The purpose of this investigation is to demonstrate the safety and effectiveness of the MED-EL Electric-Acoustic System (EAS), a medical device that combines the use of a cochlear implant with an external electric-acoustic processor designed to provide benefit in speech perception and sound quality to individuals with a sensorineural hearing loss with minimal changes in residual hearing. The acoustic component of the processor will aid residual acoustic hearing in low frequency ranges, while the cochlear implant and electric component of the processor will be used to electrically stimulate the auditory nerve across a wide range of frequencies necessary for speech perception.

Inclusion Criteria:

• Audiologic tests suggest a moderate sloping to severe/profound sensorineural hearing loss in the ear to be implanted. The non-implanted ear may fall outside of these criteria; however, threshold levels cannot be better than the indication criteria (for example, the non-implanted ear must be 60 dB or poorer at 1000 Hz, 70 dB or poorer at 2000-8000 Hz).

• Pure-tone air-conduction threshold levels for the ear to be implanted shall fall at or within the following levels:

  250 Hz - 500 Hz hearing loss less than or equal to 65 dB HL 750 Hz hearing loss less than or equal to 75 dB HL 1000-1500 Hz hearing loss less than or equal to 60 dBHL (ie. 60-110+ dB HL) 2000 Hz - 8000 Hz hearing loss less than or equal to 70 dB HL

• Pure-tone air-conduction threshold levels shall fall at or within the levels listed in the following chart Frequency (Hz): 250 500 750 1000 1500 2000 4000 8000 Lower Limit: 10 10 10 60 60 70 70 70 Upper Limit: 65 65 75 *110+ *110+ *110+ *110+ *90+

• Pure-tone air-conduction thresholds for both ears are within 20 dB of each other at 250, 500 and 1000 Hz.

• Air-bone gap at 500, 1000, 2000 and 4000 Hz should be <10 dB at two or more of these frequencies.

• Minimal benefit from optimally fit hearing aid/s, preferably bilateral, with monosyllabic word scores in quiet of ≤50% in the best-aided condition.

• Normal middle ear anatomy and function (based on clinical assessment of tympanometry and acoustic reflex results). No prior middle ear surgery or history of postadolescent, chronic middle ear infections or inner ear disorders (i.e. vertigo or Meniere's syndrome).

• No evidence that hearing loss origin is retrocochlear.

• Current user of bilateral acoustic hearing aids for at least 3 months. Note: If the subject has not been a successful hearing aid user (i.e. improper fit, feedback and/or discomfort from high frequency amplification), he/she must complete a hearing aid trial in the ear to be implanted for at least 1 month. If the ear to be implanted is poorer than the contralateral ear, then the subject must complete a hearing aid trial in both ears for at least 1 month to ensure that he/she is tested in the best-aided condition.

• Adults 18-70 years of age at time of implantation.

• Persons who currently exhibit sufficient understanding and communicative skills to comprehend general conversation in an "oral/aural mode" through normal conversation channels.

• English as primary language.

• Appropriate motivation and expectation levels.

Expanded Criteria:

Study Arm 2 candidates are those who meet the inclusion criteria listed above with the exception that:

In the ear to be implanted, the pure-tone air conduction threshold(s) at 250, 500, and/or 750 Hz will be greater than or equal to 0 dB HL and less than 10 dB HL, and/or In the ear to be implanted, the pure-tone air conduction threshold(s) at 1000 and/or 1500 Hz will be greater than or equal to 0 dB HL and less than 60 dB HL and/or In their best-aided condition, they score 51-60% on monosyllabic word scores.

Exclusion Criteria:

• Conductive, retrocochlear or central auditory disorders.
• Hearing loss in the ear to be implanted that has demonstrated a recent fluctuation at two or more frequencies of 15 dB in either direction in the last 2 years as demonstrated by serial audiograms. Note: In cases where 2-year documentation is missing, documentation of at least 18 months is necessary along with patient report of no fluctuation in the past 2 years. If 18-month data is unavailable, the patient must be monitored until 18-month data is available.
• Any physical, psychological, or emotional disorder that would interfere with surgery or the ability to perform on test and rehabilitation procedures.
• Developmental delays or organic brain dysfunction.
• Physical or geographic limitations that may interfere with the completion of scheduled follow-up evaluations.
• Skin or scalp conditions that could preclude magnetic attachment of the speech processor or use of the acoustic hearing aid.