Individualized, precise threshold-level fitting improves speech recognition and loudness differentiation in cochlear implant users

Precise fitting of threshold (T-) levels in cochlear implant (CI) sound processors is crucial for optimizing speech recognition and audibility for CI users. This study systematically investigated the effects of T-level fitting on speech recognition in quiet in adult CI users. Thirty-five adult, experienced CI users with a Nucleus system (Cochlear, Sydney, Australia) were included in a prospective exploratory study. T-levels for each electrode were individually fitted to each participant's electrical hearing threshold using a standardised adaptive procedure. Speech recognition at sound pressure levels of 50, 65, and 80 dB SPL and the Oldenburg Categorical Loudness Scaling (CLS) were measured, and the baseline and post-intervention values were compared. The preoperative outcome prediction for speech recognition was compared with the results obtained before and after the intervention. Precise T-level fitting to the electrical hearing threshold resulted in overall changes ranging from a reduction of 4.8CL to an increase of 9.9CL per electrode (mean: 0.5CL, SD: 15.7 CL). Mean monosyllabic word recognition improved significantly at all measured sound pressure levels. At 65 dB SPL, it increased from 64% to 74% (p < 0.001). With precise T-level fitting, more CI users achieved their preoperative outcome prediction for speech recognition. Precise, individualized T-level fitting to the electrical hearing threshold enhances speech recognition and increases the percentage of CI users who achieve their predicted clinical outcomes. Individualised, electrode-specific threshold adjustment is important for achieving an optimal CI outcome. Further research is needed to refine the threshold detection methods and fitting procedures for broad clinical application.