LYCOS RETRIEVER 
Deafness: Bilateral Deafness
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Deafness is the loss of ability to hear normally. There are two types of deafness conductive and sensori-neural. These may occur separately or together and one (unilateral) or both ears (bilateral) may be involved. All degrees of hearing loss, from slight to total, may occur. Conductive deafness is due to interference with the conduction of sound across the middle ear to the inner ear. Sensori-neural deafness is due to reduced function of the inner ear (cochlea) or the nerve fibres leading to the brain.
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Deafness occurs in many breeds. The highest incidence is in the dalmatian, with studies from the US and the UK showing that 18 to 30% are unilaterally deaf, and 5 to 10% are bilaterally deaf. The condition is slightly more common in female dalmatians. Other breeds with a relatively high incidence of deafness are the English setter, Australian shepherd, border collie and Shetland sheepdog.
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Deafness can be assessed by observing the animal's behavioral response, such as lifting or turning the head, after making a noise out of the animal's view. Dogs suspected of being bilaterally deaf can be challenged with sounds of increasing intensity from different directions. Be careful not to make sounds that can be "felt" through vibrations.
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Neurofibromatosis 2 (NF2) is associated with a rare, potentially treatable type of deafness. The hallmark of NF2 is hearing loss secondary to bilateral vestibular schwannomas. The hearing loss usually begins in the third decade, concomitant with the growth of a vestibular schwannoma, and is generally unilateral and gradual, but can be bilateral and sudden. A retrocochlear lesion can often be diagnosed by audiologic evaluation, although the definitive diagnosis requires magnetic resonance imaging (MRI) with gadolinium contrast. Affected persons are at risk for a variety of other tumors including meningiomas, astrocytomas, ependymomas, and meningioangiomatosis. Mutations in NF2 are causative.
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A wide variety of breeds of dogs have been reported to have congenital deafness (see Table 1); not all of these cases of deafness have been shown to result from hereditary causes. The breeds for which the prevalence is known to be high are highlighted in bold, although similar high rates may occur in other breeds that do not yet routinely receive testing. Prevalence rates measured by the author are shown in Table 2 for the canine breeds most often presented for hearing testing services. The highest rates are seen in Dalmatians, of which 30% are deaf in one or both ears; rates in other strongly affected breeds range from 8% to 20%. Typically, there are two to three unilaterally deaf animals for every bilaterally deaf animal. In breeds with white versus non-white phenotypes (Bull Terrier, English Cocker Spaniel), there is a clear increased prevalence in the white phenotype.
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There is a strong association of deafness with increased amounts of white colour in the coat and blue eyes (heterochromia iridis) in breeds with merle and piebald coat colouring. Merled breeds include the merled collie and Shetland sheepdog, Australian heeler, dapple dachshund (18% bilaterally deaf, 36% unilaterally deaf), harlequin great Dane, Norwegian dunkerhound, and old English sheepdog. Breeds with the piebald gene (which affects the amount and distribution of white areas) include the beagle, bull terrier, samoyed, great Pyrenees, Sealyham terrier, greyhound, English bulldog, English setter, and of course dalmatian.
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Bilateral Deafness