Electrocochleography in CI Subjects

The field of ECochG in CI subjects is expanding at a high rate. Three projects that bear on this topic will be described. The ECochG is to sound stimulation, and responses are obtained from nearly all (95%) of CI subjects.

1. Sources of ECochG potentials
The ECochG signal is complex, with a variety of components produced by hair cell and neural elements. Using neurotoxins in gerbils we have separated these components, and further separation was achieved with furosemide/kanamycin which selectively removes outer hair cells. In human CI subjects each ECochG components is highly variable, so individual characterization is required to interpret the results.

2. Advances in ECochG monitoring during insertion
Monitoring using ECochG can potentially identify and avoid trauma to cochlear tissue to improve hearing and speech perception outcomes. Recording sites can be extracochlear or intracochlear via recordings from the array itself. Each location has strengths and weaknesses that will be discussed. An important result is that reductions in response magnitude can be an unreliable indication of trauma, due to interactions between sources that influence the phase as well as the amplitude of the response to tones.

3. ECochG Correlations with speech perception outcomes (adults and children)
The ‘total response’, or summed magnitude of the responses to a series of tone frequencies, accounts for a high proportion of variability in adults (46% in 6-month CNC scores) and older children (>5 years, 36% using PB-K scores). The total response is primarily composed of responses from hair cells disconnected from spiral ganglion cells. In these subjects where the hearing loss is progressive, the proposed mechanism is that the total response is a measure of cochlear health that correlates with neural survival.

In young children the total response accounts for less of the variability in speech perception outcomes (15% using PB-K scores). The interpretation of the lower correlation is that children with congenital hearing loss who have cochlear functional disorders will show poor total responses but have good speech perception outcomes, while those with neural developmental abnormalities will show good total responses but poor speech perception outcomes.