Michael Podell,     Chicago, Illinois

Cecilia Friberg,     Chicago, Illinois

Causes

Deafness can result from either failure of conduction of sound waves through the outer and middle ear structures (conduction deafness) or failure of transformation of mechanical to electrical energy for signal generation through the auditory nerve to the brain (sensorineural deafness). The true prevalence or incidence of either cause of deafness in veterinary medicine is poorly documented. A summary of potential causes of deafness is listed in Box 112-1; some of these causes have been reported in the dog and cat.

The most common cause of conductive hearing loss in the dog is chronic otitis externa and otitis media, with associated ceruminous or purulent exudates; mucoid effusions; canal stenosis and hyperplasia; and changes to the bony and epithelial structures of the middle ear. Mild conductive hearing loss of up to 20 dB may occur with tympanic membrane rupture from acute effusions or trauma, or after myringotomy, but hearing should return as the membrane heals (Steiss et al, 1990).

Sensorineural hearing loss occurs most commonly as a congenital, and suspected inherited, disease (Strain, 2011). Congenital deafness is apparent early in a dog’s life, usually within the first months of life. The most common condition is a complete unilateral hearing loss, but may affect both ears. The cause of this hearing loss is a lack of formation or degeneration of small receptors in the inner part of the ear. Certain breeds are predisposed to congenital deafness, including, but not limited to, dalmatians, English setters, bull terriers, and Jack Russell terriers.

Ototoxicity is the result of chemical- or noise-induced toxic changes to inner and/or outer hair cells. Close to 200 ototoxic drugs and chemicals have been identified in a variety of mammalian species. In most cases, the hearing loss is irreversible and often progressive even after cessation of exposure to the toxic agent. The more common ototoxic agents include aminoglycoside antibiotics, loop diuretics, anticancer drugs, topical cerumenolytics (Mansfield et al, 1997), antiseptic preparations (Mills et al, 2005), and heavy metals (Merchant, 1994). Further insight into the molecular pathogenesis of ototoxicity may help lead to clinical management (Yorgason et al, 2006). Reversible noise-induced hearing loss also occurs after brief exposure to extremely loud sounds (over 100 dB), but hearing loss can become permanent with repeated and long-lasting exposure. Typically, the hearing loss is in the range of normal speech.

Presbycusis is an age-related hearing loss that is not associated with a specific pathologic process. Six categories of presbycusis have been described in people depending on the site of degeneration: sensory (predominant loss of outer hair cells), neural (loss of afferent neurons), metabolic (atrophy of the stria vascularis), mechanical (i.e., cochlear conductive), mixed (sensory, neural, and metabolic), and indeterminate. Sensorineural presbycusis is thought to be the most common form to occur in small animals, although in one study of 10 aged dogs age-related hearing loss was identified as a mixed type of presbycusis (ter Harr et al, 2009). Documentation on a larger-scale population is lacking.

In a longitudinal study of age-related hearing loss in dogs, frequency-specific brainstem auditory evoked response thresholds were determined once yearly or every other year for 7 years starting at around age 6. Age-related hearing loss was found to begin at around 8 to 10 years of age and was most pronounced in the middle to high frequency (8- to 32-kHz) range (ter Harr et al, 2008). Many owners of older dogs with suspected presbycusis-related hearing loss describe acute-onset deafness. A recent study of postanesthetic deafness reported a low prevalence based on data gathered from owners’ telephone communications, cases discussed on a veterinary information website, and a survey of general practice and dental specialists. Although prevalence was low, deafness was permanent in dogs and cats following anesthesia and occurred mainly in older dogs and cats undergoing dental and ear-cleaning procedures (Stevens-Sparks and Strain, 2010). This has been the author’s experience as well. Prospective studies should be conducted using brainstem auditory evoked response testing before and after ear cleanings and dental work performed under anesthesia to validate the prevalence of this hearing loss and identify the cause.