Timothy C. Hain, MD. Page last modified: December 13, 2011
ECochG is a variant of brainstem audio evoked response (ABR) where the recording electrode is placed as close as practical to the cochlea.
It is intended to diagnose Meniere's disease, and particular, hydrops (swelling of the inner ear). ECochG may also be abnormal in perilymph fistula and in sudden hearing loss (Filipo, 1997). The cochlear microphonic of ECochG may be normal in auditory neuropathy (Santarelli and Arslan 2002) as well as other disorders in which the cochlea is preserved but the auditory nerve is damaged (Yokoyama, Nishida et al. 1999). ECochG's have also been used to as a indicator of the temporary threshold shift that may follow noise injury (Nam et al, 2004).
The conventional explanation for the summating potential (SP) of ECochG is that there is a nonlinear response in Reissner's membrane caused by elevated endolymphatic pressure and distension. Another name for the elevated pressure is "Hydrops". Other sources of nonlinear responses are rarely considered and also may be responsible for generation of the SP in some situations.(Cheng et al. 1994).
|Figure 1: Normal ECochG. The ECochG is interpreted by comparing the height of the SP to the AP.|
|Figure 1: Equipment used to record an ECochG, a Bio-Logic Navigator Pro||ECochG electrode -- the blue tip is placed so that it is next to the ear drum. After an ECochG is done, there often is a little bit of blue gel that remains in the ear canal.|
The technique involves placing an electrode into the outer ear canal, so that it is close to the ear drum. Our preference here in Chicago is to use a commercial wick electrode (sold by Bio-Logic). These are very expensive -- but they work nicely. Other variants include a gold sponge (Nicolet Tiptrode), a wire or spring placed in the ear canal, or a needle that transfixes the ear drum. In general, these are either less effective or more painful. .
Needle type ECochG's have fallen out of favor because they are generally judged to be unreasonably invasive and are painful too (Bonucci and Hippolito, 2009). Their main use would seem to be in a person who has a perforation, as in this case, the needle can be place through the perforation and provide a better signal without doing any additional damage to the ear. Complicationsof needle ECochGs done on intact eardrums mainly include perforation and pain(Ng, Srireddy et al. 2001). Nevertheless they are still used in some settings (Ge and Shea, 2002). In our experience, the wick electrodes work very well and it is simply not generally necessary to use an invasive methodology.
The ear is stimulated with alternating polarity clicks (although tone bursts can also be used). The objective is to record wave-1 (there are 5 waves), with greater accuracy and to detect the "summating potential", which is a shoulder on wave 1. In some instances, the cochlear microphonic is inspected. A person with an intact cochlear microphonic but reduced or absent wave 1, might have a 8th nerve site of lesion. In other words, the sound gets to the cochlea (creating the microphonic), but doesn't make it into the brainstem (i.e. no waves 1-5)
|Figure 2: Abnormal ECochG on right side. Note the higher SP on the right.|
ECochG results are reported as an SP/AP ratio, for which a ratio of 0.5 or greater is considered abnormal. While some labs use 0.35 instead, (Chung et al, 2004), considering all of the technical problems with ECochG (see below), we think it best to be more conservative. The sensitivity of the SP/AP ratio to Meniere's disease 60-71%. The sensitivity is said to be higher if it is performed during a symptomatic period (Devaiah et al, 2003), but it is difficult to get patients scheduled for the test when they are symptomatic. Also, some authors disagree that sensitivity is increased when patients are symptomatic (Levine, Margolis et al. 1998). ECochG is vulnerable to operator bias (see below), which adds to the confusion.
|Figure 3: Noisy (useless) ECOG on L, obtained in person with severe hearing loss on L. Note that this ECOG is recorded "upside down" compared to the other traces in this document. They simply reversed the electrode input settings. We prefer it shown the other way around. A good thing about this recording is that the operator did not discard traces that he/she did not like. One can look at the left side and say - -this has no value. Sometimes operators are "sneaky" and eliminate noise traces that don't have much resemblance to an ECOG. This causes trouble.|
ECochG testing is technically difficult, has a questionable rationale, lacks reliable norms, lacks standards, and is vulnerable to operator bias.
The main technical problem plaguing ECochG is noise (see above). It requires getting a small electrode deep inside a small cavity (the external ear canal). The ear must be clean. There must be hearing or nothing will be recorded. Practically, it is unrealistic to get an ECOG on someone with more than a 40 DB sensorineural hearing loss.
Considerable patience is required. The test takes at least an hour. Certain electrodes have a better signal to noise ratio than others. We have not had much luck with the gold foil "tiptrodes". Gelled cotton wick electrodes, such as can be obtained from Biologic, work better. While needle electrodes have the best signal to noise ratio, these can be poorly tolerated by patients
Rationale for test: As about 6% of the population has hydrops on autopsy (Honrubia, 1999; Rauch et al, 2001), one would expect that 6% of the otherwise normal population would have a positive ECochG. Considering that only 1/2000 people have Meniere's disease (0.2%), one would expect a high number of false-positives in ECochG testing. One would also expect a very low number of false negatives.
From this perspective, the main utility of ECochG testing should be to detect hydrops, and to rule out Meniere's (as if one doesn't have Hydrops, then Meniere's shouldn't be the diagnosis). Furthermore, if one doesn't have hydrops, then the low-dose entailing protocol for Meniere's disease might be something to avoid.
The main difficulty with this logic is that ECochG testing is difficult, and often uninterpretable in the very population that is of interest (persons with Meniere's), because of hearing loss.
So, to summarize, good judgment, experience, and clinical knowledge are a must to properly interpret ECochGs.
Lack of reliable norms: It is important when interpreting ECochG to consider the noise level, which is generally assessed by obtaining multiple trials. If they are all similar, then the standard deviation should be small and the result is likely to be correct. If they vary widely, the reliability of the average SP/AP ratio may be questionable. Similarly, if multiple trials are done and the SP/AP ratio varies wildly, one's confidence in the result should be lower than if it is very tight. At the present writing, typically little nor no consideration is given in reports to the confidence intervals for ECochG results. We would like to see some research done on this, and a tightening up of standards for reporting of ECochG's.
Practically, standards for ECochG testing are sketchy, and ECochG is vulnerable to operator bias . We have observed some labs acquire multiple ECochG runs, select out the ones that look "similar", average them, and delete the rest. The individuals who acquire ECochG's are often understandably reluctant to have the outside world see their "bad" runs. Unfortunately, this procedure can easily create a "response" from random noise, and also gives the operator (generally an unblinded audiologist who is given the working diagnosis of Meniere's disease) the opportunity to create abnormal results out of noise. We advise a methodology where all ECochG runs are at least displayed, so that the interpreter can determine the noise level.
|Figure 4A. Audiogram with reduced high frequency hearing on right||4B. ECochG for individual shown on the left -- nothing obtained on the right side due to high frequency hearing loss.|
Persons with poor high-frequency hearing , such as most older individuals, are likely to have higher noise levels, and therefore the limit of normal for their ECochG should be set higher. It is also quite possible for there to be no ECochG at all on the side of hearing loss -- as shown above in figure 3.
This creates a "catch-22" situation with ECochG. ECochG is generally not considered unless there is a hearing loss, and hearing loss makes ECochG less reliable. The best candidate for ECochG is probably someone with a monaural low-frequency sensineural hearing loss, such as is often found in early Meniere's disease. The preservation of high-frequency responses makes the ECochG more likely to be valid and the low-frequency hearing loss also makes Meniere's more likely. Unfortunately, ECochG results are rarely reported as a function of hearing (e.g. Chung et al, 2004), making it difficult to interpret most studies.
The "area curve" SP/AP ratio has recently been suggested as a more sensitive method of detecting Meniere's disease (Deviah et al, 2003). In this method, the SP area is determined from response onset (also called "baseline"), to the first point after the AP where the response returns to baseline. In other words, it actually includes not only the SP but the entire AP. The AP area is determined from the onset of the AP through it's peak, and to the first opposite polarity peak. Although called the SP/AP area ratio, more accurately, it is the SP+AP/AP area ratio. It is difficult to say whether this method improves the diagnostic performance of ECochG, as one needs to know both sensitivity and specificity. It is possible that with this method one has just traded sensitivity for specificity. Only a small number of controls were reported. Time will tell whether this method is useful.
Gamble and others (1999) reported that salt-loaded ECochG may be useful in patients who have normal ECochG's but a history suggestive of Meniere's. Similarly it has been suggested that ECochG may be useful in detecting allergic Meniere's disease. ECochG is performed before and after challenge with an allergen (Noell et al, 2001). The status of the challenge tests is presently uncertain.
The author's experience with ECochG is mixed. It is a difficult test to perform. It is neither 100% sensitive nor 100% specific. In other words, you can have Meniere's disease but still have a normal ECochG, and you can also have an abnormal ECochG but not have Meniere's disease. We think it is a modestly useful test, most helpful in situations where other tests have failed to produce a clear answer as well as in situations where one is planning an invasive treatment such as gentamicin. It not indicated as a screening test because of the false-positive problem.
ECochG sometimes sometimes documents "Hydrops" (i.e. high pressure) , in a person who does not have a hearing loss. When this occurs, we will generally treat using the same protocols as we use for Meniere's disease. Whether or not this is useful is presently unknown.
Advice for patients: The most important consideration is who/where the test is done.
ECochG's are usually done in subspecialty otology practices. For example, we think that our practice in Chicago is one of just a few that do them well in a large metropolitan area. Pick a clinical setting where ECochG testing is done routinely.
We suggest avoiding having ECochG done in community settings, as well as having ECochG's done by persons with little experience in neurophysiology. It is not the sort of test that one would want someone to do who just finished their training, or who learned it last week. Technicians can certainly do ECochG's very well and it is not necessary to have an audiology or "AuD" degree to do an ECochG. Experience, aptitude and training (roughly 8 hours) is what is needed to do a good job. It is also not a test that can reasonably be done in a setting that cannot remove ear wax (ie. by the technician that does evoked resposnes in a Neurology testing center). We don't think that ECochG's done using needle electrodes are generally a good idea, and we suggest refusing the test unless the reason to use a needle (over a "Bio-Logic" electrode) is justified (for example, a perforation would be an acceptable reason).
Be sure that you get copies of the ECochG, including all traces. A type-written report is not sufficient (as there is no way to determine quality).
ECochG is usually covered by health insurance, e.g. Aetna and BCBS. ECochG may be indicated for diagnosis of hearing loss, but the main reason is for vertigo.
ECochG has been well studied. In Pubmed, there are approximately 300 publications with ECochG in their title, going from 2003 to 1970.
Heather Rudisill, AuD contributed to an earlier version of this web page.
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