Extracochlear Stimulation of Electrically-Evoked Auditory Brainstem Responses (eABRs) Remains the Preferred Pre-Implant Auditory Nerve Function Test in an Assessor-Blinded Comparison

Background
Electrically-evoked auditory brainstem responses (eABRs) can be used as a pre-operative test to verify auditory nerve function in cases of auditory nerve hypoplasia.

Objective
To compare three pre-implant eABR stimulation techniques in adults with normal auditory nerves during CI surgery: (i) extracochlear electrode placed at the round window niche; (ii) two different electrodes on a recetly developed Intracochlear Test Array (ITA), and (iii) two different electrodes on a cochlear implant (CI) electrode array.

Methods
New adult implantees (n=16) were recruited. Primary outcome measures: (a) charge (μC) required to elicit a threshold response, and (b) wave V latencies (ms) in the threshold responses. Secondary outcome measures were: (a) morphologies of responses at supra-threshold stimulation levels, and (b) wave V growth patterns.

Results
eABRs were successfully measured from 15 participants. The charge required to elicit a response using the extracochlear electrode was approximately six times larger than other electrodes. The latency of wave V was approximately 0.5 ms longer with the extracochlear electrode. Intracochlear test array responses were highly variable. Most ITA responses contained one more waveform than extracochlear responses. However, ITA responses were rated poor quality in 33% of recordings and in 2 instances did not allow for data collection.

Conclusions
Both the extracochlear electrode and the ITA can be used to produce an eABR representative of the CI eABR. In the majority of cases, extracochlear stimulation was the preferred approach for pre-implant auditory nerve function testing because it consistently produced results for all patients, and did not require a posterior tympanotomy.