Brainstem Auditory Evoked Responses (BAER)
Introduction
Auditory evoked potentials are critical for monitoring the integrity of the brainstem and cortex. BAERs are a series of positive and negative waveforms that arise from neural structures associated with the ascending auditory pathway, including the cochlea, brainstem, and auditory cortex. Short latency BAERs (< 10 ms) are used to monitor brainstem function, whereas long‐latency BAERs (> 10 ms) are used to monitor cortical function.
Auditory pathways
When sound enters the ear, the sensory information is transduced by the receptors in the cochlea (inner ear). The cochlea is composed of fluid-filled compartments separated by membranes. The membranes express hair cells that are 'tuned' to a particular frequency. Sufficient movement of the hair cells will depolarize individual fibers of the auditory portion of the VIII cranial nerve. There are inner and outer hair cells; the inner hair cells are involved in signal transduction and can connect to multiple axons of the auditory nerve.
The cochlear nucleus in the brainstem is the first relay station.
Stimulation
In the clinical setting, stimuli that consists of click sounds are emitted in the frequency range of 1000–4000 Hz to each ear through small earphone stimulators or shielded headphones. Tone bursts are also used as auditory stimuli.
Recording Techniques
To record BAERs, a recording electrode is placed near the ear above the mastoid process or the ear lobe. The reference electrode can be placed at the forehead (Fpz) or CZ.
Waveform
There are different types of electric potentials with varying inter-peak latencies. Each peak represents the activity of a specific neural generator. For example, the cochlea generates cochlear microphonics and summating potentials.
- Peaks and neural generators
- Latencies and interpeak latencies
- Normal values