There have been a number of postings recently on how one should mark the
waveforms obtained from electrocochleographic recordings. However, a more
fundamental question is what do these things mean and how are they diagnostic?
For example, it appears that in many cases, what has been called SP is nothing
more than uncancelled CM. There is the distinct possibility that AP amplitude
might be reduced in the presence of hearing loss because of slight latency
differences between condensation and rarefaction, which would also result in a
larger SP/AP ratio. But does that mean anything. Finally, SP is generated by
hair cells which presumably are damaged when cochlear hearing loss exists. Why
then would their response be enlarged.
There has been a fair bit of work at Minnesota (Margolis and colleagues) that
should be considered when doing ECochG clinically. In our experience, the only
real benefit from recording with a TM or canal electrode is that we get a
bigger wave I, making a neuro-otologic ABR assessment more interpretable in
cases of hearing loss. Unfortunately, MRI has all but eliminated the use of
ABR in this regard, at least locally. However, we have had virtually no
success in using ECochG as a test in teh Diagnosis of Meniere's, an experience
that is shared by many others. Finding a good place to measure the baseline is
unlikely to rectify the problems.
Name: Michael P. Gorga
555 North 30th Street
Omaha, NE 68131
e-mail: gorga at boystown.org