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 (Dallos, 1983).
The presence of a cochlear microhpnic along with an absent or abnormal ABR is used in the diagnosis of auditory neuropathy spectrum disorder (ANSD). When a click ABR or CE-Chirp® LS is not present or abnormal, it is common to perform a cochlear microphonic test. Determining the presence or absence of the cochlear microphonic (OHC response) is an important part of the ANSD test battery.
The patient should be relaxed or sleeping in a quiet environment and lying down during the procedure.
It is possible to obtain cochlear microphonics with a standard ABR electrode montage, however the strongest signal will be obtained with electrodes positioned from a point as close to the site of generation as possible. The most commonly used electrodes for this purpose are the gold foil TipTrodes or TM-trodes.
Below are two examples of electrode placements
For both examples the TM-trode and the test ear must be prepared prior to placing the TM-trode on the TM. To reduce impedance a solution of saline can be used. Drain the ear prior to inserting the TM-trode. The TM-trode can be placed in a saline solution for a few minutes prior to placing it on the TM and should be dipped in electrode contact gel (e.g. Sonaville) prior to placing it at the TM.
Example of electrode placement using the TM-trode and EPA4
Example of electrode placement using EPA3 and with a TM-trode electrode
The EPA3 is a simple alternative when only 1-channel is desired and using the TM-trode.
Insert phones must be used, as they allow you to perform a baseline of the recordings. This is done by clamping or pinching the insert phone silicone tubes and then measuring the response. This eliminates the stimuli to the patient’s ears allowing you to distinguish electrical artifacts from a true CM response.
Note the transducers should be placed away from the measurement electrodes and its cables.
The Eclipse comes with a pre-programmed protocol for CM testing (license) so the system is ready to use immediately. Protocols can be created or modified easily to fit your clinic needs. Consult your Eclipse Additional Information to learn how to create or modify a protocol. The procedure described below is a guideline for CM testing.
Below are two examples of cochlear microphonic responses. The upper is a case of ANSD, while the lower is measured on an individual with normal cochlear microphonic function.
The upper example shows a cochlear microphonics of a baby with ANSD, showing rarefaction, condensation and base line with tube clamped (Stevens et al, 2011). The lower example shows a cochlear microphonic from an infant considered normal.
Patients with ANSD show an abnormal cochlear microphonic, seen as greater than normal amplitude of the response within the first milliseconds. In addition the latency of the cochlear microphonic duration is often longer than expected. Please note that the cochlear microphonic response itself is not sufficient documentation for ANSD and must be supported with an ABR recording to examine if the ABR response is present or absent.
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Dallos P. (1983) Some electrical circuit properties of the organ of Corti. I. Analysis without reactive elements. Hear Res;12:89-120.
Stevens et al. (2011) Guidelines for Cochlear Microphonic Testing, NHS v. 2.0 Edited 2014.