Anatomy & Physiology of the Inner Ear

The ear consists of 3 parts, the outer, the middle, and the inner ear. The outer ear and the middle ear conduct sound to the cochlea, acting as a mechanical transmission system, converting sound (air pressure waves) into fluid waves in the inner ear.^1,2 The inner ear is located within the osseous labyrinth of the petrous part of the temporal bone. The membranous labyrinth consists of 3 parts: the cochlea, vestibule, and the semicircular canals. The sensory transduction of sounds occurs in the organ of Corti, which is situated in the scala media and separated from the scala vestibuli and the scala tympani by Reissner's membrane and the basilar membrane respectively.² 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 brainstem.^1,2

Like all wave phenomena, sound waves have four major features: waveform, phase, amplitude and frequency which determine our perception of sound. 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. By changing the frequency and/or amplitude of a sound, a different stimulation of the ear and thus perception, will occur.²

Along the cochlea, all small groups of hair cells act like miniature amplifiers, they have their own specific frequency by which they are stimulated maximally.² The hair cells are thus a set of frequency filters, ordered spatially within the cochlea; 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 (caused by a sound wave with a higher amplitude), the more sensory receptor and neurons that are stimulated, leading to increased sound intensity. A sound with a low frequency causes displacement of a more apical situated portion of the cochlea.

Hearing Loss in Dogs

Peripheral hearing loss in dogs has been classified as inherited or acquired, conductive or sensorineural, and congenital or late onset.¹ The most frequently observed forms are acquired conductive hearing loss as a result of chronic otitis externa and media, congenital (inherited) sensorineural hearing loss (SNHL) and acquired SNHL including age-related hearing loss (ARHL) or presbycusis, noise-induced hearing loss (NIHL), and ototoxicity.¹ With a thorough physical examination including otoscopy, the differentiation between conductive and sensorineural hearing loss can usually be determined. Advanced imaging with CT or MRI is necessary, however, to definitely rule out conduction deafness and to devise a plan for treatment. Though essential for the diagnostic work-up of all patients with hearing disorders, these two techniques can only identify morphological abnormalities of the petrous bone, middle ear, and inner ear. Diagnosis of functional abnormalities requires hearing tests.

Behavioral hearing studies have been performed on a very small scale in dogs and although the sensation of hearing cannot be determined in this species, studies have shown that dogs can hear frequencies up to 45 kHz, which is considerably higher than heard by humans. Hearing can be assessed with greater objectivity by several methods but brainstem evoked response audiometry (BERA) is the technique most commonly used in veterinary medicine.³ With this technique, the consistent changes in electrical activity in the brainstem following auditory stimulation can be recorded from scalp electrodes.

Diagnosis with Brainstem-Evoked Response Audiometry

Auditory stimulation evokes electrical responses in the auditory pathway in a consistent manner.^1,3 When a nerve impulse is generated in the cochlea, the signal travels along the auditory nerve to the cochlear nuclei in the brainstem. From the cochlear nuclei, many projections lead to other nuclei in the brainstem and ultimately to the primary auditory cortex.^1-3 These electrical changes can be recorded from scalp electrodes. When amplified and averaged, they typically consist of a complex wave having several distinct peaks, usually 5–7, occurring during the first 10 ms after the presentation of a transient sound, numbered with Roman numerals. Most authors use the following criteria for labeling the wave peaks in dog: wave peak I is the first recognizable wave with a positive deflection and wave peak V is the positive peak occurring immediately before the deep negative trough in the second half of the recording.³ In dogs, factors known to influence the latencies and amplitudes of the elicited waves are related to the gender, age, head size, and body temperature of the animal being examined and to the stimulation and recording protocols, whereas the effect of anesthesia on the evoked responses is generally considered to be negligible.³ Most authors have used click stimulation to assess auditory function. Click stimuli are very brief (0.1 ms) and most of their energy is in the range of 500 to 4000 Hz. Click stimulation is valuable for differentiating sensorineural from conductive deafness, detecting brainstem lesions, and for intraoperative monitoring.³ In dogs, BERA with click stimulation has been used most extensively and successfully in the diagnosis of congenital forms of SNHL. No frequency-specific thresholds are required, since hearing loss is complete in these cases.¹ Frequency-specific information is needed however to determine the extent of loss of auditory function in ARHL, NIHL, and ototoxicity.³ Tone burst stimulation (tones with short duration) is relatively easy to perform, and has been shown to provide reliable information about pure-tone thresholds. In our laboratory, a method was developed to deliver tone bursts ranging from 1–32 kHz for frequency-specific assessment of cochlear function in dogs.³ Thresholds of brainstem auditory evoked responses to click stimulation(CS) and to different tone burst stimulations (TS; 1–32) were determined in a group of healthy dogs. There were marked differences in the thresholds for the different stimulations, the lowest being for CS and for TS at 12 and 16 kHz, demonstrating that the highest sensitivity of the dog's ear is between 12 and 16 kHz.³

ARHL

ARHL is the most common form of acquired hearing loss in dogs, but little is known about its prevalence, etiology, and audiometric characteristics.^4,5 In both dogs and humans presbycusis reflects the cumulative effects of heredity, disease, noise, and ototoxic agents superimposed upon those of the ageing process itself.^4,6 Cross-sectional studies in people of differences between age groups have shown that pure-tone hearing thresholds increase with age, particularly at higher frequencies.⁶ Longitudinal studies provide a better description of the course of changes with age within individuals and they show that a significant reduction in hearing capacity occurs from the age of 60 years onward.⁶ This hearing loss begins at higher frequencies (6–16 kHz), but gradually progresses to encompass the entire frequency range. Longitudinal and cross-sectional studies of ARHL have also been reported for various animal species, including dogs.⁴ A cross-sectional study revealed significantly higher thresholds at all frequencies tested (1–32 kHz) in geriatric dogs as compared to young and middle-aged dogs.⁴ The highest absolute thresholds were found in the middle- to high-frequency region (8–32 kHz) and the increase in thresholds was significantly greater in this region than in the low-frequency region (1–4 kHz). The audiograms of the dogs in a longitudinal study reveal a progressive increase in hearing thresholds with ageing, starting at 8–10 years.⁴ The effect is again most pronounced at the middle to high frequencies (8–32 kHz). While there were considerable differences in the severity of hearing loss among these dogs, statistical analysis of the average increase in thresholds revealed that the thresholds at 8, 12, 16, 24, and 32 kHz were significantly higher at a mean age of 12 years than at a mean age of 6 years.⁴

References

1.  Strain GM. Aetiology, prevalence and diagnosis of deafness in dogs and cats. Brit Vet J 1996;152:17–36.

2.  Purves D, Augustine GJ, Fitzpatrick D, et al. The auditory system. In: Purves D, Augustine GJ, Fitzpatrick D, et al. Neuroscience, 2nd Ed. Sinauer Associates, Sunderland, MA, 2001:275–296.

3.  Ter Haar G, Venker-van Haagen AJ, de Groot HNM, et al. Click and low-, middle, and high-frequency toneburst stimulation of the canine cochlea. J Vet Intern Med 2002;16:274–280.

4.  Ter Haar G, Venker-van Haagen AJ, van den Brom WE, et al. Effects of aging on brainstem responses to toneburst auditory stimuli: a cross-sectional and longitudinal study in dogs. J Vet Intern Med 2008;22:937–945.

5.  Ter Haar G, de Groot JCMJ, Venker-van Haagen AJ, et al. Effects of aging on inner ear morphology in dogs in relation to brainstem responses to toneburst auditory stimuli. J Vet Int Med 2009;23:536–543.

6.  Gates GA, Mills JH. Presbycusis. Lancet 2005;366:1111–1120.