Aetiology, prevalence and diagnosis of deafness in cats

 By Doug Campbell, Maelstrom NFC's

(Doug has very kindly given me permission to reproduce his in depth article here).

- inherited or acquired

- congenital or later-onset

- sensorineural ( nerve deafness ) or conductive

There are three common presentations seen in cats :

- inherited congenital sensorineural

- acquired later-onset sensorineural

- acquired later-onset conductive

Inherited congenital sensorineural deafness is usually, but not always, associated with pigmentation genes responsible for white in the coat.

Acquired later-onset sensorineural deafness is most often associated with ototoxicity or aging-related hearing loss (presbycusis ), but can also result from otitis interna, noise, and other causes.

Acquired later-onset conductive deafness is associated with chronic otitis externa and media or excess wax production.

No forms of inherited late-onset deafness, either sensorineural or conductive, have been identified in cats, but the conditions are seen in humans.

Acquired congenital deafness, either sensorineural or conductive, may result from malformations, intrauterine infections or drug toxicity, or anoxia, but these are not common.

Central deafness can theoretically result from a variety of retrocochlear lesions, but in practice is rare. The auditory pathways combine information from both ears from the level of the cochlear nuclei rostrally, so it is difficult to produce total unilateral central deafness, and bilateral central deafness requires lesion of a significant portion of the brainstem or midbrain, or a bilateral lesion of auditory cortex. Significant signs beyond deafness would accompany lesions of this sort.

Deafness can also be classified as either syndromic or non-syndromic. It has not been established whether deafness in cats associated with white hair coat colour and blue irises can be considered a form of syndromic deafness. The deafness seen in white cats is often likened to Waardenburg syndrome, a dominantly-inherited condition in humans with incomplete penetrance consisting of deafness, blue irises, a stripe of white in the hair and beard with premature greying, and minor structural facial deformities.

Aetiologies of deafness

Congenital sensorineural deafness

The earliest studies of deafness in animals were in the Dalmatian in the last century; most studies have been performed with Dalmatians or white cats.

Deafness does not develop in cats until the first few weeks of life, with normal functional development occurring to that point. Studies have shown that Dalmatians do not go deaf until weeks 3-4 after birth. The histological pattern that occurs in white cats is known as cochleo-saccular, or Scheibe, type of end organ degeneration. Histological studies of deaf Dalmatians have shown that the degeneration begins as early as one day after birth, and is clearly evident histologically by four weeks.

Degeneration begins in the middle coil of the cochlea, followed by the basal then apical coils. The cause of the strial degeneration is not known, but there is an observed absence of melanocytes in the strial tissue of many deaf animals. The function of melanocytes in normal stria is not known but appears to tie in with hair pigment associations with deafness.


Conductive deafness

Conductive deafness may result from developmental defects affecting the ossicles ( malleus, incus and stapes ), such as fusion, from failure of the ear canal to completely open after birth, or otosclerosis ( fusing together of the ossicles ), but these events have not been documented in cats and are probably rare.

Congenital tympanic membrane ( ear drum ) absence occasionally occurs, but does not produce deafness. Conductive deafness is most often a result of chronic otitis externa and media, where stenosis and eventual occlusion of the external canal results, or impaction from excess wax accumulation. Chronic otitis externa may ultimately result in mineralization and ossification of the external ear canal, requiring lateral ear resection or other remedies.


Ototoxicity

Ototoxic agents may cause hearing loss or deafness by direct effects on cochlear and/or vestibular hair cells, or may cause damage to the stria vascularis with secondary hair cell loss. Ototoxicity in humans is frequently accompanied by tinnitus, a high pitched ringing in the ears. Ototoxicity in cats may likewise be accompanied by behaviour suggesting the presence of similar sensory phenomena.

Over 180 compounds and classes of compounds have been identified as ototoxic. In some cases the ototoxic effects are reversible if caught early, such as with salicylates e.g. aspirin, but in most instances the deficit is permanent by the time of detection. The best recognized, and perhaps most frequent, agents of ototoxicity are the aminoglycoside antibiotics, especially gentamicin. Gentamicin and neomycin are the most commonly used aminoglycosides in veterinary practice, especially as topical agents, they are the drugs most likely to produce cochlear ototoxicity in companion animals.

Route of administration may affect ototoxicity, with systemic exposure providing better access of drugs to the cochlea than topical administration in ears with intact tympanic membranes. However, tympanum rupture frequently accompanies otitis externa, increasing access of drugs to the oval and round windows of the cochlea, through which absorption occurs. As a result, care must be exercised in topical drug application when visualization of the tympanic membrane is not possible.

Age, concurrent infection, anaesthesia, or pre-existing cochlear damage may potentiate drug ototoxicity, and repeated courses of antibiotic treatment may produce cumulative effects that are initially clinically inapparent.

Presbycusis

Presbycusis is the decline in hearing associated with various types of auditory system dysfunction that accompany aging, and cannot be accounted for by ototraumatic, genetic, or pathological conditions.

The pathologic change in most cats appears to be sensorineural, although decreased tympanum and ossicle joint articulation flexibility can potentially contribute. Presbycusis is common in geriatric dogs but prevalence rates or other related data are not available. Although it is a progressive disorder, owners usually report an acute onset because of the ability of the animal to compensate for hearing loss until nearly complete deafness occurs.

Hearing aids have been successfully utilized in dogs with some residual auditory function, but not all dogs will tolerate the presence of the ear plug. The primary determinant of the success of hearing aids is the ability of the owner to train the animal to accept the presence of a foreign body in the ear canal. Because of this training requirement and the sensitivity of the cat to ear contact it is unlikely that these devices would be successful in cats.

There is no known way of retarding the progression of the deafness. In humans, men are affected more at high frequencies, while women are affected more at low frequencies, but it is not known if similar patterns hold in cats.


Noise

Noise-induced hearing loss or deafness can be temporary or permanent. Temporary increases in hearing threshold occur after brief exposure to intense sounds, with gradual recovery of function occurring over periods ranging from minutes to two weeks.

Noise induced hearing loss is thought to result from either disarrangement or breakage of hair cell cilia, but can also result from damage to the tympanum and ossicles. Continuous or repeated exposure to noise results in a progressive loss of hair cells and a corresponding deafness. In humans the greatest hearing loss is at the middle frequency range near 4000 Hz, but progresses to higher and lower frequencies with continued exposure. Dogs used to hunt with firearms, like their human companions, may develop noise induced hearing loss.



Other

Hearing loss or deafness may also result from anoxia, anaesthesia, trauma, or infections, such as otitis interna and meningitis. On occasion, interactive effects may be expected to produce loss or deafness when the individual causes would have been insufficient alone to produce an effect.


Prevalence

Few cat breeds are noted for congenital deafness. Those reported with congenital deafness, or with the potential for it, include all those carrying the dominant white (W) gene and perhaps the white spotting or piebald gene.

Although cat owners in these breeds are familiar with the problem of deafness, little specific published information is available by breed. Several studies have examined deafness in mixed-breed white cats. Out of 256 white cats from three studies, 12.1% were unilaterally deaf and 37.9% were bilaterally deaf, or a total of 50% were affected. When cats that were the offspring of two white parents were examined, the prevalence of deafness (unilateral or bilateral) ranged from 52% to 96%.

When examining the effect of blue eye colour on deafness, two studies found, respectively, a prevalence of deafness (unilateral and bilateral combined) of 85% and 64.9% in cats with two blue eyes, 40% and 39.1% in cats with one blue eye, and 16.7% and 22% in cats with no blue eyes. There have been no studies of deafness in cats by specific breed. Purebred white cats are said to have a lower prevalence of deafness than mixed-breed white cats, but supporting data is unavailable.


Genetics

It is usually impossible to determine the cause of congenital deafness unless a clear problem has been observed in a breed or carefully planned breedings are performed. In affected breeds, deafness has often been long established but kept hidden from outsiders to protect reputations.

Hereditary deafness can potentially result from any of several mechanisms : autosomal dominant or recessive, X-linked, mitochondrial, or polygenic; in most instances the mechanism is unknown. Incomplete penetrance, where not all aspects of a deafness syndrome are expressed in an affected individual, frequently complicates an understanding of the mode of inheritance.

No known X-linked or mitochondrial deafness has been reported in cats. With a few known exceptions, hereditary deafness is usually associated with pigmentation patterns, where increasing amounts of white in the hair coat increase the likelihood of deafness.

The white (W) pigment gene in cats is autosomal dominant over colour, and is unrelated to albinism. Cats carrying the W gene are not always solid white, often having coloured spots on their heads that may disappear with age.

Unlike dogs with the merle gene, homozygous white cats do not have visual or reproductive defects, but they are more prone to the occurrence of blue irises and deafness, either unilateral or bilateral, and deafness occurrence increases with the number of blue eyes.

Whether the cat is heterozygous or homozygous for W, the blue eyes and deafness have incomplete penetrance. Long-haired cats have a higher prevalence of blue eyes and deafness than short-haired cats. White cats carrying the underlying Siamese dilution pigment gene can have blue eyes without deafness, and it has been suggested that the presence of this gene explains why purebred white cats are less often deaf than mixed-breed white cats. The white gene is present in many cat breeds but no data is available on relative rates of occurrence of deafness between them.

A dominant piebald gene is also found in various cat breeds but there has been no report of deafness associated with its presence.


Diagnosis

Since the ear canal does not open until approximately 5 days in cats and deaf kittens cue off of the responses of litter mates, it is not uncommon for deafness to go unrecognised for many weeks.

In some breeds, bilaterally deaf puppies will display more aggressive play with litter mates because they do not hear cries of pain, and kittens after weaning will not waken at feeding time unless jostled.

Bilateral deafness can usually be detected by behavioural testing with sound stimuli presented outside of the visual field or with the animal blindfolded, taking care to avoid visual or vibratory cues.

The minimum  desired response is a twitch of the ears in response to the sound; many animals will also orient to the sound source.

However, hearing animals, especially the young, quickly adapt and stop responding, resulting in    equivocal results. Further, unilateral deafness cannot be detected by these measures; at best such animals may demonstrate difficulty in localizing the origin of a sound.

Objective assessment of the presence of auditory function requires a test known variously as the brainstem auditory evoked response ( BAER ), brainstem auditory evoked potential ( BAEP ), or auditory brainstem response ( ABR ).

 

What is the baer test ?

The hearing test detects electrical activity in the cochlea and auditory pathways in the brain in much the same way that an antenna detects radio or TV. The response waveform consists of a series of peaks numbered with Roman numerals. Peak I is produced by the cochlea and later peaks are produced within the brain.

The response from an ear that is deaf is an essentially flat line. In the sample recordings shown below, Puppy 1 heard in both ears, Puppy 2 was deaf in the left ear, Puppy 3 was deaf in the right ear, and Puppy 4 was deaf in both ears.

Because the response amplitude is so 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 ( brain, muscle activity, etc. ).

The response is collected with a special computer through three extremely small electrodes placed under the skin of the scalp. One in front of each ear and one between and behind the eyes. The cat does not show any evidence of pain from the placement of the electrodes. If anything the cat objects to the gentle restraint and the irritation of wires hanging in front of its face. Sedation of cats is usually required to avoid electrical interference due to movement.

The stimulus click produced by the computer is directed into the ear with a foam insert earphone. Each ear is tested individually, and the test is usually complete in 10-15 minutes. Assessment of hearing loss is usually evaluated at several intensity's to gauge the extent of loss. A printout of the test results, showing the actual recorded waveform, is provided at the end of the procedure.

With complete peripheral  deafness, peak I of the BAER is totally absent, as are the subsequent peaks. With partial hearing loss, as is seen with presbycusis and some cases of ototoxicity, the time to occurrence of peak I is increased and the amplitude of the peaks is diminished.

The BAER changes during postnatal development of the auditory system, so appropriate reference values must be used when evaluating young animals.

UK Baer test sites

This list is for information and is not a general endorsement of the abilities of the individuals listed.

Mrs. Celia L. Cox
The Hearing Assessment Clinic
Red Lane House, Shawford
Winchester, Hampshire SO21 2AA
Tel : 01962 713155 ( and Fax )

Email : cox.hear@btinternet.com

Dr. Julia Freeman
Centre for Small Animal Studies
Animal Health Trust, PO Box 5
Newmarket, Suffolk CB8 8JH
Tel : 01638 55 2700 Fax : 01638 55 5600

Email : julia.freeman@aht.org.uk

Dr. Susan N. Fitzmaurice
Neurology & Neurosurgery Referrals
3(4) Park Avenue
Bedford, Bedfordshire MK40 2JY
Tel : 07970 387027 Fax : 07971 047279

Mrs. Mary Greening
British Dalmatian Club - BAER Coordinator
Tollcross, Church Lane, Moreton
Newport, Shropshire TF10 9DS
Tel : 01952 691518

Dr. Judith Skerritt
Cranmore Veterinary Referrals
140 Chester Road
Childer Thornton, Cheshire CH66 1QN
Tel : 0151 3399141

Email : JOSkerritt@aol.com



Management

Cats with unilateral deafness make excellent pets, with owners often unable to detect any impairment. However, owners of these animals should be discouraged from breeding them to prevent further affected animals and an ultimate increase in the prevalence of the disorder.

Some cats will show directional localization deficits and may not awaken to sounds if sleeping with the good ear against the ground. Cats with late-onset acquired deafness generally adapt well, but precautions must be observed to prevent vehicular injury or death, and bite injuries to humans, especially children, when deaf cats are startled.

Cats bilaterally deaf from both congenital and acquired causes place greater reliance on visual and vibratory sensory information to cope with the loss of auditory input. Despite the worry of animal owners and those concerned with animal rights, the quality of life of deaf cats is not demonstrably diminished. Likewise, these animals do not have diminished mental capacities, any more than the average deaf or blind human has diminished mental capacity.

The brain responds to the loss of a sensory modality by various forms of plasticity, whereby CNS structures that would have received input from that sensory modality constrict and adjacent structures expand to take advantage of the available space.

Genetic counselling for owners of at risk breeds will be difficult until the mechanisms of inheritance are identified or a DNA blood test is developed. In general, unilaterally deaf animals should not be bred, since they have the genetic defect and will pass it on to their offspring.

Reproduced by kind permission of Dr Doug Campbell.