Cochlear Implantation in Children With Asymmetric Hearing Loss or Single-Sided Deafness Clinical Trial

Inclusion Criteria: * At least 4 ys of age and up to 14 yrs, 11 mos of age and able to complete all investigational procedures. * Parents and child fluent in English. Parents desire bilateral hearing for their child, are willing to comply with study requirements, including the pre-implant HA phase, and are able to provide informed consent. * Poor ear (AHL/SSD): PTA at .5, 1, 2 kHz \> 70 dB HL; Aided CNC word recognition score at 60 dB SPL \< 40%; SPHL duration ≥ 6 mos and ≤ 10 yrs at time of CI surgery; If \> 5 yrs of age, evidence of non-congenital SPHL onset (e.g., passed newborn hearing screening); If ≤ 5 yrs of age, no restrictions on SPHL onset. * Better ear * AHL: PTA at .5, 1, 2, 4 kHz \> 25 and ≤ 60 dB HL; Aided CNC word score at 60 dB SPL ≥ 55%; Currently using a HA; Stable hearing for the past 6-mo period. "Stable" is defined as thresholds that have not declined by more than 20 dB at three or more octave-interval audiometric frequencies. * SSD: PTA at .5, 1, 2, 4 kHz ≤ 25 dB HL; CNC word score at 60 dB SPL ≥ 70%; Stable hearing for the past 6-mo period. * Both ears of NH participants: PTA at .5, 1, 2, 4 kHz ≤ 25 dB HL. * To continue with cochlear implantation and into the CI phase of the study: Indication, via imaging, of a normal cochlear nerve and of cochlear anatomy in the ear to be implanted that allows full insertion of the electrode array. Imaging modality is at the discretion of the surgeon. In addition, better ear hearing must be stable throughout the HA phase. Exclusion Criteria for AHL/SSD Participants: * Medical condition that contraindicates surgery; Actively using an implantable device in the ear to be implanted; Inability to complete study procedures; Unrealistic expectations related to the benefits and limitations of implantation; Unwillingness or inability to comply with all investigational requirements. * Exclusions for cochlear implantation and the CI phase of the study: Cochlear malformation or obstruction that would preclude full insertion of the electrode array in the ear to be implanted; Abnormal cochlear nerve in the ear to be implanted; Hearing loss of neural or central origin.