ARHL; Histology

ARHL is the most common form of acquired hearing loss in dogs.^1-4 The audiometric characteristics have been described in a cross-sectional and a longitudinal study.¹ Little information is available on the histological changes in the cochlea associated with ageing and on the relationship between these changes and the audiometric characteristics. The most widely referenced framework for describing the histological changes related to ageing in humans is that proposed by Schuknecht. He divided ARHL into four types: sensory (predominantly loss of outer hair cells [OHCs]), neural (loss of afferent neurons and spiral ganglion cells [SGCs]), metabolic (atrophy of stria vascularis), and cochlear conductive.⁵ Later, he added two more categories: mixed (sensory, neural, and metabolic) and indeterminate (no morphological findings at cochlear level). Although > 25% of cases can be classified as indeterminate, in most cochleas from aged humans there is a mixture of histopathological changes.⁵ Age-related loss of hair cells and SGCs has been reported in several animal species, including dogs. The findings in these studies imply a high-frequency hearing loss of the mixed type in dogs, comparable to that found in most cases of human ARHL. However, frequency-specific hearing thresholds were not determined in these studies. In a study of the effects of ageing on inner ear morphology in dogs and their brainstem responses to tone burst auditory stimuli, elevated hearing thresholds and cochlear lesions were found in all geriatric dogs, aged 11–14 years.² Together with loss of OHCs and IHCs there was a significant loss of SGCs and a reduction in the cross-sectional area of the stria vascularis (SVCA). The histological changes were primarily in the basal turn, which is consistent with the occurrence of the largest threshold shifts and lowest absolute thresholds in the middle- to high-frequency regions. It was concluded that the concomitant degeneration of OHCs and SGCs in the basal turn was primarily responsible for the elevated hearing thresholds, similar to findings in humans.² The auditory thresholds found in these dogs do not indicate whether histological changes were primarily sensory, neural, or strial. This is most likely because there were mixed lesions in all cases and the severity of OHC loss, IHC loss, SGC loss, and reduction in SVCA also varied in all cases. Individual audiograms did, however, reflect the severity and location of the histological changes. In general, histological changes were more extensive in dogs with more advanced hearing loss. It is concluded that tone audiograms can be used to diagnose and characterize ARHL in dogs, because they not only indicate the severity of hearing loss but also the extent and location of the histological changes.² This information is helpful in planning treatment of ARHL with hearing aids or middle ear or cochlear implants.

ARHL; Treatment Options

Providing a remedy for presbycusis makes an important contribution to improving the quality of life of geriatric patients. For hearing-impaired individuals, modern hearing aids, assistive listening devices, middle ear implants, cochlear implants, and brainstem implants are valuable aids for communication. Generally, in humans with mixed SNHL and average hearing thresholds of > 40 dB on the audiogram, amplification is indicated. Amplification is primarily accomplished with conventional hearing aids and most hearing impaired human patients are helped by these devices. Conventional hearing aids have been used in dogs, but not with great clinical success and there have been no clinical reports on their efficacy. Implantable hearing devices (middle ear and cochlear implants) have been developed for elderly people with moderate to severe SNHL who do not benefit from conventional external amplification. Such patients with moderate to severe SNHL are typically not considered good candidates for cochlear implantation because their relatively good residual hearing may be damaged by the procedure. Hence these implants are only indicated in patients with bilateral severe-to-profound hearing loss that is not improved by other means. Middle ear implants provide acoustic amplification of residual hearing and transmission of sound energy by coupling a vibratory element (implanted transducer) directly to the middle ear ossicular chain or to the round window membrane. The Vibrant Soundbridge (VSB) middle ear implant is the only middle ear-implantable hearing device with US Food and Drug Administration approval that is currently available.^3,4 Several studies have been reported on the successful short-, medium- and long-term use of it in patients with moderate to severe SNHL or both conductive and sensorineural hearing loss. A feasibility study demonstrated that the VSB can be implanted successfully in dogs as well.³

Vibrant Soundbridge Implantation

The two main components of the Vibrant Soundbridge Middle Ear Implant (MED-EL Corp.) are the Audio Processor (AP) and the Vibrating Ossicular Prosthesis (VORP).^3,4 The AP is the external part of the VSB, worn on the head. It is held over the implant by magnetic attraction and converts sound to a signal that is transmitted to the implant. The VORP is surgically implanted and contains a magnet within a receiving coil, a demodulator, a conductor link, and the Floating Mass Transducer (FMT). The VORP receives the signal from the AP to drive the FMT, which mechanically stimulates the ossicles or the round window membrane, the stimuli being perceived as sound.

The left side of the head and anterior cervical region were prepared for aseptic surgery. The dogs were placed in lateral recumbency.³ The first surgical site was created by a curvilinear skin incision from the lateral wing of the atlas to the transition between the vertical part of the external ear canal and the horizontal part, and then diagonally over the horizontal part.³ Using sharp and blunt dissection the ear canal was approached, freed from the surrounding tissues and followed to the bony acoustic meatus. The caudodorsal quadrant of the tympanic bulla was freed from its periosteum and a small hole was drilled in the tympanic bulla, to be used later for introduction of the FMT of the VSB. Under microscopic guidance the fibrous attachment of the ear canal to the bony acoustic meatus, together with the overlying epithelium, was sharply dissected from the bony acoustic meatus in the medial direction, toward the tympanic membrane, leaving the external ear canal intact. In opening the tympanic bulla, the tympanic membrane was reflected cranially to allow manipulation inside the bulla. The second surgical site was created by an incision cranial to the auricle in the skin overlying the cranial border of the parietal bone.³ The fascia of the temporal muscle was incised and a pocket was created within the muscle to incorporate the VORP of the implant. A deep subcutaneous tunnel was created between the two surgical sites for the wire connecting the VORP to the FMT. The fascia of the temporalis muscle was closed over the VORP in an interrupted pattern with PDS 3-0 to hold the implant in place. Again under microscopic guidance, the FMT of the VSB was inserted through the hole drilled in the tympanic bulla and manipulated via the bony acoustic meatus to place it in position near the round window niche. The FMT was positioned perpendicular to the round window and in direct contact with its membrane. An air-tight seal was achieved with tissue glue (Tissucol, Baxter B.V.). The tympanic membrane and ligament with epithelial lining were then carefully folded back into position within the bony acoustic meatus. The subcutaneous tissues were closed routinely. The two skin incisions were closed with continuous subdermal sutures of Monocryl 4-0.

Results

Three Beagle dogs with a mean age of 11.1 years and diagnosed with ARHL were assessed pre- and postoperatively by brainstem-evoked response audiometry (BERA) using tone bursts, by otoscopy, and by CT scans of the ears.⁴ A VSB middle ear implant was implanted unilaterally. Three months later the functionality of the implants was assessed by auditory steady-state responses (ASSRs). The VSB was implanted successfully in all 3 dogs. Recovery from surgery was uneventful, except for transient facial nerve paralysis in 2 of the 3 dogs. The implantation procedure did not affect residual hearing as measured by BERA. ASSRs showed improved hearing with a maximum mean decrease in threshold of 20.7, 13, and 16.3 dB at 1, 2, and 4 kHz, respectively. It was concluded that implantation of the VSB with the FMT positioned in the round window niche resulted in lower ASSR thresholds, but only at the higher gain settings of the AP. As in humans, a more powerful AP is required to treat SNHL of the mixed type exceeding 20 dB in dogs.⁴

References

1.  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.

2.  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 Intern Med 2009;23:536–543.

3.  Ter Haar G, Mulder JJ, Venker-van Haagen AJ et al. 2011. A surgical technique for implantation of the Vibrant Soundbridge middle ear implant in dogs. Vet Surg 40,340–346.

4.  Ter Haar G, Mulder JJ, Venker-van Haage, AJ, et al. Treatment of age-related hearing loss in dogs with the Vibrant Soundbridge middle ear implant: short-term results in 3 dogs. J Vet Intern Med 2010;24:557–564.

5.  Schuknecht HF, Gacek MR. Cochlear pathology in presbycusis. Ann Otol Rhinol Laryngol 1993;102:1–16.