gms | German Medical Science

Objective of this ongoing study is to investigate feasability of intraoperative and postoperative electrically evoked auditory response telemetry. Further objectives were to record amplitude growth functions as a function of cochlear stimulation site, post-stimulus neural recovery functions as a function of cochlear site, and to correlate both intraoperative stapedius reflex thresholds and behavioural measures with evoked responses.

Twenty four adult and paediatric subjects received the Med-El PULSARci100 cochlear implant and underwent both intraoperative and postoperative auditory response telemetry measurements. Postoperative measurements were performed one month and three months after first activation. Stapedius reflex thresholds were measured intraoperatively. The adult subjects scaled loudness perception on a 0 to 50 scale. Stimulation pulses were presented with 30 us pulse duration in a kathodic phase first scheme. Stimulation sites were the electrodes 2, 4, 6, 8 and 10, while recording electrodes where those one electrode further to the apex. Neural recovery functions were recorded at most comfortable loudness levels, both with the second stimulation pulse at 50 cu lower and at the same amplitude. Time distance between first and second pulse was varied from 300 us to 8000 us.

Duration of intraoperative measurements is at an average of 6 minutes for the stapedius reflex threshold measurement before closing the wound and at an average of 18 minutes for the neural response telemetry after closing the wound. Standard electrode impedance measurement takes less than 1 minute. Amplitude growth functions could be observed on at least one electrode in all cases intraoperatively. Postoperatively, amplitude growth functions could be measured with the same reliability and exhibited no asymptotic behaviour. Compound action potentials tend to be higher in amplitude towards the apex. Neural recovery functions exhibit asymptotic behaviour from a stimulation pulse distance of 3000 us on.

Intraoperative measurements are feasible, reliable, tolerable in time consumption and can be reassuring for the surgical team by extending the standard procedure of device testing to testing the integrity of neural structures. Postoperative measurements are equally reliable, yet much less tolerable in time consumption when performed together with a series of corresponding behavioural measures. There is hope, that with growing experience at least some of the behavioural measurements can be replaced by auditory response telemetry through the implant.