Integrity testing of cochlear implants in the awake child

Cochlear implantation is becoming a routine rehabilitation process for profoundly deaf adults and children. Often children are implanted at just two or three years of age and therefore the subsequent tuning of the device is challenging. Although some children demonstrate quick and reliable responses to electrical stimulation, there are others who do not respond consistently thus causing concern about the functioning of the device. It is therefore desirable to have an objective test of the integrity of the implanted electrodes. The principle of the integrity test is the measurement of voltages generated by the biphasic current pulses at the electrode array; this is accomplished using surface electrodes placed around the implanted ear, in conjunction with recording and averaging equipment typically used for evoked response testing. Traditional integrity testing usually requires a general anaesthetic in young children, however this study demonstrated a simple, quick and reliable method of obtaining results in the normally active child using ear-clip electrodes. Results are presented from 12 children tested in this way, and compared with results from 20 children who were tested in theatre using a different electrode configuration. The tests were performed in common ground stimulation mode, but some measurements were also made in bipolar + 1 and pseudo-monopolar modes. The three stimulation modes were compared, with the conclusion that common ground mode provides an efficient check of implant function in the awake child, while pseudo-monopolar mode may be preferable for anaesthetized patients. In addition, measurements were made in vitro using a functioning cochlear implant in a saline tank in order to investigate the current flow during stimulation. The standard procedure in this department is to perform a full intra-operative integrity test on all implanted children. The simplified technique is used to repeat the measurement post-operatively if required.