In pediatric cochlear implantation subjective measurement play a major role in finding functions of electrode. Impedance measurements are carried out in both intraoperative and postoperatively during switch-on. These measurements make possible to check, whether all electrodes functions correctly. The analysis of the impedances changes makes possible to guess, what processes happen in tissues and liquids of the inner ear around the electrode. In Cochlear implant the communication between the internal component of implanted electrode and the external speech processor signal is monitored or checked via Custom Sound Software. Neural Response Telemetry (NRT) gives us clear information about the impedance of the electrode, electrodes position, electrical problems like short circuit and open circuit, failure of electrode during the time of surgery and post-surgery can be monitored . In Neural Response Telemetry (NRT) the impedance of the electrodes is a method of measuring resistance encountered by electricity passing through wires, electrodes and biological tissue . It is calculated as the ratio of the effective voltage applied to a particular circuit and the actual amount of electrical power intensity absorbed by the circuit. The unit of impedance is the ohm. The electrodes impedance does not confirm the electrode placement and neither replace the radiographies after the implant .
The aim of the study was to measure the change in impedance values of cochlear implant children during the time of surgery, and the cochlear implant Sound/Speech processor switch on (1 month postoperatively).
In current study 100 children whose age range are from 18 to 60 months with bilateral profound hearing loss has been operated with Cochlear Nucleus CI24RE (ST) Implants on one side. The impedance values are measured for all 22 electrodes with help of Custom Sound EP, Custom Sound software and Nucleus CP802 Speech Processor. The impedance of electrodes
in cochlear implants users was measured during time of surgery and postoperative while switching on the speech processor.
We observed raised values of the impedance during the time of activation of the speech processor when compared to intra-operating measurement. Below chart explains current level of each electrodes function includes MP 1, MP2, Mp1+2 and common ground (Figures 1-3), (Table 1).
Impedance decreases after first fitting but less than intraoperative
measurement. Increasing values of impedance during
first fitting of the speech processor support the hypothesis that
a layer of fibrous tissue forms around the electrode because
of the inflammatory changes or due to exudation of protein.
This may suggest that electrical stimulation prevents adverse
changes in the cochlea. The intra-operative impedance values
and postoperative change in values are part of the common
trend specific to each brand separately. Besides the parameters
related to the device, the individual organic features of the inner
ear may play an important role, as confirm by the patients from
the latest study.