Hypothesis: Trauma caused by cochlear implant electrode insertion is attributable to the combination of direct physical trauma and the delayed cell death of oxidative stress-injured auditory sensory cells.
Background: Histologic evaluation of cochlear implant electrode trauma has demonstrated that the extent of sensory cell losses is proportional to the degree of injury. However, the impact of delayed oxidative stress within injured cochlear tissues and the progressive loss of injured hair cells by way of apoptosis are at present unknown.
Methods: Laboratory rats were evaluated for hearing acuity before and after electrode insertion, before and after round window membrane incision only. Hearing was measured before trauma or incision and over the next 7 days. Objective measurements of hearing function were distortion products of otoacoustic emissions (DPOAEs) in the frequency range of 2 to 32 kHz and tone-burst (i.e., 4-32 kHz) evoked auditory brain stem responses (ABRs).
Results: For the experimental cochleae, there were progressive increases in ABR thresholds and decreases in ABR amplitudes. The amplitude of the DPOAEs in the experimental cochleae also showed progressive decreases. For the contralateral control and round window membrane surgical control ears, there were no significant changes in either DPOAE or ABR thresholds.
Conclusion: These results document a progressive loss of hearing acuity postimplantation and strongly suggest that electrode insertion trauma generated oxidative stress within injured cochlear tissues.