ECOG TESTING (Electrocochleography)

Timothy C. Hain, MD

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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. 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).

Mechanism

The conventional explanation for the summating potential (SP) of ECOG is that there is a nonlinear response in Ressner's membrane caused by elevated endolymphatic pressure and distension. 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)

Method

The technique involves an electrode consisting of a gold sponge, wire, wick or spring placed on the ear canal, or a needle that transfixes the ear drum. 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.

Needle type ECOG's have 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 main problem plaguing ECOG is noise. 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.

Generally, the results are reported as an SP/AP ratio, for which generally a ratio of 0.5 or greater is considered abnormal (some labs use 0.35). (Chung et al, 2004). 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).

It is important when interpreting ECOG however to consider the noise level, which is generally assessed by obtaining multiple trials. If they are all similar, then the standard error is 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 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.

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. This creates somewhat of 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. 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.

Variant ECOG procedures

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.

Challenge ECOG procedures

Recently 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).

Dr. Hain's take on ECOG

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. The status of the challenge tests is presently uncertain.

Insurance.

ECOG is usually covered by health insurance, e.g. Aetna and BCBS. ECOG may be indicated for diagnosis of hearing loss or vertigo.

Research

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

• Cheng J, He L and Gu R (1994). "[Relationship between summating potentials and perilymphatic fistula]." Zhonghua Er Bi Yan Hou Ke Za Zhi 29(5): 278-81.
• Chung WH and others. Clinical usefulness of extratympanic electrocochleography in the diagnosis of Meniere's disease. Otol Neurotol 25:144-149, 2004
• Devaiah AK, Dawson KL, Ferraro JA, Ator GA Utility of area curve ratio electrocochleography in early meniere disease. Arch Otolaryngol Head Neck Surg 2003 May;129(5):547-51
• Filipo and others. Electrocochleographic findings: Meniere's disease vs. sudden sensorineural hearing loss. Acta Otol 526:21-3, 1997
• Gamble BA, and others. Salt-load electrocochleography. Am J. Otol 20:325-330, 1999
• Ge X, Shea JJ Jr. Transtympanic electrocochleography: a 10-year experience.Otol Neurotol 2002 Sep;23(5):799-805 .
• Levine S, Margolis RH and Daly KA (1998). "Use of electrocochleography in the diagnosis of Meniere's disease." Laryngoscope 108(7): 993-1000.
• NAM EC, Won JY. Extratympanic electrocochleographic changes on noise-induced temporary threshold shift. Otolaryngol Head Neck Surg 2004;130:437-42.
• Ng M, Srireddy S, Horlbeck DM and Niparko JK (2001). "Safety and patient experience with transtympanic electrocochleography." Laryngoscope 111(5): 792-5.
• NOELL CA, Roland PS, Mabry RL, Shoup AG. Inhalant allergy and Meniere's disease: Use of electrocochleography and intranasal allergen challenge as investigational tools. Otolaryngol Head Neck Surg 2001; 125: 346-50.
• Santarelli R and Arslan E (2002). "Electrocochleography in auditory neuropathy." Hear Res 170(1-2): 32-47.
• Yokoyama K, Nishida H, Noguchi Y and Komatsuzaki A (1999). "Hearing impairment in patients with acoustic neuroma--analysis by electrocochleography." Auris Nasus Larynx 26(4): 401-9.