Deafness
Canine deafness is diagnosed with increasing frequency, primarily as a result of heightened awareness of the disorder among owners, breeders and clinicians. Quick, non-invasive, objective and unequivocal diagnosis of deafness, especially when unilateral, is achieved by means of the Brainstem Auditory Evoked Response (BAER). This is an electrodiagnostic test where the electrical activity of the brain in response to an auditory (click) stimulus is recorded and displayed on a computer screen.
First of all, to understand deafness, it is helpful to know how hearing works:
Sound waves enter the external ear canal and strike the eardrum, which begins to vibrate. This in 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, which may be defined as (1) conductive, (2) sensorineural, (3) congenital versus late onset, (4) inherited versus acquired.
- Conductive deafness is 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.
- Sensorineural deafness results from damage or defect in any part of the auditory pathway from the cochlea in the inner ear, via the auditory nerve to the auditory cortex of the brain.
- Congenital deafness is present at birth, whilst late onset deafness, as its name suggests, occurs later in life, such as hearing loss associated with old age.
- Inherited deafness is passed down through one or both parents, whereas acquired deafness is due to external factors such as injury or disease.
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. More than 80 breeds of dog have been identified as suffering from congenital deafness. Affected individuals may be completely deaf in both ears (bilaterally deaf), or completely deaf in one ear and able to hear normally in the other (unilaterally deaf). There is no “partial deafness” in these cases and the deafness is irreversible and permanent. Whilst unilaterally deaf animals can lead normal lives, bilaterally deaf animals can sometimes prove difficult to manage. Brainstem Auditory Evoked Response testing is a technique that can be used to assess the hearing status of adult animals prior to breeding and puppies before they go to their new homes. It gives a clear, unequivocal diagnosis of an animal’s hearing ability.
It has been suggested that inherited deafness linked with white coat colouration could be due to some abnormality in the migration of pigment cells within the embryo, or their interaction with other structures in the inner ear during development of the embryo.
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 English Sheepdog. Absence of pigment in one or both irises resulting in blue eye colouration may also be present in merle dogs, as illustrated in figure 1, a blue merle Border Collie with two blue eyes.
Figure 1
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.
The lower prevalence of deafness in Dalmatians tested at the Animal Health Trust (19.5% affected), where blue eyed dogs are not bred from, when compared with the US population (29.7% affected), where blue eyed dogs are used as breeding stock, seems to imply that removing blue eyed individuals from the breeding population could reduce the prevalence of affected offspring. This is supported by recent information from the US, which shows a statistically significant relationship between deafness and blue eyes. Data collected at the Animal Health Trust shows that the prevalence of deafness in individuals with a blue eye or eyes is 54.3% (33.1% unilaterally deaf, 21.2% bilaterally deaf), i.e. more than half of blue-eyed individuals tested at the Animal Health Trust are either unilaterally or bilaterally deaf.
The anatomical changes that take place in the inner ear due to this disorder have been extensively studied. Microscopic examination has shown that the deafness that affects Dalmatians is caused by degeneration of the blood supply to the cochlea in the first few weeks of life. This is followed by destruction of the hair cells and ultimately further deterioration of components of the inner ear and nerve degeneration. The loss of the hair cells is permanent and irreversible. The passage of sound to the auditory nerve is interrupted, i.e. sound cannot “get in” to the auditory pathway, resulting in the dog becoming deaf. Affected puppies can be detected by 4 weeks old and one study has showed no evidence of progressive hearing impairment beyond this age. Deafness may occur in both ears (bilateral deafness) or in one ear only (unilateral deafness). There is no partial deafness in these cases.
It is not possible to assess hearing in puppies before the ear canals open at 12 to 14 days, as sound waves cannot enter. After this age, it is sometimes possible for a skilled observer to identify bilateral deafness (total deafness in both ears), as totally deaf animals often show very characteristic behaviour patterns.
Suspicion that an animal cannot hear may be confirmed by observing the animal's response to an auditory stimulus such as banging on a saucepan, a hand-clap or jingling keys. However, this subjective method of testing hearing ability is very open to misinterpretation. Some hearing animals are unreactive, others adapt quickly and stop responding, and highly stressed animals may also not respond. Deaf animals may sense the presence of an unseen examiner, or respond to other cues (vibration, air current, body smell), which are undetectable to the person conducting the test.
A unilaterally deaf dog is very difficult to identify, as it hears perfectly in the non-affected ear, and so usually behaves normally. Occasionally, a dog with unilateral deafness may seem unable to localise the source of a sound, for example running in the wrong direction when called, or having to look round and actually see the owner before coming when called. Even if it is suspected that a dog is unilaterally deaf, it is almost impossible to confirm without performing a more objective test.
An electrodiagnostic test first used to assess hearing in dogs in the US has been available since 1992 in the UK. It is known as the Brainstem Auditory Evoked Response or BAER test, and offers a quick, non-invasive, objective and, above all, unequivocal assessment of an individual's hearing status.
The BAER test
When a sound enters the ear, tiny electrical impulses are generated by components of the auditory pathway in the inner ear and brain. Recording electrodes positioned on the head pick up these signals, and they are in turn passed into a specialised electrodiagnostic machine, (Figure 2). A series of approximately 500 stimuli, usually clicks, is passed into the ear through a headphone. In a normal-hearing dog a series of peaks and troughs is produced which is displayed on a small TV-type screen. The resulting traces may be printed out and attached to a Hearing Certificate confirming the dog’s hearing status.
Figure 2: Showing a puppy undergoing a BAER test, and the electrodiagnostic machine in the background
Thus, the BAER can be defined as the electrical response of the brain to a series of auditory stimuli.
The BAER test does not require patient co-operation, and is performed without sedation in 5-7 week old puppies. They tolerate the test extremely well, often remaining asleep throughout. Older dogs may require a light sedative to relax them and enable a good trace to be obtained, although some dogs may be tested without sedation if they are calm enough. Each ear is assessed in turn, and if the expected waveform is generated, that ear is deemed to be normal.
In the Dalmatian, and other predominantly white-coated dogs, deafness occurs due to degeneration of structures in the cochlea. A normal BAER trace therefore confirms that these structures are intact, as the waveform is generated by structures of the brainstem anatomically found beyond the cochlea (i.e. they are further along the auditory pathway). Therefore the BAER is a valid screening technique for this type of deafness, (Figure 3).
Deafness from cochlear damage eliminates all peaks in the BAER waveform, in the affected ear or ears, (Figures 4 and 5) so a straightforward, instantaneous yes-or-no assessment of hearing ability is possible.
 |
 |
Figure 3: Normal BAER printout, showing very similar waveforms from the right and left ear at 80dB. |
|
Some dogs produce a waveform from one ear only, and the other ear generates a trace where the required peaks and troughs are absent (Figure 4). These animals are identified as being unilaterally deaf. Many unilaterally deaf dogs will have gone through life unrecognised as such, because their behaviour is completely normal. At present, approximately 14.1% of Dalmatians tested at the Animal Health Trust are unilaterally deaf.
 |
 |
Figure 4: 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. |
|
When it is not possible to elicit a waveform in either ear, the dog is unfortunately bilaterally deaf (Figure 5). The deafness is complete, there is no "partial hearing" in this case, and any perceived hearing by the owner is probably due to the dog's reaction to cues picked up by other senses, such as vibrations or scent.
 |
 |
Figure 5: Printout from a bilaterally deaf dog, where the traces on the left are from the left ear at 80 and 100dB respectively and the traces on the right are from the right ear at 80 and 100 dB. No waveform is elicited from either ear. The slight deflection at the start of each of the bottom traces is the stimulus artefact, generated by the delivery of the larger 100dB stimulus, not by the patient. |
|
To date, 5.4% of Dalmatians tested by the Animal Health Trust are bilaterally deaf.
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.
Although it is possible to keep a bilaterally deaf dog, the potential problems associated with such a decision need to be considered. A deaf dog is a big responsibility and could be a liability both to itself and others in the wrong circumstances. It is hoped eventually that a DNA test may be available to identify carriers of the disorder, and the Animal Health Trust is 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.
How to book a BAER test
If you have a litter from a breed that is “at risk” of congenital deafness, such as Dalmatians, English Setters, Border Collies or Bull Terriers (for full list see www.lsu.edu/deafness/breeds.htm), the best age to test is around 51/2 to 61/2 weeks of age. 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. Also, at this age, the pups very predictably have an active period followed by a period of sleep, which is the perfect time to carry out the test. It may be necessary to sedate a pup over the age of 8 weeks, but this is a last resort; it is always attempted unsedated 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.
To book a deafness screening test, call the Small Animal Centre at the Animal Health Trust on 01638 552700 and give the details of your litter. An appointment slot is then booked for a convenient date and time when the pups are an appropriate age.
However, 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 veterinary surgeon. If they think that a hearing test is required, they will be able to refer you for an appointment to the Animal Health Trust. 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 veterinary surgeon.
What happens during the test?
 |
Figure 6: Border Collie Pups having a |
The test takes place in a dedicated room, which has a “playpen” set up, with plenty of clean newspaper covering the floor. The pups are brought in from the car and placed in the pen. This allows them to stretch their legs after the journey, as they will most likely have slept for a good part of their time in the car. (This bit can get messy, but there’s plenty of spare, clean paper!) At this point, some owners like to offer the pups some food, and/or a drink.
The pups need some time to quieten down, so this is factored in to the time allocated for each litter. It is much easier to perform the test once they are calm.
 |
Figure 7: English Setters settle down before their hearing test |
Once the puppies settle down and are comfortable, they usually go to sleep. This is the optimum time to begin the testing.
The owner selects a puppy, and identifies it. This is noted and the owner then holds the pup on their lap ready for the test.
The recording cables are placed, and the headphone positioned against the ear (figure 8) and the test begins. When the results for that ear have been obtained, the other ear is then tested. The traces are printed out, the puppy is returned to the pen, and the next puppy is picked up.
 |
Figure 8: Dalmation puppy (still asleep!) ready for hearing test |
Once all the pups have been tested, their details are entered into a computer, and the certificates printed out. Each puppy receives a certificate with his or her recorded traces attached to it. The certificates are verified and signed by a veterinary surgeon.
The certificates are then given to the owner to take with them when they leave.
The pups are then returned to the car, and the test is over!
Different types of animals can have their hearing tested using the BAER, but the other most common species tested at the Animal Health Trust is the cat. White cats, especially those with a blue eye, as mentioned earlier, are also prone to congenital deafness, so can be tested in the same way as dogs (figure 9). The test is booked in the same way: if a simple screening test is required please call the AHT on 01638 552700, but again, if you are worried that the deafness is not congenital, please consult your veterinary surgeon.
 |
Figure 9: Kitten having a BAER test |
Cats may not be as receptive to performing the hearing test without sedation, but, as illustrated in image, 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 8 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.
To discuss any further questions you may have, please contact
Julia Freeman
Centre for Small Animal Studies,
Animal Health Trust,
Lanwades Park,
Kentford,
NEWMARKET,
Suffolk,
CB8 7UU.
Tel: 01638 552700