Summating Potential as a Marker of Intracochlear Position of Electrodes

Andrej Kral 2,4 Laya Rahbar Nikoukar 1,2 José Santos Cruz Andrade 1,3 Thomas Lenarz 1 Peter Baumhoff 2
1Department of Otorhinolaryngology and Head and Neck Surgery, Hannover Medical School, Hannover, Germany
2Institute for AudioNeurotechnology (VIANNA), Dept. of Experimental Otology, Hannover Medical School, Hannover, Germany
3Department of Otorhinolaryngology and Head and Neck Surgery, Federal University of São Paulo (UNIFESP/CAPES Foundation), São Paulo, Brazil
4Cluster of Excellence "Hearing 4 All“, Deutsche Forschungsgemeinschaft, Germany

Background

To preserve residual hearing during cochlear implantation (CI), intraoperative control of insertion depth is crucial .

Objective

We determined the intracochlear position of CI with regard to cochleotopy using bipolar electrocochleograms (ECochG) recorded through CI in guinea pigs.

Methods

Normal-hearing guinea pigs were implanted bilaterally (N=18 ears) with CI and exposed to band-filtered noise of either 8-12 kHz (N=10 ) or 14-18 kHz (N=8). Intracochlear ECochG in bipolar configuration was recorded in response to pure-tone stimulations. Based on changes of summating potential (SP) amplitude across frequencies, we defined the zero crossing between positive and negative SPs as “Turning frequency” (Ft). These data were compared to frequency reconstructions from micro-CT images (N=11).

Results

We observed that the Ft varies systematically with the intracochlear electrode position, with mean Ft=5.71 kHz for the most apical channels (mean apical distance =66.20%) and mean Ft=21.51 kHz for the most basal channels (mean apical distance =82.41%) at 60 dB SPL. The Ft correlated well with the position frequency calculated by an adapted Greenwood function (r2=0.90). This correlation was preserved after high frequency noise trauma with a focal hearing loss of 16-22.6 kHz. The correlation strength was reduced for hearing loss in lower frequencies (8-22.6 kHz, r2=0.59).

Conclusions

From the relation of Ft to intracochlear position of the electrode we conclude that bipolar intracochlear recording of SPs can be used to determine the insertion depth of CI in regard to cochleotopy in the region with residual hearing.

Supported by Deutsche Forschungsgemeinschaft (EXC 1077) and MedEl Company, Austria









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