Is threshold shift after implantation and local corticosteroid treatment correlated with tissue growth in the cochlea? An electrophysiological and histological evaluation

Preservation of residual hearing during cochlear implantation has become increasingly important. Aside from improvement of implant devices and surgical techniques that minimise the mechanical trauma, treatment with corticosteroids is also deemed to reduce hearing loss due to suppression of inflammatory processes and reduction of growth of connective or bone tissue at the implantation site. In the present study two different corticosteroids were administered directly into the cochleae of guinea pigs in order to analyse their effects on hearing preservation after cochlea implantation. The extent of tissue growth was investigated histologically and correlated with the treatment. Three groups of guinea pigs were implanted with guinea pig electrodes (supplied by MED EL). Either triamcinolone, dexamethasone or artificial perilymph (AP) was infused with a micro-syringe directly through a cochleostomy prior to implantation. The respective other ears were treated likewise omitting implantation and served as additional pharmacological controls. Click-evoked and frequency-specific compound action potential (CAP) audiograms were recorded via round window electrodes and hearing thresholds were measured before and after drug/AP application during the following 3 months. Threshold shifts were determined with respect to the pre-surgery CAP-thresholds for each individual ear. Three months after surgery paraffin-embedded cochlea sections were cut longitudinally (7m) and Azan-stained. For each section the area of tissue filling the Scala tympani at the implantation site was measured in relation to the area left free. Hearing loss was most pronounced in implanted ears treated with AP. CAP-audiograms revealed lower threshold shifts at all frequency ranges for steroid-treated animals. No correlation between mean threshold shift and extent of tissue growth was found regardless of the treatment. Furthermore, acute corticosteroid treatment did not suppress long-term tissue growth within the scala tympani. The results indicate that the two steroids reduced implantation-induced hearing loss even after an acute application. They did not however suppress tissue growth significantly compared to AP-treatment.