A guide to the positioning of brainstem implants using intraoperative electrical auditory brainstem responses.

The number of electrodes that elicit usable auditory sensations with an auditory brainstem implant varies significantly between subjects. For those with only very few, movement of the array by only a few millimetres could make a significant improvement to their outcome, but yet the point at which this is normally discovered is during activation, weeks after the surgery. The number of the electrodes that are able to stimulate the auditory system can be more reliably assured by the use of electrophysiologic guidance in the placement of the implanted electrode array. This chapter describes a procedure for the use of electrophysiology to aid placement in the operating room. The procedure involves stimulating the individual electrodes and recording electrical auditory brainstem responses (EABR) as an aid in positioning the electrode array. This procedure makes it possible to position the majority of electrodes over the surface of the cochlear nucleus thus minimising stimulation of other cranial nerves, which might result in undesirable side effects. Correctly positioned electrodes elicit an EABR response with between 1 and 4 peaks with average latencies approximately (in ms) 0.7 (0.4-0.9), 1.5 (1.2-1.9), 2.7 (2.1-3.4) and 3.7 (3.4-4.0). These waves likely correspond to waves III-VI of the traditional ABR (and wave II if an excitable stump of the auditory nerve is present). No further peaks within a 10-ms window should be seen nor should activation of other cranial nerves occur. The response to stimulation of bipolar combinations of electrodes covering lateral, medial and distal positions provide information about the insertion depth. In individuals with a large lateral recess, measuring other combinations may assist in sideways and rotational orientation of the electrode array.