Effect of experimentally induced otitis media on cochlear implants.

Cat cochleas implanted with scala tympani prostheses were investigated histologically after inoculating the bullae with a suspension of group A streptococci. The prosthesis was passed through the round window membrane in one ear. In the other the prosthesis bypassed the round window via an opening anteroinferior to the round window niche. Before death, horseradish peroxidase was administered as a tracer for possible pathways of infection. Results showed that group A streptococci were pathogenic to the cat and caused inflammation in the bulla. The unimplanted round window membrane and the seals around the electrode entry points prevented infection from entering the cochlea. The seals around electrodes inserted either through the round window membrane or an opening drilled anteroinferior to the niche were equally effective. The horseradish peroxidase tracer studies showed, however, that a gap existed between the electrode and membranous seal, and this could be a potentially vulnerable site under certain conditions. Drilling an anteroinferior opening into the cochlea resulted in bony sequestra entering the cochlea. This can be avoided by blue-lining the opening and removing bone with picks before making an opening through the endosteum.