This trial is a pilot study to assess the feasibility of using Electrocochleography recorded from a cochlear implant intra-operatively in real time to monitor the progress of its insertion. Correlation between:

1. Observed changes in this signal during surgery and residual hearing loss post-operatively and
2. Recordings of this signal post-operatively and actual pure tone audiometry results will also be assessed.

Cochlear Implants (CIs) can provide deaf individuals the ability to hear. CIs are electronics packages inserted into the cochlea (hearing organ). An external processor digitises sound and transmits it to the implant. An electric stimulus from the implant mimics the change in electrical potential normally caused by the movement of stereocilia in the cochlea when a sound occurs. This change generates an impulse along the auditory nerve to the brain resulting in perception of sound.

CIs are available in the UK to people who have profound hearing loss. The improvement of CI technology means individuals with less significant losses, or profound losses at only some pitches may now benefit from a CI over a hearing aid. Patients having useful residual hearing has led to interest in better preserving that hearing during surgery. Thus allowing benefit from traditional acoustic amplification in addition to electrical stimulation from the implant.

The investigators propose recording a measurement via the implant during surgery and at audiology appointments called a cochlear microphonic using a technique called electrocochleography (ECochG). This is essentially recording the electrical signal generated by the movement of hair cells in the cochlea in response to a sound. It is hypothesised that reduction of this signal during implantation may correspond to cochlea damage occurring and that with development this signal might be useful feedback for surgeons. The signal is known to be correlated to actual hearing ability so a better understanding of this may allow us to fit the acoustic amplification portion of implants in individuals who are unable to respond reliably to a normal hearing test.

The purpose of this study is a pilot to assess the correlation between observed changes in this signal during surgery and residual hearing loss post-operatively and the degree correlation between recordings of this signal post-operatively and actual hearing test results.

Inclusion Criteria:

• Meets normal candidacy requirements for cochlear implantation
• At least 1 year of age at time of consent being given
• Patent cochlea as verified by CT or MRI scan
• No cochlear abnormality that might prevent insertion of the electrode array
• Measurable residual hearing in the ear to be implanted
• No additional complex needs that would prevent study procedures or normal clinical follow-up protocol being followed
• AB Implant selected by patient or MDT
• Written informed consent obtained

Exclusion Criteria:

• Medically complex cases where minimal duration of surgery is required