Deafness in small animals: a retrospective study of 122 cases. Bianchi E., Callegari D., Ravera M., Dondi M. Unit of Internal Medicine, Animal Health Department University of Parma - Italy

Introduction  Auditory abnormalities are common in companion animals.  One or both ears affected.  Complete or partial hearing loss.  Classification: inherited or acquired, congenital or later-onset, sensorineural or conductive. Mostly inherited congenital sensorineural (ICS), acquired later-onset sensorineural (ALS), and acquired later-onset conductive (ALC).

Introduction  Objective auditory investigation: Brainstem auditory-evoked potentials (BAEPs), impedence audiometry (Tympanometry etc.), transient evoked otoacustic emissions.  BAEPs: recording of electrical activity in the cochlea and auditory pathway evoked by acoustic or vibratory stimuli.  The response waveform consists of peaks produced by the cochlea (I) and nervous structures of brainstem (II-V).

Introduction: BAEPs  Light sedation is often required in animals for obtaining clear tracings.  BAEPs can be recorded using similar methodologies in all companion animals.  Waveforms are similar in different species.

1 uV/Div; 1 ms/Div.

Green Iguana (Iguana iguana) Bearded Dragon (Pogona vitticeps) 0.62 uV/Div; 1 ms/Div. 1 uV/Div; 1 ms/Div.

Aim  Review medical records and BAEPs tracings of small animals referred to the Teaching Hospital for evaluation of auditory function or with a history of suspect hearing loss.  Establish the prevalence of different types of deafness present in these animals.

Materials and Methods  360 dogs, 6 cats and 11 ferrets referred in the period  Medical records and BAEPs evaluated.  Inclusion criteria: unilateral or bilateral abnormalities in BAEPs consistent with partial or complete hearing loss.  Exclusion criteria: CNS signs, BAEPs as diagnostic tool for lesion of the brainstem.

Materials and Methods  112 dogs, 6 cats and 4 ferrets met the inclusion criteria.  All subjects underwent physical and otoscopic evaluation.  BAEPs tests were performed under sedation.  When needed, laboratory tests and diagnostic imaging evaluation were also included.

Materials and Methods: BAEPs  Recording: Needle electrodes inserted at the vertex (non-inverting input), at the level of the ipsilateral mastoid or of the 2nd cervical vertebra (inverting input), and at the occipital protuberance (ground).  Signal was amplified, filtered ( Hz) and averaged times. Artifact rejection was automatically performed.  At least 2 trials for each ear were performed to test waveform consistency.  Amplitudes and latencies of each wave measured.

Materials and Methods: BAEPs  Stimulation: 90 – 105 dB NHL alternate clicks delivered monaurally through insert earphones. The untested ear was masked with a wide band noise (- 40 dB).  Bone stimuli were also used if conductive deafness was suspected.  Auditory threshold was established when needed to confirm partial deafness (5-10 dB steps).  ALS + ALC: main cause of hearing problems.

Results: dogs  94/112 (83.9%): ICS deafness.  Puppies or adults of breeds predisposed to congenital deafness.  65 Dalmatians, 7 Eng. Setter, 6 Dogo Argentino, 3 Eng. Bulldog, 2 Fr. Bulldog, 2 Border Collie etc.  49 unilateral, 45 bilateral deaf.

Results: dogs  18/112(16.1%): acquired hearing loss.  11/112 (9.8%): ALS deafness.  1 unilateral, 10 bilateral.  8 partial, 3 complete deafness.  5 ototoxicity, 6 presbycusis.  7/112 (6.3%): ALC deafness.  4 unilateral, 3 bilateral.  Severe chronic otitis (External/middle ear).  Partial deafness.

Results: cats  4/6 (66.6%): ICS deafness  3 Norvegian Forest, 1 white DSH.  3 unilateral and 1 bilateral.  2/6 (33.3%): ALC deafness.  DSH with inflammatory polyps.  1 unilateral and 1 bilateral.

Results: ferrets  All 4 ferrets had ICS deafness.  2 unilateral and 2 bilateral.

Discussion  ICS most common type of deafness.  Assessment of the hearing status: subjects of breeds affected by hereditary cochleo-saccular degeneration.  Usually breeds with white pigmentation: dogs (piebald or merle genes), cats (white gene), ferrets (different coats).  Absence of melanocytes in the stria vascularis, degeneration of the stria vascularis, collapse of Reissner’s membrane and cochlear duct, degeneration of hair cells, collapse of the saccule.

Discussion  ICS deafness easy to recognize.  Usually complete (unilateral or bilateral).  Flat waveforms with acoustic and bone stimuli.  Sometimes a negative peak (N3 potential) present at high intensities (vestibular potential?).

ICS deafness  18 months old male Ferret.  Normal right ear.  Flat tracing in left ear.  Dx: cochleo-saccular degeneration.  100 dB NHL  1 uV/Div; 1 ms/Div. Right Left

ICS deafness  60 days old female Dalmatian puppy.  Left ear of a bilaterally deaf.  105 dB/NHL.  0.62 uV/Div; 1 ms/Div. Left N3 pot.

Discussion  ALS deafness: 2nd most common type in dogs.  Not recognised in cats and ferrets.  Causes: ototoxicity (aminoglycosides etc.), presbycusis.  Presbycusis very common in geriatric dogs.  Partial or complete hearing loss.  BAEPs: delayed I wave, decreased amplitudes, increased threshold, increased slope of wave V latency-intensity curve.  No improvement with bone stimulation.

ALS deafness  6 years old, male Pug dog.  Treated for 20 days with Amminosidine for Leishmaniosis.  Bilaterally increased threshold (approx 50 dB NHL).  No improvement with bone stimulation.  Dx: Ototoxicity.  0.31 uV/Div; 1 ms/Div. Left dB Acoustic Bone 90 dB

Discussion  ALC deafness: recognised in dogs and cats.  Causes: severe otitis externa/media in dogs, inflammatory polyps in cats.  Usually partial hearing loss.  BAEPs: delayed I wave, decreased amplitudes, increased threshold, wave V latency- intensity curve shifted to the right.  Marked improvement with bone stimulation.

ALC deafness  3 years old, male, DSH cat.  Inflammatory polip in right ear.  Abnormal waveform in right ear with acoustic stimuli.  Normal waveform in both ears with bone stimuli. Left ac. Right ac. Left bone Right bone

Conclusions  Hearing loss is a common problem in companion animals.  Dogs more affected / Hearing loss easier to recognize.  Many breeds affected by congenital deafness.  Increased average life expectancy (presbycusis, chronic otitis etc.).  BAEPs: easy to perform, fast and sensitive test.  Normal with cortical deafness!

Conclusions  Limitations of the study:  Small number of animals (especially cats and ferrets).  Test mainly performed in litters of breeds predisposed (Dalmatians).  Future plans:  Awaken more breeders to congenital deafness.  New therapies for sensorineural deafness (cochlear implants).

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