Tuesday, March 10, 2015
Your Audiology Tutorial: Electrocochleography (ECochG)
The ECochG requires no participation from the patient - the more relaxed they are the better- as they are laid supine on a table with sticky electrodes placed on the forehead (ground and reference) and recording electrodes that also act as transducers of a sound stimulus (click) are utilized. Where such electrodes are placed is dependent on which type is selected:
Tip Trodes: Gold (or other metal) foil covered foam inserts set usually between the first and second bend of ear canal. Easiest for patient to tolerate, but amplitude of waveforms' degradation is significant compared to.....
Tymp Trodes: Long, wick like electrode (silver wire enclosed within Silastic tube), the tip (soft foam sponge filled with conductive gel) of which makes contact with the tympanic membrane (eardrum). This is a more invasive method, requiring anesthesia.
Transtympanic Electrode: Needle is inserted through eardrum to rest on the promontory, a rounded hollow prominence near the first turn of the cochlea (organ of hearing), and the round window, an opening into the inner ear. TT electrodes are used while patient is sedated and anesthetized. As this needle is closest to the voltage generated by inner ear, the response will yield the cleanest, highest amplitude, and most recognizable waveforms, which consist of.........
1) Action Potential (AP): A compound action potential which documents activity of cranial nerve VIII fibers on their way out of the cochlea (distal end).
2) Summating Potential (SP): Direct current activity of the hair cells (sensory receptors) in the cochlea.
3) Cochlear Microphonic: Alternating current activity for hair cells. Clinically, polarity settings are sometimes set to "alternating" to enhance the SP and eliminate presence of CM so in order to have a Baseline marker.
The ratio between the SP and AP is considered when diagnosing Meniere's Disease. A calculation of 45-50% or higher is considered positive for MD. However, the ECochG test often has high variability and can result in false positive or false negative findings. Patients with hearing loss greater than 50 dB HL in the range of 1000-4000 Hz are not good candidates for the ECochG as waveforms may be compromised.