HEALTH ISSUES
Deafness in White Cats
Some Cats Lose Hearing Almost Immediately, Others Later.
Congenital deafness in cats is primarily of the hereditary sensorineural form associated with white pigmentation genes, although acquired forms of deafness are possible. Highest prevalence is seen in white cats, especially those with blue eyes This deafness results from degeneration of the cochlear blood supply at age 3 to 4 weeks, presumably resulting from suppression of melanocytes by the white (cat) genes. Mechanism of inheritance is not understood for most breeds. Such animals should not be bred and may present liabilities for their owners. Objective diagnosis of deafness, especially when unilateral, relies on the brain stem auditory evoked response, an electrodiagnostic test where electrical activity in response to a click stimulus is recorded from the scalp using needle electrodes and a special purpose computer.
Sensory function in cats is primarily tactile, olfactory and gustatory. The visual and auditory senses, although partially functional at birth, exhibit significant postnatal development: in the dog, the eyes do not open until a puppy is 8 to 10 days of age, the ear canals do not open until it is 12 to 13 days of age, and mature system function up through the cortex is not present until it is 3 months of age or older. Similar delays are seen in the cat. As a result, disorders of these systems frequently escape early detection.
Deafness can be described as (1) congenital or late onset, (2) hereditary or acquired, and (3) conductive or sensorineural. The most commonly occurring deafness forms in cats are congenital hereditary sensorineural deafness, late onset acquired sensorineural deafness, and late onset acquired conductive deafness. Distinguishing between hereditary and acquired deafness is generally not possible without breeding trials, although an assumption of hereditary deafness can be made in breeds with a high prevalence of deafness. The most common form of deafness in young cats is congenital hereditary sensorineural deafness, with acquired conductive and acquired sensorineural appearing on rare occasions.
PATHOPHYSIOLOGY OF DEAFNESS
Perception of sound first requires transmission through the outer and middle ears to the cochlea for transduction by neural hair cells. Perception results from (1) transmission of transduced auditory information from the cochlea by the eighth cranial nerve to the dorsal and ventral cochlear nuclei, the inferior colliculus, the medial geniculate nucleus of the thalamus, and the primary and secondary cortical auditory areas on the temporal lobe; and (2) attention to the arriving information. Commingling of ipsilateral and contralateral auditory information occurs at multiple steps in the ascent up the auditory pathway. As a result, unilateral hearing loss rarely results from lesions or disease affecting auditory structures above the eighth nerve. Central deafness (unilateral or bilateral) in the absence of severe neurologic disease is clinically unknown in veterinary medicine and is not considered further here.
Conduction Deafness
Conduction deafness results from blockage of sound transmission to the cochlea as a consequence of occlusion of the ear canal or middle ear cavity, or from developmental defects. Occlusion may result from excess cerumen production, from otitis externa or media, or from foreign objects. Developmental defects, which are uncommon, may include atresia of the tympanum or ossicles, fusion of the ossicles, or collapse of the ear canal from cartilaginous weakness or incomplete development. Conduction deafness may be partial or complete and may be reduced by intervention in some cases. Clearance by the body of the mucopurulent discharge and detritus from otitis media may require weeks to months after termination of the infection; hence, recovery of auditory function is delayed. Hereditary forms of conduction deafness have not been identified in domestic species, but the appearance of such a disorder from a spontaneous genetic defect is possible.
Congenital Acquired Sensorineural Deafness
Congenital acquired sensorineural deafness, which is uncommon, can result from in utero or perinatal exposure to ototoxic compounds such as maternal treatment with aminoglycoside antibiotics, in utero or perinatal otitis or meningitis, anoxia, or even trauma. Breeders with animals belonging to breeds with a high prevalence of hereditary sensorineural deafness may suggest acquired causes of deafness rather than confront the breeding and other implications of the presence of a hereditary disorder. Sensorineural deafness, whether hereditary or acquired, is the consequence of cochlear hair cell loss through primary or secondary mechanisms.
Congenital Hereditary Sensorineural Deafness
Congenital hereditary sensorineural deafness is usually seen in cat breeds with white pigmentation. In breeds of cats carrying the white gene, the hair cell loss is secondary to degeneration of the cochlear blood supply. Figure 1 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 scala tympani 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 bed degenerates, 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 the scala 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.
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. 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.
GENETICS OF DEAFNESS
Pigment-associated congenital hereditary sensorineural deafness in the cat is linked to the white gene, which is dominant over color and is unrelated to albinism. On occasion these cats will have a head spot and usually have one or two blue eyes. Although the white gene is dominant, not all carriers are deaf; thus, deafness is not simply inherited.
BEHAVIORAL INDICATORS OF DEAFNESS
Newborn kittens with undeveloped auditory and visual function use other sensory cues for their feeding, elimination, and locomotion behaviors. As auditory development proceeds, they can detect loud noises, despite the unopened ear canal. Breeders relying on this for home testing may find themselves to have been in error at a later date. Behavioral testing of hearing after opening of the canal relies on detection of a response to sound stimuli in the absence of other detectable sensory signals. These noises should be produced outside of the visual fields, avoiding visual cues, vibratory cues, touch, and air movements. Behavioral testing has limited value; animal responses rapidly adapt even when hearing is present, stressed animals with intact hearing may fail to respond, and unilateral deafness cannot be detected. In unilaterally deaf animals, the only behavioral sign of deafness is a difficulty in localizing the source of a sound, and many animals adapt to that also. Behavioral deafness detection with young animals in the home is difficult, as the deaf young cue off the behavior of their littermates. A kitten that does not awaken in response to a loud noise is almost certainly bilaterally deaf, but the unilaterally deaf cannot be detected with any reliability. As a consequence, behavioral hearing assessment of animals in the clinic or home is of limited reliability, and electrodiagnostic tests are used for objective assessment.
ELECTRODIAGNOSIS OF DEAFNESS
The most widely used electrodiagnostic test of hearing is the brain stem auditory evoked response (BAER), also known as the brain stem auditory evoked potential (BAEP) or the auditory brain stem response (ABR). This test was first used in veterinary research applications in the 1970s, and in clinical veterinary applications in the early 1980s. The BAER detects electrical activity in the cochlea and auditory pathways in the brain in much the same way that an electrocardiogram detects electrical activity in the heart.17 The response waveform consists of a series of peaks labeled with Roman numerals: peak I is produced by the cochlea and auditory nerve, and later peaks are produced within the brain. The response from an ear that is deaf is an essentially flat line. Because the response amplitude is quite small, it is necessary to average the responses to multiple stimuli (clicks) to unmask them from the other unrelated electrical activity that is also present on the scalp (e.g., electroencephalographic activity, muscle activity).
The response is collected with a special computer through small subdermal needle electrodes: one is placed in front of each ear, one is placed at the top of the head, and a ground electrode is placed either between and behind the eyes or on the neck. It is rare for a dog to show any evidence of pain from the placement of the electrodes - if anything the dog objects to the gentle restraint and the presence of wires hanging in front of its face. A stimulus click (air-conducted) produced by the computer is directed into the ear with a foam insert earphone. Each ear is tested individually, and the test usually is complete in 10 to 15 minutes. Sedation or anesthesia is unnecessary unless the dog becomes extremely agitated, which can usually be avoided with patient and gentle handling. Sedation or anesthesia does not materially affect the BAER.
The click stimulus used contains most of the audible frequencies of the cat, with the exception of the very highest perceived frequencies. Accordingly, the BAER is a frequency nonspecific test that is more useful for detecting the presence or total absence of auditory function without quantifying hearing loss in decibels. Assessment of the normalcy of a response is based on identification of the presence of Peak I within a narrow expected time frame (which varies based on the equipment used) and the presence of the expected pattern of peaks. With progressive hearing loss, there is a reduction in the amplitude of the BAER peaks and an increase in peak latencies; thus, a subjective assessment of partial hearing loss can be made but not quantified, and differing degrees of loss in different frequency ranges cannot be determined. Diagnosis of partial hearing loss based on the BAER is done only with great caution, as a number of technical factors can affect peak amplitude and latency in subjects with normal hearing. Fortunately, partial hearing loss is rare in kittens.
The BAER demonstrates maturational changes. Because the greater portion of the BAER originates in the brain stem, there is less postnatal development than is seen in tests of other sensory modalities; however, postnatal development is greater in altricial species like the cat than in precocial species like the horse and cow. Full maturation of the BAER occurs by 40 days in the cat. The BAER can be recorded in response to loud stimuli prior to the opening of the ear canal, but this is not of use as it predates the age at which deafness is manifested.
In some circumstances, it is useful to be able to differentiate between sensorineural and conductive deafness, as this can affect breeding decisions and whether a young animal is placed in a show home or a pet home. When a BAER indicates deafness in an animal in which conduction deafness might be suspected (i.e., long-eared breed, recent ear infection), the test is repeated with a mechanical transducer that transmits the stimulus click as a vibration through bone rather than through air conduction. Because the cochlea is imbedded in bone, the bone-conducted BAER bypasses the outer and middle ears, the sites of conduction blockade, and directly activates the cochlea. The response appearance is the same as an air-conducted BAER, but the peaks occur at a shorter latency due to the shorter path traversed by the stimulus.
A limited availability of BAER testing sites blocks some potential users from access, but the number of test locations is increasing beyond the original veterinary school sites.
Listed below are the registered testing sites for the screening of hearing in White Cats:
Animal Health Trust, Newmarket - +44 (0)163 855 2700.
Edinburgh Vet School.
Celia Cox, Winchester.
Judith Skerritt, Cheshire.
Animal Medical Centre, Manchester.
Cost of the test is £20.00 + VAT.
Cat Breeds Carrying the White (W) Coat Pigment Gene are:
White Scottish Fold
European White
White Turkish Angora
Foreign White
White American Wirehair
White Cornish Rex
White American Shorthair
White Devon Rex
White British Shorthair
White Manx
White Exotic Shorthair
White Persian
White Oriental Shorthair
Figure 1
Figure 1. Cross-section of the cochlea. The organ of Corti rests on the basilar membrane, with its hair cell cilia embedded in the tectorial membrane. The stria vascularis on the outer margin of the scala media secretes the endocochlear fluid of the scala media and maintains a high K+ concentration essential to sound transduction by the hair cells. Sensorineural deafness can result from primary or secondary loss of cochlear hair cells