Relation of electrically evoked compound action potential thresholds to behavioral T- and C-levels in children with cochlear implants.

OBJECTIVE: This study examined the relation of electrically evoked compound action potential thresholds obtained using neural response telemetry (NRT) to T- and C-levels in children's speech processor programs optimized for recognition of very soft to loud sounds while ensuring tolerance of very loud sounds.
DESIGN: Forty-one children (age 2 to 14 yr) with stable electrical hearing participated. All children were Nucleus 24 System recipients and attended one of three auditory-oral schools that have on-site pediatric audiologists experienced at cochlear implant programming. Speech processor MAPs were created and adjusted over a period of months until aided warble-tone thresholds were between 10 and 30 dB HL at octave frequencies between 250 and 4000 Hz, and understanding of speech was maximized for many listening situations. At least 1 yr postactivation, visual (vNRT) and predicted (tNRT) thresholds were obtained on 9 to 11 electrodes and compared to each child's T- and C-level values on these electrodes in their MAPs. Test-retest stability of NRT thresholds was compared for two test sessions 1 mo apart.
RESULTS: NRT-based evoked compound action potential thresholds could be obtained from 36 of the 41 children. vNRT and tNRT test-retest reliability was high; average correlation coefficients (r) across subjects were 0.90 (range: 0.64 to 0.99) and 0.88 (range: 0.31 to 1.00), respectively. Group average correlation coefficients between vNRT and T-level, vNRT and C-level, tNRT and T-level, and tNRT and C-level were low (0.18, 0.21, 0.24, and 0.26, respectively). Group mean tNRT thresholds were four current levels lower than the group mean vNRT thresholds. Subsequent analysis was performed with the vNRT thresholds because the range of test-retest correlation coefficients for individual subjects was narrower than with tNRT. Hierarchical linear modeling was used to determine if vNRT could be used to predict T- and C-levels. This analysis indicated a significant average relation between vNRT and T-levels and between vNRT and C-levels, but significant heterogeneity in the individual-level estimates of those relations. In other words, subjects varied significantly in the size of the relation between their individual vNRT values and both T- and C-levels. Attempts to account for that heterogeneity did not identify any subject characteristics that were significantly related to the individual-level parameters.
CONCLUSIONS: The position of the group average vNRT and tNRT thresholds in the upper half of the dynamic range between Ts and Cs agrees with previous studies. The fact that the profile of vNRT thresholds did not parallel the profiles of Ts and Cs across electrodes for most children suggests that simply shifting the NRT profile to select T- and C-levels in initial MAPs is likely to result in a loudness imbalance for certain speech frequencies and/or tolerance issues for many children. This was verified by the hierarchical linear modeling analysis, which showed substantial and significant heterogeneity in the relations between vNRT and T-levels and between vNRT and C-levels. In summary, vNRT is not related to T- or C-levels in a simple and uniform way that would allow it to guide MAP fine tuning with any precision. Consequently, it is recommended that MAP fine tuning be based on the child's behavioral responses on individual electrodes.