Evaluation of Spiral Ganglion Lesions by Electrophysiological Measures

Background: Through the direct electrical stimulation of spiral ganglion neurons (SGNs) of the hearing nerve, cochlear implants overcome functionally impaired or missing hair cells in patients with profound to severe hearing loss. In routine clinical fitting, regions with severe local SGN degeneration (modiolar "dead regions") cannot be identified. As a result, the electrical fields of neighboring electrodes are broadened, which can lead to increased channel interaction and, consequently, poorer speech understanding and hearing. The objective of this study was to ascertain whether neural health status can be evaluated by using cochlear implants' inbuilt measures. Methods: Electrode impedance (MP1-, MP2-, MP1/MP2-, common ground mode), transimpedance matrix (TIM) and electrically evoked compound action potential (eCAP) measurements were performed before and after laser-induced induction of lesions on the modiolus of the guinea pig. Laser treatment-related shifts in impedance, TIM, and eCAP characteristics (threshold, amplitude, and a modified version of the failure index, referred to as the efficiency index (EI)) were correlated with the histologically assessed damage in three predefined areas of the basal modiolus within the electrode region. Results: Modiolar damage resulted in a significant reduction in the electrode impedance in MP2- and MP1/2-mode, the eCAP amplitude, and the EI. In contrast, TIM values and eCAP thresholds were significantly elevated. MP1, MP1/MP2 electrode impedance, TIM, and the eCAP thresholds were not correlated with the extent of modiolar damage. The shifts in eCAP amplitudes and the EI were significantly correlated with the damage at all regions of the basal modiolus. Conclusions: The eCAP amplitude and the EI are both capable of objectively evaluating the neural health status of the cochlea. Thus, a modiolar dead region could be expected from a local drop in eCAP amplitude values or the modified EI within the electrode array.