Observations on mastoid versus ear canal recorded cochlear microphonic in newborns and adults

The cochlear microphonic (CM) may play an important role in the diagnosis of auditory neuropathy (AN) in newborns. However, since the CM tends to mirror the waveform of the acoustic stimulus, conscientious recording methodology must be applied to separate true response from artifact. The difficulty in achieving this separation has limited the clinical usefulness of the CM. In an effort to call attention to the importance of recording protocol when measuring the CM, the present study was designed to optimize CM recordings in humans by investigating the following parameters: (1) secondary minus electrode recording site (mastoid versus ear canal [EC]), (2) stimulus parameters, and (3) grounding and shielding conditions. Normative data were collected in full-term newborns (n = 7) and adults (n = 4) with no known risk factors for cochlear or retrocochlear pathology. Results suggest that the CM is easier to separate from stimulus artifact using an EC electrode and toneburst stimuli. In addition, electromagnetic shielding and grounding of the electrode cables and the acoustic transducer were effective in reducing and/or eliminating stimulus artifact. Results from this normative study may be helpful in improving the diagnostic utility of the CM in AN and other hearing-related disorders.