Brainstem auditory evoked response (BAER) or BERA (brainstem evoked response audiometry). It is an objective, non-invasive test to assess the structural integrity and functional status of the auditory pathway from the spiral ganglia to the level of the lateral lemniscus in the midbrain. The test was introduced by Jewitt & Williston.
Principal : The series of electrical potentials generated by the activation of different parts of the auditory system are recorded by placing electrodes on the scalp. But since these cortical potentials are small and buried in the background of spontaneous electrical activity (EEG waves), an amplifier is required to study the waves in detail.
- The test is conducted in a sound-proof room.
- The patient is asked to be in a supine posture with eyes closed, relaxed and preferably asleep(to reduce myogenic potentials). However, accurate assessment in children may require sedation.
- Test one ear at a time.
- Three surface electrodes are used:
- Active electrode kept on the vertex (best location) and if not possible may be placed on the lop of the forehead just below the hairline.
- Reference electrode kept on ear lobe/mastoid of the tested ear.
- Ground electrode kept on ear lobe/mastoid of the opposite ear.
- The electrical potentials generated in response to a series of 1000-1200 clicks (given at a rate of 5-50/sec) at an intensity of 50 to 60dB above the average pure tone hearing level are picked up from the vertex by the active electrode and is plotted graphically.
- These neurogenic potentials elicited are recorded for the first 10 milliseconds. This is the time taken for the electrical responses to be carried to the brainstem alone
- In a normal person, seven waves are produced in the first 10-15 milliseconds. The first, third and fifth waves are most stable and are used in measurements.
Characteristics of waves:
- Latency – absolute, inter-wave (usually between wave i and v) and interaural.
- Absolute latency is the time interval (in millisec) between the onset of the stimulus and the peak of the wave. The absolute latency of the wave V is most important, as it is common, easily identifiable and is least affected when intensity increases.
- lnterwave latency is the time interval between two different waves in the same ear and in the same BERA tracing.
- Interaural latency is the difference in the time interval of the same wave between the two ears e.g. the latency of wave IV of left ear and right ear
Upper limits of normal values
- Latency of wave V – 5.9 m.s.
- I – V interval – 4.4 m.s.
- I – V interaural difference – 0.29 m.s.
|Wave I||Distal part of CN VIII|
|Wave II||Proximal part of CN VIII near the brainstem|
|Wave III||Cochlear nucleus|
|Wave IV||Superior olivary complex|
|Wave V||Lateral lemniscus|
|Waves VI and VII||Inferior colliculus|
Remember as mnemonic EE COLI (eight, eight, cochlear nucleus, olivary complex, lateral lemniscus, inferior colliculus) compare E COLI-MA in pathways of hearing.
- Objective hearing assessment test in infants, young children, non-cooperative adults, in malingerers, comatose and unconscious patients.
- To diagnose the site of lesion in retro cochlear pathologies particularly acoustic neuroma.
- To diagnose brainstem pathology, e.g. multiple sclerosis or pontine tumours.
- To monitor and preserve the auditory nerve intraoperatively during surgery of acoustic neuromas
- There is no standardisation at present for BERA.
- Wave V is not recorded if the hearing level is 75 dB at 3 kHz.
- Normally, the latency of wave V increases in old age, conductive hearing loss and pure sensorineural hearing loss.
- Wave I is not easily identifiable in BERA.
- ABR testing is highly sensitive(90 %) for detecting large tumours; however, small tumours (<1 cm) may be missed. MRI is a more sensitive and specific test for an acoustic neuroma.
- ABR waves are usually absent when a patient has a severe or profound hearing loss.
- A conductive hearing loss also attenuates cochlear stimulation and increases ABR wave latencies.
- It does not give frequency-specific information. Low-frequency hearing losses are undetectable by the BERA. Hence additionally ASSR is needed.
- The test is practically insensitive for hearing losses above 75·dB.
- This test can only be done on awake, conscious and cooperative adult patients. It can be done in infants and young children under sedation as the latter has no effect on BERA.
——— End of the chapter ———
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Dr. Rahul Kumar Bagla
MS & Fellow Rhinoplasty & Facial Plastic Surgery.
Associate Professor & Head
GIMS, Greater Noida, India
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