Method and device for intracochlea impedance measurement

A method of measuring a tissue response arising from a stimulus, the method comprising:

• applying an electrical stimulus to the tissue using at least two stimulating electrodes;
• after the stimulus, open circuiting the stimulating electrodes and measuring a residual two point potential between the stimulating electrodes;
• after the stimulus, measuring a residual potential between two non-stimulating electrodes, referred to as a residual four point potential;
• and
• comparing the measured two point potential to the measured four point potential to determine at least one of a residual tissue potential and a residual electrode/tissue interface potential.

A cochlear implant system for measuring a tissue response arising from a stimulus, the cochlear implant system comprising:

• an implantable electrode array comprising a plurality of electrodes for applying Metrical stimuli to intracochlea tissue;
• and
• a controller adapted to apply an electrical stimulus to the tissue using at least lating electrodes;
• after the stimulus, open circuit the stimulating electrodes ire a residual two point potential between the stimulating electrodes;
• after the stimulus, measure a residual potential between two non-stimulating electrodes, referred to as a residual four point potential;
• and compare the measured two point potential to the measured four point potential to determine at least one of a residual tissue potential and a residual electrode/tissue interface potential.

A method for estimating modiolus currents induced in a cochlea by electrical stimulation, the method comprising:

• determining an intra-cochlea tissue impedance Zt1 between a first intra-cochlear electrode and a second intra-cochlear electrode by a four point impedance measurement;
• determining an intra-cochlea tissue impedance Zt2 between the second intra-cochlear electrode and a third intra-cochlear electrode by a four point impedance measurement;
• while passing a current between a fourth intra-cochlear electrode and an extra-cochlear electrode, measuring respective voltages V1, V2 and V3 arising at the first, second and third intra-cochlear electrodes;
• determining respective intra-cochlea tissue currents It1 and It2 flowing through the intracochlea tissue impedances Zt1 and Zt2 as being It1=(V1−V2)/Zt1 and It2=(V2−V3)/Zt2;
• and determining that a modiolus current Im2 arising in the proximity of the second electrode due to the current between the fourth intra-cochlear electrode and the extra-cochlear electrode is Im2=It2−It1.

A method of automated initiation of re-mapping of a cochlear implant, the method comprising:

• obtaining measurements of tissue response to electric stimulation;
• comparing the tissue response measurements with previously obtained tissue response measurements, to identify any changes which have occurred over time in the tissue response to stimulation;
• and
• upon identification of a change in tissue response, initiating re-mapping of the cochlear implant.