Histopathology of Inner Ear Malformations: Potential Prognostic Factors and Their Influence in the Decision-Making Between CI and ABI

Importance Histopathological findings are important to guide our decision between cochlear and auditory brainstem implant in inner ear malformations (IEM) with cochlear hypoplasia (CH), incomplete partition (IP), and common cavity. Objective Cochlear implantation (CI) has variable outcome in IEM. Histopathology of IEM is evaluated to understand the causes of this variation. Design Evaluation of human temporal bone specimens with IEM. Participants Twenty temporal bone specimens from 10 donors who had IEMs were evaluated. Main outcome measures Classification of IEM and analysis of cochlear morphology inner and outer hair cells, spiral ganglion cells, and cochlear nerve (CN) to understand the functional outcome of implantation. Results Among the temporal bones with IEMs, 16 were classified as CH, 2 were IPs type II, and 2 had a normal cochlear anatomy, but absent CNs. The CN was hypoplastic in 3 specimens with CH-III. The nerve was lying adjacent to the bony wall, which would be difficult to demonstrate with current magnetic resonance imaging (MRI). The mean percentage of loss of spiral ganglion cells in CH-III among our specimens was 62%. Some specimens showed asymmetric development, which will play a crucial role in the benefit from CI. Two specimens from the same donor with a fully-developed cochlea demonstrated absent CN. Conclusion Better MRI sequences are needed to demonstrate the hypoplastic CN adjacent to the wall of the internal auditory canal. Audiological examination is of paramount importance to demonstrate the presence of neural connection between the cochlea and the brainstem. In asymmetric developments between 2 sides, more developed side should be chosen for CI. Depending on the findings cochlear or auditory brainstem implants may be chosen for the contralateral findings. If there is only one implant available, the side with the better developed CN should be picked.