Objectives. Bimodal auditory stimulation represents a possibility of treating asymmetric hearing loss with the aid of simultaneous use of a cochlear implant on one ear and a conventional hearing aid on the other ear. Researches by foreign authors show that such binaural listening enables, above all, better sound localization and speech recognition, especially in noisy environment or when conversing with multiple conversational partners. Furthermore, it contributes to a better quality of life. The aim of this study was to determine the influence of bimodal hearing on speech comprehension in noise in cochlear implant recipients and to check whether other factors also indirectly effect speech comprehension in noise with bilateral listening. Factors such as the time of onset of hearing loss, the time of cochlear implant use, the level of hearing loss in the non- implanted ear, the cause of hearing loss, comorbid illnesses, hearing and speech rehabilitation following implantation, the daily singular/combined use of a cochlear implant and a hearing aid, the bimodal users' subjective perception of auditory perception parameters – the natural qualities of listening to environmental sounds, the ability of orientation and sound localization in a certain place, the need for lip reading and use of sign language.
Methods. The study included 34 adult cochlear implant recipients (that is 57,6 % bimodal hearing individuals of entire representative sample) with progressive asymmetric hearing loss who, alongside using a cochlear implant for a minimum of one year, also utilize a conventional hearing aid in the contralateral ear, to communicate. The patients were treated (and still undergo follow-up care) at the Clinic of Otorhinolaryngology and Cervicofacial Surgery at the University Medical Centre Ljubljana. For testing speech recognition in noise, a shortened screening version of the original Slovenian speech test was used (an adaptation of Freiburg speech test, adjusted to Slovenian speakers). 50 semantically different, monosyllabic words were used in a speech audiometry testing, which was carried out in three different listening conditions – the unilateral use of a cochlear implant, the unilateral use of a hearing aid and bimodal auditory stimulation. For the purpose of this study a questionnaire exploring the effect of bimodal listening on speech understanding in noise was created.
Results. Considering the success of the speech audiometry testing with bilateral stimulation, we notice a significantly better result with patients with a postlingual hearing loss (55,9 % study participants) compared to the prelingually deaf group (p = 0,004). We also see a correlation between greater hearing loss levels in the non-implanted ear with a statistically significant lessened performance of binaural listening in noisy environment (p = 0,007), whereas we've found that the result of the speech test in noise in bimodal hearing individuals is not statistically significant in connection to the duration of regular use of a cochlear implant (p = 0,421), hearing and speech rehabilitation after implantation (p = 0,616), the daily regular use of a cochlear implant and a hearing aid combined (p = 0,197), comorbid illnesses (p = 0,671) and the cause of hearing loss (p = 0,630). Nonetheless, we find that a simultaneous use of both hearing aids – a cochlear implant and a conventional hearing aid – gives a statistically significant improvement in the users' subjective evaluation of speech understanding in noise (p = 0,005) and individual auditory perception parameters – the more natural quality of listening to environmental sounds (p = 0,008), the ability of orientation and sound localization in a certain place (p = 0,000), and a lessened need for lip reading (p = 0,003). Although the comparison of speech audiometry results of both unimodal ways of listening does not show a statistically significant difference in speech recognition in noise (p = 0,291), the percentage of recognized monosyllabic words when using bimodal stimulation significantly improves when using monaural listening with a hearing aid (p = 0,0001) and a cochlear implant (p = 0,001), however better results in binaural listening compared to the unilateral use of a cochlear implant do not eliminate the person's need for lip reading (p = 0,007).
Conclusion. The participants' subjective perception as well as the acquired results of speech audiometry testing show that a bimodal auditory stimulation provides better speech recognition in noise compared to the monaural way of listening. Bimodal stimulation also helps to improve the person's subjective evaluation of auditory perception parameters – natural quality of listening to environmental sounds, localization of auditory stimuli in a certain place and eliminates the need for lip reading.
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