Electrophysiological status indexed by early changes in impedance after cochlear implantation: A literature review

Cochlear implantation is a major treatment option for severe-to-profound hearing loss. By insertion into the cochlea and stimulation of the cochlear nerve, cochlear implantation can improve the performance of hearing and speech performance of the implantees. The microenvironment of the cochlea is innate and gets disturbed in response to the insertion of a foreign body. However, real-time changes inside the cochlea in terms of electrophysiology at the molecular level can never be investigated in vivo in human beings. Thus, impedance is a good guide that reflects the electrophysiology inside the cochlea. Because the initial measurement of impedance cannot be performed earlier than the traditional interval of 1 month postoperatively, early changes in impedance have not been explored until recently; however, surgeons are now trying the initial switch-on earlier than 1 month after implantation. This review discusses the scenario of electrophysiological variation after early switch-on in <1 day postimplantation. Evidence has shown that fluctuations in impedance after implantation depend on the interplay between cell cover formation, fibrosis, electrode design, and electrical stimulation. Further studies addressing the correlation between impedance and clinical parameters are required to develop reliable biomarkers for better performance of cochlear implantation.