Why are penetrating electrodes for the cochlear nucleus not significantly superior to superficial implants?

摘要

OBJECTIVE: It was assumed that penetrating auditory brainstem implants (PABI) would be significantly superior to surface implants with respect to auditory perceptions in profoundly deaf patients. However, this is not yet the case. Data on the anatomy and variability of the human cochlear nucleus complex (CNC) may provide some reasons for this lack of superiority. METHODS: The location and the size of 33 CNC (17 right-sided, 16 left-sided) were examined in cross-axial sections of 20 formalin-fixed specimens with the aid of a light microscope. Length, size, width, and the relationship to the intramedullary portion of facial nerve have been assessed for both the ventral (VCN) and dorsal portion (DCN) of the nucleus on either side. AH measurements were corrected by a factor of 1.13 (12%) for transverse and 1.21 (17%) for longitudinal shrinkage. RESULTS: The outline of the cochlear nuclei is complex, creating a distorted X-shaped silhouette in a sagittal view of the brainstem. Mean maximal dimensions obtained for the DCN were 3.60 x 0.73 x 2.21 mm (anteroposterior x mediolateral x rostrocaudal) on the right side and 4.14 x 0.72 x 1.59 mm on the left side, for the VCN 4.50 x 1.65 x 3.30 mm on the right side and 4.7 x 1.8 x 3.06 mm on the left side with significant inter-individual differences. While the outer posterolateral surface of the DCN runs almost parallel to the surface of the brainstem in a mean depth of about 0.3 mm, this was true only for the caudal third of the VCN. In rostroventral direction the VCN gradually dives into the depth to end up at a mean minimum of 7 mm off the surface. CONCLUSION: The complex morphology of the CNC, its interindividual variability, and its varying location with respect to the surface of the brainstem and outer landmarks constitute a major challenge in implantation of a PABI.