Electrocochleography (ECochG) is a measure of the electrical potentials of the cochlea. Typically, the measurement is characterized by the stimulus onset (baseline), the response of the cochlea to the stimulus (summating potential - SP), and response to the synchronous firing of nerve fibers (action potential -AP). The AP is also known as Wave I. The cochlear
microphonic (CM) is also part of the ECochG and has its own protocol. Measuring the CM requires slightly different test parameters than the SP and AP and for this reason it is described in a separate quick guide.
Certain vestibular and auditory conditions may be diagnosed with ECochG. The ECochG is primarily used to diagnose Meniere’s Disease, particularly Cochlear Hydrops. The SP and AP amplitudes, latencies and their relationship are used to diagnose these conditions. Perilymph Fistula, sudden hearing loss and other pathologies may result in abnormal ECochG results. Recent studies indicate that Superior Canal Dehiscence (SCD) may also result in elevated SP/AP ratios (Devaiah et al., 2009).
Surface electrodes are not adequate for recording ECochG. It is recommended to use Tiptrodes, TM-trodes or Transtympanic electrodes to measure the electrocochleogram. While transtympanic electrodes will result in the most robust response but are not feasible for most clinics. Gold foil Tiptrodes are sometimes used but TM-trodes will produce larger responses as it is closer to the site of generation. The following is an example of preparation and electrode placement performed with a TM-trode. Note the procedure should only be performed by trained professionals.
The patient must lie down and should be relaxed or sleeping in a quiet environment during the procedure. An examination of the ear canal and TM must be performed prior to performing the test.
The electrode sites must be prepared and cleaned in order to obtain acceptable low skin impedance. It is recommended to have impedance values be 5kΩ or lower for Tiptrodes. The impedance values between one another should be balanced or similar in value. For TM-trodes the impedance should be 20kΩ or lower. It may be quite difficult to obtain such low impedance on the ECochG test ear electrode and higher levels may be accepted.
The ECochG test leads must be used to acquire the waveform. Below is an example of the electrode placement using the TM-trode and Tiptrode with the EPA4 and an example of the EPA3 with a TM-trode and Tiptrode.
The TM-trode or Tiptrode and the test ear must be prepared prior to placement. 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. If using a Tiptrode, use a cotton swab with a bit of nuprep on it, and gently clean the ear canal. Then cover the Tiptrode in conductive gel, and gently squeeze the Tiptrode, and insert it in the patient's ear.
EPA4 TM-trode example
When using EPA4 together with a TM-trode the red TM-trode cable is moved when switching ear.
EPA3 TM-trode example
Only 1-channel is needed to perform an ECochG with the TM-trode and for simplicity EPA3 can be used.
EPA4 Tiptrode example for right and left ears
EPA3 Tiptrode example, right ear
The procedure discussed below is simply a suggested process to be used as a guideline. Consult your Instruction for Use or Eclipse Additional Information to learn how to create or modify a protocol.
Choose the protocol ECochG Click
Manual Mode: To begin the manual mode, choose the intensity and select the ear to test on the Record sheet.
Next choose Start (or hit F2).
During testing monitor the EEG to assure a collection with minimal noise. The EEG levels should be low and consistent. As averaging commences, the waveform will appear on the screen.
HINT Waveform Scaling can be increased or decreased by using the arrows on the top left side of the recording window or on your keyboard.
HINT Window sizing may be changed during testing by selecting one of the arrow keys on the bottom, right side of the recording window or using the arrows on your keyboard.
Waveforms are marked from the Edit sheet during or after testing either manually or automatically. Amplitude Ratio or Area Ratio Calculation will automatically be computed once the required labels are assigned. The ratio selection is found in the General Setup.
To mark a selected waveform, click the appropriate waveform marker in the Edit sheet (or select 1-6 on the keyboard). Now bring the mouse to the correct position on the waveform and click to place the marker (or hit Enter).
HINT You can use the digital filters to “clean up” noisy data even after a completed test or run. You’ll find this feature in the bottom of the Edit sheet.
PBSL=Baseline, SP= Summating Potential, AP=Action Potential, BLst=start of baseline, Blend=end of baseline, AP1=start of AP, AP2=end of AP
Amplitude ratio is simply marked with the baseline, the summating potential and the action potential. A ratio between the BSL/SP and BSL/AP is calculated automatically by the system. Abnormal SP/AP amplitudes are exceeding a ratio of 0.53 as the critical value (Devaiah et al., 2009).
Area ratio is marked by first marking the start of the baseline (BLst). The BL end will be marked automatically at the next point in the waveform where the amplitude crosses this baseline. If the waveform does not allow this, you can place the BL end manually. Now mark the SP and the AP1 (the beginning of the AP). Next mark the AP peak. Finally mark the AP2, which is where the AP ends and “changes direction”. A ratio is calculated automatically by the system. Abnormal SP/AP area ratios are exceeding a ratio of 1.94 as the critical value (Devaiah et al., 2009).
Choose the Report Icon. When complete, choose Save and Exit.
Devaiah, A.K., Dawson, K.L., Ferraro, J.A., & Ator, G.A. (2009). Utility of area curve ratio electrocochleography in early meniere disease. Arch Otolaryngol Head Neck Surg, 129, 547-551.