Graeme Clark Collection
Permanent URI for this collection
Search Results
1 - 4 of 4
-
ItemCochlear pathology following chronic electrical stimulation of the auditory nerve. I: Normal hearing kittens( 1992)The present study examines the histopathological effects of long-term intracochlear electrical stimulation in young normal hearing animals. Eight-week old kittens were implanted with scala tympani electrode arrays and stimulated for periods of up to 1500 h using charge balanced biphasic current pulses at charge densities in the range 21-52 µC cm^-2 geom. per phase. Both click and electrically evoked auditory brainstem responses were periodically recorded to monitor the status of the hair cell and spiral ganglion cell populations. In addition, the impedance of the stimulating electrodes was measured daily to monitor their electrical characteristics during chronic implantation. Histopathological examination of the cochleas showed no evidence of stimulus induced damage to cochlear structures when compared with implanted, unstimulated control cochleas. Indeed, there was no statistically significant difference in the ganglion cell density adjacent to the stimulating electrodes when compared with a similar population in implanted control cochleas. In addition, hair cell loss, which was restricted to regions adjacent to the electrode array, was not influenced by the degree of electrical stimulation. These histopathological findings were consistent with the evoked potential recordings. Finally, electrode impedance data correlated well with the degree of tissue growth observed within the scala tympani. The present findings indicate that the young mammalian cochlea is no more susceptible to cochlear pathology following chronic implantation and electrical stimulation than is the adult.
-
ItemCochlear pathology following chronic electrical stimulation using non charge balanced stimuli( 1991)During the course of a chronic intracochlear electrical stimulation study using charge balanced biphasic current pulses, one animal inadvertently received a short period of direct current (DC) stimulation at a level of approximately 1 µA. Subsequent, the animal was chronically stimulated using a poorly charge balanced waveform that produced a DC level of approximately 2 µA. Extensive pathological changes were observed within the cochlea. These changes included widespread spiral ganglion cell loss and new bone growth that extended throughout all turns of the cochlea. Significant changes in the morphology of the electrically evoked auditory brainstem response (EABR) were associated with these pathological changes. EABRs recorded prior to the DC stimulation exhibited a normal waveform morphology. However, responses recorded during the course of the DC stimulation were dominated by a short latency response believed to be vestibular in origin. The response thresholds were also significantly higher than levels recorded before the DC stimulation. In contrast, the contralateral cochlea, stimulated using charge balanced stimuli, showed no evidence of adverse pathological changes. Furthermore, EABRs evoked from this cochlea remained stable throughout the chronic stimulation period. Although preliminary, the present results illustrate the adverse nature of poorly charge balanced electrical stimuli. These results have important implications for both the design of neural prostheses and the use of DC stimuli to suppress tinnitus in patients.
-
ItemClinical results using a multiple-channel cochlear prosthesis( 1984)A total of eight profoundly deaf patients have been implanted with the Nucleus Limited multiple-channel cochlear prosthesis at the University of Melbourne since it became available in late 1982. All patients are everyday users of the device. Speech testing using the device alone has shown consistently high scores for a variety of closed set tests and significant levels of open set speech understanding in most of the patients. Lipreading assessment with phoneme, word, sentence and speech tracking material shows significant improvement when using the prosthesis for all patients tested. Other benefits reported are recognition of environmental sounds, decrease in tinnitus. increased confidence in social and vocational situations and improved voice control. One patient is able to cope with interactive conversations over the telephone and three others are able to use the telephone in a limited way without special codes.
-
ItemCochlear implant round window sealing procedures in the cat: an investigation of autograft and heterograft materials( 1984)In this series of experiments it was shown that a round window seal produced with a muscle autograft or a Teflon felt disc glued to the electrode prevented a Staphylococcus aureus infection in the bulla extending to the cochlea. The seal following a fascial autograft also prevented β-haemolytic streptococci type A spreading to the basal turn of the cochlea. On the other hand Dacron® velour is contra-indicated as it was associated with a strong inflammatory response and a high incidence of infection. The studies also indicated that infection can extend to the cochlea before the round window membrane has healed. The junctional area between the graft and the margin of the round window niche is a potentially vulnerable site.