Comparison of the Bone Anchored Hearing Aid Implantable Hearing Device with Contralateral Routing of Offside Signal Amplification in the Rehabilitation of Unilateral Deafness

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Abstract
Monaural hearing imposes constraints under many listening conditions. The authors compared the effects of a semi-implantable bone conductor, the Entific bone anchored hearing aid, with conventional contralateral routing of offside signal amplification to assess rehabilitative benefit in adults with unilateral deafness. Prospective trials of subjects with unilateral deafness using benefit surveys, source identification testing, and hearing in noise testing. Tertiary referral center, outpatient surgical and audiologic services. Adults with unilateral deafness (pure tone average >90 dB, SD <20%) after acoustic neuroma excision (n = 7), meningitis (n = 1), sudden sensorineural hearing loss (n = 1), and sudden sensorineural hearing loss with chronic suppurative otitis media (n = 1). Entry criteria included normal hearing in the contralateral ear (pure tone average 80%). Subjects were fitted with contralateral routing of offside signal amplification devices for 1 month and tested with contralateral routing of offside signal before mastoid implantation of the deaf ear, fitting, and testing for bone anchored hearing aid. Subjects' assessment of experience with their devices and patterns of use, 2) source azimuth identification in noise test, and 3) speech discrimination in quiet and in noise under conditions of noise-front, noise-to-normal-ear, and noise-to-deaf-ear. There was consistent satisfaction with bone anchored hearing aid implantation and amplification, and poor acceptance of contralateral routing of offside signal amplification. Sound localization was poor at baseline and with both bone anchored hearing aid and contralateral routing of offside signal. Relative to baseline, contralateral routing of offside signal and bone anchored hearing aid produced significantly better speech recognition in noise under most conditions. The bone anchored hearing aid enabled significantly better speech recognition than contralateral routing of offside signal in quiet and in a composite of noise conditions. The advantages may relate to averting the interference of speech signals delivered to the better ear, as occurs with conventional contralateral routing of offside signal amplification. Preliminary data in subjects with normal monaural hearing indicate that vibromechanical stimulation with the bone anchored hearing aid overcomes some of the negative head shadow effects in unilateral deafness. The bone anchored hearing aid system, when placed on the side of a deaf ear, yields greater benefit in subjects with normal monaural hearing than does contralateral routing of offside signal amplification. It seems that this rehabilitative approach can expand the sound field of monaural listeners in further enhancing speech understanding. Observations suggest that further understanding of bone conduction as implemented in transcranial stimulation will guide further options for patients with monaural hearing. Longer follow-up will help to determine whether communicative skill improvements with the bone anchored hearing aid outweigh the disadvantages of implantation surgery, costs, and device maintenance.

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