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1070.e2 Brainstem Auditory Evoked Response (BAER) Test

Brainstem Auditory Evoked Response (BAER) Test
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• Suspected brainstem disease (neoplasia,
Difficulty level: ♦♦♦
sedation is required, light sedation is usually
encephalitis, infarction) and if it will lie still for 10-15 minutes. If
Synonyms • Confirmation of brainstem death (complete sufficient, and it does not significantly
Hearing test, auditory brainstem response loss of brainstem function) alter the characteristics of the recorded
(ABR) waveforms.
Contraindications
Overview and Goal None Anticipated Time
• A noninvasive way to evaluate hearing and Usually 15-30 minutes, depending on the
brainstem function by recording the electrical Equipment, Anesthesia animal’s temperament and behavior
response in the brainstem to an external • A computer with a signal averager and
auditory stimulus software capable of labeling and analysis of Preparation: Important
• An average of 256 to 1000 time-locked waveforms Checkpoints
recordings result in a characteristic waveform • Subdermal electrodes • Computer check to see that established
with five to seven discernible waves (I-VII). • Foam insert microphones, headphones, or protocols have been loaded appropriately
• Wave I is generated by the ipsilateral cochlear a bone conductor and settings are correct
nerve. Waves II-V are generated by nuclei • Otoscope • Impedance check (with computer software
within the medulla, pons, and midbrain. • Sedation may or may not be required, program) to ensure that all the electrodes
• The brainstem auditory evoked response depending on how quiet the animal is are operating effectively
(BAER) test does not assess conscious percep-
tion of hearing; however, because there are
very few diseases that interrupt the hearing
pathways after they have left the brainstem,
it is accepted as a valid test of hearing.
• Conductive (obstruction to sound in the
ear canal or middle ear) and sensorineural
(abnormality of the nerves or receptors in the
inner ear) deafness can result in an abnormal
BAER test result.
• Congenital sensorineural deafness typically
results in a flat BAER recording.
• With conductive disorders, the latency
from stimulus to the appearance of wave
I is delayed, and there is often decreased A
amplitude that increases when the intensity
is raised from 80 to 100 dB.
• With brainstem lesions, the interpeak laten-
cies (I-III, I-V) may be prolonged relative
to normal reference range on the opposite
side, and the wave V/I amplitude ratio may
be reduced.
• Brainstem death results in a BAER recording
that is flat or has only a wave I.
• Although BAER testing is commonly used
for identifying congenital deafness (an
absolute yes/no assessment), it can also
be used for assessing the level of hearing B
function by performing threshold analyses.
Most dogs can hear sounds at a level of sound
intensity of 0-5 dB. By gradual reduction
of the sound intensity during testing (from
an initial level of 80 dB), the examiner can
establish whether the threshold is abnormally
increased.
Indications
• Screening for congenital deafness is the
most common reason for BAER testing. It
is frequently performed in puppies of the
breeds commonly affected with congenital
deafness (e.g., Dalmatian, bull terrier, English
setter). C
• Suspected deafness or hearing loss (unilateral BRAINSTEM AUDITORY EVOKED RESPONSE (BAER) TEST A, Normal study. B, Unilateral deafness
or bilateral) in the left ear. C, Bilateral deafness. (Courtesy Dr. Peter Foley.)

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