In noninvasive brainstem audiometry, the neurologist or otolaryngologist performs an objective hearing performance measurement under auditory stimulation using impulses from the auditory nerve pathways that can be traced to the middle brainstem. This procedure is one of the few methods for objective assessment of hearing performance that can be performed on young children or otherwise unwilling patients. The testing method is used particularly for differential dignostics of cochlear and retrochlear damage to hearing, in assessment, and as part of the ERA, a hearing screening procedure for newborns.
What is brainstem audiometry?
In noninvasive brainstem audiometry, the neurologist or otolaryngologist performs objective hearing performance measurement under auditory stimulation using impulses from the auditory nerve pathways that can be traced to the middle brainstem. Brainstem audiometry is also known as BERA (brainstem evoked response audiometry)and is a non-invasive hearing assessment procedure. It is a neurological and otorhinolaryngological examination method, which is primarily intended to help in the differential diagnosis of hearing disorders. In principle, the procedure involves the measurement of brain waves under acoustic stimulation to assess objective hearing ability. The impulses of the auditory nerve pathways are traced to the middle brainstem by means of targeted stimulus transmission and are registered and recorded as individual waves. The evaluation of the measurement data refers to the latency of the waves, which may provide information about the origin of hearing impairment. The recorded data of a brainstem audiometry are therefore mostly used for differential diagnosis of hearing disorders, but can also be collected during a general hearing screening.
Function, effect, and goals
The otolaryngologist or neurologist uses brainstem audiometry primarily for differential diagnosis. For example, impaired auditory function, which can be detected by disturbed brain waves, could indicate hearing damage due to multiple sclerosis or a tumor on the auditory nerve. The most common tumors of this type are, for example, acoustic neuroma and cerebellopontine angle tumor. In terms of differential diagnosis, ABR can thus be used to distinguish primarily between cochlear and retrochlear damage to the auditory system. Another area of application for the objective test method is in the field of assessment. Hearing thresholds can be detected with ABR entirely without the patient’s assistance, and can thus be determined even for children who resist testing. It can even be used to screen hearing in newborns using brainstem audiometry. Ultimately, the basic principle of ABR is always the graphical representation of electrical potentials in waveform. Five to six waves are recorded during the test. This recording occurs exclusively during successful processing of acoustic stimuli. The displayed potentials thus illustrate normal or disturbed activity of the auditory pathway. Electroencephalography (EEG) derives the potentials during acoustic stimulation with a latency greater than or equal to 10 ms between the middle of the vertex and the mastoid. For this purpose, three adhesive electrodes are attached to the patient’s head. The patient wears one electrode on each side behind the ear and a neutral electrode on the center of the forehead. Acoustic stimulation is provided by means of clicks, which are given via headphones at regular intervals of 20 seconds. The response potential is derived and summed up via the electrodes, while other EEG signals are filtered out. In this way, only the response of the brainstem to the acoustic click signals is ultimately displayed. Waves I, III and V can usually be clearly identified and are thus suitable for determining an absolute latency to an acoustic stimulus. In addition, a so-called inter-peak latency is recorded. This is a latency difference between several waves, which can provide information about retrochlear processes. In adults, for example, inter-peak latencies with latencies greater than or equal to 4.4 ms in waves I to V provide an indication of retrochlear damage caused by MS or tumors. For infants, a delayed latency is generally considered the norm.
Risks, side effects and dangers
Because ABR does not require patient assistance and can even be performed while the patient is anesthetized, this procedure is one of a few hearing measurement procedures that can be performed on unwilling patients such as children. Often, the ABR is also used as one of three components of the ERA (Evoked response audiometry) and is completed by ECochG and CERA. While the former measures the potentials of the cochlea and auditory nerves, the latter measures the potentials of the cerebral cortex. Thus, in a comprehensive hearing screening, ECochG, CERA and ABR are used to collect all hearing-relevant potentials. For adults, brainstem audiometry usually does not require any additional precautions. However, prior to the measurement, the patient must participate in an extensive educational interview designed to ensure the accuracy of the measurements. During this discussion, patients are therefore given precise rules of conduct for the period of the measurement. For example, if they do not lie down in a relaxed position or move around a lot, this can significantly falsify the results. Newborns and children usually have to be placed under anesthesia for the measurement, as they rarely behave completely calmly. Patients who are otherwise unwilling are also anesthetized. Complications are usually not to be expected. However, there is always a risk associated with anesthetizations during the measurement, as the anesthesia itself is slightly risky. After the measurement, no special precautions need to be taken and the patient can go home again. Depending on the evaluation findings, however, further diagnostic procedures may be indicated in subsequent weeks to further confirm or rule out a possible diagnosis.
