What Is BAER (Brainstem Auditory Evoked Response) Testing All about?
The process of hearing is extremely intricate. Whether the sound is loud or soft, high or low an incredible number of very intricate biological processes have to occur in a strictly choreographed manner. When you think about it, it is amazing that so many of us DO hear. There are such a multitude of places along the biological path of hearing where malfunctions can occur, it’s amazing that there are relatively few individuals that are deaf.
Sound begins its journey into our senses, our consciousness, as waves transmitted through the air. These air waves then cause vibrations in the tympanum or ear drum. These sound waves move the ear drum and then move three tiny bones (the smallest bones in the body) that sit within the middle ear. This moves liquid within the cochlea, a spiral, sea shell shaped organ, of the inner ear. Another series of membranes are moved by the liquid which causes small, pigmented hair cells to move. These are receptors for the 8th cranial nerve (CN VIII). At this point the movement triggered by the sound waves is now conducted through the nerve and the brainstem as electrical activity within the skull.
Dogs and cats, just like people, can be born deaf. This is typically due to sensorineural deafness or a dysfunction of the sensors that allow sound to enter the nervous system as electrical impulses (the hair cells).
BAER testing is a hearing test. In veterinary medicine we cannot rely on the pets response to sound to check their ability to hear. Dogs and cats with hearing deficiencies can depend heavily on vibration and visual input to compensate for their hearing loss. It can be very difficult to tell if a pet is completely deaf or even deaf in one ear. They are very capable in adapting and compensating for their disabilities. Rather than depending on our observations (subjective analysis) we use BAER tests (objective analysis) to follow the sound as it is conducted from the inner ear to the brainstem.
Few dogs are trained to raise their paw (or finger as a person would) when they hear a to a certain tone. BAER testing helps us analyze their ability to hear without relying on their conscious responses. BAER testing is easily performed on either an awake or anesthetized pet. We recommend that the pet be at least 6 weeks old and preferably 12 weeks old for this test. When they are less than 12 weeks old there is a slight risk of false negative tests (this means that a hearing ear might tests as deaf).
Each ear is tested separately and 3 small needles are placed under the pets skin, one at the top of the head, the next at the base of the ear being tested and the last, a ground lead, is somewhere out on the pets body. The needles at the base of the ear and the top of the head will record the electrical activity as it is conducted from those little hair cells into the brain stem. Then an ear phone or ‘clicker ‘ is placed in the pets ear and it will emit a clicking tone at a volumes of 80-100 decibels. We use an 87 decibel sound. The machine then records each click as it is conducted along the 8th cranial nerve and on through the brain stem.
A pet that has a hearing response on the BAER test will demonstrate a series of peaks that occur at certain time intervals on a graph. This is a normal BAER test.
If the pet has an inherited deafness or deafness as a result of damage to the inner ear and the hair cells within the inner ear , the test would have no peaks and would be flat(see below).
BAER testing can be used to check for other interruptions in the hearing process such as tumors of the 8th cranial nerve and damage of areas within the brainstem.
At our office, BAER testing is performed on awake patients, we seldom find that we need to sedate or anesthetize puppies and kittens that are tested. Sometimes when more detailed tests are indicated or a pet is particularly uncooperative anesthesia can be used.
BAER testing should be performed on any pet suspected of being deaf or one of the breeds listed below, prior to their being adopted into a new home. Deaf animals can make great pets and companions, but owners must be aware of their special needs so they can learn to communicate with their new pet and train that animal adequately. This will allow the pet to avoid injury and learn to function well within the household.
Dog and Cat breeds associated with reported congenital (inherited deafness)
From: Dewey CW. A practical guide to canine and feline neurology 2003;Ames: Iowa
|Am. Staffordshire Terrier
|Australian Blue Heeler
|Australian Cattle Dog
||Walker American Foxhound
||Jack Russel Terrier
||West Highland White Terrier
||White Cornish Rex
||Mixed Breed Dog
||White Devon Rex
|Cardigan Welsh Corgi
||Nova Scotia Duck Tolling Retriever
|Catahoula Leopard Dog
||Old English Sheepdog
||White Scottish Fold
|Cavilier King Charles Span.
||White Turkish Angora
||Pit Bull Terrier
||White American Shorthair
||White British Shorthair
||White Exotic Shorthair
||White Oriental Shorthair
|English Cocker Spaniel
||Soft Coated Wheaten Terrier
State Press: 232.