hearing clinic testing for DEAFNESS in dogs
Our quick and non-invasive diagnosis of deafness in dogs is achieved through Brainstem Auditory Evoked Response (BAER); a test where the electrical activity of the brain in response to an auditory (click) stimulus is recorded and displayed on a computer screen.
BAER testing is a technique that can be used to assess the hearing status of adult animals where there is a concern about deafness, or prior to breeding and puppies before they go to their new homes.
HOW HEARING WORKS
Sound waves enter the external ear canal and strike the eardrum, which begins to vibrate. This is turn causes vibration of the tiny bones in the middle ear (the ear ossicles). The vibrations set up waves in the fluid of the cochlea, the spiral-shaped structure in the inner ear. The change in pressure ultimately causes movement of the hair cells inside the cochlea that are connected to the auditory nerve. This triggers a nerve impulse via the auditory pathway to the brain.
Any interruption in the delivery of sound to the brain results in one of several types of deafness:
1. Conductive deafness – due to interference in the transmission of sound waves to the inner ear, for example caused by a foreign body or wax in the ear canal, rupture of the ear drum or infection in the middle ear.
2. Sensorineural deafness – results from damage or defect in any part of the auditory (hearing) pathway from the cochlea in the inner ear, via the auditory nerve to the auditory cortex of the brain.
3. Congenital deafness – present at birth, while late onset deafness, as its name suggests, occurs later in life, such as hearing loss associated with old age.
4. Inherited deafness - passed down through one or both parents, whereas acquired deafness is due to external factors such as injury or disease.
WHICH DOGS CAN BE AFFECTED BY DEAFNESS?
Congenital deafness (deafness at birth) is recognised as a problem in many breeds of dog that carry the extreme piebald gene, demonstrated by a predominantly white coat. Such breeds include Dalmatians, English Setters, white Boxers and white English Bull Terriers. Deafness in these breeds can occur due to degeneration of structures in the cochlea. BAER testing can provide an instantaneous yes-or-no result through the assessment of the waveforms generated through the test.
More than 80 breeds of dog have been identified as suffering from congenital deafness; for the full list of ‘at risk’ breeds, please click here.
Deafness has also been reported to be associated with the gene that causes the merle coat colour, for example in the Border Collie, Australian Shepherd, Dappled Dachshund or Old Sheepdog.
Figure 1. Absence of pigment in one or both irises resulting in blue eye colouration may also be present in merle dogs.
The merle gene is not reported to be present in breeds with the extreme piebald gene, for example the Dalmatian, but blue eyes do occur in these breeds. Because of the recognised relationship between blue eyes, white coat and deafness in cats, an association between blue eyes and deafness in dogs was suggested. Blue iris colour is caused by a lack of pigment in the iris, and it is thought that this may reflect a lack of pigment cells in the inner ear, which is linked with this kind of deafness.
For further information about deafness in Dalmatians, please click here.
If you have an animal you think may have become deaf due to illness such as infection, or from a breed not usually associated with congenital deafness, it is important that your dog is seen by your local vet. If they think that a hearing test is required, they will be able to refer you to us for an appointment. In this instance, you are not able to make an appointment for a hearing test directly. If you are in any doubt, please contact your local vet.
BAER TESTING PROCEDURE
If you have a litter from a breed that is at risk of congenital deafness (see link above for full list), the best age to test is around 5 ½ to 6 ½ weeks of age. It is not possible to assess hearing in puppies before the ear canals open at 12-14 days as sound waves cannot enter. The test can be carried out at any age after this, including on adult dogs; however, many breeders wish to know the hearing status of their pups before they go to their new homes.
Our dedicated room for BAER testing has a ‘playpen’ where your puppies can stretch their legs after the journey, and owners can offer them food and drink. The optimum time to begin the testing is when they have settled down, therefore a quietening down period is factored into the time allocated for each litter.
Recording electrodes are positioned on the puppy’s head and the test begins. A series of clicks are passed into the ear through a headphone and they are in turn passed into a specialised electrodiagnostic machine. It may be necessary to sedate a pup over the age of eight weeks, but this is a last resort; it is always attempted un-sedated in the first instance. Older animals are requested to come in without having eaten on the day of the test, just in case sedation is required.
When the result for one ear has been obtained, the other ear is then tested. The traces are printed out, the puppy returned to the pen, and the next puppy is tested.
Each puppy receives a certificate with his or her recorded traces attached to it. The certificates are verified and signed by one of our vets ready to take away with you.
BAER TESTING RESULTS
In a normal-hearing dog, a series of peaks and troughs is produced which is displayed on a screen. BAER testing can identify whether your animals are deaf in both ears (bilateral deafness) or in one ear (unilateral deafness). There is no partial deafness in these cases and the deafness is irreversible and permanent.
Figure 2: Normal BAER printout, showing very similar waveforms from the right and left ear.
Figure 3: BAER printout from a puppy that is unilaterally deaf. Trace 1 on the left is from the deaf ear at 80dB, trace 2 at 100dB, showing contralateral recordings that are eliminated by masking the other (hearing) ear with "white noise" in trace 3. The trace on the right shows a normal recording at 80dB.
Figure 4: BAER printout from a bilaterally deaf dog. No waveform is elicited from either ear. The slight deflection at the start of each of the bottom traces is the stimulus artefact.
Unilaterally deaf dogs make excellent pets, but they carry the genetic material for deafness. As the condition is hereditary, it makes sense not to use affected individuals for breeding. Work in the US showed that in unilateral-to-normal matings, the prevalence of affected offspring is approximately double that obtained from matings where both parents have bilaterally normal hearing. It may therefore be possible to reduce the prevalence of unilaterally or bilaterally deaf puppies if only bilaterally normal hearing parents are used for breeding.
It is hoped eventually that a DNA test may be available to identify carriers of the disorder, and we are currently collecting samples for prospective study and for this purpose, we encourage breeders to always bring over the entire litter if possible. Until we achieve a DNA test, the only way to know with certainty the hearing status of each individual, and go some way towards reducing the percentage of affected dogs, is to evaluate each animal using the BAER test.
FELINE BAER TESTING
Different types of animals can have their hearing tested using the BAER, but the other most common species we test is the cat. White cats, especially those with one or more blue eyes, are also prone to congenital deafness, and may be tested in the same way as dogs.
Cats may not be as receptive to performing the hearing test without sedation, but some will allow it with no problems. However, owners are requested to bring cats and kittens in without having had food on the day of the test, just in case. Kittens may be tested from eight weeks of age onwards and it is always attempted without sedation first.
Each cat gets a certificate with his or her trace attached, and this is again verified and signed by a veterinary surgeon. As with dogs, if you are concerned that the deafness is not congenital, please consult your veterinary surgeon.