[Effect of cochlear implantation on sound localization for patients with unilateral sensorineural hearing loss]

The aim of this review was to examine the current literature regarding application of cochlear implantation on patients with unilateral sensorineural hearing loss (USNHL) for improvement on sound localization. The literature were searched in the PubMed database with 'cochlear implantation AND single-sided deafness' or 'cochlear implantation AND unilateral deafness' as keywords. The publication date of the articles was up to 2015-2-12. A total of 12 articles were included. The results show that the ability of sound localization for most of the USNHL subjects (90%) with cochlear implantation was significantly improved than that without CI, which suggests that CI is a superior auditory rehabilitation treatment than BAHA and CROS hearing aids for patients with USNHL, because of the re-establishment of the benefits of binaural hearing. In addition, the benefit of CI for USNHL requires a period of auditory experience or training. About 30% subjects showed significantly improvement on sound localization ability after CI worked for three months. For most of the patients (90%), the sound localization ability improved after CI worked for six months. When CI worked for nine months, all the subjects would show improvement on sound localization ability. Sound localization of the USNHL subjects with a CI is based primarily on interaural level differences (ILD) while interaural time differences (ITD) provide little advantage or probably not perceptible at all. The younger subject suffers from USNHL, the stronger the plasticity of the auditory center shows, which results in more obvious degeneration of the affected side and adaptive enhancement of the contralateral side of the auditory pathway. Similarly, the longer duration of USNHL lead to more obvious degeneration of the affected side and adaptive enhancement of the contralateral side. An adaptive enhancement of auditory pathway corresponding to the healthy ear will rely more on the monaural spatial cues that available to the intact ear to improve the sound localization in the horizontal plane. Contrarily, the degeneration of auditory pathway corresponding to the USNHL may be limited to re-reorganize from auditory deprivation even after CI, which may increase the risk that a few subjects cannot re-establish the binaural benefits after CI. Therefore, patients with USNHL should accept CI as soon as possible to obtain good binaural benefits, especially for sound localization.