Influence of Cochlear Microanatomy on Hearing Preservation in Cochlear Implant Recipients

Objective: Identify cochlear microanatomic variables that influence hearing preservation in adult cochlear implant (CI) recipients. Study design: Retrospective review. Setting: Tertiary academic referral center. Patients: Forty-two CI recipients with preoperative unaided thresholds ≤45 dB HL at 250 Hz. Interventions: Cochlear implantation with a 24, 28, or 31.5-mm straight electrode array. Main outcome measures: Preoperative and postoperative imaging were used to extract scala tympani (ST) volume, cochlear duct length (CDL), and cross-sectional measurements (height, width, area, and diameter), as well as angular insertion depth (AID) and cross-sectional measures at the most apical electrode (E A ). Shift in low-frequency pure tone average (LFPTA; 125, 250, and 500 Hz) was computed at activation and at 6 months post activation. Results: A linear mixed-effects model analyzed the effects of cochlear microanatomy (ie, ST volume, CDL, and AID), cross-sectional measurements at E A (height, width, area, and diameter), as well as interval, age at surgery, and biological sex on the shift in LFPTA. Significant effects included biological sex ( P = 0.01) and ST volume ( P = 0.005). Age demonstrated a trend toward significance ( P = 0.052). Interval ( P = 0.26), CDL ( P = 0.80), AID ( P = 0.40), ST height ( P = 0.71), ST width ( P = 0.65), ST diameter ( P = 0.56), and ST cross-sectional area ( P = 0.14) did not significantly influence shift in LFPTA. Conclusions: ST volume and biological sex are significantly predictive of hearing preservation for adult CI recipients. Hearing preservation may be more affected by overall ST volume than by interactions of the electrode with the local environment. Indirect mechanisms of trauma, such as hydraulic pressure as the electrode is inserted, may predominate in damage to intracochlear structures. Future studies are needed to validate these findings with measurement of intraoperative fluid pressure transients in relation to ST volume during array insertion.