Optimization of Cochlear Implant MAP-parameters in Children

Cochlear implants (CI) enable hearing for persons who otherwise would have been deaf. A CI substitutes the sensory cells in the cochlea and stimulates the auditory nerve directly with weak electrical pulses. For children born deaf, or become deaf at an early age, prompt detection of this condition is important to since it enables the possibility of an early implantation of the CI and facilitates the child's development of a spoken language. When it comes to detecting deafness in newborns, Sweden has a well-functioning and widespread hearing screening program for infants, permitting children born deaf, or with severe hearing impairments, to be discovered very early. In general, children that are born deaf can get a CI implanted at about one year of age, if there are no other contraindications. Internationally it has become increasingly common for implants to be inserted at even lower ages, sometimes even as young as five months of age.

For small children who receive CI's at an early age it is not possible to perform measurements based on the child making subjective assessments of how the sound is perceived when the electrical stimulation is altered through the implant. Also, small children cannot judge and report back the quality of the sound. Instead, objective measurements are commonly used since they do not require the active participation of the child, along with observations of the child and how it spontaneously reacts to the sound stimulation. A common objective measurement is to find the lowest level of electrical stimulation required to obtain a response from the auditory nerve, i.e. the electrically evoked action potential (ECAP) threshold. The CI manufacturer Cochlear Ltd has developed an automatic system for this measurement. Based on such measurements, as the automatic ECAP-measurement, the electrical stimulation from the implant can adjusted to the so-called MAP, where the minimum and maximum stimulation levels (T- respectively C-levels) are set for each individual electrode on the implant to create the dynamic range of electrical stimulation. This MAP then becomes the basis for the child's ability to perceive sound.

Clinically, today (at Sahlgrenska University Hospital, Gothenburg, Sweden) a basic programming of the MAP is performed based on ECAP measurements, after which adjustment of only the overall level is carried out based on the child's reactions. However, no subjective measurement is made regarding threshold levels of stimulation through the implant or that the child actually perceives medium sound equally across the different channels. When the child can participate in sound field audiometry, adjustments of the MAP settings are made to get a straight audiogram, but basically, the children continue to use the setting based on the ECAP measurement. Adult patients with acquired deafness receiving CI are managed in a different manner than the children. The programming of the implant for adults is based on subjective measurements of how stimulation via individual electrodes is perceived. By measuring thresholds for individual electrodes, i.e. minimum audible stimulation (T-levels) and levels where stimulation is immediately below unpleasant strong (C-levels), then the MAP is programmed with an individual dynamic range for each electrode.

To our knowledge no previous study have explored how the MAP based on an ECAP measurement endures over time or if a re-programming should be carried out when the child reliably can participate in behavioral measurements, which is about seven years of age in terms of measuring T-levels. Studies have shown that there is a correlation between ECAP-thresholds and T- and C-levels for people who can participate actively. However, the relationship is relatively weak and some researchers have even considered it too weak for ECAP thresholds to be the basis for programming the implant. However, in most clinics, these thresholds are used as a basis for programming CI to small children.

Several of those who work with programming the MAP in small children claims to witness an adaptation in sensitivity of the auditory nerve to the applied stimulation; indicating that the results of the objective measurement could gradually change if there is an alternation in stimulation levels. However, there are no studies at this time that neither confirm nor deny this.

The aim of this project is to investigate and clarify how children who receive CI at an early age should be monitored and to evaluate the objective ECAP measurement as a basis for setting stimulation levels. It also aims to investigate if there is any change in the result of the ECAP measurement after adjustment has been made in the MAP. After completion of the study, the goal is to offer a better and more knowledge-based (re-) habilitation for children who receive CI's, for whom there are currently no definite guidelines for programming and follow-up.