Impact of Frequency Mismatch and Modiolar Proximity on Speech Perception in Single-sided Deafness Patients Implanted With Perimodiolar Cochlear Electrodes

Introduction: This study evaluated the impact of frequency mismatch and electrode proximity to the modiolus on speech perception in patients with single-sided deafness (SSD) implanted with perimodiolar electrodes (PMEs). We aimed to determine whether frequency mismatch or electrode positioning more strongly predicts speech outcomes, and whether these associations change over time. Methods: A retrospective cohort of 34 SSD patients implanted at the Royal Victorian Eye and Ear Hospital between 2014 and 2022 was analyzed. Frequency mismatch was calculated using the Stakhovskaya spiral ganglion map, and electrode proximity was measured from postoperative CT. Speech perception was assessed with consonant-nucleus–consonant phoneme (CNCP) scores and adaptive speech-in-noise testing (SRT) at 3 and 12 months. Five nonusers (<1 h/day) were included. Results: CNCP scores improved from 10% preoperatively to 53% at 3 months and 58% at 12 months. Basal distance to the modiolus showed the strongest and most consistent correlations with CNCP scores (3M: r=–0.616, P <0.001; 12M: r=–0.514, P =0.004). Frequency mismatch also correlated negatively with CNCP; however, only basal frequency mismatch remained significant at 12 months. Mismatch and proximity were strongly interrelated, suggesting mismatch largely reflects electrode positioning. No associations were observed for apical mismatch or speech-in-noise measures. Conclusion: In SSD patients with PMEs, basal electrode proximity to the modiolus was most strongly associated with speech perception, while frequency mismatch appeared secondary. These findings highlight the importance of basal positioning but should be interpreted with caution given the sample size and retrospective design.