Electrical impedance measurements with the CI24M cochlear implant for a child with Mondini dysplasia

Electrical impedance measurements can give useful information about the status of individual electrodes of a cochlear implant. Impedances within the normal range (when measured in the common ground mode of stimulation) suggest that current flow occurs in the tissue and fluid of the cochlea. Low impedance measurements may suggest that particular electrodes are short circuiting, whereas high impedances might be due to a broken electrode wire or an electrode only in contact with air. In the case discussed, low impedance measurements were recorded intra-operatively from a child with Mondini dysplasia on electrodes 1-13 of a Nucleus CI24M device. Post-operatively the impedances had returned to the expected range and were comparable with other patients implanted with the CI24M device. Possible reasons for this are discussed. It was thought that impedances were low intra-operatively due to a larger-than-normal proportion of fluid surrounding the electrode array, rather than short circuits occurring along the array, as suggested by the dps7 software.