Middle ear disease and implantation: age-related hearing loss

Introduction

Hearing loss is a common disorder in many dog breeds and auditory dysfunction and its clinical consequences can vary from mild to severe. Dogs with unilateral hearing loss can experience difficulty in localizing the source of a sound¹ . These animals are not suited as working dogs for blind and deaf people or rescue and police work² , but are not severely handicapped. In case of bilateral hearing loss however, dogs are unable to anticipate dangers such as motor vehicles and they may consequently fall victim to injury or death¹ . In addition, these animals seem to be easily startled and have an increased tendency to bite³ . Furthermore, deaf puppies require specialised training and are therefore usually euthanized³ .

Like in humans, hearing loss in dogs and cats can result from central or peripheral causes. Central deafness can theoretically result from a variety of retrocochlear lesions but is very rare in veterinary practice^1,4. Bilateral central deafness requires bilateral lesions of the auditory cortex or lesions of such a significant portion of the brainstem or mid-brain, that significant clinical signs beyond deafness are to be expected^1,4. Peripheral hearing loss in dogs is much more common and has been classified as inherited or acquired, conductive or sensorineural and congenital or late-onset^1,3,4. Conductive deafness results from a lack of presentation of sound to the inner ear, usually secondary to otitis externa or media or middle ear effusion, while sensorineural deafness occurs with abnormalities of the cochlear system, cranial nerve VIII or auditory pathways and higher brain centers⁵ . The most frequently observed forms are acquired conductive hearing loss as a result of chronic otitis externa and media or middle ear effusion (such as in Cavalier King Charles Spaniels and other brachycephalic breeds), congenital (inherited) sensorineural hearing loss (SNHL), and acquired SNHL including age-related hearing loss (ARHL) or presbycusis, noise-induced hearing loss (NIHL), and ototoxicity^5,6. With a complete physical exam including otoscopy, the differentiation between conductive and sensorineural hearing loss can usually be made5 . Advanced imaging with CT or MRI is necessary however to definitely rule out conduction deafness, middle ear abnormalities and for treatment planning. 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 structures. Functional abnormalities have to be diagnosed with hearing tests, such as brainstem-evoked response audiometry⁷. [...]