We are very pleased to announce that we have added hearing testing (B.A.E.R.) to our diagnostic capabilities at our Dublin, Redwood City and Campbell facilities.
BAER stands for brainstem auditory evoked response. In this test we play tones through earphones at sound levels between 70 and 105dB (which is roughly equivalent sound level from a vacuum cleaner and a rock concert) in each ear and record the nervous system’s electrical response from the level of the inner ear through the brainstem.
The clicks produce between four and five waves in a normal animal. In many animals wave IV is not apparent or is seen as only a small notch as it is superimposed on the larger wave III. It is generally agreed that wave I comes from the cochlear nerve. The exact generators of the remainder of the waves are less certain: wave II is thought to be generated in the cochlear nuclei in the lateral medulla and the remainder of the waves are likely composites of several different brainstem structures as the electrical activity of auditory sensation moves cranial through the brainstem. The last wave usually observed, wave V, is thought to be generated at the level of the caudal colliculus and medial geniculate nucleus of the diencephalon.
The most common utilization of this test is in certification of puppies and breeding animals through OFA. Congenital deafness is present in many dog breeds, especially those with merle or piebald color variants. Exact prevalence varies widely by breed, with some breeds like the Dalmatian having as much as 30% of the population affected in at least one ear.
The test can be performed in dogs as young as 5 weeks old (35 days) to be certified by OFA, but in general we prefer them to be slightly older (3-6 months old). Testing can be performed awake, but as the in-ear earphones are irritating and the patient must be fairly still for the procedure, light sedation is often used. The test is unaffected by sedation or even general anesthesia.
The ability of the test to detect unilaterally deaf animals is particularly useful as these dogs are often not clinical for their deafness, and can be difficult to detect otherwise. This can lead to the persistence of these traits in the breeding population, and therefore certification is very valuable in breeding and show animals. In the non-breeding animal, identification of deafness can assist in training and life style changes. We recommend a book written by Susan Becker called Living with a Deaf Dog: A Book of Advice, Facts and Experiences About Canine Deafness to owners with questions about living with and training a deaf animal.
The test can also be used in neurologically abnormal animals to help determine disease location (i.e. brainstem vs. ear) and can also assist in determining the level of brain function (i.e. in brain death). A complete lack of waveforms demonstrates that an animal is completely deaf in that ear or in the comatose animal signals brain death. Decreases in amplitude in the wave forms, or a delay in the appearance of the first wave suggests conduction deafness- or a problem in the structure of the middle/inner ear (i.e. otitis media/interna). An increase in the timing between the waves is suggestive of a brainstem disorder, and occasionally we can even lateralize the lesions with this testing.
We are very excited to add this and other electrodiagnostics including nerve conduction and electromyography to our practice. Please feel free to contact our neurologists if you have any questions about the use of electrodiagnostics in your patents!
References and suggested reading:
*Orthopedic Foundation for Animals Congenital Deafness site http://www.offa.org/
*Dewey, CW A Practical Guide to Canine and Feline Neurology. pg 103-105, 271-272
*Dr. George Strain’s web site (a professor of Neuroscience at LSU) Deafness in Dogs & Cats: Information on Deafness Prevalence, Causes, & Management For Owners, Breeders, and Researcher http://www.lsu.edu/deafness/deaf.htm