WHAT'S A BAER?
Betsy Dayrell-Hart, VMD, Dip ACVIM

A Brainstem Auditory Evoked Response is a neurodiagnostic test that records the response to an external auditory stimulus. In other words, it examines the ability of the animal's hearing receptor, auditory nerve, and auditory pathways in the brain to hear a noise that we make outside the animal's head.

The procedure is performed on a dog or cat that is sedated, because it is necessary for the animal to hold still while it is wearing a device that looks like a foam-rubber ear plug (where the sound comes from) in each ear, and to have some very small recording electrodes placed under the skin (these are EEG electrodes, and they record activity from nerve cells in the inner ear and the brain).

Young puppies can be safely sedated using promazine (sparine), morphine, and atropine, and although they remain sleepy for several hours afterward, they tolerate the sedation and the BAER very well. In advance of the appointment, the animals are fasted for 8 hours, then prior to sedation, it is weighed, a brief physical exam is performed, and the BAER follows. The test usually takes 5-10 minutes, once the animal is sedated.

For young adult animals, other sedatives are often used, since they do not relax as well as puppies when they are given morphine and promazine, but otherwise the process is the same. Except for being sleepy for a few hours, there are no side effects to the test, and it is safe for an animal of any age provided that it can be sedated. Some very ill animals have been tested without sedation, because they do not struggle with the ear pieces and electrodes.

The test works in the following way: An animal is sedated, the stimulating ear pieces are placed in the ear canals, and the recording electrodes are placed under the skin over the top of the head ("vertex"), and beneath each ear ("bullae"). The Dantex Cantata is the machine that coordinates the stimulus, which is a click of a given sound strength (usually between 80-130 decibels), and the recording period afterward.

Animals with normal hearing and normal brainstem function usually develop a characteristic set of peaks in response to the click - the earliest peaks on the screen represent activity in the hearing receptors, the acoustic nerve, and its collection of cell bodies called the ganglion, and the later peaks represent depolarization of neurons in the brainstem nuclei.

The BAER is an excellent "yes or no" test for congenital deafness - an animal either has a response that can be clearly seen on the screen, or it has a flat line response, showing that the hearing receptors are not operating at all.

In instances where an animal is believed to have severe brain disease, the BAER can sometimes provide evidence that the brainstem is not transmitting impulses properly. The test is performed in the same way, but the peaks are analyzed differently. Results are not always as "clear-cut" when the test is used for this purpose, but the BAER can provide evidence to help confirm brain disease, or in rare instances, to indicate "brain death" in a severely compromised animal.

Betsy Dayrell-Hart, VMD, is a graduate of the University of Pennsylvania School of Veterinary Medicine and is board-certified in Neurology by the American College of Veterinary Internal Medicine. Dr. Dayrell-Hart brings over 10 years of experience in veterinary neurology to SouthPaws. Dedicated to providing the highest quality care for her patients, Betsy also places the teaching of her specialty high on the short list of her professional priorities. Dr. Dayrell-Hart offers a full compliment of neurology services including electrodiagnostics, and CT and MRI scans.