This clinical study will test a newly developed cochlear implant known as CI622D. This experimental cochlear implant has been designed to slowly release a drug called dexamethasone. Dexamethasone works to ease inflammation, which is common after any surgical procedure. The goal is to learn if there are added benefits in implant performance and hearing outcomes with the dexamethasone-releasing cochlear implant (CI622D) vs. the standard cochlear implant (CI622) without dexamethasone. The study will be conducted in adults with sensorineural hearing loss, a type of hearing loss caused by damage to the inner ear or auditory nerve. The study participants will undergo a series of tests that include testing their implant and their hearing. They will also complete questionnaires to see how they rate their hearing ability and their overall general health.

Inclusion Criteria:

• Aged 18 years or older (at time of consent)
• Clinically established post-linguistic bilateral moderately severe to profound sensorineural hearing loss
• Meets local candidacy criteria for cochlear implantation
• Compromised functional hearing with a hearing aid in the ear to be implanted
• Willing to participate in and comply with all requirements of the protocol, including willingness to be randomised to either arm
• Evidence of pneumococcal vaccination (e.g., Pneumovax) according to local guidelines prior to randomisation
• Candidate is proficient in the language used to assess speech perception performance
• Willing and able to provide written informed consent.

Exclusion Criteria:

• Planned for a partial insertion of the electrode array
• Intra-axial (within the brain) lesions or deafness due to lesions of the acoustic nerve affecting the ear to be implanted
• Middle ear infection (including acute otitis media, chronic otitis media, suppurative otitis media, or serous drainage) in the ear to be implanted either at the time of surgery or within 3 months prior to enrolment
• Active autoimmune disease or active immunosuppressive therapy
• Any of the following in the ear to be implanted within 3 months prior to enrolment: history of prior otologic surgery including grommets, or tympanic membrane perforation
• Previously reported diagnosis of auditory neuropathy, in the ear to be implanted
• Previously reported diagnosis, in the ear to be implanted, of Large Vestibular Aqueduct Syndrome (LVAS), Meniere's disease, or cochlear hydrops
• Ossification, otosclerosis, malformation or any other cochlear anomaly, such as common cavity, that might prevent complete insertion of the electrode array, as confirmed by imaging, in the ear to be implanted
• Current cerebrospinal fluid (CSF) shunts or drains, existing perilymph fistula, temporo- parietal skull fracture or CSF leaks
• History of bacterial meningitis
• Known allergic reaction or contraindication to dexamethasone or corticosteroids
• Severe, or poorer, bilateral sensorineural hearing loss prior to 5 years of age, as reported by the subject
• Severe, or poorer, sensorineural hearing loss of more than 20 years, as reported by the subject, in the ear to be implanted
• Prior cochlear implantation, in either ear
• Medical plan for cochlear implantation during the clinical investigation, contralateral to the ear to be implanted
• Medical or psychological conditions that contraindicate general anaesthesia, surgery or participation in the clinical investigation, as determined by the investigator
• Women who are pregnant
• Unrealistic expectations on the part of the subject, regarding the possible benefits, risks, and limitations that are inherent to the surgical procedure(s) and prosthetic devices as determined by the Investigator
• Additional disabilities that may affect the subject's participation or safety during the clinical investigation, as determined by the Investigator
• Unable or unwilling to comply with the requirements of the clinical investigation as determined by the Investigator
• Investigator site personnel directly affiliated with this study and/or their immediate families; immediate family is defined as spouse, parent, child, or sibling to the study candidate
• Cochlear employees or employees of Contract Research Organisations or contractors engaged by Cochlear for the purposes of this investigation
• Current participation, or participation in another interventional clinical study/trial in the past 30 days, involving an investigational drug or device (unless Cochlear sponsored and determined by investigator or Sponsor to not impact this investigation)
• Subjects recruited at a French site who are not affiliated with social security
• Subjects recruited at a French site who are under legal protection.