B,. vet . /. (1992) . 148, 2 7 5
GUEST EDITORIAL BRAINSTEM AUDITORY EVOKED POTENTIALS IN VETERINARY MEDICINE
Until recent years, the assessment of auditory function in animals relied on the evaluation of a behavioural response to sound stimuli produced outside of the visual fields . The interpretation of the animal's response was highly subjective and could not reliably detect a unilateral deficit . Little importance was attributed to loss of auditory function in an animal except to the extent that it might indicate a brainstem lesion . Nevertheless, the auditory sense serves an important function in the performance of everyday behaviour in most domestic and wild animals, and its loss jeopardizes the domestic animal's ability to perform its `societal' duties and greatly endangers the wild animal's very life . House pets are generally able to function quite satisfactorily with unilateral or even bilateral deafness, but their owners are often distressed by empathy over a perceived disruption of the animal's quality of life as well as the loss of whatever guard function the animal may have provided . Advances in the 1960s in electronics and computer technology resulted in the development of instrumentation able to provide objective evaluation of auditory (as well as visual and somatosensory) function in a clinical setting with a minimum of technical skill and patient preparation and cooperation . The resulting test methodology, known variously as the brainstem auditory evoked potential (BAEP), brainstem auditory evoked response (BAER), or auditory brainstem response (ABR), was enthusiastically adopted in many quarters of human medicine . Its value was soon clearly evident and the requisite instrumentation became commonplace in hospitals and speciality practices . In brief, scalp electrodes detect small but robust and highly repeatable electrical potentials in response to auditory stimuli, either air- or bone-conducted, with the peaks in the response corresponding to loci in the ascending auditory brainstem pathways . Although the anatomical correlates of the later peaks of the BAEP are controversial, the accepted association of the first peak with cochlear structures, and the second and third peaks with the cochlear nucleus and the superior olivary nucleus allow confirmation of peripheral sources of deafness and permit indirect localization of brainstem lesions impinging on the brainstem auditory nuclei . The human audiology, neurology, and neurosurgery literature is replete with resources providing technical detail for utilization and application of audit )rti , evoked potentials in appropriate clinical settings (Chiappa, 1990) . These have resulted from literally thousands of investigations in hundreds of laboratories over several decades, from which it has been possible to optimize stimulus and recording parameters and to validate the disease states for which the tests provide
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useful diagnostic and prognostic information and disease progression tracking . Veterinary medicine has been slower to adopt the various evoked potentials, witli reports first appearing in the veterinary literature about 1980, in part due to the cost of the instrumentation and the effort of documenting normative responses in different domestic species . Slower still has been the validation of the use of these techniques with different nervous system diseases . Clinical assessment of a BAEP recording relies on measurement of peak latency times and amplitudes (Sims, 1988) . Due to differences in brain geometry, neural pathway lengths, skull thickness, and other factors, the normal population responses will differ by species, so that utilization of the test first requires acquisition of species population norms . These studies have been most extensively performed with dogs, where adult (see, for example, Sims & Moore, 1984) and developmental (Strain el (I t., 1991) data are established . Data on a much more limited scale have been reported for the adult and developing cat (Jewett & Romano, 1972 ; Arbor & Starr, 1980), cow (Strain et al., 1989a, b), and horse (Marshall, 1985 ; Steiss et al., 1991) . The equine data reported by Mayhew & Washbourne in this issue, building on previous reports of equine BAEPs, puts the diagnostic capabilities for the BAEP in horses on a par with, or exceeding, its use in dogs . A high degree of similarity exists among the BAEPs recorded from all examined mammalian species, as well as for lower species . Thus, as a diagnostic tool the BAEP has an enormous capacity for contributing to the practice of veterinary medicine . Cost may preclude its use in most private practices, but the increasing recognition of its value mandates its availability for referrals at schools of veterinary medicine and neurology speciality practices . An abnormal BAEP is not, in and of itself, diagnostic for any disorder except possibly deafness . Instead, it contributes to the clinical data hase, as does a CSF analysis or blood lead level measurement, increasing the likelihood of a correct clinical diagnosis . As might be expected, the greatest number of veterinary clinical applications have been in assessing deafness, either congenital (Sims & ShullSelcer, 1985 ; Marshall, 1986 ; Strain et al., 1992) or acquired, especially due to ototoxic drugs (Morgan el al., 1980 ; Crowell et al., 1981 ; Marshall et al., 1981 ; Nostrandt et al., 1991 ; Merchant et al., 1992) . This application has enabled the identification of a 30% incidence of congenital deafness (unilateral or bilateral) in Dalmatian dogs (Holliday et al., 1992 ; Strain el (il ., 1992) . Applications with CNS diseases have been limited, but include scrapie (Strain et al., 1986) and brain abscesses (Strain et (il., 1987) . Clearly, possible changes in the BAEP associated with the many CNS diseases of domestic animals are yet to he examined . Among the diseases that might profitably be examined are equine protozoal myelitis and other parasitic and mycotic diseases, bovine and other spongiform encephalopathies, brain tumours, demyelinating diseases, encephalitis, distemper, rabies, feline infectious peritonitis, gangliosidoses, hydrocephalus, hypovitaminoses, leucoencephalomalacia, central motor disorders, vascular malformations, vestibular disease, and metabolic disorders, among others . For diseases where clear parallels exist with human disease, we can benefit from their findings to guide our studies . However, many diseases have no such parallel and we must begin from scratch . Progress is likely to be slow due to the current number of active investigators, low incidence of neurological diseases at many research centres, and limited financial
BRAINSTEM AUDITORY EVOKED POTENTIALS IN VETERINARY MEDICINE
resources. Nevertheless, we owe it to our patients to pursue these studies to serve better their medical needs . GEORGE M . STRAIN
Professor of Neuroscience, Louisiana State University, School of Veterinary Medicine, Baton Rouge, Louisiana 70803, USA
AcHOR, l . J . & STARR, A . (1980) . Auditory brain stem responses in the cat . I. Intracranial and extracranial recordings . Electroenceph . clin . Neurophysiol. 48, 154-73 . CHIAPPA, K. H . (1990) . Evoked Potentials in Clinical Medicine, 2nd edn . New York : Raven Press . CROWELL, W . A., DRIERS, T . J., BYARS, T . D ., MARSHALL, A . E ., NUSBAUM, K. E . & LARSEN, L . (1981) . Neomycin toxicosis in calves . Am . J vet . Res. 42, 29-34 . HOLLIDAY, T . A ., NELSON, H . J., WILLIAMs, D . C . & WILLITS, N . (1992) . Unilateral and bilateral brainstem auditory evoked response abnormalities in 900 Dalmatian dogs . J vet . intent . Med. 6, in press . JEWETT, D . L . & RoMANO, M . N. (1972) . Neonatal development of auditory system potentials averaged from the scalp of rat and cat . Brain Res . 36, 101-15 . MARSHALL, A . E . (1985) . Brain stem auditory-evoked response in the non-anesthetized horse and pony . Am . J. vet. Res . 46, 1445-50 . MARSHALL, A . E . (1986) . Use of the brain stem auditory-evoked response to evaluate deafness in a group of Dalmatian dogs . J. Am. vet . med. Ass . 188, 718-22 . MARSHALL, A. E ., BYARS, T . D ., WHITLOCK, R . H . & GEORGE, L. W . (1981) . Brainstem auditory evoked response in the diagnosis of inner ear injury in the horse . J. Am. vet. med . Ass. 178,282-86 . MAYHEW, I. G. & WASHBOURNE, J . R . (1992) . Short latency auditory evoked potentials recorded from non-anesthetized thoroughbred horses . Br. vet. J. 148, 315-27 . MERCHANT, S . R ., NEER, T . M., STRAIN, G . M ., TEDFORD, B . L ., TWEDT, A. C . & CHERAMIE, P. M . (1992) . The effects of chlorhexidine acetate on vestibular and cochlear function in the dog . Proc. 2nd World Conga Vet. Derm. ; Montreal, May, 1992 . MORGAN, J. L ., COULTER, D . B., MARSHALL, A . E . & GOETSCH, D . D . (1980) . Effects of neomycin on the waveform of auditory-evoked brain stem potentials in dogs . Am . J. vet . Res . 41, 1077-81 . NOSTRANDT, A . C ., PEDERSOLI, W . M ., MARSHALL, A . E ., RAvis, W. R . & ROBERTSON, B . T. (1991) . Ototoxic potential of gentamicin in ponies . Am. J. vet. Res . 52, 494-8 . Slus, M . H . (1988) . Electrodiagnostic evaluation of auditory function . Vet . Clin . N. Am : small anim . Pract . 18, 913-44. Slus, M . H . & MOORE, R . E . (1984) . Auditory-evoked response in the clinically normal dog. Am .]. vet. Res . 45, 2019-27 . SIMS, M . H . & SHULL-SELCER, E . (1985) . Electrodiagnostic evaluation of deafness in two English Setter littermates . J. Am. vet . med. Ass . 187, 398-404 . STEISS, J. E ., BRENDEMUEHL, J. P ., WRIGHT, J. C . & STORRS, D . P . (1991) . Nerve condition velocities and brain stem auditory evoked responses in normal neonatal foals compared to foals exposed to endophyte-infected fescue in utero. Progr. Vet . Neurol . 2, 252-60 . STRAIN, G . M ., OLCOTT, B . M . & BRAUN, W. F. Jr . (1986) . Electroencephalogram and evoked potentials in naturally occurring scrapie in sheep . Am . j vet . Res. 47, 828-36 . STRAIN, G . M ., CLAXTON, M . S ., TuRNQuIST, S . E . & KREEGER, J . M . (1987) . Evoked potential and electroencephalographic assessment of central blindness due to brain abscesses in a steer. Cornell Vet. 77, 374-82 .
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STRAIN, G . M ., GRAHAM, M . C ., CLAXTON, M . S . & Oimri, B . M . (1989a) . Postnatal develop-
ment of brainstem auditory-evoked potentials, electroretinograms, and visual-evoked potentials in the calf. J. vel. intern. Med . 3, 231-7. STRAIN, G . M ., Oi .cor'r, B . M ., TnorhsoN, D . R . & GRAHAM, M . C . (1989b) . Brainstem auditory-evoked potentials in Holstein cows . J. vet . intern . Med . 3, 144-8 . STRAIN, G . M ., TEDFORD, B . L . & JACKSON, R . M . (1991) . Postnatal development of the brainstem auditory-evoked potential in dogs . Am. J. vet . Res . 52, 410-15 . STRAIN, G . M ., KEARNEY, M . T ., GIGNAC, I . J ., et al. (1992) . Brainstem auditory evoked potential assessment of congenital deafness in Dalmatians : associations with phenotypic markers . J. vet. intern . Med . 6, in press .