Inclusion Criteria:

• Post-lingual, bilateral, moderate (≥ 40 dB HL) to profound sensorineural hearing loss in the low (250 and 500 Hz) frequencies and profound (≥ 90 dB HL) hearing loss in the mid (1000 Hz) to high frequencies.
• Preoperative aided sentence perception score is 50% correct or less in the ear to be implanted (60% or less in the best aided condition).
• 18 years or older at time of consent.
• Willing to be randomised into either a treatment (CI632D) or control (CI632) arm.
• Evidence of pneumococcal vaccination (e.g., Pneumovax) according to local guidelines prior to randomisation.
• Fluent speaker in the local language used to assess clinical performance as judged by the investigator.
• Willing and able to provide written informed consent.

Exclusion Criteria:

• Deafness due to lesions of the acoustic nerve or central auditory pathway.
• Diagnosed active middle-ear infections or history of middle ear infection within past two years prior to randomisation.
• Diagnosis of auditory neuropathy.
• Diagnosis of Large Vestibular Aqueduct Syndrome (LVAS), Meniere’s disease, or cochlear hydrops.
• Prior history of surgery in the ear to be implanted (excluding grommets).
• Current use of grommets or evidence of unhealed tympanic membrane perforation.
• 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.
• Existing cerebrospinal fluid (CSF) shunts or drains, existing perilymph fistula, skull fracture or CSF leaks.
• History of bacterial meningitis.
• Known allergic reaction or contraindication to dexamethasone or corticosteroids.
• Use of ototoxins and/or steroids (does not include topical or inhaled steroids) up to 30 days prior to randomisation.
  • Note: Systemic ototoxin and/or steroids therapy must be completed at least 7 days prior to Screening/Baseline audiometric and speech testing.
• Evidence of severe or greater sensioneural hearing loss prior to five years of age, as reported by the subject.
• Severe to profound sensorineural hearing loss for more than 20 years, as reported by the subject.
• Existing contralateral cochlear implant.
• Medical plan to implant a contralateral cochlear implant during the clinical investigation.
• More severe than mild cognitive impairment as assessed by a score worse or equal to 25 on the Hearing-Impaired Montreal Cognitive Assessment (HI-MoCA) within 30 days prior to randomisation.
• Medical or psychological conditions that contraindicate general anaesthesia, surgery or participation in the clinical investigation, including uncontrolled diabetes.
• Women who are pregnant or plan to become pregnant during the course of the investigation.
• 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.
• Unable or unwilling to comply with all 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.
• Cochlear employees or employees of Contract Research Organisations or contractors engaged by Cochlear for the purposes of this investigation.
• Currently participating, or participated within the last 30 days, in another interventional clinical investigation/trial involving an investigational drug or device.