Development of an algorithm for testing a hearing implant during cochlear implantation surgery

Currently, cochlear implantation is the only and unique method which helps people with sensorineural hearing loss (including congenital deafness) gain the opportunity to hear. The core of the method is the high-tech device – cochlear implantation system consisting of internal and external parts. The internal part is the hearing implant with an electrode array installed into the cochlea of the inner ear. The external part is the constantly worn sound processor which receives acoustic signals and transforms them in a certain way into electric impulses for further transmission on the electrode array to stimulate the auditory nerve receptor thus to evoke auditory sensations by human brain and is interpreted by it as a sound. The main stage of the cochlear implantation including the medical and technical approaches is cochlear implant surgery and subsequent testing of the implant to check performance of the implant electrodes installed into the cochlea as well as the registration of the electrically evoked action potential of the auditory nerve dependent on time. Registration of the latter is carried out mainly by the computer-assisted algorithms in the software of the specific implant manufacturer. In some cases, it is not possible to fix the neural response because of the artifact emergence during the receptor stimulation by the implant intracochlear electrodes which requires correction of the electric charge transmitted by the nerve receptor, and also accountability of all possible technical parameters of the implant (amplification factor to record the potential and the time between the stimulation and registration). These parameters cannot be fully taken into account in the computer-assisted algorithms, so it is necessary to create a template of the computer-assisted measurements in the manufacturer software for all technological peculiarities of the registration process accountability. Lately, electrophysiological measurement method based on the auditory nerve telemetry has been used to assess bend and bend tip fold over of the electrode array. The gist of the method is the electric stimulation of a selected electrode and electric potential stimulation registration on all the other electrodes of the array. The process is to be repeated until every electrode has been stimulated. To illustrate the measurements and calculation results, color coding in the form of 2D chart (temperature map) including the calculated transimpedances values is introduced. Analysis of the given transimpedance matrix (TIM) leads to the conclusion about the presence or absence of contacts (electrode array installation into the cochlea is considered to be correct) between the electrodes. The aim of the present research is development of an algorithm which allows to automatize the testing process of cochlear implant CI 612 from Cochlear® manufacturer taking into account all its peculiarities: impedance measurements of the intracochlear electrodes to assess the short circuit or open circuit (the first stage), registration of the electrically evoked action potential of auditory nerve with machine algorithm Auto™NRT (the second stage), in case of the second stage malfunction, carrying out measurements according to the created template in the Advanced NRT mode (the third stage), TIM formation to assess bend and bend tip fold over of the electrode array (the fourth stage). 20 patients aged from 1 to 5 years old were under research. Testing of the implants was carried out in the software from Custom Sound EP (v. 6.0) manufacturer. Five (out of twenty-two) intracochlear electrodes placed almost evenly along the array have been tested for every patient; testing of impedances has been carried out; action potential with Auto™NRT has been registered (the main parameters: gain is 50 dB of the amplifier; the time delay between stimulation and registration is 122 mcs, Forward Masking artifacts suppression technology) and with the help of the template in Advanced NRT mode at the same relatively high current (the main parameters: amplifier gain is 40, 50, 60 and 70 dB, time delay is 44, 68, 93, 117 and 122 мкс, Forward Masking artifacts suppression technology), TIM has been formed along all electrodes of the array. Action potential has been determined automatically on 99 electrodes with Auto™NRT; it has been done with the created template in the Advanced NRT mode on 100 electrodes. Out of 2000 measurements with the template, 463 time dependencies of the potential have been fixed automatically, 22 dependencies have been determined manually taking into account the generally accepted rules in the clinical audiology. Numerical characteristics analysis of the time dependence of the potential showed uniformity of results and their low degree of diversity. Matrix TIM analysis for all patients showed absence of bend and bend tip fold over of the array. Measurements results have been processed in MS Excel. The developed algorithm has been introduced into the clinical practice of the medical center for improvement of the operations quality. Usage of TIM in color coding based on the electrophysiological measurements of the hearing implant parameters might eliminate a computed tomography scan performance, as well as X-ray during the operation, if necessary. This might reduce the intraoperative testing time significantly, hence – the under general anesthesia time for patients and excludes additional financial expenses.