INTRACORPOREAL CORTICAL TELEMETRY: THE COCHLEAR IMPLANT AS EEG DEVICE?

Beynon AJ, Luijten BL, Snik AFM.  

Auditory Evoked Potential Lab.Radboud University Nijmegen Medical Centre. Donders Institute for Brain, Cognition and Behaviour. The Netherlands.

Background: Until now, the only EP directly recorded by a cochlear implant is the electrically-evoked compound action potential (ECAP). However, the predictive value of ECAPs on electrical thresholds (T/C-levels) seems to be limited. Several groups focus on electrically-evoked auditory cortical responses (EACRs).

Although longer latency responses are usually obtained with conventional EEG systems, it would be more convenient for CI patient and clinician when EPs are directly recorded and averaged by the implant itself without the discomfort and disadvantages of (extracorporeal) EEG setup, e.g. fixed scalp wirings, environmental noise. The development of an implant that could stimulate, obtain, ánd average longer latency EPs might enhance the development of automatic objective fitting procedures.

Aims: A feasibility study was carried out to develope a recording paradigm based on neural response telemetry (NRT) functionality of a CI system to (intracorporeally) record EACRs.

Materials and methods: A Nucleus Freedom CI system was used for recording and averaging. A new recording paradigm that concatenates multiple NRT windows was used to enlarge recording time window up to 240 ms. Subcutaneous reference electrodes were used to record EEG, while intracochlear electrodes were used for bipolar stimulation. First, the feasibility of a CI as an EP recording system was investigated by using a second (extracorporeal) implant system to measure EPs of normal hearing subjects, 'in vitro'. Secondly, 'in vivo', data intracorporeally-obtained from CI users were compared to data that was simultaneously extracorporeally-obtained from conventional EEG recordings in same subjects.

Results and Conclusions: Data show that it is feasible to record cortical potentials with a cochlear implant. In contrast to 'in vivo' data, 'in vitro' recordings were more sensitive for external noise. All data showed reproducible responses. Based on the present findings, role of location of subcutaneous recording electrodes, response morphology, clinical application, e.g. automatic processor fitting, and future implant design are addressed.

E-mail: a.beynon@kno.umcn.nl