Extensive monitoring during auditory brainstem implant surgery
In patients with reduced auditory nerve function, for example due to tumour removal or an accident, hearing rehabilitation can be elicited by an auditory brainstem implant (ABI). The electrode array of the ABI manufactured by Cochlear Ltd., Sydney, consists of 21 circled contacts in a silicon carrier. This is inserted in the lateral recess of the fourth ventricle. Since 1996, in Hannover eight patients have been implanted with a cochlear ABI Nucleus 21 + 1. All of them were profoundly deaf on both sides due to neurofibromatosis type 2 (NF2). To find the optimal electrode position during surgery, a multimodal monitoring by auditory evoked potentials (AEP), electromyography (EMG) and somatosensory evoked potentials (SEP) was performed. When monitoring AEPs, the function of the implant can be checked first by the stimulus artefact. By analysing the AEPs in more detail, the optimal positioning of the electrode on the cochlear nucleus can be found. If systems other than the auditory system are stimulated this will be revealed in one or more of the AEP, EMG and SEP recordings. According to the literature, AEPs stimulated by an ABI consist of three vertex positive peaks with latencies shorter than 4 ms. Typical AEPs are correlated with good post-operative hearing sensation. Comparing these AEPs with AEPs stimulated acoustically or electrically at different sites of the auditory system, it can be assumed that the first peak corresponds to J3, the second to J4 and the last to J5. From this comparison it can also be concluded that no potentials should occur later than 5 ms. This corresponds to our findings. Post-operatively, side-effects occurred when areas of the electrode array were stimulated that showed potentials with latencies longer than 5 ms intra-operatively. Our results indicate that monitoring is an essential aid for the surgeon in finding the optimal electrode position. Positioning solely with reference to anatomical landmarks may not be enough to find the optimal functional position.