Electrocochleography (ECochG)


Written by Travis M. Moore
Last edited 19-Jul-2020


Survey of Auditory Evoked Potentials

Congratulations, you are now at a point where you can begin learning specific electrophysiological tests! The next several lessons will cover various auditory evoked potentials, including: their proposed site of origin, recording parameters, stimulus parameters, waveform morphology, and clinical applications. We will organize our survey of clinical assessments by starting with tests that examine more peripheral structures, and ending with tests that record the most central potentials from the cortex itself.

Electrocochleography

The cochlea is the most peripheral structure in the auditory system that gives rise to electrical potentials. Believe it or not, we can measure the tiny electrical activity from the cochlea in response to sound. The audiological test for doing so is called electrocochleography, abbreviated ECochG. Electrocochleography is also sensitive to the firing of auditory nerve fibers (first order neurons) activated by inner hair cells (IHCs).

Let's be clear though, the ECochG potentials that we measure in clinic arise from the OHCs and the auditory nerve fibers (ANFs). While the inner hair cells cause ANFs to depolarize, we're not actually measuring IHC potentials here. You can think of the hair cell potentials from ECochG as an electrophysiological correlate of DPOAEs. The ANF activity recorded using ECochG can also be seen as Wave I of the auditory brainstem response (ABR).


Potentials of the ECochG

The output of ECochG is called an electrocochleogram, and is unfortunately also abbreviated ECochG. (You could also see ECochGm.) You'll know from context clues which word is meant.

Electrocochleography produces three waveforms: two from the OHCs and one from the ANFs. The potentials measured from the OHCs are called the cochlear microphonic (CM) and the summating potential (SP). The OHC potentials are from a class of potentials known as receptor potentials, which means the CM and SP arise from sensory cells. In other words, they show sensory transduction happening (i.e., OHCs depolarizing). Because the activity of receptor potentials is tied to sensation, their waveforms tend to "follow" the shape of the stimulus, rather than produce peaks and troughs related to more abstract neural firing like most potentials. The potential measured from the ANFs is called the compound action potential (CAP), and represents the initial activity of the auditory nerve to sound.

TABLE 1. Summary of ECochG potentials.
Potential Abbreviation Origin
Cochlear Microphonic CM Outer hair cells
Summating Potential SP Outer hair cells
Compound Action Potential CAP Auditory nerve fibers

Cochlear Microphonic

If we present a 250-Hz tone burst, we should see a roughly sinusoidal-looking CM on the computer screen at 250 cycles per second. Not convinced? Let's think about why this happens. A 250-Hz sine wave enters the cochlea, containing a series of peaks and troughs. At each peak, the basilar membrane moves, and OHCs fire. Since that's what we're recording, well see a change in voltage. The the sine wave drops into a trough, the basilar membrane moves again, and OHCs fire again. We see another change (i.e., a drop) in voltage. So really it makes sense that we see a waveform that follows the stimulus if we're recording from the sensory cells involved in transduction. See Figure 1 below for a look at the CM.

TM electrode
FIG. Isolated cochlear microphonic.

In fact, the waveforms recorded from auditory neurons mimic the stimuli so closely, if you play the recorded electrical waveforms through a speaker, you can reproduce them! Listen to the audio clip below from Dr. Nina Kraus' lab, which starts playing recorded music, then switches to neural output recorded from the brain(stem). The evoked potential used for this recording was not the CM, but the frequency-following response (related in principle, but from the brainstem).

Playing back neural recordings
FIG. © Nina Kraus

If you don't think that's cool, you're too hard to impress. Let's make an observation based on what we just heard. You know that far-field neural recordings are messy due to unwanted electrical noise. Those recordings were just played through a speaker. Notice all the extra acoustic noise in the signal when the sound clip switches to the electrophysiological recording? Now instead of seeing noise in recorded potentials, you're hearing it played as sound. Again, if you don't think that's cool, you need to recalibrate.

Summating Potential

The SP is a little less exciting. Instead of following the peaks and troughs (i.e., fine structure) of the stimulus, the SP shows a change in voltage when a stimulus begins that is maintained until the stimulus ends. It's sort of like an "on/off" indicator. If there is no stimulus, the SP is at baseline, if a stimulus is played, we'll know when and for how long due to the change in voltage from the SP. Figure 3 below shows the brief SP in response to a click. Note that the CAP immediately follows the cochlear activity indicated by the SP.

Click-evoked summating potential and compound action potential
FIG. The summating potential (and compound action potential) evoked by a click.2

Are you starting to sense a problem with recording the CM and SP? If the CM mimics the stimulus and the SP is "on" during the stimulus, that means both potentials occur at the same time. And yes, that can be problematic for obvious reasons. The waveforms we record are messy enough without having to deal with simultaneous signals. What typically happens is the CM "rides" on the SP (Figure 4). It turns out the CM isn't very diagnostically useful at the time of this writing, so ideally we could just get rid of it. Luckily, there is a simple but elegant solution. Can you guess what it is?

Cochlear microphonic obscuring the summating potential
FIG. Cochlear microphonic obscuring the summating potential.2

Isolating the Summating Potential

Remember the CM mimics the acoustic stimulus. And how do we cancel out acoustic stimuli? Just like the differential amplifier, and as discussed in the lesson on additive synthesis, if we combine two waves that are 180 degrees out of phase ("flipped"), they cancel each other out. If we apply this logic to our ECochG stimulus, we can alternate presenting clicks that start with a peak (i.e., rarefaction) and clicks that start with a trough (i.e. condensation). Averaging over many sweeps, the alternating polarities cancel out the CM. That is, OHCs' response to the peaks and troughs, which are positive and negative, average to zero. But, the SP responds the same way (on or off) regardless of the polarity, so it is preserved. Take a minute to bask in the intersection of so many principles you have learned from various fields (e.g., acoustics, neurophysiology, electromagnetism) and realize how complex but simple this concept is.

Playing back neural recordings
FIG. Removing the cochlear microphonic.2

Compound Action Potential

The CAP is more straightforward than the cochlear potentials. We present a brief click, thousands of ANFs depolarize in synchrony, and we record the summed voltage that reaches the scalp. If you're wondering why this is referred to as the compound action potential (and not just action potential), it's to highlight the fact that thousands of fibers contribute to the response we see on the computer screen. This is the type of evoked potential we have been discussing all along, so there's not much else to say.


Interpreting the ECochG

Now that you know what the waveforms of the ECochG look like, it's time to learn how to quantify them. Recall that the two main pieces of information from evoked potential testing are (1) latency and (2) amplitude. The expected latency of the CAP is around 1 to 2 ms (the same) as Wave I of the ABR). So in the sea of squiggles you're likely to encounter, start your search at 1 ms. The SP is cochlear in origin (almost entirely from the OHCs), so it makes sense the SP should appear immediately before the CAP. In fact, it typically appears on the left-hand slope of the CAP (e.g. Figure 3). What you're seeing is the cochlea respond, then the auditory nerve.

There are two ways to report ECochG amplitude: peak-to-trough and using a prestimulus baseline. The latter seems to be used most often so we'll focus there, but both methods are shown in Figure 6. Remember that a prestimulus baseline involves recording a little "quiet" time before the stimulus is presented. The idea is that there won't be any huge voltage fluctuations if the patient is sitting quietly. Simply choose a point that is clearly before the SP (in a flat area if possible), and use that as your reference point for both the SP and the CAP amplitude. That means in order to find the SP amplitude, you would measure the change in voltage from the baseline, to the tough of the SP. To find the CAP amplitude, you would measure the change in voltage from the baseline to the trough of the CAP.

Playing back neural recordings
FIG. Peak-to-trough (left) and baseline (right) ECochG measurements.

Recording the ECochG

Recall that the inner ear is housed in the petrous part of the temporal bone, which is the densest bone in the body. (It's so dense it's often what CSI techs use to extract DNA when trying to identify a body.) So it should come as no surprise that extra dense, extra thick bone presents even more difficulty recording from the scalp. In fact, the voltage is so attenuated that we cannot measure ECochG from the scalp. Instead, we must measure even closer to the source (i.e., the cochlea). There are three possible sites: the external auditory meatus (EAM), the tympanic membrane (TM) and the cochlear promontory. Recall the promontory is a bulge in the medial wall of the middle ear caused by the first basal turn of the cochlea. It appears between the oval and round windows. These different sites require specialized electrodes.

TABLE 2. ECochG recording sites and corresponding electrodes.
Site Electrode Type Typical Amplitude6
External auditory meatus Foil electrode / tiptrode 1.5 μV
Tympanic membrane TM electrode 3 μV
Promontory Transtympanic / needle electrode 15 μV

Gold Foil Electrodes

When we record from the EAM, we're using the same principles as when we record from the scalp, we're just using the skin inside the ear canal. Now, there's no need to reinvent the wheel when deciding how to get an electrode in the EAM; we already have something that fits snugly in there: the foam tip of an insert earphone. We'll need an foam insert anyway to deliver the stimulus. Because foam doesn't conduct electricity, these foam tips are wrapped in thin gold foil (recall that gold is an excellent conductor. The idea is that the insert will expand and press the metal foil against the EAM (as well as pass sound). The electrode leads clip onto the gold-wrapped stem of the insert with tiny alligator clips. You can see what these gold foil electrodes (sometimes called tiptrodes) look like in Figure 7.

Gold foil electrodes
FIG. Gold foil electrodes for ECochG (or celebrity patients).

Gold foil electrodes seem to be the most common in U.S. clinics because they are non-invasive. You'll need to slather the foil with an electrolyte gel (similar to the stuff in the foam on a regular electrode), so it will feel damp, but otherwise no more inconvenient than a typical foam tip. The tradeoff is that recordings look fairly noisy and low-amplitude.

TM Electrodes

TM electrodes are just what they sound like: modified electrodes that rest against the TM. If you're wondering whether that requires local anaesthetic, the answer is no. It's not the most comfortable thing in the world, and placing them takes practice, but they're livable. The increased invasiveness gets us closer to the cochlea, and therefore produces a larger, cleaner recording. Take a look at Figure 8 to see what a TM electrode looks like.

TM electrode
FIG. © dizziness-and-balance.com

There's not really much to see here - it consists of a length of wire that is fed down the ear canal, and ends in a small blob of foam and electrode gel. The opposite end is business as usual and provides a connector to plug the electrode into the terminal box.

The answer to your other question is "yes," audiologists can and do place these electrodes. You need to use an otoscope so you can guide the TM electrode down the ear canal and make sure the end is resting nicely on the TM. These electrodes are quite expensive for a disposable tool, so these are relatively rare in clinics. Also that whole placing something on the eardrum thing.

Transtympanic / Needle Electrodes

The most invasive ECochG site in humans is the cochlear promontory. This site can be reached using a transtympanic electrode (trans- = through). This electrode is also called a needle electrode, which describes it very well (see Figure 9). Audiologists do NOT place this type of electrode, and it does require a local anaesthetic. Figure 10 illustrates how the needle is placed against the promontory.

Transtympanic electrode
FIG. Transtympanic electrode.
TM electrode
FIG. © Transtympanic recording setup.

Recording Parameters

The tables below provide the electrode montage, amplifier settings, and stimulus parameters typically used for ECochG with gold foil and Ag/AgCl electrodes.

TABLE 3. Single channel ECochG electrode montage.
Electrode Site
Non-inverting (active) Tiptrode in test ear
Inverting (reference) Contralateral mastoid
Ground (common) Fpz (low forehead)
TABLE 4. ECochG amplifier parameters.
Parameter Value
High-pass filter 10 Hz
Low-pass filter 1500 Hz
Gain 50,000
TABLE 4. ECochG stimulus parameters.
Parameter Value
Stimulus Click
Duration 100 μs
Intensity 90 dB nHL
Polarity Alternating
Rate 7.1 /s
Sweeps ~1500

Clinical Applications of ECochG

This section follows the Frontiers in Neuroscience review paper by Gibbons (2017).

Hearing Testing

Before less invasive tests became available, ECochG provided an objective method of assessing hearing function in children and other populations unable to respond behaviorally. Keep in mind the only information gained from ECochG for assessing hearing is the presence/absence of the CAP. So ECochG is not the same as a behavioral hearing test, but rather a test of OHC and ANF function. Futhermore, because ECochG is so difficult to measure using noninvasive electrodes (see Electrodes subsection below), the only way to find the CAP threshold is to record from an electrode touching the bony labyrinth.

Auditory Neuropathy Spectrum Disorder

Generally speaking, auditory neuropathy spectrum disorder (ANSD) is characterized by intact OHCs, but damaged ANFs and/or auditory brainstem. Therefore, clinical tests for identifying ANSD include otoacoustic emissions and the CM of the ECochG to assess OHCs, and the CAP and auditory brainstem response (ABR) to assess the auditory nerve and brainstem.

Endolymphatic Hydrops

Endolymphatic hydrops (ELH) refers to increased pressure in the membranous labyrinth of the inner ear due to an excess of fluid. That extra fluid pressure causes changes in how the basilar membrane and vestibular organs operate, which can lead to a variety of symptoms. Most commonly, ELH is associated with Ménière's disease (MD), but the two are not synonymous. ECochG testing typically reveals a larger than normal response (e.g., SP/AP ratio) in the presence of hydrops, and is therefore sometimes ordered when attempting to diagnose MD (which is not easy). What you need to keep in mind is that (1) ELH does not equal MD, and (2) the sensitivity of click ECochG to hydrops is not that great. (Tone burst ECochG appears to be more sensitive.)

Figure 11 shows the results of an example patient. The left panel reveals a larger than normal SP, leading to a larger SP/AP ratio (i.e., greater than 0.3). The right panel shows a normal SP amplitude. Thus, these results suggest the patient has ELH in the left ear, but not the right.

Endolymphatic hydrops in the left ear
FIG. Endolymphatic hydrops present in the left ear.3

Intraoperative Monitoring

Let's first define intraoperative monitoring. The prefix intra- in Latin means "within," so intraoperative means something that happens during surgery. In this case, we are monitoring responses from the cochlea (typically the CM) during a surgical procedure. Before this technology was available, surgeons could only take their best guess in trying to avoid damaging the inner ear/auditory nerve. With the advent of ECochG, audiologists can now continuously record the CM while the surgeon performs surgery. If the amplitude of the CM begins to decrease, the audiologist can alert the surgeon they are getting too close to important auditory structures and to try another approach. If the CM completely disappears, then damage has occurred and the audiologist works with the surgeon to try and restore function. These types of monitoring procedures have greatly improved hearing preservation during surgery, and are legally required in some areas.

Common procedures that can be monitored with ECochG:

  • Cochlear implant surgery
  • Perilymphatic fistula repair
  • Stapedectomy

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Additional Resources

The American Speech-Language-Hearing Association (ASHA) has a good page about short latency auditory evoked potentials, including general principles, ECochG, and ABR.

Test Your Understanding

Answer One
Answer Two
Answer Three


REFERENCES

Dallos, P., & Wang, C.-Y. (1974). Bioelectric correlates of kanamycin intoxication. Audiology, 13(4), 277-289.
Santarelli, R., Starr, A., Michalewski, H. J., & Arslan, E. (2008). Neural and receptor cochlear potentials obtained by transtympanic electrocochleography in auditory neuropathy. Clinical Neurophysiology, 119(5), 1028-1041.
Ferraro, J. A., & Durrant, J. D. (2006). Electrocochleography in the evaluation of patients with Meniere's disease/endolymphatic hydrops. Journal of the American Academy of Audiology, 17(1), 45-68.
Kulstein, G., Hadrys, T., & Wiegand, P. (2018). As solid as a rock—comparison of CE-and MPS-based analyses of the petrosal bone as a source of DNA for forensic identification of challenging cranial bones. International journal of legal medicine, 132(1), 13-24.
Tiefenbach, M & Shehata-Dieler, Wafaa & Cebulla, M. (2015). [Electrocochleography using Transtympanic, Ear Drum and Ear Canal Electrode in Diagnosis of Morbus Menière]. Laryngo-rhino-otologie. 94. 10.1055/s-0035-1547286.
Picton, T. W. (2010). Human auditory evoked potentials: Plural Publishing.
Gibson, W. P. (2017). The clinical uses of electrocochleography. Frontiers in Neuroscience, 11, 274.
Hood, L. J. (2015). Auditory neuropathy/dys-synchrony disorder: diagnosis and management. Otolaryngologic Clinics of North America, 48(6), 1027-1040.
Ziylan, F., Smeeing, D. P., Stegeman, I., & Thomeer, H. G. (2016). Click stimulus electrocochleography versus MRI with intratympanic contrast in Ménière's disease: A systematic review. Otology & Neurotology, 37(5), 421-427.
Hornibrook, J., Flook, E., Greig, S., Babbage, M., Goh, T., Coates, M., . . . Bird, P. (2015). MRI inner ear imaging and tone burst electrocochleography in the diagnosis of Ménière’s disease. Otology & Neurotology, 36(6), 1109.
hydrops. (n.d.) Farlex Partner Medical Dictionary. (2012). Retrieved July 7 2020 from https://medical-dictionary.thefreedictionary.com/hydrops
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