Relations between cochlear histopathology and hearing loss in experimental cochlear implantation

This study reviews the cochlear histology from four hearing preservation cochlear implantation experiments conducted on 73 guinea pigs from our institution, and relates histopathological findings to residual hearing. All guinea pigs had normal hearing prior to surgery and underwent cochlear implantation via a cochleostomy with a silastic-platinum dummy electrode. Pure tone auditory brainstem response (ABR) thresholds from 2 to 32 kHz were recorded prior to surgery, and at one and four weeks postoperatively. The cochleae were then fixed in paraformaldehyde, decalcified, paraffin embedded, and mid-modiolar sections were prepared. The treatment groups were as follows: 1) Systemic dexamethasone, 0.2 mg/kg administered 1 h before implantation, 2) Local dexamethasone, 2% applied topically to the round window for 30 min prior to cochlear implantation, 3) Local n-acetyl cysteine, 200 μg applied topically to the round window for 30 min prior to implantation, 4) inoculation to keyhole-limpet hemocyanin (KLH) prior to implantation, and 5) untreated controls. There was a significant correlation between the extent of the tissue reaction in the cochlea and the presence of foreign body giant cells (FBGCs), new bone formation and injury to the osseous spiral lamina (OSL). The extent of the tissue response, as a percentage of the area of the scala tympani, limited the best hearing that was observed four weeks after cochlear implantation. Poorer hearing at four weeks correlated with a more extensive tissue response, lower outer hair cell (OHC) counts and OSL injury in the basal turn. Progressive hearing loss was also correlated with the extent of tissue response. Hearing at 2 kHz, which corresponds to the region of the second cochlear turn, did not correspond with loco-regional inner hair cell (IHC), OHC or SGC counts. We conclude that cochlear injury is associated with poorer hearing early after implantation. The tissue response is related to indices of cochlear inflammation and injury. An extensive tissue response limits hearing at four weeks, and correlates with progressive hearing loss. These latter effects may be due to inflammation, but would also be consistent with interference of cochlear mechanics.