Adult Auditory Brainstem Implant Outcomes and Three-Dimensional Electrode Array Position on Computed Tomography

Factors contributing to auditory brainstem implant (ABI) outcomes are poorly understood. The aims of this study are to (1) characterize ABI electrode array position on postoperative imaging and (2) determine if variability in position is related to perceptual outcomes.

Retrospective cohort study. Subjects were selected from the adult ABI recipient population at Massachusetts Eye and Ear. Postoperative three-dimensional (3D) computed tomography (CT) reconstruction of the head was used to measure ABI array position in 20 adult ABI recipients (17 with Neurofibromatosis Type 2 (NF2) and three non-NF2 recipients). Three-dimensional electrode array position was determined based on angles from the horizontal using posterior and lateral views and on distances between the proximal array tip superiorly from the basion (D1), laterally (D2P) and posteriorly (D2L) from the midline. Array position was correlated with perceptual data (in 15 of the 20 recipients who used their ABI). Perceptual data included the number of electrodes that provided auditory sensation, location and type of side effects, level of speech perception (from no sound to open-set word recognition of monosyllables) and the amount of charge required for auditory perception.

Although the 3D orientation of the ABI array exhibited a variety of angles, all arrays were posteriorly tilted from the lateral view and most were medially tilted from the posterior view. ABI position relative to the basion from posterior showed mean distances of 1.71 ± 0.42 and 1.1 ± 0.29 cm for D1 and D2, respectively, and a mean D2 of 1.30 ± 0.45 cm from the lateral view. A strong linear negative correlation was found between the number of active electrodes and the distance of the proximal array tip laterally from the basion (D2P; rs = -0.73, p = 0.006) when measured in the posterior view. Although side effects were experienced in all recipients and varied in type and location across the array, electrodes in the middle part of the array tended to elicit auditory sensations while the proximal and distal tips of the array tended to elicit nonauditory side effects. Arrays with and without low charge thresholds appeared to generally overlap in position. However, the two recipients with the best (open-set) speech perception had low charge thresholds and had arrays that were tilted superiorly in the posterior view.

ABI recipients with better speech perception appear to share a profile of arrays that are tilted superiorly as compared to recipients with lower speech perception levels. These ABI recipients have a high number of active electrodes (10 or more) and require less electrical charge on individual electrodes to achieve optimal stimulation.