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Cochlear Microphonics (CM)

Table of contents

  1. What is a cochlear microphonic?
  2. Why measure cochlear microphonics?
  3. Preparing for the test
  4. Test procedure
  5. Cochlear microphonic results
  6. References
 

What is a cochlear microphonic?

The cochlear microphonic is a response from the cochlea that mimics the input stimulus and is believed to be a response primarily from the outer hair cells [1].

 

Why measure cochlear microphonics?

The presence of a cochlear microphonic along with an absent or abnormal auditory brainstem response (ABR) is used in the diagnosis of auditory neuropathy spectrum disorder (ANSD).

 

Preparing for the test

Before testing, you need to consider the following:

  1. Patient preparation
  2. Electrode placement
  3. Transducer selection

 

1. Patient preparation

This is very important.

The patient should be relaxed or sleeping in a quiet environment and lying down during the procedure.

 

2. Electrode placement

You can measure cochlear microphonics with a standard ABR electrode montage.

But you will get the strongest signal with the electrodes positioned as close to the site of generation as possible.

The most used electrodes for this purpose are the gold foil TipTrodes or TM-trodes.

Below are two examples of electrode placements:

  1. Electrode placement using the EPA4 cable collector with a TM-trode (Figure 1).
  2. Electrode placement using the EPA3 cable collector with a TM-trode (Figure 2).

For both examples, you must prepare the TM-trode and the test ear before placing the TM-trode on the tympanic membrane.

This includes:

  1. Placing the TM-trode in a saline solution for a few minutes and dipping it in electrode contact gel to reduce impedance.
  2. Draining the test ear.

Man shown with electrode placement for right and left test ears. TM-trode on tympanic membrane in test ear, electrode on opposite mastoid, and active and ground electrodes on high forehead and low forehead, respectively.

Figure 1: Example of electrode placement using the TM-trode and EPA4.

 [Man shown with electrode placement for right and left test ears. TM-trode on tympanic membrane in test ear, and active and ground electrodes on high forehead and low forehead, respectively.

Figure 2: Example of electrode placement using the TM-trode and EPA3.

 

3. Transducer selection

You should use insert phones, as they allow you to perform a baseline recording.

You do this by clamping or pinching the insert phone silicone tubes and then measuring the response.

This eliminates the stimuli to the patient’s ear allowing you to distinguish electrical artifacts from a true cochlear microphonic.

Note that you should place the transducers away from the electrodes and their cables.

 

Cochlear microphonic test procedure

The procedure is outlined below step-by-step, using the Eclipse with the licensed cochlear microphonic protocol.

 

Step 1

Enable the A/B buffer in the toolbar menu.

Using an alternating polarity stimulus, the A/B buffer will separate the condensation and rarefaction components after the measurement.

You can also change the polarity to measure one curve as condensation and one curve as rarefaction in ‘Temporary Setup’.

 

Step 2

Perform a baseline measure with the tube clamped.

Make sure not to move the transducer when doing so.

 

Step 3

Select ear and intensity and start the measure.

You should use clicks at the intensity level of 85 dB nHL with a stimulus repetition rate of 87.1 per second [2].

This is because the cochlear microphonic is a pre-neural response.

It is thus not susceptible to neural adaptation seen when recording responses from the central auditory system.

Remember to check the EEG during testing to make sure noise is minimal.

 

Step 4

Check the response on the screen in the first few milliseconds.

2000 sweeps are often enough when measuring with the A/B buffer enabled.

But you should not use the number of sweeps as a stop criterion on its own.

You should continue recording until the noise floor becomes sufficiently low.

Because of the high repetition rate the test time should be very short.

 

Step 5

If a cochlear microphonic is present it is important to ensure that it is not a stimulus artifact.

You can confirm the presence of the cochlear microphonic by a sinusoidal segment in the stimulus polarity waveforms beginning within one millisecond of the stimulus, which can last up to five or six milliseconds.

To check that this is not a stimulus artifact, perform a control run using the same parameters.

But this time, clamp the insert earphone tubing, which will stop the sound from reaching the ear.

If the measured potential remains it is due to the stimulus artifact and is not a true cochlear microphonic.

If the response disappears then this indicates a true response.

 

Cochlear microphonic results

Below are two examples of cochlear microphonic responses (Figures 3 and 4).

Cochlear microphonic in a baby with ANSD. Rarefaction (top curve), condensation (middle curve), and baseline with tube clamped (bottom curve). Sinusoidal segment in both rarefaction and condensation curves, while response disappears in the baseline, indicating a true response.

Figure 3: Cochlear microphonic in a baby with ANSD. Rarefaction (top curve), condensation (middle curve), and baseline with tube clamped (bottom curve) [3].

Cochlear microphonic in a normal infant. Rarefaction (top curve), condensation (middle curve), and baseline with tube clamped (bottom curve). Sinusoidal segment in both rarefaction and condensation curves, while response disappears in the baseline, indicating a true response.

Figure 4: Cochlear microphonic in a normal infant.

Patients with ANSD show an abnormal cochlear microphonic, seen as a greater than normal amplitude of the response within the first milliseconds.

Also, the duration of the cochlear microphonic is often longer than expected.

Please note that the response itself is not enough documentation for ANSD and you must support it with an ABR recording to examine if the ABR response is present or absent.

 

References

[1] Dallos P. Some electrical circuit properties of the organ of Corti. I. Analysis without reactive elements. Hear Res. 1983 Oct;12(1):89-119. doi: 10.1016/0378-5955(83)90120-x. PMID: 6319350.

[2] British Society of Audiology (2019). Recommended Procedure Cochlear Microphonic Testing [Online]. Available from: https://www.thebsa.org.uk/wp-content/uploads/2019/01/FINAL-JAN2019-Recommended-Procedure-for-Cochlear-Microphonic-Testing-GL21-01-19.pdf.

[3] Stevens et al. (2011). Guidelines for Cochlear Microphonic Testing [Online]. Available from: https://www.thebsa.org.uk/wp-content/uploads/2015/02/CM_Guidance_v2_2109111.pdf.

July 2016
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