Question
What do you expect to see in ECochG testing using rarefaction and condensation clicks in normal individuals?
Answer
Most normal hearing persons have very small cochlear microphonic (CM) waves on separate stimulation with rarefaction and condensation clicks. With a typical earlobe or mastoid electrode, even the ECochG AP component (wave I of the ABR) is only about 0.2 or 0.3 microvolts, so the CM is almost impossible to see unless there's no noise in the recording. Using a TM electrode, however, it's possible to see clear CM in a normal hearing subject and it's very easy to see the CM with the trans-tympanic approach (although it's hard to find normal hearing subjects who will participate in a study).
The CM in many ANSD patients is abnormally enlarged (maybe due to dysfunction of the efferent system) and often easy to detect even with an ABR recording technique.
This Ask the Expert was asked as a follow up question to Dr. Hall's course, Clinical Applications of Electrocochleography Today, which can be accessed in the AudiologyOnline Course Library.
