Cochlear prosthesis: morphological considerations

Anatomy of the round Window with Respect to Cochlear Implant Surgery

Twenty-three human temporal bones were dissected to study the anatomy of the round window with respect to cochlear implantation. Information was also gathered concerning the passage of a 22-channel electrode array along the cochlear duct by using cochlear surface dissection preparations. The insertion length and any insertion trauma were noted both by observation at the time of insertion and by careful analysis of video recordings. The crista semilunaris did not interfere with smooth electrode insertion. The recommendation is made that enlarging the round window anteroinferiorly is unnecessary and potentially hazardous in most cases.

Brainstem auditory pathway degeneration associated with chronic cochlear implants in the monkey

The form and pattern of first-order and transsynaptic degeneration in the central auditory pathway was studied in monkeys following inner ear stimulation by a cochlear implant. Multielectrode, scala tympani, and modiolar systems were implanted; in some cases, neomycin was perfused into the cochlea to destroy the organ of Corti at the time of implantation. The monkeys were maintained chronically for 5 to 120 weeks, then the cochleas and brainstems were examined histologically. The extent of spiral ganglion cell loss across animals showed variability, reflecting the different procedures and devices used. The degree and distribution of spiral ganglion cell loss was related to the degree and distribution of neural degeneration seen in the cochlear nucleus in all cases. Peripheral damage progressed toward the cochlear apex as survival time increased, and this progression was reflected in the cochlear nucleus by a ventrolateral shift in the locus of degeneration over time. In addition, evidence for transneuronal degeneration was seen at the superior olive, the lateral lemniscus and the inferior colliculus. Our findings indicate that several factors inherent in the use of a cochlear prosthesis, i.e., insertion trauma, host reaction, and/or electrical stimulation, may be associated with a long-term, continuing process of central degeneration visible at several levels of the auditory system.

Cochlear prostheses: stimulation-induced damage

The effects of 4 weekly, three-hour exposures to continuous sinusoidal (l kHz) electrical stimulation of the inner ear at various current levels were assessed in the chronically implanted guinea pig. With scala tympani stimulation, histopathological damage, including new bone growth, was observed for currents at and above 100 microA rms. No changes were observed in similarly implanted, but not stimulated cochleas. At equal current levels, less damage was found in subjects stimulated via electrodes placed on the round window and promontory, as compared to the scala tympani. Consistent reversible changes in threshold and suprathreshold features of the electrically evoked auditory brainstem response (EABR) were found. The magnitude of EABR change was directly related to exposure stimulus current level and to cochlear stimulation site. Suprathreshold features of the EABR were more sensitive to continuous stimulation exposures than threshold measures. Reversible EABR changes were found in the presence and absence of stimulation-induced histopathology.

Amplitude and pulse rate difference limens for electrical stimulation of the cochlea following graded degeneration of the auditory nerve

Experimentally deafened cats with differing populations of residual spiral ganglion cells were implanted with cochlear electrodes and were electrically stimulated. They were conditioned to respond to changes in electrical pulse rate and amplitude, and both electrical pulse rate and amplitude difference limens were determined. It was found that although there were some variations in difference limens between animals, these showed no correlation with residual cell populations over the range 8-44%.