Deafness in small animals: a retrospective study of 122 cases.

1. 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

2. 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).

3. 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).

4. 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.

5. 1 uV/Div; 1 ms/Div.

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

7. 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.

8. Materials and Methods 360 dogs, 6 cats and 11 ferrets referred in the period 1998-2007. 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.

9. 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.

10. 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 (100-3000 Hz) and averaged 500-1000 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.

12. 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.

13. 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.

14. 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.

15. 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.

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

17. 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.

18. 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?).

19. 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. ICS deafness Right Left

20. 60 days old female Dalmatian puppy. Left ear of a bilaterally deaf. 105 dB/NHL. 0.62 uV/Div; 1 ms/Div. ICS deafness Left N3pot.

21. 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.

22. 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. ALS deafness Left dB 90 70 50 40 Acoustic 90 dB Bone

23. 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.

24. 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. ALC deafness Left ac. Right ac. Left bone Right bone

25. 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!

26. 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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