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Summating Potential as a Marker of intracochlear Position of ElectrodesHearing4all
18 April 2018 (online)
Intraoperative control of insertion depth is crucial during cochlear implantation (CI) in the presence of residual hearing. We determined the intracochlear position of CI with regard to cochleotopy using bipolar electrocochleograms (ECochG) recorded over a CI in guinea pigs.
Normal-hearing guinea pigs (N = 8) were implanted uni- or bilaterally with a CI. After implantation, intracochlear ECochG in response to frequency-resolved acoustic stimulations was recorded in bipolar configuration and further analyzed. Based on changes of summating potential (SP) amplitude relative to frequency, we defined “Turning frequency” (Ft) as turning point between positive and negative SP. These positional data were compared to a frequency reconstruction from micro-computer tomography (µCT) slices.
Mean of insertion depth of apical electrodes were 5339.56 µm (± 306.45, 6 contacts inserted) and 4447.75 µm (± 290.23, 5 contacts inserted). We illustrated that frequency-dependent amplitude of acoustic-evoked SPs varies systematically with the intracochlear position of the electrode. The calculated Ft was between 4.5 kHz apical and 19.6 kHz basal in 6 inserted contacts and between 9.0 kHz apical and 24.0 kHz basal in 5 inserted contacts. These values are in a range comparable to that of frequency reconstruction from µCT data.
From the relation of Ft to the position of the electrode in the cochlea we conclude that intracochlear recording of SPs in bipolar mode can in principle be used to determine the insertion depth of a CI in regard to cochleotopy.