Classification of deafness

Peripheral deafness (or hearing loss), defined as that being due to abnormalities outside the central nervous system, can be characterized by three pairs of descriptors: inherited or acquired, congenital or later-onset, and sensorineural or conductive. This results in eight classifications of deafness, but only three are commonly seen in dogs and cats: inherited congenital sensorineural, acquired later-onset sensorineural, and acquired later-onset conductive. Inherited congenital sensorineural deafness is usually, but not always, associated with pigmentation genes responsible for white in the coat. Acquired later-onset sensorineural deafness is most often associated with ototoxicity or aging-related hearing loss (presbycusis), but can also result from otitis interna, noise, and other causes. Acquired later-onset conductive deafness is associated with chronic otitis externa and media or excess cerumen production. No forms of inherited late-onset deafness, either sensorineural or conductive, have been identified in dogs or cats, but the conditions are seen in humans. Acquired congenital deafness, either sensorineural or conductive, may result from malformations, intrauterine infections or drug toxicity, or anoxia, but these are not common. Causes of acquired deafness are listed in Table I.

Central deafness can theoretically result from a variety of retrocochlear lesions, but in practice is rare. The auditory pathways commingle information from both ears from the level of the cochlear nuclei rostrally, so it is difficult to produce total unilateral central deafness, and bilateral central deafness requires lesion of a significant portion of the brainstem or midbrain, or a bilateral lesion of auditory cortex. Significant signs beyond deafness would accompany lesions of this sort.

Deafness can also be classified as either syndromic or non-syndromic. It has not been established whether deafness in dogs and cats associated with white hair coat color and blue irises can be considered a form of syndromic deafness. The deafness seen in Dalmatians and white cats is often likened to Waardenburg syndrome (Foy et al., 1990; Baldwin et al., 1992), a dominantly-inherited condition in humans with incomplete penetrance consisting of deafness, blue irises, a stripe of white in the hair and beard with premature graying, and minor structural facial deformities.

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Deafness in Dogs & Cats