No easy target: anatomic constraints of electrodes interfacing the human cochlear nucleus.

BACKGROUND: Auditory brainstem implants have failed to produce consistent clinical results comparable to those with the cochlear implant, both with surface and penetrating electrodes.
OBJECTIVE: To determine neuromorphological constraints of the auditory brainstem implant interface.
METHODS: The size, shape, surface depth, and spatial orientation of 33 human cochlear nuclei in 20 brainstem specimens obtained at autopsy were systematically analyzed in 792 slices each with a thickness of 8 μm. Three-dimensional renderings of the cochlear nucleus complex were obtained from a true-to-scale model, and the resulting photographic views were arranged according to the axes of the brainstem.
RESULTS: The dimensions of the ventral and dorsal cochlear nuclei in the axial, coronal, and sagittal planes correlated linearly with each other. There were no significant side differences. Maximum dimensions of the whole cochlear nuclear complex were 8.01 × 1.53 × 3.76 mm. The appearance of the ventral and dorsal nuclei combined resembles a distorted X shape from a lateral view and an angulated wedge shape when viewed from above. Slanted into the depth of the brainstem above the facial nerve entrance, the superior boundary of the ventral nucleus is located more than 7 mm off the surface of the brainstem on average.
CONCLUSION: In the absence of appropriate surface landmarks and imaging guidance, to gain tonotopic access to the human cochlear nucleus with surface and depth electrode remains a major challenge. Due to its location close to the surface, the dorsal cochlear nucleus is vulnerable to surgical manipulation and to tumors.