Restoration Of Binaural Hearing By Means Of Bilateral Cochlear Implantation

Müller, J, Schön, F, Helms, J

Today, cochlear implants (CI) are supplied monolaterally as a standard. However, with monolateral implantation there comes the loss of binaural hearing presumably resulting in a deterioration in speech understanding and in the ability to localize sounds. In an attempt to at least partly restore binaural hearing, we started with bilateral cochlear implantation in 1996 and have bilaterally implanted 18 adults and 53 children to date. To evaluate the benefit of bilateral implantation, a series of studies has been conducted. These studies served the purpose to provide a broad image on binaural hearing in bilateral CI users.

To assess binaural and monaural effects in adults, speech reception was measured (9 subjects) using a setup where speech was presented from the front and noise from either left or right. At 10 dB SNR, the average sentence score was 31.1 % higher with both CIs compared to the CI ipsilateral to the noise, and 10.7 % higher with both CIs compared to the CI contralateral to the noise. The average monosyllable score was 18.7 % higher with both CIs than with one CI.

To primarily assess binaural effects in adults, speech reception was measured (9 subjects) using a symmetrical four-loudspeaker set-up that largely reduces any head shadow effect. All subjects showed a substantial gain in SRT of around 4 dB on average. This gain was essentially stable over a period of up to 4.4. years.

To assess directional hearing in adults, sound localization experiments in the frontal horizontal plane where conducted (9 subjects). In addition, sensitivity to interaural cues was investigated. 8/9 subjects significantly showed the ability to localize sounds with a mean deviation in azimuth of 22.6°. These subjects also showed a significant sensitivity to ILDs (mean rate oft shift towards the louder side: 1.4° per unit on the Würzburg loudness scale) and ITDs (average time difference required for complete lateralization: 1187 ms).

To assess the bilateral benefit in children, speech reception was measured (18 subjects) using the same four-loudspeaker as in adults, and sound localization in the frontal plane was measured (14 subjects) in a simplified set-up. At 15 dB SNR, the mean score was 18.4 % higher when listening with both CIs as compared to listening with one CI only. 12/14 children significantly showed the ability to localize sounds (mean deviation in azimuth: 24,8°).

From our results we conclude that bilateral cochlear implantation provides a significant benefit in speech understanding for both children and adults. Bilateral CI users seem to benefit from all binaural and monaural effects that are known from normal hearing. In addition, bilateral cochlear implantation potentially restores spatial hearing.

Literature:
Speech Understanding in Quiet and in Noise in Bilateral Cochlear Implant Users of the Med El Combi 40/40+ Cochlear Implant System; J. Müller, F. Schön, J. Helms; Ear and Hearing;23, Vol. 3, 2002 Speech Reception Thresholds Obtained in a Symmetrical Four Loudspeaker Arrangement from Bilateral Users of Med El Cochlear Implants; F. Schön, J. Müller, J. Helms; Otology and Neurootology, 23, 2002

Contact Information

Department of Otorhinolaryngology
University of Würzburg, Germany
E-mail:Joachim.m@mail.uni-wuerzburg.de