BERA (Brainstem evoked response audiometry), ABR (Auditory brain stem response), BAER (Brainstem auditory evoked response audiometry).
BERA is an electro-physiological test procedure which studies the electrical potential generated at the various levels of the auditory system starting from cochlea to cortex. This investigation was first described by Jewett and Williston in 1971.
Procedure: The stimulus either in the form of click or tone pips is transmitted to the ear via a transducer placed in the insert ear phone or head phone. The waves of impulses generated at the level of brain stem are recorded by the placement of electrodes over the scalp.
Electrode placement: Since the electrodes should be placed over the head, the hair must be oil free. The patient should be instructed to have shampoo bath before coming for investigation. The standard electrode configuration for BERA involves placing a non inverting electrode over the vertex of the head, and inverting electrodes placed over the ear lobe or mastoid prominence. One more earthing electrode is placed over the forehead. This earthing electrode is important for proper functioning of preamplifier.
BERA is resistant to the effects of sleep, sedation, sleep and anesthesia. Its threshold has been found to be within 10dB as elicited by conventional audiometry.
The waves detected in BERA tests
1. Auditory nerve
2. Cochlear nucleus
3. Superior olivary complex
4. Lateral lemniscus
5. Inferior colliculus
6 and 7. Medical geniculate body
Uses of BERA:
1. It is an effective screening tool for evaluating cases of deafness due to retrocochlear pathology i.e. (Acoustic schwannoma). An abnormal BERA is an indication for MRI scan. The BERA test helps us not only in identifying lesions in the 8th cranial nerve, but also the lesions in the brainstem region which affect the auditory pathway. The BERA response obtained in a particular case will depend upon the nature, location, and size of the lesion; e.g. if there is a small unilateral intracanalicular acoustic neuroma the BERA response will be of a particular type (increase in wave I and wave III interpeak latency). If there is a large acoustic neuroma in the CP angle region pressing upon the brainstem there is an increase in wave III and V latency on the contra lateral side, etc.
2. Used in screening newborns for deafness
3. Used for intraoperative monitoring of central and peripheral nervous system
4. Monitoting patients in intensive care units
5. Diagnosing suspected demyelination disorders
BERA findings suggestive of retrocochlear pathology:
1. Latency differences between interaural waves 5 (prolonged in cases of retrocochlear pathology)
2. Waves I – V interaural latency differences – prolonged
3. Absolute latency of wave V – prolonged
4. Absence of brain stem response in the affected ear
BERA has 90% sensitivity and 80% specificity in identifying cases of acoustic schwannoma. The sensitivity increases in proportion to the size of the tumor.
Criteria for screening newborn babies using BERA:
1. Parental concern about hearing levels in their child
2. Family history of hearing loss
3. Pre and post natal infections
4. Low birth weight babies
5. Hyperbilirubinemia
6. Cranio facial deformities
7. Head injury
8. Persistent otitis media
9. Exposure to ototoxic drugs
Limitation
1. All waves are absent in severe hearing loss as well as in a large acoustic neuroma.
2. A normal BERA response virtually rules out an acoustic neuroma; but doesn\’t at all rule out intrinsic brainstem lesion or even non-acoustic tumor of the CP angle e.g. Meningioma.
There are many electrophysiological tests available, for localizing the site of the pathology in sensori-neural lesion; but none of these tests are foolproof. A detailed history, thorough clinical examination with the battery of audiometric tests available, help a clinician to come to a correct diagnosis.
