Reduced Spread of Electric Field After Surgical Removal of Intracochlear Schwannoma and Cochlear Implantation.

The primary aim of the study was to explore whether reduced spread of electrical field is observed after partial or subtotal cochleoectomy and cochlear implantation compared with standard cochlear implantation. Secondarily, the influence on speech perception was explored comparing both groups. Nonconcurrent cohort study. Monocentric study at a tertiary referral center. Twenty adult cochlear implant (CI) users after tumor resection with cochleoectomy of varying extent and 20 electrode-matched CI users with standard electrode insertion. Partial and subtotal cochleoectomy for tumor removal and CI. Trans-impedance, electrically evoked compound action potentials, and word recognition were measured. Relative impedance was computed as a function of distance between the stimulation and recording electrode. Trans-impedance was smaller and more homogeneous in patients with partial or subtotal cochleoectomy than in the control group. In the tumor group, the mean relative impedance decreased to 0.20 (standard deviation [SD] = 0.03) at a distance of 1 electrode and to 0.25 (SD = 0.04) in the control group. After excluding seven patients with a second tumor in the internal auditory canal or cerebellopontine angle, with transmodiolar tumors, after near total cochleoectomy, or only extended cochleostomy, word recognition was 61% (SD = 19%) at 3 months and 75% (SD = 19%) at 12 months after activation of the audio processor in the tumor group. At 12 months, it was significantly (p < 0.05) better than in the control group (3 mo: 45%, SD = 25%; 12 mo: 53%, SD = 26%). A smaller trans-impedance is associated with a better word recognition. We conclude that the surgical technique used for CI surgery after subtotal cochleoectomy reduces the spread of the electric field and overcomes the potential drawbacks in structure preservation associated with that technique.