Abstract
Age-related hearing loss (ARHL) is the most common cause of acquired hearing impairment in dogs. Diagnosis requires objective electrophysiological tests (brainstem evoked response audiometry [BERA]) evaluating the entire audible frequency range in dogs. In our laboratory a method was developed to deliver tone bursts ranging in frequency from 1 -
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32 kHz for frequency-specific assessment of the cochlea in dogs. Brainstem auditory evoked responses 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 (SPL) stimulus intensity in 10 clinically-healthy dogs and thresholds were determined for all specific frequencies tested. Marked differences in the thresholds for the different stimulations were found, the lowest being for the click and for 12 and 16 kHz toneburst stimulations. The thresholds for all other toneburst stimulations were significantly higher than for the click stimulation. A cross-sectional and a longitudinal study on age-related changes in audiograms in dogs, was conducted using the technique described above. Auditory thresholds at all frequencies tested were significantly higher in elderly than in young and middle-aged dogs. The longitudinal study demonstrated a progressive increase in thresholds associated with aging, starting around 8-10 years of age and being most pronounced in the middle- to high-frequency region (8-32 kHz). Having found audiometric proof of frequency-specific age-related changes in hearing in dogs, histological cochlear lesions in dogs with audiometric ARHL were studied. Cochlear lesions were found in all geriatric dogs. Not only was significant loss of spiral ganglion cells found, but also of outer hair cells and a reduction in stria vascularis cross-sectional area. The histological abnormalities found were primarily in the basal turn, which was consistent with the occurrence of the largest threshold shifts and lowest absolute thresholds in the middle- to high-frequency regions (8-32 kHz). Individual audiograms reflected the severity and location of cochlear lesions in all cases. The use of hearing aids in dogs has only anecdotally been mentioned but hasn’t met with great clinical success. The Vibrant Soundbridge (VSB) is an active, semi-implantable, middle ear hearing device that has been used in people who cannot benefit from conventional hearing aids. In a feasibility study in three cadavers we demonstrated that the VSB could be successfully implanted using a lateral approach to the tympanic bulla and with positioning of the floating mass transducer of the VSB in the round window niche. To evaluate the technique in dogs with ARHL, three dogs with audiometric ARHL were implanted unilaterally with a VSB implant. The implantation procedure did not affect residual hearing as measured with BERA. The functionality of the implants was demonstrated by the finding of decreased auditory thresholds on auditory steady-state evoked response testing. Measurable benefit from the VSB on hearing thresholds in the implanted ear was demonstrated. Owner satisfaction and results in client-owned dogs with ARHL have yet to be established.
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