The current study is a randomized multi-center clinical trial that investigates the role an intraoperative hearing monitoring system (electrocochleography) has on helping to save residual hearing in patients undergoing cochlear implantation (CI).

The clinical importance of residual acoustic hearing in CI remains unclear. It is estimated that only five percent of adult candidates undergo CI. A major barrier is the loss of residual hearing associated with surgery, meaning most patients lose any or all residual hearing as a result of the CI surgery, especially since the majority of today's candidates do not suffer from complete deafness. Hence, many candidates are faced with the decision to relinquish their remaining acoustic hearing in exchange for electric hearing via the CI, as CI surgery often results in complete loss of any residual hearing. However, recent developments in hearing monitoring systems during CI surgery have shown promise for providing the surgeon with live feedback of how the cochlear implant insertion process is proceeding, allowing the surgeon to make intraoperative adjustments during the insertion, potentially minimizing any trauma caused by the insertion of the implant, ultimately helping preserve residual hearing. This ultimately could lead to improvements in the rates of hearing preservation. Therefore, the current study seeks to investigate: 1) the advantage of using a hearing monitoring system (electrocochleography) monitoring guided CI surgery on hearing preservation rates, 2) determine the benefit that hearing aid/cochlear implant combined hearing provides participants over non-hearing preservation CI (electric-only stimulation from the CI) on measures of speech perception and quality of life.

Inclusion Criteria:

1. Provision of signed and dated informed consent form.

2. Adults, males and females, ages 18 -79 who have a bilateral sensorineural hearing loss with postlingual onset.

3. Minimum of 30 days experience with appropriately fit binaural amplification (standardized National Acoustic Laboratories (NAL) fitting method) verified with real ear measurements within 5 dB SPL of targets.

4. Limited benefit from conventional amplification in the best aided condition as defined by test scores of:

   1. The ear to implanted: Consonant-nucleus-consonant (CNC) words ≤ 60% or AzBio sentences (+10, +5 dB SNR ≤ 60% correct)
   2. Contralateral ear: ≤ 80% on CNC words or AzBio sentences (+10, +5 dB SNR ≤ 80% correct)

5. Low frequency Pure Tone Average (PTA- 125, 250, 500 Hz) ≤ 55 dB HL in the ear to be implanted.

6. Severe to profound mid to high-frequency sensorineural hearing loss (threshold average of 1000, 2000, 3000, and 4000 Hz ≥75 dB HL) in the ear to be implanted.

7. Low frequency PTA ≤ 55 dB HL sloping to moderately severe to profound mid-to high frequency sensorineural hearing loss (threshold average of 1000, 2000, 3000, 4000 Hz ≥ 60 dB) in the contralateral ear.

8. Proficient in English.

9. Undergoing implantation with a current generation CI device from either Cochlear Limited or Advanced Bionics AG.

   1. Cochlear Limited devices include: Nucleus CI612, CI622, CI632, CI624
   2. Advanced Bionics AG devices include: HiFocus SlimJ, Mid-Scala

10. Stated willingness and ability to complete testing and all associated study visits.

Exclusion Criteria:

1. Previous cochlear implantation.
2. Prelingual onset of hearing loss.
3. Abnormal inner ear anatomy on CT imaging.
4. Auditory neuropathy spectrum disorder.
5. Retrocochlear pathology such as a vestibular schwannoma or stroke.
6. Unwillingness or inability to comply with all investigational requirements including the randomization process.
7. Additional medical, or social barriers that would prevent completion of all study requirements.
8. Medical condition contraindicated for surgery.
9. Device selection of Med El CI (per the patient's selection).