Congenital Deafness

Congenital Deafness
Congenital Hereditary Sensorineural Deafness
Congenital hereditary sensorineural deafness is usually seen in dog and cat breeds with white pigmentation. In the small number of canine breeds where it is not associated with white pigmentation (Doberman and other dog breeds not carrying piebald or merle genes),23 deafness results from the type of hair cell loss that is aprimary event with unknown cause. In breeds of dogs carrying the piebald or merle genes and breeds of cats carrying the white gene, the hair cell loss is secondary to degeneration of the cochlear blood supply. shows a cross section of one turn of the cochlea, demonstrating the separation of the cochlea into three parallel ducts: the scala vestibuli, the scala media (or cochlear duct), and the scalatympani which joins at the apex of the cochlea with the scala vestibuli. The outer margin of the scala media is covered by a vascular bed, the stria vascularis. The stria is responsible for secretion of endocochlear fluid and maintenance of its high K+ concentration which is essential to sound transduction by the sensory hair cells. In pigment-associated hereditary deafness, this vascular beddegenerates, resulting in secondary loss of hair cells and deafness. The cause for the strial degeneration is unknown, but histologic studies have demonstrated an absence of strial melanocytes, whose presence or postnatal development is suppressed by the piebald or merle genes. The function of melanocytes in the stria is unknown, but they appear to be critical to maintenance of elevated K+ levels in thescala media and survival of the stria. Whether hair cell death is from primary or secondary mechanisms, the loss is permanent, as mammals are unable to regenerate cochlear neuronal tissue.
In the Dalmatian, postnatal auditory function development has been shown to proceed normally up until 3 weeks of age, at which point the strial degeneration produces rapid loss of hair cell function.6 A similartime course is likely in other breeds of dogs and cats with pigment-associated deafness, but it has not been studied. Likewise, the time course of congenital deafness in canine breeds not associated with white pigmentation has not been documented, but it is likely that deafness is present at birth or shortly thereafter. As a consequence of the documented 3 to 4 weeks of age at which time deafnessappears, hearing testing, as described below, is typically not performed until an animal reaches at least 5 weeks of age.
A wide variety of breeds of dogs have been reported to have congenital deafness; 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 ratesmay occur in other breeds that do not yet routinely receive testing. Prevalence rates measured by the author are shown in 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 animalsfor 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. Prevalence rates for pure feline breeds have not been measured, but are highest for the breeds carrying the white gene especially in cats with blue eyes. Deafness in 256 mixed breed white cats was reported as 12%unilateral and 38% bilateral, for a total of 50% of cats being affected (reviewed in Delack3). The prevalence of deafness increases as the number of blue eyes increases from zero to two, but not all blue-eyed white cats are deaf. Deafness prevalence (unilateral and bilateral) in mixed breed white cats was 17%, 40%, and 85% for zero, one, or two blue eyes, respectively.9
Pigment-associated...