Schusser, Gerald Fritz, Scheidemann, Wolfgang, Blanke, Annemarie, Recknagel, Stephan, Dögl, Tina, Röhricht, Angela, and Fuchs, Michael
The evaluation during the neurologic examination of hearing in horses generates only subjective values because the horse is turning the head and moves the ears towards the noise. Therefore the brainstem auditory evoked response (BAER) technique is very useful for evaluating the hearing in horses objectively. The aim of this study is to measure the BAER in horses with altered behaviour or lack of responsiveness to loud sounds. This has been effected on older horses, also on a horse with ceruminal occluded external ear canals and on a horse with colic surgery and bad prognosis during general anesthesia and euthanasia using T61®. Horses were sedated using detomidine (20 μg/kg bw i.v.) and placed in a stock or silent area. Needles were inserted subcutaneously on the median place of the vertex between the lateral canthus of the eyes, on the ventrocaudal area of the osseus part of the external ear canal on the top of Processus retrotympanicus on the left and right side, on the midline between the Os occipitale and Processus spinosus of the second cervical vertebra (ground). A click stimulus at 80 and 60 dB normal hearing level was applied ipsilateral and noise-masking sound at –35 dB normal hearing level was applied contralateral. An average of 2 000 responses of BAER was recorded over a 10 milliseconds epoch operated by the AEP-diagnostic system Corona. The latencies of peaks I, III and V were measured and the interpeak latencies between the waves I-III, III-V and I-V were calculated and saved by the AEP-diagnostic system. These measurements of five horses with altered behaviour and of one horse during euthanasia were compared with a control group of normal Warmblood horses (12 mares, 2 geldings, 2 stallions; mean age of 15 years, min 3 and max 19 years). The external ear canals were examined endoscopically before and they had normal findings (grade I) and the tympanic membranes were intact and transparent. The tympanohyoid and stylohyoid in the guttural pouches were normal too. The latencies in ms and interpeak latencies of the control group stimulated with 80 and 60 dB nHL ipsilateral are: peak I (left 1.49 ± 0.05, 1.79 ± 0.13; right 1.53 ± 0.09, 1.86 ± 0.11), III (left 2.81 ± 0.16, 3.27 ± 0.18; right 2.86 ± 0.20; 3.25 ± 0.18), V (left 4.60 ± 0.21, 5.05 ± 0.27; right 4.62 ± 023, 4.94 ± 0.26); I-III (left 1.32 ± 0.15, 1.48 ± 0.24; right 1.33 ± 0.15, 1.38 ± 0.20), III-V (left 1.79 ± 0.14, 1.78 ± 0.19; right 1.76 ± 0.24, 1.69 ± 0.28), I-V (left 3.11 ± 0.20, 3.26 ± 0.28; right 3.09 ± 0.19, 3.07 ± 0.27). A 20 years old Haflinger mare turned her head only to the left side when the owner called her. The peak amplitudes (I, III, V) and interpeak latencies (I-III, I-V) were reduced ipsilateral of the right ear. These are signs of a conductive hearing deficit and cochlear dysfunction or neural problem. The 25 years old Warmblood gelding which turned his head only towards very loud noises had reduced peak amplitudes and increased latencies (I, III, V) ipsilateral left and right. These are signs of conductive hearing deficit on both sides. No BAER (80 and 60 dB nHL) were detected ipsilateral on the left side on a 1.5 years old Warmblood mare which reacted only to very loud noises. These diagnostic findings explain deafness on the left ear. A five months old Warmblood stallion which responded only to very loud noises too, had prolonged latencies (I, III, V; 80 dB nHL) and unchanged interpeak latencies ipsilateral on the right ear. No peaks were detected stimulated by 60 dB. These are findings of a severe hearing deficit and cochlear dysfunction on the right ear. A five years old Warmblood mare had an altered behaviour with startling due to environmental noises. A complete obstructed osseous part of the right ear and a partial obstructed osseous part of the left external ear canal were seen endoscopically. Decreased peak amplitudes stimulated by 80 dB nHL and no peaks stimulated by 60 dB ipsilateral on the right side in comparison to the normal horse are findings with severe conductive hearing deficit on the right ear based on the complete obstruction. The applicated drug T61® used for euthanasia (30 ml/250 kg bw i.v.) under general anesthesia of a 19 years old Shetland pony gelding which had colic surgery and a very bad prognosis induced a complete loss of BAER which was a diagnostic finding of brain death within three minutes. In conclusion the testing of BAER in horses is an objective measurement of hearing and evaluates the physiology of the auditory pathway. The endoscopy of the external ear canal has to be done generally before BAER measurements! BEAR testing is indicated in horses with altered behaviour, head tilt, head shaking, temporohyoid osteoarthropathy, guttural pouch empyema, othematoma, melanoma, sarcoid or squamous cell carcinoma in the pinna or external ear canal, after surgery on the pinna or in the external ear canal, after treatment of Otitis purulenta externa or after surviving an encephalitis (West-Nile-Virus-, EHV-1-, Sarcocystis neurona-Infection). The BAER testing ought to be included into the pre-purchase exam plan/requirements! [ABSTRACT FROM AUTHOR]