Congenital Hereditary Sensorineural Deafness in Pedigree Dog Breeds. Background:
Deafness in dogs may be peripheral (associated with the external, middle or inner ear) or central (associated with changes in the brain). Deafness may be inherited or acquired, congenital (present at birth) or later in onset, and may be conductive (associated with changes in the external and middle ear) or sensorineu
ral (caused by pathological changes in the cochlea of the inner ear), or occasionally in the auditory nerve). Congenital hereditary sensorineural deafness (CHSD) is one type of sensorineural deafness, and this occurs in at least 99 dog breeds as reported by Dr George Strain 2011. Much research is being carried out on this condition.The hereditary nature of CHSD has not yet been proven in all breeds, it can be assumed in many, based on a high prevalence in various breeds and characteristic lesions in some. Congenital hereditary sensorineural deafness affects dog behaviour. Dogs that are bilaterally deaf need specialised training, and are unsuitable for work, and dogs that are deaf in one ear have difficulty localising sources of sound. Dogs with hearing loss are more likely to be involved in traffic accidents and need to be kept in a secure environment, as they may be startled by sudden movement or an unseen approach. Owners of deaf dogs may face legal consequences if the deafness leads to the injury of a third party. Ideally, all unilaterally or bilaterally deaf dogs should be neutered so that they are not used for breeding. How CHSD affects a dogs hearing:
With normal hearing, sound reaches the cochlea in the inner ear through the air in the external and middle ear canals (air conduction), and also through vibration through the bones in the skull (bone conduction). Nerve signals are then transmitted to the brain by the auditory nerve. Both air conduction and bone conduction hearing can be tested by Brainstem Auditory Response Evoked testing (BAER). There seem to be two types of CHSD in different dog breeds: 1) non-pigment related and 2) pigment-related. The non-pigment related form occurs in some dog breeds with solid coloured coats, including the Doberman Pinscher and the Shropshire Terrier. In this form, direct death of the cochlear hair cells is seen; these cells transmit the sound stimulus by stimulating the auditory nerve. In other breeds, CHSD is related to indirect death of the hair cells and other cochlear structures. This pigment-related form of CHSD is caused by a reduction in pigment-containing cells (melanocytes) in the cochlea. The melanocytes, once they reach the cochlea, become energy producing cells supporting the health of the outer and inner hair cells. If these energy producing cells are not present in sufficient numbers, the cochlear structures and hair cells die, so the puppy is deaf. The lesions seen in this form of CHSD have been described in a range of dog breeds, including the Dalmatian, Border Collie, Australian Sheepdog, Norwegian Dunkerhound, Bull Terrier, Great Dane, Old English Sheepdog and Dachshund. A likely pigment-related form of CHSD in the Australian Cattle Dog and Australian Stumpy Tail Cattle Dog is under investigation. In these breeds, a lack of pigment in the skin, hair, iris of the eye and in the inner ear cochlea can be associated with CHSD. Testing for CHSD. Brainstem auditory evoked response testing:
BAER testing is a reliable, minimally invasive method of assessing hearing through air conduction BAER and if necessary by bone conduction BAER. Air conduction testing assesses the whole auditory pathway from the external ear to the brainstem and this is used to diagnose CHSD. Bone conduction testing can assess the path from the cochlea to the brainstem and can be used to diagnose problems in the external and middle ear of dogs causing conductive deafness. The test for CHSD is non-invasive and at Veterinary Hearing air conduction BAER is performed with light sedation in puppies from 6 weeks of age. The reason for sedation is to keep the puppy still for the test and therefore to allow a clear tracing to be obtained. All puppies undergo a full physical examination and temperatures and weights are recorded. If there is any contraindication for sedation then the pup will not be sedated, however testing may be more difficult to interpret.
