Latrisha Tate, LVT and Diana
A Brainstem Auditory Evoked Response (BAER) is a test that is most commonly used to screen for deafness, but also has applications in determining the health of the brainstem in conditions such as neoplasia, strokes, head trauma, and vestibular disease. The BAER is an electrophysiologic study. Electrophysiologic studies are common within veterinary medicine and record the spontaneous or stimulated electrical activity in various body systems. The most common example would be the ECG that records the spontaneous electrical activity of the heart and creates the familiar PQRS complexes that are monitored during anesthesia. The BAER works on the same principles and records the electrical activity of the auditory pathway (nerves and brainstem) when stimulated by sound.
According to the OFA (Orthopedic Foundation for Animals), the BAER test is the only accepted method of diagnosis for deafness2. Deafness is a failure in auditory function caused by either a disturbance in sound conduction or in sound sensation1. The disturbance may be acquired from various causes (Table 1) or congenital in nature4. Breeds that are susceptible to congential deafness include the Dalmatian, Australian Shepherd, Boxer, Collie, and French bulldog with breeds of white pigmentation being the most affected overall4. Animals can be affected unilaterally (affecting one ear) or bilaterally (affecting both ears) and owners often notice vague symptoms such as inattentiveness, being difficult to wake up, or a change in obedience. As these behavioral signs are subjective in nature and non-localizing, the BAER test is a good objective way to identify hearing abnormalities. Clients need to understand, however, that BAER testing only determines whether an ear can hear or not and does not identify the inciting cause.
Table 1 – Causes of Sudden Onset of Deafness
Source: Strain, GM, Causes of Sudden Onset of Deafness, Comparative Biomedical Sciences
School of Veterinary Medicine, Louisiana State University, June 2006, Web.
BAER testing is a short and relatively painless procedure. At BVNS, we perform outpatient BAER testing on patients routinely. Patients must be at least 35 days old or have fully developed hearing capability before they qualify for the BAER1. While sedation is not required for the procedure, a light amount may be used in order to
remove the potential for movement artifact. Dosages may vary according to the patient and veterinarian in charge but a good starting point at BVNS would be 2-8 ug/kg of Dexdomitor. The test can usually be completed in 10-15 minutes. Equipment needed for measuring the BAER includes headphones or tubal inserts, 3 electrodes (recording, ground, and reference), an electrode board (channel board), and a computer that serves as the amplifi , signal averager, and stimulator5.
Proper setup and technique is extremely important for accuracy and interpretation of results. Three small gauge needle electrodes are placed subcutaneously on the patient’s head—one is placed at the top of the head (the vertex), another below the ear being examined, and the third electrode (the ground) is placed either in front of the opposite ear or over the dorsal spinous process of the first thoracic vertebra5. The computer generates an acoustic signal, or a click, of a specific frequency and strength which is transferred to the ear being tested by an insert or headphones. Waveforms are generated in response to the auditory stimulus. Minimally, OFA requires that each ear must be tested at a stimulus intensity of 70 dB for at least 200 total clicks2.
Inferences about the animals’ ability to hear are ultimately made by evaluating the generated electrical waveforms. The response waveform consists of a series of peaks numbered with Roman numerals from I-VII (Figure 1) The presence or absence of waveforms, their width (latency) and their height (amplitude) are all considered in the first evaluation by the neurologist3. In a deaf animal, there will be no waveforms and only a straight line (Figure 2). Each of these waveforms is associated with a particular structure or region along the hearing pathway. In particular, the earliest waves are generated in the auditory nerve and brain stem so assessment of these waveforms can be used to diagnose brain death in emergency situations1.
1Jassy, Andre. Small Animal Neurology: An Illustrated Text. Hannover: Schlütersche, 2010. 161-162. Print.
2Orthopedic Foundation for Animals. “BAER testing protocol.” OFA, 2010. Web. 6 April 2010. < http://www.off g/deaf_baer.html>
3Platt, Simon R. and Natasha J Olby. “Electrophysiology.” BSAVA Manual of Canine and Feline Neurology. 3rd edition. Luc Poncelet. Gloucester: British Small Animal Veterinary Association, 2010. 54-66. Print.
4Strain, GM. “Canine Deafness.”Vet Clin North Am Sm Anim Prac. 42.6 (2012):1209-24. Web. 10 April 2014.
5Webb, Aubrey A. “Brainstem auditory evoked response (BAER) testing in animals.”The Canadian Veterinary Journal 50.3 (2009): 313-318. Web. 8 April 2015.