Inherited congenital sensorineural (ICS), acquired later-onset sensorineural (ALS), and acquired later-onset conductive (ALC) are the types of hearing loss usually found in clinical practice in companion animals. The aim of this study was to retrospectively evaluate the abnormalities found in dogs, cats and ferrets referred to the Veterinary Faculty of Parma for evaluation of auditory function tested with brainstem auditory evoked potentials (BAEPs).

The medical records and BAEPs of 360 dogs, 6 cats and 11 ferrets referred in the period 1998-2007 for auditory evaluation were reviewed. Inclusion criteria were: the presence of unilateral or bilateral abnormalities in BAEPs consistent with partial or complete hearing loss. One hundred-twelve dogs, 6 cats and 4 ferrets met the inclusion criteria.

All animals were tested under sedation using auditory stimuli of intensities varying from 90 to 105 dB NHL. Bone stimuli were also used to identify conductive deafness. Tracings were recorded, amplified and averaged using standard methodologies. Auditory threshold was established when needed to confirm partial deafness.

ICS deafness was the prevalent abnormality among dogs (94/112), most of these subjects were puppies of predisposed breeds. Forty-nine of these dogs were unilateral deaf, 45 bilateral deaf. In 20.2% of the dogs (18/112) an acquired form of hearing loss was found. Eleven of these subjects had ALS deafness (1 unilateral, 10 bilateral) and 7 had ALC deafness (4 unilateral, 3 bilateral). In 4 of the cats of this study a diagnosis of ICS deafness was made. Three of these cats were Norwegian Forest and 1 a white DSH (3 unilateral and 1 bilateral). The remaining 2 cats had ALC deafness (1 unilateral and 1 bilateral). All 4 deaf ferrets had an ICS form (2 unilateral and 2 bilateral).

Hereditary cochlear degeneration is common in subjects of predisposed breeds and generally associated with some pigmentation patterns. In the present study ALS hearing loss could be demonstrated only in dogs, and was generally a consequence of ototoxicity or presbycusis. ALC deafness is usually noted by owners when dramatic abnormalities of external/middle ear anatomy are present. In this study ALC hearing loss was associated with chronic otitis in dogs and inflammatory polyps in cats. More data are needed to better define the prevalence of different types of deafness in companion animals.