Deafness that is present at or soon after birth may have either an acquired or a hereditary etiology and may occasionally occur in any puppy whether pure bred or mixed breed. Acquired deafness may be caused by viral infections, anoxia, or the ototoxic side effects of drugs or other materials. Because dogs and cats are born deaf, deafness in a puppy or kitten is not abnormal up to a certain age. In cats the earliest discriminating hearing tests were performed at the age of 7 days. Cochlear potential measurements from a round-window electrode were found to be conclusive about the presence or absence of hearing in cats over 7 days of age (). In dogs, hearing tests were performed from the age of 4 weeks () by means of cochlear potential measurements from round-window electrodes () or brainstem auditory evoked responses (BAERs) ().
Testing the Hearing of Young Puppies
In our laboratory, two Irish wolfhound puppies and two beagle puppies were investigated for hearing from the third day after birth. Brain-stem auditory evoked potentials (BAERs) were recorded from surface electrodes (Dantec) on the pinnae and the skin over the parietal bone on the midline. For the recording of air-conducted BAERs, each pup was placed in a plexiglass cage (30 X 40 X 40 cm) with two microphones at opposite ends of the cage. No sedation was used. The puppies were free to move when in the cage. The equipment used was as described previously (), and the stimulus was a click produced by a rectangular wave of 0.2 ms duration with an intensity of 80 dB SPL and 90 dB SPL, as measured in the center of the cage (Sound Level Meter). The stimulus repetition rate was 10 per second. The sound was delivered via both microphones simultaneously. For subsequent recording of bone-conducted BAERs, the puppies were taken out of the cage while leaving the electrodes in place. The same stimulus was used (80 dB SPL) except that it was now applied via the bone conductor (Bone Vibrator B.71B, Radioear Corporation) placed on the stop (). The bone conductor was pressed against the skull by hand, maintaining the pressure that resulted in the best response signals. The tests were conducted every other day until BAERs for both the air-conducted and the bone-conducted stimuli were recorded.
In both the Irish wolfhound puppies brainstem evoked potentials were first recorded on the 11th day after birth with the bone-conducted stimulus. In both beagle puppies the first brainstem evoked potentials were recorded on the seventh day after birth with the bone-conducted stimulus (). The first recording of brainstem evoked potentials with air-conducted stimuli was on the 25 th day after birth in both Irish wolfhound puppies and on the 26th day in both beagle puppies (). During the test period (from the 3rd to the 27th day after birth) none of the puppies showed signs of sound perception.
The results indicate that bone-conducted BAERs detected cochlear activity at a younger age than air-conducted BAERs. The method does not discriminate between unilateral and bilateral cochlear activity, but with bone-conducted BAERs bilateral cochlear deafness can be elucidated before 2 weeks of age in dogs.
Congenital Deafness Caused by a Genetic Defect
Congenital deafness caused by a genetic defect has been thoroughly investigated for the white cat () and the Dalmatian dog (). These investigations were in part motivated as animal model studies for the Waardenburg syndrome in humans. This syndrome is characterized by hereditary deafness associated with disorders of pigmentation of hair, iris, and skin (). Congenital deafness associated with disorders of pigmentation was described in the cat () and in the dog (). In dogs, deafness is associated with merle pigmentation (e.g., Shetland sheepdog, collie, harlequin Great Dane) and carriers of the piebald gene (e.g., bull terrier, Samoyed, Great Pyrenees mountain dog, Sealyham terrier, greyhound, bulldog, Dalmatian dog). In this form of inherited deafness one or both ears are completely deaf.
Most investigators of the histologic development of the cochlea in deaf white cats agree that the primary degeneration of the epithelial and sensory elements occurs in the first weeks after birth, after which secondary degeneration of the neural structures follows (). Normal neural structures were found together with an advanced stage of degeneration of the epithelial elements in deaf white cats (). In a later study, however, it was reported that the spiral ganglion neurons in 2 of 11 white kittens (7 and 16 days old) were completely degenerated. It was concluded that the degeneration of the cochlea in white cats may be considered to be a process affecting both sensory and neural structures with a variety of features and a very variable timing ().
The earliest postnatal histologic events in deaf Dalmatian dogs were studied at the age of 4 weeks. At that age a volume reduction of the saccule and of the cochlear duct had already occurred (). This volume reduction is a consequence of the descent of Reissner’s membrane toward the organ of Corti and, in a later stage, the covering of the organ of Corti by this membrane. In die cochlear duct, other signs of degeneration were a decrease in thickness of the stria vascularis and of the cellular components of the organ of Corti. In older deaf Dalmatian dogs the degeneration was more severe and was accompanied by loss of ganglion cells (). In a histologic study comparing cochleas of hearing and deaf Dalmatian dogs at the age of 6 weeks, the findings of Mair () were confirmed for the deaf dogs, whereas the cochleas’ of the dogs with normal hearing were completely normal (). The relation between a variance in pigmentation and inner ear dysfunction is recognized in several hereditary syndromes in several species of mammals, including humans. The pigmentation abnormalities are always of the “white spotting” kind, also known as hypopigmentation. When the entire coat is white, the animal must be regarded as having one very large spot rather than as being an albino.
The difference between the two types of whiteness, albinism and hypopigmentation, is fundamental. In the albino, melanocytes can be identified in the hair follicles, but they are incapable of forming melanin due to a biochemical block. In hypopigmented animals or in spotted regions of pigmented animals no melanocytes can be identified (). In the albino there is no pigmentation at all, whereas in hypopigmented animals pigment is present in the retina, showing that the genetic capacity to produce melanin is not lacking (). The abnormalities in the inner ear are confined to the cochlea and saccule. There is a severe degeneration of the organ of Corti, stria vascularis, spiral ganglion, and the macula of the saccule. The relation between hypopigmentation and inner ear abnormalities was investigated by Deol () in mice. His main conclusions were that “spotting” of the coat was always associated with hypopigmentation of the stria vascularis in the cochlea, usually a heavily pigmented area. Hypopigmentation of the stria vascularis was always associated with degeneration of the cochlea. There was no correlation between the severity of hypopigmentation of the coat and the hypopigmentation of the stria vascularis of the cochlea (). These finding may be applicable to the dog and the cat as well.
The relation between the piebald gene and “spotting” was established in mice (), and the abnormality was localized in the neural crest. The abnormal parts develop abnormal melanoblasts in the differential areas, resulting in local hypopigmentation. The abnormal areas varied in location and size, as did the pigmentation in the stria vascularis. The degeneration of the cochlea varied in severity within the cochlea. Hearing was not tested in these animals.
From these findings it may be concluded that when an animal with “spotting” or hypopigmentation is born with unilateral or bilateral deafness, the deafness is based on a hereditary defect. Furthermore, when, as in dogs, hypopigmentation is a breed characteristic, deafness will occur in the breed.
The labyrinth contains the sensory cells for both hearing and equilibrium. In labyrinthitis and ototoxicity both of these functions are impaired. When the sensory cells are destroyed, they are not replaced, and hence the hearing loss is permanent. The loss of function of the semicircular canals and utricle may be partially compensated by the central nervous system ().
The ototoxicity of a drug or any other chemical is its toxic effect on the inner ear, including the cochlea, the vestibule, and the semicircular canals. The toxic materials can reach the inner ear via local application in the middle ear or hematogenously via absorption from the digestive tract, respiratory tract, or wounds or via parenteral administration. In veterinary practice, ototoxic drugs (in young dogs and cats antiparasitic drugs are the most obvious candidates) usually pass a perforated (iatrogenic?) tympanic membrane and enter the perilymph via the membranes of the oval and round windows. There is direct contact between the perilymph and the hair cells.
The toxic effect is the degeneration of the sensory cells within the membranous labyrinth, causing dysfunction of the cochlea (deafness) and vestibular dysfunction (loss of equilibrium). When the ototoxicity is unilateral, the signs of vestibular dysfunction are most impressive (). In the acute phase there is a head tilt with the affected ear under, horizontal nystagmus with the fast phase away from the affected side, rolling over toward the affected side and inability to stand, nausea, and refusal of food. Within 3 days central compensation results in diminishing and eventual disappearance of the nystagmus, gradual attempts to stand, and beginning efforts to eat and drink, but the head tilt is unchanged. Within 3 weeks the situation improves, but jumping and walking down stairs often result in falling. The compensation is optimal after about 3 months, but the head tilt is then still obvious.
Many substances used for local application in the ear canal contain ototoxic components, including disinfectants, antimicrobial agents, antiparasitics, and even the vehicle in some ear drops. None of these medications should ever be instilled into the external ear canal unless it is certain that the tympanic membrane is intact.
Selections from the book: “Veterinary pediatrics: dogs and cats from birth to six months”. Johnny D. Hoskins. (2001)