To be eligible to participate in this trial as a study subject, an individual must meet all of the following criteria:
1. Provision of signed and dated parental permission form
2. Unilateral severe-to-profound sensorineural hearing loss, congenital or acquired prior to the age of 2 years, defined as:
1. Unaided residual hearing thresholds that yield a 3 frequency pure tone average (PTA; average at 500, 1000, and 2000 Hz) of ≥70 dB hearing loss (HL) in the ear to be implanted
2. Hearing thresholds in the contralateral ear of ≤20 dB HL (3 frequency PTA of 500, 1000, and 2000 Hz).
i. Thresholds should be measured using inserts wherever possible, or in the sound field to record the better-hearing ear alone. Required thresholds will include 250, 500, 1000, 2000, and 4000 Hz for air conduction. All other octave frequencies should be attempted. Bone conduction thresholds should be obtained at 500, 1000, 2000, and 4000 Hz. Masking should be attempted where appropriate. As participants are very young and masking is difficult for this population, suspected shadow audiograms will be sufficient if objective testing has confirmed a severe-to-profound unilateral hearing loss. Testing should confirm a severe-to-profound hearing loss, defined as a 3-frequency PTA (500, 1000, and 2000) \>70 dB HL in one ear and ≤20 dB HL in the contralateral ear. The Primary Investigator and Lead Diagnostic Audiologist must agree on this diagnosis for enrollment.
3. Insufficient functional access to sound with appropriately fit amplification and aural habilitation (based on best practices and standard of care) defined as:
1. PEACH + parent questionnaire scores below the expected value for children who lack the requisite language to complete open-set word recognition testing or
2. Recorded Multisyllabic Lexical Neighborhood Test (MLNT) scores ≤ 30% in the ear to be implanted for children with the requisite language to complete open-set word recognition testing.
4. Between 7 months and 2 years, 11 months of age at implantation
5. Normal cochlear nerve as evaluated by imaging, required imaging by MRI brain/Internal auditory canal (IAC) without contrast with heavily weighted T2 images. CT optional at the physician's discretion.
6. No malformation of the cochlea
7. No evidence of progressive hearing loss of the contralateral ear
8. Willing to/has undergone hearing aid trial as warranted based on achieving an aided speech intelligibility index (SII) of \> .65 when fitted via Desired Sensation Level (DSL) methods.
9. Realistic parental expectations: a verbal acknowledgment of the potential benefits and risks, and postoperative variation in performance. For instance, cochlear implantation will not restore normal hearing
10. Willing to obtain recommended meningitis vaccinations per Centers for Disease Control (CDC) recommendations
11. Development and cognition within the normal range as measured by the Developmental Assessment of Young Children (DAYC-2).
12. Parental commitment to study parameters including being able and willing to participate in the evaluation schedule, involvement in prescribed therapy, and travel to the investigational site and study-related activities. Parents must be willing to encourage wearing implant during waking hours.
To participate as an SSD control subject, an individual must meet all the following criteria:
1. Provision of signed and dated parental permission form
2. Unilateral severe-to-profound hearing loss - congenital or acquired prior to 2 years of age.
1. Unaided residual hearing thresholds that yield a 3 frequency PTA (500, 1000, and 2000 Hz) of ≥70 dB HL in the affected ear.
2. Hearing thresholds in the contralateral ear of ≤20 dB HL (3 frequency PTA of 500, 1000, and 2000 Hz)
3. Unable to use or benefit from traditional amplification
3. Five years of age at the time of testing
4. Development and cognition within the normal range as measured by the Early Stanford Binet 2 (SB2).
To participate as a TH control subject, an individual must meet all the following criteria:
1. Provision of signed and dated parental permission form
2. Bilateral hearing thresholds that yield a PTA of ≤20 dB HL (3 frequency PTA of 500, 1000, and 2000 Hz).
3. Five years of age at the time of testing
4. Development and cognition within the normal range as measured by the Early SB2.
An individual who meets any of the following criteria will be excluded from participation in this study as a study subject:
1. English is not primary language of the home
1. Speech perception materials are presented in English
2. Parental questionnaires are administered in English
2. Conductive hearing loss in either ear
3. Cochlear nerve deficiency
4. Ossification/Fibrosis of the cochlea precluding implantation
5. Inability to participate in follow-up procedures (i.e., unwillingness, geographic location)
6. History of refractory chronic otitis media or condition that contraindicates anesthesia
An individual who meets any of the following criteria will be excluded from participation in this study as a control subject:
1. English is not primary language of the home
1. Speech perception materials are presented in English
2. Parental questionnaires are administered in English
2. Inability to participate in testing (i.e., unwillingness)
