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Deafness
Summary:
Congenital deafness is recognised as a problem in many breeds of dog that carry the extreme piebald gene. Such breeds include Dalmatians, White Boxers and White English Bull Terriers. More than 60 breeds of dog have been identified as suffering from congenital deafness (deafness at birth). Deafness may be either complete or unilateral. Whilst unilaterally deaf animals can lead normal lives, bilaterally deaf animals can be a liability, and are sometimes euthanased. Brainstem Auditory Evoked Response (BAER) tests may be used to assess the hearing status of adult animals prior to breeding and puppies prior to sale.
Hearing and Deafness in Dalmatians
How hearing works
Sound waves enter the external ear canal and strike the ear drum, which begins to vibrate. This, in turn, causes vibration of the tiny bones in the middle ear (the ear ossicles). These vibrations set up waves in the fluid of the cochlea, the spiral-shaped structure in the inner ear. The change in pressure ultimately causes vibration of the hair cells inside the cochlea which 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, defined below.
Types of deafness
Deafness can be defined as (1) conductive, (2) sensorineural, (3) congenital or late onset, (4) inherited or 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.
It has been recognised that the Dalmatian breed is affected by inherited, congenital sensorineural deafness since the last century.
In dogs, deafness is often associated with the gene that causes merled coat colour, for example in the Border Collie or Old English Sheepdog. Absence of pigment in one or both irises resulting in blue eye colouration may also be present. Predominantly white coat colouration is also thought to predispose certain breeds to deafness, for example in the Boxer or English Bull Terrier. 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. The genes governing coat colour in the Dalmatian are the extreme piebald gene, expressed as solid white coat colour, combined with the dominant ticking gene to give the spotting on the white background.
The merling gene is not reported to be present in the Dalmatian, but blue eyes do occur in the breed. Because of the relationship between blue eyes, white coat and deafness in cats, an association between blue eyes and deafness in the Dalmatian was suggested. The lower incidence of deafness in the UK population (21.6% affected), where blue eyed dogs are not bred from, when compared to 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 reduces the incidence of affected offspring. This is supported by recent information from the US, which shows a statistically significant relationship between deafness and blue eyes.
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 which 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, resulting in the dog becoming deaf. Affected puppies can be detected by 4 weeks old and one study 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, bilateral deafness (total deafness in both ears) can often be identified by the skilled observer, 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.
Showing a puppy undergoing a BAER test, and the electrodiagnostic machine in the background.
A dog which is unilaterally deaf is very difficult to identify as it hears perfectly in the non-affected ear, and so usually behaves normally. Even if it is suspected that a dog is unilaterally deaf, it is almost impossible to confirm without performing a more objective test. A 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 and, above all, unequivocal assessment of an individual's hearing status.
When a sound enters the ear, tiny electrical impulses are generated by components of the auditory pathway in the inner ear and brain. These signals can be picked up by recording electrodes positioned on the head, and are in turn passed into a specialised electrodiagnostic machine, (Figure 1). A series of approximately 500 stimuli, usually clicks, are 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. Thus, the BAER can be defined as the electrical response of the brain to a series of auditory stimuli.
Normal BAER printout, showing very similar waveforms from the right and left ear at 80Db.
In the Dalmatian, deafness occurs due to degeneration of structures in the cochlea. A normal BAER trace therefore confirms that these structures are intact, and is a valid screening technique for this type of deafness, (Figure 2). Deafness from cochlear damage eliminates all peaks in the BAER waveform, in the affected ear or ears, (Figures 3 and 4) so a straightforward yes-or-no assessment of hearing ability is possible.
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.
Printout from an animal which is unilaterally deaf in the right ear. Trace1 is at 80dB and trace 2 at 100dB show contralateral recordings which are eliminated by masking the left ear with "white nose" in trace 3. Trace 4 is from the left ear at 80 and 100dB. No waveform is elioited from either ear.
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 3). 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% of the Dalmatians tested in the UK are unilaterally deaf.
When it is not possible to elicit a waveform in either ear, the dog is unfortunately bilaterally deaf (Figure 4). 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. Approximately 7% of the Dalmatians tested in the UK are diagnosed as being 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 incidence 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 incidence 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 Dalmatian, the problems associated with such a decision cannot be emphasised too highly. A deaf dog is an incredible responsibility and can be a liability both to itself and others. Unilaterally deaf animals make wonderful pets, but should not be bred. It is hoped eventually that a blood test may be available to identify carriers of the disorder. Until then, 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 every animal using the BAER test.
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