamicin, which can also be nephrotoxic. Despite the known potential
toxicity of gentamicin, it is still probably the most commonly used antibiotic for topical
treatment of otitis externa, in part because of its high efficacy, broad spectrum of
gram-negative activity, and low cost. Its toxicity is unpredictable and does not
seem to reliably be related to dose, frequency, or route of administration.
45
The mech-
anism of toxicity is a sequence of iron chelation, followed by free radical formation,
and then caspase-dependent apoptosis.
46
Although drug manufacturers may state
that gentamicin-induced ototoxicity can be reversible, this has not been the author’s
experience.
Human studies have shown that coadministration of aspirin or other antioxidants
during dialysis prevents or reduces gentamicin ototoxicity,
47
but it is not known
whether postdeafness treatment has any value in restoring hearing loss. Ototoxicity
is covered in more detail elsewhere in this issue.
Noise-Induced Hearing Loss
NIHL, or noise trauma, increasingly affects humans and animals. Military dogs
exposed to loud percussive sounds, hunting companion dogs (especially Labrador
retrievers), dogs housed in kennels with high ambient noise levels,
48,49
and even
dogs shipped by airplane in cargo compartments where there are high ambient noise
levels may all develop NIHL.
44
Protective reflexes exist where middle ear muscles
contract in response to loud sounds to reduce sound levels reaching the inner ear,
but the reflexes are too slow for percussive sounds, such as gunfire, and only provide
limited protection against very loud noises. Hearing loss is primarily caused by loss of
hair cells from mechanical disruption, but extreme sounds can also damage the
tympanum and ossicles. NIHL is a cumulative process where sound intensity and
duration of exposure determine the damage. The US Occupational Safety and Health
Administration and federal agencies in other countries have established regulations for
permissible workplace noise exposures for people.
50
Separate standards have not
been developed for dogs, but human standards provide reasonable criteria unless
future studies suggest more sensitive standards. It should be noted that Occupational
Safety and Health Administration standards refer to continuous noise exposure;
percussive sounds have the potential to produce significant damage, so efforts should
Canine Deafness 1219
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be used to protect the hearing of hunting dogs exposed to gunfire and military dogs
exposed to explosions, just as hunters protect their own hearing with protective
devices. However, effective devices for noise protection are not yet commercially
available for dogs.
Noise exposure can produce temporary and permanent hearing loss. Gradual
recovery may occur with some exposures if they are brief; this type of loss is called
a temporary threshold shift. Repeated or continued exposures result in permanent
hearing loss, or permanent threshold shift. NIHL is greater in older subjects, and there
seem to be genetic differences in susceptibility, including increased sensitivity in
subjects with white pigmentation.
51
Damage is produced in the organ of Corti, stria
vascularis, spiral ganglion cells, and spiral ligament. Temporary threshold shift is asso-
ciated with effects on the stria vascularis, whereas permanent threshold shifts are
associated with damage to hair cells in the organ of Corti,
51
including disruption of
stereocilia and the cuticular plate (their attachment point to the hair cell body), gener-
ation of reactive oxygen species, apoptosis, and necrosis.
Dogs seem to be more often affected than cats, perhaps simply because of greater
opportunities for exposure. Associated behavior is a gradual reduction in the distance
at which a dog responds to a voice or whistle command. The hearing loss is gradual
and cumulative, so owners often report an apparent acute onset even though the dog
was undergoing progressive damage. Animals adapt to the loss until a point is
reached where they can no longer compensate. NIHL often is accompanied by
ARHL, and in humans there can also be the presence of tinnitus. Studies have shown
that concurrent administration of antioxidants, such as N-acetylcysteine, have protec-
tant effects, and some agents, such as adenosine A
1
receptor agonists, may even be
effective postexposure.
52
Anesthesia-Associated Deafness
Although uncommon, some dogs or cats that undergo anesthesia, especially for
dental cleaning procedures, recover from anesthesia with bilateral deafness that in
most cases is permanent.
53
In a study of 62 reported cases in dogs and cats, no asso-
ciation was observed between deafness and breed, gender, anesthetic drug used, or
dog size.
53
Forty-three of the reported cases occurred after dental procedures, and 16
cases after ear cleanings. Geriatric animals seemed more susceptible to the hearing
loss, which might reflect a bias because of dental procedures being performed
more often on older animals. In at least one case, deafness was the result of a persis-
tent otitis media with effusion, suggesting possible eustachian tube dysfunction
subsequent to vigorous jaw manipulation during a dental procedure (unpublished
observation). For most cases the cause is unknown and ongoing studies are pursuing
possible mechanisms.
Otitis Interna-Associated Deafness
Infections that reach the internal ear have the potential to produce sensorineural deaf-
ness, and are frequently accompanied by vestibular signs because of connections
between the fluid-filled compartments of the cochlea and vestibular organs. Otitis
interna is common, and usually results from extension of otitis media. Factors that
influence or determine whether an infection results in permanent deafness have not
been identified, but the duration of the infection may be a factor. The appearance of
vestibular signs that suggest otitis interna should suggest immediate initiation of
appropriate diagnostic tests to determine the cause of the signs, and if they are related
to an infectious otitis antimicrobial therapy should be instituted to minimize the poten-
tial for hearing loss.
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OTHER HEARING CONDITIONS
Tinnitus
Tinnitus is the perception of sounds in the absence of actual external sounds.
54
Objec-
tive tinnitus is the perception of actual sounds generated by the ear, from turbulent
blood flow near the cochlea or abnormalities in the muscles near the ear or eustachian
tube, and in rare cases generated by the cochlea itself. They are low in intensity, but
can be heard by others when in close proximity or by means of a stethoscope. Objec-
tive tinnitus has been reported in dogs and cats, usually as a high frequency tone that
does not seem to bother the animal. Subjective tinnitus (ringing in the ears) is the
perception of phantom sounds when no actual sound is present; this is the form typi-
cally meant when tinnitus is used as a term without a modifier.
In humans, subjective tinnitus exhibits with variable features that can be so severe
as to be disabling. It may seem to originate in one or both ears or seem to originate in
the center of the head; may be continuous or intermittent; and may be perceived as
a single or multiple pure tones or any number of various other sounds that include
hissing, roaring, whistling, chirping, clicking, and buzzing. Tinnitus most often results
from exposure to loud noises, especially gunfire or loud music, or presbycusis, and
less often from ototoxic drugs, meningitis, encephalitis, stroke, and traumatic brain
injury.
Because subjective tinnitus is subjective, it is not possible to know if dogs experi-
ence it. Some dogs with an acute onset of deafness exhibit anxious behavior that
may indicate tinnitus, but may instead just reflect a response to the loss of hearing.
The behavior is typically short lived, suggesting that the dogs adapt if it is indeed
a continuing sensation.
Hyperacusis
Increased sensitivity to sounds that otherwise would not be bothersome is called
hyperacusis. It can result from peripheral auditory disorders, central nervous system
disorders, and hormonal and infectious disorders, but the cause is often unknown.
55
Noise-induced hearing loss and facial nerve damage are common origins. Hyperacu-
sis often accompanies tinnitus and can precede its appearance. Some dog owners
report an apparent hypersensitivity to sound, but BAER results have been normal.
There have been no studies of this condition in dogs or cats.
Otoacoustic Emissions
Although not a hearing disorder, the ear can generate sounds (otoacoustic emissions
[OAE]), either spontaneously or in response to introduced sounds. OAE are thought to
reflect the function of outer hair cells of the cochlea, which contain contractile proteins
that allow the cells to shorten or lengthen at a rapid frequency.
56
Spontaneous OAE
are present in most normal ears at very low levels, and can occasionally be loud
enough to be detected by the unaided listener.
57
Evoked OAE provide a sensitive early
indicator of loss of auditory function. Evoked OAE are used clinically to assess audi-
tory function in humans,
56
and have begun to be reported for deafness screening
applications in dogs.
58
Two types of evoked OAE are in use: transient evoked OAE
and distortion product OAE.
56
OAE are discussed in more detail elsewhere in this
issue.
SUMMARY
Deafness is common in dogs and has a variety of causes. The most frequent is
congenital hereditary sensorineural deafness associated with white pigmentation.
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Conductive deafness often may be resolved, whereas sensorineural deafness is, at
present, permanent. However, it has recently been demonstrated that hair cells can
be generated from mouse stem cells in culture,
59
so therapies may become available
in the future. In addition to hereditary causes of sensorineural deafness, hearing loss
can also result from aging (presbycusis), ototoxicity, noise trauma, otitis interna, anes-
thesia, and several less common causes. Because pigment-associated congenital
deafness is hereditary, affected animals should not be bred. Definitive diagnosis of
deafness requires BAER testing, because behavioral testing has limited reliability.
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