Introduction
CHAPTER ONE
Introduction
20
1.6
Auditory Brainstem Response
Auditory Brainstem Response (ABR) is an electrophysiologic response
that measures the auditory system’s response to sound. Three sensors are
placed on the patient and a probe is placed in the test ear. The equipment
sends a soft clicking sound to the ears and the sensors pick up the nerve’s
response to that sound.
The cochlea or inner ear converts sound into electrical signal. These
signals travel in sequence to the acoustic nerve, the brainstem, and finally
to the cortical areas of the brain. These electrical responses are commonly
known as auditory evoked potentials. One type of auditory evoked potential
is the Auditory Brainstem Response, which occurs within approximately
3-20 ms of the onset of the stimulus (depending on frequency and
intensity of the stimulus). Voltages (potentials) can be measured at the
skin with surface electrodes; the electrode montage consisting of three
such electrodes. As the amplitude of the ABR is very small compared to
the ‘noise’ of other brain electrical activity (EEG), the signal to noise ratio
is enhanced by averaging. The amplitude of the ABR is also quite small
compared to voltages generated by myogenic (muscle) activity; therefore,
ideally, children should be tested when sleeping. The ABR consists of a
series of positive waves (at the vertex of the scalp) that are named by their
relative order (waves I through V). ABR is typically elicited by click, brief
tone, or chirp stimuli.
If EEG and myogenic artefacts are below a tolerable (noise reject) level,
the Otoport ABR firmware will detect the ABR waves and give a simple
Pass/Fail response to the user. The resultant waveforms may also be
viewed, and results interpreted, manually if so desired.
