Hearing Assessment In Dogs and Cats
Celia L. Cox BvetMed CertVRFRCVS
RCVS Specialist in Small Animal Surgery (ENT)
INTRODUCTION
In last ten years, there have been major advances in the methods used to assess hearing loss in dogs and cats. These tests are capable of determining both bilateral deafness and also subtle reductions of hearing in one or both ears. This knowledge allows veterinary surgeons and owners to make informed decisions regarding the treatment and management of their pets. For example:
INCIDENCE
Hearing loss in dogs is far more widespread than is commonly recognized. In
an early study, one in 3,000 puppies was thought to have congenital deafness.
However, the increased number of tests being performed today has revealed that
the true figure is much higher than this.
Over 60 breeds of dogs and ten breeds of cats have now been recognized with hearing impairment. Dog breeds with the highest incidence include the Dalmatian (22% in the U.K.), Setter (14.3%), Border Collie (10%), Australian Cattle Dog (12.6%) Cocker Spaniel (6.8%) and White Bull Terrier (19.1%) (Strain 1999). In the U.K., out of the 4,500 Dalmatians tested, 21% had congenital hearing loss. Of these, approximately 14% were deaf in one ear (unilateral) and 7% in both (bilateral). It has also been established that dogs with blue eyes were more likely to be deaf.
IMPORTANCE OF HEARING
Hearing is important in dogs and cats for their own social interaction, and to make them more pleasurable companions. It is also vital that they can hear well, in order to do their job effectively. Dogs who are deaf in one ear have difficulty localizing sound, which may affect their performance as an agility or sheepdog.
ANATOMY
The ear is divided into an outer and, middle and inner ear. The outer ear consists of the pinna, canal and part of the tympanic membrane. The middle ear consists of an air filled cavity, which contains a chain of three small moveable bones - the ossicles. The inner ear is a system of fluid filled cavities which houses the cochlea and the vestibular apparatus. The function of the outer and middle ear is to conduct sound vibrations to the receptor organ, the cochlea. The cochlea is a tightly coiled up tube, a few millimeters in diameter, whose function is to convert the mechanical vibrations into neural impulses.
It is divided longitudinally into three fluid-filled tubes. The two outer tubes
(the scala vestibule and scala tympani) are connected by a small hole (the helicotrema)
at the apex, and are filled with perilymph. The central tube (scala media) is
filled with endolymph produced by the vascular bed called the stria vascularis.
It is the differing chemical composition of the perilymph and endolymph which
provides the energy source for the generation of neural impulses.
On one side of the scala media is the Reisners and tectorial membrane, and on the other the basilar membrane supporting the Organ of Corti containing the hair cells with their tiny projections called the cilia. Branis et al (1985) estimated that there were about 13,000 hair cellsin each dog’s ear arranged along the length of the cochlea. These are responsible for initiating the nerve impulses in response to sound vibrations, which then travel to the brain to produce the sensation of hearing.
MECHANISM OF HEARING
In the normal ear, sound waves travel down the canal to the tympanic membrane, causing it to vibrate. This movement is picked up by the malleus and passed to the ossicular chain. The footplate of the stapes, strikes the oval window setting up traveling waves along the cochlea fluid. Fluid is compressed in the scala vestubuli and moves via the helicotrema to the scala tympani. The round window acts as a pressure relief valve bulging outwards in response to the oval window bulging inwards. The wave travels up the flexible basilar membrane and causes it to vibrate. It moves toward the rigid tectorial membranes causing bending of the cilia of the hair cells. This causes electrochemical changes, which are transmitted as neural impulses along acoustic nerve VIII to the brain.
LOUDNESS
The level of loudness is expressed in decibels using a logarithmic scale. The higher the number, the louder it sounds. A tenfold increase in sound energy is called a bel and hence 1/10 of this is a decibel (dB). A sound emitted at 50-60 dBnHL is equivalent to someone talking loudly.
FREQUENCY AND HEARING SENSITIVITY
Frequency refers to the rate of vibration of air particles which constitute the sound. It is measured in Hz, equal to one cycle per second. The higher the number, the higher the pitch. A normal hearing adult human can hear sounds up to 67kHz. As both man and dog are especially sensitive around the 1-8 kHz range, the tests used to assess hearing loss in man can be applied to dogs.
The ability to discriminate between different frequencies of the incoming sound is partly due to the analysis in the neural pathways and also to the cochlea. High frequencies are sensed at the basal end and low frequencies near the apex (helicotrema end).
DIAGNOSIS OF HEARING LOSS
History and Clinical Signs
Dogs, which are bilaterally deaf, are difficault to arose from sleep and do not respond to loud noises. They may be more aggressive and have a higher pitched cry. Dogs with unilateral or partial hearing loss are harder to train and may have difficulty localizing sound.
Otoscopy
This identifies / eliminates disease in the outer ear which could account for a hearing loss.
Behavioural Tests
A positive response is a turn of the head or a twitch of the ear in response to an auditory stimulus. This technique is cheap and quick to perform, but the results are often invalid because:
Brainstorm Auditory Evoked Response Tests (BAER)
Fortunately, there are objective tests, which can be used to overcome some of these difficulties. The BAER test is a non-invasive, repeatable, objective method of assessing dogs’ hearing. It does require patient co-operation. It can be performed from four weeks of age onwards. The test should be carried out in a quiet room. The equipment used costs around 14,000 British Pounds. A headphone is used to deliver an acoustic signal as a series of broadband clicks with a center frequency 2-4 KHz to the auditory meatus. The neural impulses generated are recorded as small changes in electrical activity by a specialized machine via three fine sub-dermal scalp electrodes placed in specific sites subcutaneously on the cat or dog’s head.
The response comprises five or more waves of known morphology which occur within 10ms of the on set of the stimulus. They represent a complex summation of neuro-electrical activity in the auditory pathways of the brainstem up to the level of the inferior colliculus. Each ear is assessed separately. As a result it provides information about the severity and site of the disorder, which in turn will determine the prognosis and treatment options. The results are not affected by sedation or general anaestesia.
Air Bone vs Conduction
If the sound signal travels to the cochlea via the outer ear, then the method
used is called “air conduction BAER”. If the signal bypasses the outer
and middle ear and stimulates the cochlea directly by placing a bone vibrator
on the skull, then it is called “bone conduction BAER”. By comparing
the results from air and bone conduction, the tester can tell if the hearing
loss is sensori-neural, conductive or mixed. The disadvantage of the test is
that it stimulates both cochlea at once and so the status of only the better
cochlea is recorded (Munro et al 1997).
Tympanometry, Radiography and MRI
These are used to assess the status and integrity of the middle ear.
CLASSIFICATION AND AETIOLOGY
Hearing loss can be classified into conductive or sensori-neural deafness depending on the site of the disorder. If the pathology affects the outer and or middle ears, then the loss is a conductive one. If the pathology affects the inner ear or nerves to the brainstem, then a sensori-neural is diagnosed.
Deafness can be further subdivided into inherited or acquired, congenital or late onset. The most commonly occurring combinations are dogs with inherited sensori-neural deafness(e.g. the Dalmatian), acquired sensori-neural deafness e.g. due to aging (presbycusis) and acquired conductive hearing loss e.g. due to chronic otitus externa/media. Other causes include ototoxicity, infections, excess noise and anoxia.
TREATMENT AND MANAGEMENT
Dogs with conductive hearing problems may be treated successfully using medicine or surgery with restoration of hearing in some cases. Dogs who undergo surgery of the vertical canal (LWR or VCA) should have normal hearing after surgery. Hearing loss after a TECA/LEBO results in a moderate hearing loss with a threshold of 65-70 dB nHL. However, many owners will not notice a change as the presence of discharge +/- mass pre operatively has already caused a significant hearing loss. Even if a bilateral TECA/LBO is performed, some loud noises and vibration can be picked up by bone conduction. These dogs would not be totally deaf.
A sensori-neural hearing loss is generally not reversible and management advice would include the following.
Dog Breeders: Dogs that pass the BAER test with both ears may be used for breeding.
This has been statistically shown to reduce the number of deaf puppies born
in the litter. Ideally, the dog’s pedigree should include as many other
dogs as possible tested clear using BAER. Some breed clubs advocate euthanasia
for bilaterally deaf dogs on the basis that they make poor pets, are more prone
to biting, require more care and can die from misadventure. Dogs with inherited
unilateral hearing loss should not be used for breeding.
Trainers: Selection should only include dogs with “normal” BAER hearing to avoid loss of time, frustration and money.
Owners: Deaf dogs are harder to train and require more patience and time. Should an owner inadvertently acquire a totally deaf dog, their options include:
AETIOLOGY OF INHERITED SENSORI-NEURAL DEAFNESS
The melanocytes in the stria vascularis play a role in maintaining the ionic environment needed by the cochlear hair cells. When these are absent, the stria degenerates, after which cochlea-saccule degeneration occurs with irreversible death of hair cells. In inherited sensori-neural deafness, it is believed that the melanocyte is presence is suppressed by genes e.g. the extreme piebald gene in the Dalmatian, the merle gene in the Collie and Sheltie or the white pigment gene in cat. As these melanocytes also migrate to the skin and to the iris, failure of their migration can result in blue eyed dogs. A DNA blood test will eventually be developed to identify the gene carrying deafness, but is still several years away. From a practical point of view, dogs with white coat color +/- blue eyes have an increased likelihood of being deaf.
CONCLUSION
Hearing tests should be performed on dogs and cats from breeds with high incidence of inherited hearing loss or those who acquire a problem later in life. The tests should be performed in centers with both proper testing equipment and expertise in this subject to ensure correct interpretation of the results and appropriate advice.
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