Brainstem Auditory Evoked Potential
BAEP measures the function of the auditory nerve and auditory pathways in the brain stem The short-latency BAER generally is used for clinical purposes. The above named patient is complaining of off balance.The test can be performed regularly, under sedation or under general anesthesia. Standard broadband click stimulation is used on the ear tested, while the contralateral ear receives masking noise of 30- to 40-dB lesser intensity. Monoaural stimulation is used. The click intensity should be 65-70 dB above click perception threshold. A repetition rate of about 10 Hz should be used.
An electrode is placed on each ear lobe and at Cz. Whether nuclei or tracts, or both, generate the peak latencies is not known. Generators currently are postulated to be as follows:
- Wave I - Action potential of the cranial nerve (CN) VIII
- Wave II - Cochlear nucleus (and CN VIII)
- Wave III - Ipsilateral superior olivary nucleus
- Wave IV - Nucleus or axons of lateral lemniscus
- Wave V - Inferior colliculus
Factors influencing peak latencies of BAERs include age, sex, auditory acuity stimulus repetition rate, intensity, and polarity. Rarefaction (ie, earphone diaphragm moves away from the eardrum) produces an increase in wave I amplitude. In severe hearing loss, all waveforms may be delayed, wave I may be absent with waves II through V delayed, or all waveforms may be absent. Note that in patients with hearing loss BAER still can be obtained to assess central conduction time by increasing stimulation intensity.
BAEPs are useful in estimating or aiding in the assessment of hearing loss. The most commonly used method for this is evoked response audiometry. The frequency of stimulation is 50-70 Hz, Wave V latency shifts are used to estimate the amount of hearing loss.
In children, especially those younger than 2 years, the BAEP can be used to screen those who might benefit from auditory amplification in order to achieve more normal speech and language development. However some children with a normal BAER have abnormal hearing. The role of BAEP nevertheless is to identify those patients who could benefit from a hearing aid. Obviously with normal BAEP a hearing aid would not be useful to correct the hearing loss.