Anatomy & Physiology

The inner ear is located within the osseous labyrinth of the petrous part of the temporal bone. The membranous labyrinth consists of three parts: the cochlea, vestibule and the semicircular canals. The sensory transduction for hearing occurs in the organ of Corti, which is situated in the scala media en separated from the scala vestibuli and the scala tympani by reissner's membrane and the basilar membrane respectively. It is here where the hair cells interact with supporting elements to convert fluid waves into the bending of hair bundles and resultant ion influxes.

The release of neurotransmitter from the basal portions of stimulated hair cells leads to neural impulses, action potentials. Once the nerve impulse is generated in the cochlea, the signal travels along the acoustic nerve to the cochlear nuclei. From here, many projections lead to the olivary nuclei at the same level. The axons of the olivary neurons project via the lateral lemniscus to the inferior colliculi, where they synapse on neurons that project to the primary auditory cortex.

Sounds

Like all wave phenomena, sound waves have four major features: waveform, phase, amplitude and frequency. These four determine our perception of sound, especially the frequency and amplitude of the waves. The frequency of a sound, expressed in cycles per second or Hertz roughly corresponds to the pitch of a sound, whereas the amplitude, usually expressed in decibels, determines the loudness of a sound. The receptors, the hair cells, act like miniature amplifiers and provide a maximal electrical response when vibrated at a particular frequency by the fluid waves of the inner ear.

Along the cochlea, all small groups of hair cells have their own specific frequency by which they are stimulated maximally; those with high-pass frequencies occupy the base and those with low-pass frequencies occupy the apex. Therefore, a sound with a high frequency will cause maximal displacement of a portion of the basilar membrane at the base of the cochlea. The greater the displacement of the basilar membrane, the more sensory receptor and neurons that are stimulated, leading to increased sound intensity. A sound wave with a higher amplitude leads to a greater basilar membrane displacement. A sound with a low frequency causes displacement of a more apical situated portion of the cochlea.

Hearing Assessment

Several methods have been employed to test hearing ability in dogs, ranging from behavioural studies to measurement of electrical responses after auditory stimulation, using impedance audiometry (tympanometry, acoustic reflex testing), evoked response audiometry (brainstem (BAER) and middle latency (MLAER) auditory evoked responses), and cochlear microphony.

The brain responds to auditory stimuli by consistent changes in electrical activity and these changes can be recorded from scalp electrodes. It is generally agreed that the recorded brainstem evoked potentials represent the passage of auditory input from the inner ear through the various structures of the brainstem towards the auditory cortex. The last two decades, brainstem auditory evoked responses have been used increasingly to test hearing ability in veterinary medicine. The acoustic signal usually consisted of a click stimulus, which stimulates a large part of the cochlea.

Brainstem evoked response audiometry using clicks will suffice for differentiating neurologic from conduction deafness and is of use in assessing some brainstem pathologic changes. Frequency-specific information, however, is needed in assessing the extent of neurologic deafness, e.g., noise-induced deafness, deafness caused by ototoxicity and presbycusis, which can all be partial and frequency-specific.

Results of Hearing Assessment using Brainstem-evoked Response Audiometry

In our laboratory a method was developed to deliver tone bursts ranging in frequency from 1-32 kHz for frequency-specific assessment of the canine cochlea.

Brainstem auditory evoked responses (early latency responses, 0-10 ms) to a click (CS) and to 1, 2, 4, 8, 12, 16, 24, and 32 kHz tone burst stimulations (TS) were compared at 80 dB sound pressure level stimulus intensity in 10 clinically-healthy dogs, 3.5 to 7.0 years of age (mean, 5.7 years) weighing 12.5 to 21.3 kg (mean, 17.8 kg). The responses were obtained with the animals under a light plane of anaesthesia.

All stimulations yielded a 5-7 positive wave pattern, with the exception of the 1 kHz TS, which evoked a frequency-following response. Thresholds were lowest for CS, 12, and 16 kHz TS.

We concluded that specific tone burst stimulation of the canine cochlea using low to high frequencies yields reproducible results and that results are in agreement with results of behavioral studies on frequency thresholds and hearing sensitivity in dogs. To reliably determine the extent of neurological damage due to ototoxic drugs, presbycusis, or noise, frequency-specific assessment should be done. Our report provides a normative database for parameters necessary to evaluate these frequency-specific hearing losses in dogs.

Age-related Hearing Loss

Deafness is classified as inherited or acquired, conductive or sensorineural and congenital or late onset. Conductive deafness results from a lack of presentation of sound to the inner ear, usually secondary to otitis externa or media and therefore amenable to therapy. Sensorineural deafness occurs with abnormalities of the cochlear system, cranial nerve VIII or auditory pathways and higher brain centers. The most common forms of sensorineural deafness are the congenital forms of deafness, deafness as a result of ototoxicity and presbycusis or age-related hearing loss. To evaluate hearing in old dogs, animals older than 12 years of age were examined with brainstem-evoked response audiometry using the same method as in the previously mentioned group. The results showed that their hearing range did not differ much from the first group, but their thresholds were significantly higher, especially in the high-frequency area. The histology of the cochlear changes was studied and preliminary results show profound hair cell loss and neuronal loss, most prominent in the basal coils of the cochlea, comparable to presbycusis in humans.

There is no cure for sensorineural hearing loss, but ongoing research in human and veterinary medicine regarding middle ear implants, cochlear implants, neurotrophic factors and stem cell research is yielding promising results.