Page last modified:
December 31, 2007
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Timothy C. Hain, MD and Heather Rudisill, AuD.
Page last modified:
December 31, 2007
ECOG is a variant of brainstem audio evoked response (BAER) 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). ECOG may also be abnormal in perilymph fistula and in sudden hearing loss (Filipo, 1997). The cochlear microphonic of ECOG 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). ECOG'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 ECOG 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).
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| Figure 1: Normal ECOG. The ECOG is interpreted by comparing the height of the SP to the AP. |
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| Figure 1: Equipment used to record an ECOG, a Bio-Logic Navigator Pro | ECOG electrode -- the blue tip is placed so that it is next to the ear drum. |
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). Other variants include a gold sponge, 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 ECOG's have particularly fallen out of favor because they are generally judged to be unreasonably invasive. Complications include perforation and pain (Ng, Srireddy et al. 2001). Neverthless they are still used with success in some settings (Ge and Shea, 2002).
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.
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| Figure 2: Abnormal ECOG on right side. Note the higher SP on the right. |
ECOG 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 ECOG (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). ECOG is vulnerable to operator bias (see below), which adds to the confusion.
ECOG testing is technically difficult, has a questionable rationale, lacks reliable norms, lacks standards, and is vulnerable to operator bias.
The main technical problem plaguing ECOG is noise. It requires getting a small electrode deep inside a small cavity (the external ear canal). The ear must be clean. 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 ECOG. Considering that only 1/2000 people have Meniere's disease (0.2%), one would expect a high number of false-positives in ECOG testing. One would also expect a very low number of false negatives.
From this perspective, the main utility of ECOG 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 gentamicin protocol for Meniere's disease might be something to avoid.
The main difficulty with this logic is that ECOG 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 judgement, experience, and clinical knowledge are a must to properly interpret ECOGs.
Lack of reliable norms: It is important when interpreting ECOG 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 ECOG results. We would like to see some research done on this, and a tightening up of standards for reporting of ECOG's.
Practically, standards for ECOG testing are sketchy, and ECOG is vulnerable to operator bias . We have observed some labs acquire multiple ECOG runs, select out the ones that look "similar", average them, and delete the rest. The individuals who acquire ECOG'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 audiologist who is given the working diagnosis) the opportunity to create abnormal results out of noise. We advise a methodology where all ECOG runs are at least displayed, so that the interpreter can determine the noise level.
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| Figure 3A. Audiogram with reduced high frequency hearing on right | 3B. ECOG 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 ECOG should be set higher. It is also quite possible for there to be no ECOG at all on the side of hearing loss -- as shown above in figure 3.
This creates a "catch-22" situation with ECOG. ECOG is generally not considered unless there is a hearing loss, and hearing loss makes ECOG less reliable. The best candidate for ECOG 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 ECOG more likely to be valid and the low-frequency hearing loss also makes Meniere's more likely. Unfortunately, ECOG 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 dermined 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 ECOG, 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 ECOG may be useful in patients who have normal ECOG's but a history suggestive of Meniere's. Similarly it has been suggested that ECOG may be useful in detecting allergic Meniere's disease. ECOG 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 ECOG 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 ECOG, and you can also have an abnormal ECOG 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.
Advice for patients: We suggest avoiding having ECOG done in community settings, as well as having ECOG'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 graduated from their training, or who learned it last week. Pick a clinical setting where ECOG testing is done routinely. Be sure that you get copies of the ECOG, including all traces. A type-written report is not sufficient.
ECOG is usually covered by health insurance, e.g. Aetna and BCBS. ECOG may be indicated for diagnosis of hearing loss or vertigo.
ECOG has been well studied. In Pubmed, there are approximately 300 publications with ECOG in their title, going from 2003 to 1970.
Links:
http://www.menieresinfo.com/ecog.html
| © Copyright May 11, 2008 , Timothy C. Hain, M.D. All rights reserved. Last saved on May 11, 2008 |