Radical Scavenging of Nanoceria in Minimizing the Oxidative Stress-Induced Loss of Residual Hearing: A Review

Among the several unresolved issues, profound/total loss of low-frequency residual hearing after cochlear implant fixation is the most frequent event. Even after several attempts such as modifications in the design of electrodes, improvement in the surgical procedures and use of protective drugs to minimize the trauma and its after-effects, residual hearing has been retained in less than 50% patients only. Surgical procedure and mechanical trauma at the electrode insertion site are thought to be responsible for excess generation of reactive oxygen species (ROS) following initiation of inflammatory cytokines resulting into loss of residual hearing due to programmed cell death of essential inner ear structures. Though very recent studies have reported the use of conventional antioxidants to preserve the residual hearing, they have their own limitations. With the emerging need of better and effective antioxidants, nanoceria has spurred immense research interest on utilizing its unique catalytic characteristics for ROS-associated diseases. Nanoceria has shown effective protection against several ROS-induced damages compared to conventional antioxidants such as vitamin C and vitamin E. The objective of the present work is to develop an understanding about the underlying mechanism of loss of residual hearing and propose a novel method based on delivery of nanoceria to minimize it. The first part of the article highlights the failure of cochlear implants, nature of failures and revised surgeries due to loss of residual hearing. Subsequently, the article explores the relation among surgical/mechanical trauma, excess generation of ROS at electrode insertion site, progressive death of hair cells and loss of residual hearing. Finally, effectiveness of radical scavenging characteristics of nanoceria along with controlling parameters and involved mechanisms has been reviewed. The present work also focuses on the limitations and challenges of nanoceria in clinical applications. Based on the literature review, it is hypothesized that the residual hearing loss is associated with excess generation of ROS and it is proposed that the delivery of effective antioxidants/radical scavengers having a good longevity and regenerative ability is expected to reduce the excess level of ROS and retain the residual hearing.