Hearing loss is the most common sensory deficit in humans, and may also be so in companion animals. It affects all mammalian species. Deafness in an animal is usually not a life-threatening disorder in the way of some renal disorders, and it is not a painful condition like hip dysplasia, but it does put an animal at risk from undetected dangers such as motor vehicles or predators. Deaf animals create their own liabilities and present non-trivial training challenges to their owners. Because large numbers of deaf puppies get put down, it is important to identify animals affected by the hereditary forms of deafness - both unilaterally and bilaterally deaf - and remove them from the potential breeding pool to reduce the number of future deaf animals. A bilaterally deaf puppy (can only hear out of one ear) is a genetically deaf puppy. A BAER (Brainstem Auditory Evoked Response) test to determine if hearing is normal in both ears or if the animal is deaf in one ear or in both ears. This test may be given to dogs after six weeks of age. Results of this test are valid for the dog's lifetime. It is recommended that all animals used for breeding should be BAER tested and have bilateral hearing. No bilaterally deaf dogs should ever be bred.

Anatomy and Physiology

The ear has three compartments:
(1) the outer ear, including the pina and the ear canal down to the tympanic membrane,
(2) the air-filled middle ear, including the three ossicles (malleus, incus, and stapes) and the connection to the pharynx (the auditory canal), and
(3) the fluid-filled inner ear or cochlea, which contains the organ of Corti and the initial portion of the auditory nerve. The cochlea is imbedded in solid bone, which protects it from all but the most severe trauma. Air vibrations (sound) in the ear canal vibrate the tympanum; these oscillations are amplified and transmitted through the three ossicles to a membranous opening (the oval window) into the cochlea. Oscillations of the oval window are transmitted through the fluids of the snail-shaped cochlea, causing deflections of small stereocilia on the sensory hair cells of the organ of Corti, hereby initiating nerve impulses into the brain.

The organ of Corti wraps around in the cochlea in a fluid-filled compartment. That fluid is high in potassium and low in sodium, similar to intracellular fluid. On the outer wall of this compartment is a special vascular bed, the stria vascularis, that is responsible for maintaining the high K+ and low Na+ levels. Part of the mechanism by which the stria for maintains these ionic levels involves pigment cells - melanocytes (Ref 1). This structure is a major target of mechanisms producing deafness.

Forms of Deafness

Forms of deafness can be sorted into three complimentary categories, each with two types. Deafness can be:

Inherited or acquired,

Congenital or later-onset, or

Sensorineural or conductive.

This results in eight possible combinations, such as acquired later-onset sensorineural deafness. Sensorineural (nerve) deafness is loss of auditory function because of loss of cochlear hair cells or auditory nerve neurons. Conductive deafness is blockage of sound transmission through the outer and/or middle ear without damage to the cochlea. Most congenital deafness is hereditary, and most later-onset deafness (as far as we know) is acquired, although there are human forms of inherited later-onset deafness. The most commonly seen forms of deafness in companion animals are (1) hereditary congenital sensorineural deafness, (2) acquired later-onset sensorineural deafness, and (3) acquired later-onset conductive deafness. Acquired conductive deafness may result from otitis externa, otitis media, excess cerumen production, or congenital defects in the ossicles (rare). Acquired sensorineural deafness may result from otitis interna, meningitis, drug toxicity (e.g., gentamicin), noise trauma, mechanical trauma, anoxia, anesthesia, or aging (presbycusis). In human audiology deafness is also classified as syndromic (associated with other disorders) or nonsyndromic. Allport syndrome, which presents with both sensorineural deafness and kidney disease, is an example. Finally, sensorineural deafness can be primary or secondary. Primary deafness results from destruction of hair cells in the cochlea without antecedent events. This occurs in hereditary deafness in Doberman pinchers, some forms of ototoxicity, and presbycusis. Secondary deafness occurs when hair cells die as a consequence of other damage in the cochlea, most commonly to the stria vascularis. This occurs in pigment-associated hereditary deafness and some forms of ototoxicity. Degeneration of the stria vascularis eliminates the high K+ levels in the fluids surrounding the hair cells, resulting in death of the hair cells and deafness. In pigment-associated hereditary deafness, the strial degeneration and hair cell death usually occur 2-4 weeks after birth.

Finally, sensorineural deafness can be primary or secondary. Primary deafness results from destruction of hair cells in the cochlea without antecedent events. This occurs in hereditary deafness in Doberman pinchers, some forms of ototoxicity, and presbycusis. Secondary deafness occurs when hair cells die as a consequence of other damage in the cochlea, most commonly to the stria vascularis. This occurs in pigment-associated hereditary deafness and some forms of ototoxicity. Degeneration of the stria vascularis eliminates the high K+ levels in the fluids surrounding the hair cells, resulting in death of the hair cells and deafness. In pigment-associated hereditary deafness, the strial degeneration and hair cell death usually occur 2-4 weeks after birth.

Hearing Testing

Behavioral testing of hearing is usually accomplished by making a sound outside the animal's visual field, or while the animal is sleeping, and observing for any response. Lack of a response may be interpreted as bilateral deafness, but it may instead reflect an overly stressed animal, an inattentive one, or one who has tired of this "game." A response may indicate that at least one ear hears, but deaf animals are especially attentive to other sensory cues, so it may respond from detection of visual, vibration, or air current stimuli. A unilaterally deaf animal responds to sound stimuli - the only behavioral deficit is a difficulty localizing the location of a sound source, so it may turn in the wrong direction to a stimulus. As a result, behavioral testing is subjective and limited in its reliability.

Electrodiagnostic testing provides an objective, non-invasive assessment of the presence or absence of auditory function, and is the gold standard of auditory testing in animals. It permits detection of unilateral deafness and can be performed in awake or anesthetized animals, but facilities for this testing may not be locally available. The hearing test is known as the brainstem auditory evoked response (BAER) [also known as the brainstem auditory evoked potential (BAEP) and auditory brainstem response (ABR)]. It detects electrical activity in the cochlea and auditory pathways in the brain in much the same way that an EKG detects the electrical activity of the heart (Refs 2-4).

The response waveform consists of a series of peaks identified with Roman numerals (top): peak I is produced by the cochlea and auditory nerve, and later peaks are produced within the lower brain (brainstem). The response from an ear that is deaf is an essentially flat line. In the sample recordings shown in Figure 2, Puppy 1 heard in both ears, Puppies 2 and 3 were deaf in one ear, and Puppy 4 was deaf in both ears. Because the response amplitude is very 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 (EEG, muscle activity, etc.).

The response is collected with a special computer through small needle electrodes placed under the skin of the scalp: one in front of each ear, one at the top of the head, and a ground. 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 irritation of wires hanging in front of its face. 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 usually is complete in 10-15 minutes. Sedation or anesthesia are not necessary unless the dog becomes extremely agitated, which can usually be avoided with patient and gentle handling. A printout of the test results, showing the actual recorded waveform, is provided at the end of the procedure. Because of the time after birth at which congenital deafness develops (2-4 weeks), testing can be performed any time starting at 5 weeks of age. Since there is no known hereditary late-onset deafness in dogs or cats, a single normal recording is good for the animal's life - barring error, any later hearing loss is assumed to be acquired and not hereditary. BAER tests do not provide a quantitative assessment of hearing loss (i.e., 40 dB) or frequency-specific results, but instead primarily are used to identify totally deaf ears. The click stimulus simultaneously activates receptors for most of the frequencies detected by dogs and cats; the click is similar to white light, which contains all colors.