Analysis of electrical thresholds and maximum comfortable levels in cochlear implant patients

It is well known that a proper fitting of the cochlear implant processor is relevant to provide good quality in speech perception. The aim of this study is to extract statistical information to be applied for fitting the processor.

This study is based on the programming maps of 121 patients, aged from 18 months to 68 years at the moment of implantation. All subjects were implanted with the COMBI 40+ cochlear implant at San Cecilio University Hospital, Granada (Spain). The patients were classified into groups based on their age at implantation: younger than 5 years, between 5 and 16 years, and older than 16 years. The patients in each age-based group were divided into two subgroups, considering whether they had recent hearing experience or not. A special group including patients affected by severe damages in the cochlea was also defined.

Relationships between the programming parameters and factors like the age at implantation, the hearing experience and the presence of severe cochlear damage were found. The THR levels for patients younger than 5 years were significantly lower than those for patients implanted between 5 and 16 years, and this group presented significantly lower THR levels than adults. The MCL levels were not significantly influenced by the age at implantation. A significant increment was observed for both, MCL and THR levels, when patients were affected by severe cochlear damage. A significant increment in the THR levels were observed for patients with no recent hearing experience, while no significant differences were found for MCL levels. This study also analyzes the distribution along the cochlea of the stimulation levels. In the case of patients not affected by severe cochlear damage, the most basal electrodes presented a significant increment in the stimulation levels with respect to the rest of electrodes.

This work provides information of great value for programming the speech processors, particularly when the subjective responses of the patients are not sufficient. The application in our ENT Service has reduced substantially the average time needed to obtain an acceptable fitting of the processor, especially in children. Our study also shows that electrical thresholds are a good indicator of the functionality of the auditory nerve. The analysis of this parameter highlights the importance of an early intervention as well as a deep insertion of the electrode carrier in order to obtain the maximum functionality from the cochlear implant.