Anatomy of the human cochlear nucleus in relation to auditory brainstem implants

摘要

OBJECTIVE: The performance of auditory brainstem implants is significantly inferior to that of cochlear implants with respect to auditory perceptions in profoundly deaf patients. Inconsistencies in the electrode-tissue-interface may contribute to this issue. The study fills in essential data on the anatomical variability of the human cochlear nucleus complex (CNC) that have never been systematically analyzed before.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. All measurements were corrected by a factor of 1.13 (12%) for transverse and 1.21 (17%) for longitudinal shrinkage.RESULTS: The shape of the cochlear nuclei is complex, creating a distorted X-shaped silhouette in a sagittal view of the brainstem. Mean maximal dimensions (anteroposterior x mediolateral x rostrocaudal; ± standard deviation) obtained for the VCN were 4.59 ± 0.89 × 1.53 ± 0.64 × 3.18 ± 0.69 mm, and 3.42 ± 1.21 × 0.68 ± 0.20 × 1.90 ± 0.66 mm for the DCN. Interindividual differences were highly significant and there was considerable overlap between VCN and DCN in the sagittal plane. 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, the VCN gradually dives into the depth in the rostroventral direction, ending 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 designing auditory brainstem implants.