Deafness is classified as inherited or acquired, conductive or sensorineural, and congenital or late onset. Acquired deafness may be either conductive or sensorineural, depending on the etiologic agent, which also affects the time of onset. Conductive deafness results from a lack of presentation of sound to the inner ear, usually secondary to otitis externa media. Sensorineural deafness occurs with abnormalities of the cochlear system, cranial nerve VIII, or auditory pathways and higher brain centers. Inherited deafness, ototoxicity, cochlear nerve degeneration, and presbycusis (age-related deafness) are forms of sensorineural deafness.
Diagnostic Principles Signalment can help to prioritize possible causes of deafness. Certain breeds have a high incidence of inherited deafness, and clinical signs are usually noticed at an early age if the deafness is bilateral. Most other forms of deafness occur as late-onset disorders. The history is important in establishing the nature of the deafness, because certain conditions or treatments may cause deafness (e.g. otitis externa media, ototoxic medications, head trauma, prior infectious diseases). The owner may note behavioral changes in the pet, and complete, bilateral deafness is usually easily identified. Astute owners may notice behavioral changes in puppies at an early age. Inherited deafness usually results in loss of hearing within 3 to 4 weeks of birth due to a degenerative process of the inner ear. The affected pup is often more aggressive than its littermates, because it cannot hear cries of pain during play. The puppy may also be more difficult to rouse and is more vocal than its littermates, especially when they are out of view of the affected pup.
Unilateral deafness is harder to identify, but the owner may report difficulty rousing the pet when it is sleeping in lateral recumbency or difficulty in the pet’s ability to orient to the origin of sounds. The physical, otoscopic, and neurologic examinations are important in differentiating peripheral from central disease and establishing whether conductive deafness is possible. The index of suspicion for conductive deafness is increased in dogs and cats with abnormalities of the external ear canal, tympanum, cranial nerve VII, or sympathetic innervation of the eye.
Abnormalities suggestive of central disease prompt more aggressive diagnostic testing such as advanced imaging and cerebrospinal fluid (CSF) analysis.
Hearing loss can be evaluated with the pet in the examination room. Different sounds can be used in an attempt to observe behavioral reactions to sound. A Preyer’s reflex (i.e. movement of the pinna in response to sound) is the minimal expected response. Care should be taken to avoid visual cues and air movement close to the animal’s head, which cause an apparent reaction to sound in an affected dog or cat. Unfortunately, pets may be stressed by the hospital environment and may not react to auditory cues even if hearing is normal.
Impedance audiometry and tympanometry
Impedance audiometry is based on the concept that the intensity of a sound wave is dependent on the size of the cavity in which it is generated and on the compliance of the cavity’s containing walls. The external ear canal is occluded for the test, and a sound wave generated. Changes in the pressure within the canal are measured to evaluate the compliance of the tympanic membrane. Accumulation of fluid in the middle ear canal, rupture of the tympanic membrane, immobility of the ossicles, and other causes of conductive deafness alter the results. Normal results in a deaf animal would be supportive of sensorineural deafness.
The acoustic, or stapedial, reflex test is also known as the acoustic decay test. A high amplitude sound is used to cause stapedial muscle contraction that protects the structures of the inner ear. This reflex alters tympanic compliance. The afferent (cranial nerve VII) and efferent limbs (cranial nerves V and VTI) must be intact for a normal reflex. A normal dog will maintain this reflex for a known period of time, but decay of the reflex occurs in dogs with lesions of cranial nerve VIII. Tympanometry is not commonly performed because it requires specialized equipment adapted from human kits. Ideally the generated sound affects the tympanic membrane perpendicular to its surface. This is difficult to obtain in the dog or cat due to the angle of the ear canal. The results are not sensitive or specific, but they are reliable for the diagnosis of tympanic membrane rupture.
Brain stem auditory evoked response
The brain stem auditory evoked response (BAER), or brain stem auditory evoked potential (BAEP), is an objective measure of hearing. The test identifies the presence or absence of hearing and progressive changes in hearing. Subjective determination of partial hearing loss is possible. Electrodes placed on standard sites of the head record responses to an auditory stimulus generated in one ear. Sound is generated as a series of clicks, and the contralateral ear is excluded by the presentation of white noise. The position of electrodes, stimulus intensity, and body temperature can also alter the waves produced. Electrode position is standardized and the sound generated, or stimulus intensity is presented over a range of decibels; the rate of presentation remains constant. The waves generated are subjectively and objectively evaluated. Wave I corresponds to cranial nerve VIII; wave II represents the cochlear nucleus and intracranial, extramedullary portion of cranial nerve VIII. Wave III corresponds to the dorsal nucleus of the trapezoid body, and waves IV and V originate in the rostral pons and caudal colliculi, respectively.
Wave latencies and amplitudes are used to assess hearing and conduction of impulses through the brain stem, making brain stem auditory evoked response testing valuable not only in the diagnosis of hearing loss but also for evaluation of brain stem lesions. Most pets do not require sedation or anesthesia for the procedure, which is beneficial because both cause changes in wave latency. Wave latencies and amplitudes vary with the intensity and rate of delivery of the sound. In general, amplitude increases and latency decreases with increasing sound intensity. Interpeak latencies are used to evaluate conduction in the brain stem, which should not change with stimulus intensity.
Conductive deafness reduces the intensity of sound reaching the inner ear. A lack of air-conducted hearing in the presence of bony-conducted hearing is diagnostic of conductive deafness during brain stem auditory evoked response. The hearing threshold is usually increased when sound is presented through air conduction, but bony-conducted hearing should remain intact. Increased hearing threshold may be accompanied by decreased wave amplitude and increased wave latency in cases of severe otitis externa. Sensorineural deafness alters the appearance of the brain stem auditory evoked response, depending on the site of the lesion. Alteration of waveforms, increased hearing threshold, or complete abolition of waveforms may be present in animals with sensorineural deafness.