Permanent and transient effects of locally delivered n-acetyl cysteine in a guinea pig model of cochlear implantation.

Protection of residual hearing after cochlear implant surgery can improve the speech and music perception of cochlear implant recipients, particularly in the presence of background noise. Surgical trauma and chronic inflammation are thought to be responsible for a significant proportion of residual hearing loss after surgery. Local delivery of the anti-oxidant precursor n-acetyl cysteine (NAC) to the cochlea via round window 30min prior to surgery, increased the level of residual hearing at 24-32kHz 4weeks post surgery compared to controls. The hearing protection was found in the basal turn near the site of implantation. Coincidentally, the basal turn was also the location that sustained the greatest hearing loss. As well as protecting residual hearing, NAC-treated animals demonstrated a reduction in the chronic inflammatory changes associated with implantation. While these findings indicate that anti-oxidant therapy can be used to reduce the hearing loss associated with surgical trauma, the local delivery of NAC was associated with a transient increase in hearing thresholds, and osseoneogenesis was seen in a greater number of NAC-treated animals. These side-effects would limit its clinical use through local cochlear administration. However, it is not known yet whether these effects would also be produced by other anti-oxidants, or ameliorated by using a different route of administration.