gms | German Medical Science

Impedances play an important role during the fitting of a cochlear implant. One clinical usage is to check the proper functioning of the electrode array. But impedances are also used in a more advanced setting: e.g. they form the basis of the automatic pulse width adjustment capability in the SoundWave software of Advanced Bionics Corp. Still impedances are often considered as some kind of "magic numbers". They must lie within a valid range in order to be good, but little is known about their origins. The goal of this presentation is to clarify the electrophysiological origins of the impedance. There all multiple components contributing to the total impedance as measured by the clinical software in monopolar coupling mode, mainly the electrode contacts and the cochlear tissues. The evolution of the in-built measurement capabilities present in all modern CI devices makes that nowadays these components can be accurately quantified. A precise intracochlear potential map, also known as EFI, can be captured in less than a minute. Analysis of this intracochlear potential map provides insight into the properties of the electrode contacts and the cochlear tissues.

Electrical field measurements have been performed at different times post-operation and with different types of stimuli in a set of 5 subjects, including pulsatile, sinusoidal and multipolar stimulation. Controlled experiments under laboratory conditions have been added as well. In 2 subjects loudness growth has been measured to study the relation with their impedances.

We discuss the influence of the type of stimulation, e.g. sinusoidal or pulsatile measurements and the level of stimulation on the intracochlear potential map, showing that the frequency dependence is exclusively caused by the electrode contacts. The cochlear tissues exhibit a purely resistive response. We also discuss the effect of de-activation of a contact on the impedance, giving rise to time-varying impedances. Finally we discuss how electrical field imaging relates to spread of excitation and influences threshold and comfort levels.

Accurate measurement of the intracochlear potential map can provide an electrical image of the properties of the cochlear tissues. Initial work is ongoing on understanding its relation to program levels.