Page last modified:
January 5, 2008
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Timothy C. Hain, MD
Page last modified:
January 5, 2008
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Figure 1:Brainstem auditory evoked response. Right ear responses are shown on top and left ear on the bottom half. |
Figure 2:Most contemporary evoked response machines are general purpose computers with specialized software and hardware to enable recording of physiological potentials. In the author'sclinical practice, a unit made by Bio-Logic is presently used. This consists of a small external box that interfaces with a general purpose computer. |
Brainstem auditory evoked responses (BAER), also known as auditory brainstem evoked response (ABR), test both the ear and the brain. They measure the timing of electrical waves from the brainstem in response to clicks or tone bursts in the ear. Computer averaging over time to filters background noise to generate an averaged response of the auditory pathway to an auditory stimulus Three waves (1, 3 and 5) are plotted for each ear. The waveform represents specific anatomical points along the auditory neural pathway: the cochlear nerve and nuclei (waves I and II), superior olivary nucleus (wave III), lateral lemniscus (wave IV), and inferior colliculi (wave V). Delays of one side relative to the other suggests a lesion in the 8th cranial nerve between the ear and brainstem or the brainstem itself.
The main indication for BAER is when an acoustic neuroma is suspected. This generally comes about when there is an asymmetrical sensorineural hearing loss. BAER testing is more cost effective for this purpose than MRI (Rupa et al, 2003), but MRI provides additional information. The most reliable indicator for acoustic neuromas from the BAER is the interaural latency differences in wave V: The latency in the abnormal ear is prolonged
BAER testing may also be useful in situations where an auditory neuropathy is suspected. In this case, it may be combined with otoacoustic emission testing.
BAER's are commonly abnormal in brainstem disorders such as multiple sclerosis, brainstem stroke, or brainstem degenerative disorders. These are much less common than inner ear disorders, but also are intrinsically much more dangerous.
BAER testing requires reasonable high-frequency hearing. This means that it is often not worth doing in persons over the age of 70. We recommend that either an audiogram or at least a screening test for high frequency hearing be done prior to BAER testing.
An example of an abnormal BAER (for the left ear ) is shown in Figure 1. The case history is found here.
Stacked derived-band ABR, basically band-passed computer processed ABR, may theoretically identify small, intracanalicular acoustic neuromas with greater sensitivity than a standard ABR. This technology -- a software program -- has been patented by one of the audiology equipment companies (Bio-Logic). The stacked ABR uses the amplitude of wave V as the measurement point: Wave V of each derived-band ABR is aligned and the time-shifted responses are summed. As MRI testing provides more information, we do not use stacked ABR in our practice in Chicago. VEMP testing would also seem to have more promise than ABR, because VEMP's are testing the part of the inner ear first affected by acoustics.
The BAER test is not a painful test, and in fact, it is often best if the patient goes to sleep during the test. There will be clicks heard in the ears, and wires attached to the head to record electricity from the ears. The BAER test can take as long as 1 hour.
BAER tests are commonly performed by an audiologist or an electrophysiology technician. Audiologists are often associated with otolaryngology practices (ENT doctors), while electrophysiology technicians are often associated with Neurology practices. It is not a difficult test and does not require much training for one to perform. Thus technically, either setting is quite reasonable. Because hearing testing is absolutely required to interpret the test (see above), often the most convenient process is to have an audiologist do the BAER test and audiogram in a single sitting.
Reading of the BAER test is more problematic because BAER's may be obtained in settings relatively unfamiliar with CNS disorders (i.e. most otology practices) or settings relatively unfamiliar with ear disorders (i.e. most neurology practices).
In our opinion, BAER's should always be interpreted by a physician familiar both with disorders of the ear and brain as well as the patient's clinical history.
This generally means a subspecialist, either an otologist (an "ENT" doctor who just takes care of ear diseases), or an "otoneurologist", a neurologist who specializes in ear diseases. Experienced and extremely well trained audiologists, particularly those with academic doctoral degrees (here we mean Ph.D, not Au.D.), may be also be competant to interpret BAER's. However, while audiologists are generally well informed about technical aspects of BAER testing and they are fairly well informed about many ear pathologies, they are nearly always untrained and unfamiliar with neurological conditions that cause abnormal BAER's, especially central disorders such as MS (multiple sclerosis) or brainstem stroke.
In our opinion, a "stand-alone" audiology or other non-physician practice (such as is often organized for the purpose of selling hearing aids) should not be providing intepretations of BAER tests.
This is because the testing is done without any physician oversight and/or knowledge of the patient's specifics, may not be able to interpret central BAER patterns other than in the most cursory way. Otologists (ear specialists) may also be unfamiliar with central BAER patterns. Similarly, a general neurologist, even one certified in electrophysiology, interpreting BAER tests may often be unaware of the many interactions between peripheral ear disease and BAER test results. A team combining an experienced audiologist and otoneurologist is optimal. However, this may not be available in every clinical setting and some compromises may be necessary.
BAER testing is a mature technology and nearly any device on the market can do an acceptable job. If you are in the market for a new BAER device, we think it is best to get a multipurpose machine - can it do ECOG testing ? VEMP testing ? Other types of evoked responses such as SSEPs and VEPs ? Does it do OAE's ? Does it interface with NOAH ? If it is an external device, how does it connect to your host computer (USB is best). At this writing (7/2004), a stand-alone "box" should cost about $10,000. Examples of vendors include Bio-Logic, GN otometrics, Interacoustics, and Nicolet (alphabetical order).
Be sure to consider the company's technical support, as it generally seems to be problematic in this industry. Be very cautious if you cannot reliably reach technical support when you call them, or if technical support is an option that costs more money. As evoked potential testing in general is evolving rapidly right now, it is very likely that you will need technical support. Look also to see what the device does "out of the box", and whether or not additional software is needed to do what you want. If you purchase a unit, we also suggest insisting on a 1 month return, should the unit not work out in your environment.
Rupa V, Job A, George M, Rajshekhar V. Cost-effective initial screening for vestibular schwannoma: Auditory brainstem response or magnetic resonance imaging? Otolaryngol Head Neck Surg 2003 Jun;128(6):823-8
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