Cochlear Implantation and Single-sided Deafness: A Systematic Review of the Literature

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Single-sided deafness (SSD) may cause many problems involving communication between people. Permanent acquired
unilateral several-profound hearing loss has been estimated to affect between 12–27 persons in every 100,000 among the general population, with the majority of losses being sudden or idiopathic.1 The most common impairment is difficulty in hearing sounds in the affected side due to the head shadow effect, which attenuates the highfrequency
components of sounds at the ear contra-lateral to their source.2 Other problems involved are: prejudice inword
discrimination, difficulty in understanding speech particularly in noisy environments; constantly adjusting head to try and compensate for the handicap; restriction of onés ability to localize sounds; and, in some cases, leading to social isolation.3

The cochlear implant (CI) is one of the more recent treatment options for such cases. However, there is a concern about the ability of the brain distinguish acoustic and electric stimuli and concern that the hearing from the
cochlear implant would interfere with acoustic signal processing from the good ear. Contralateral routing of sound
(CROS) and osseointegrated implants are also devices used as rehabilitative options for SSD, although they are not
able to provide binaural hearing4 or improve sound localization.5 People with binaural hearing enjoy certain advantages.

The first advantage is better speech-to-noise ratio (SNR), which improves speech understanding in noisy environments. A second advantage results from the processing of the input sound signal by the brain from both ears. The brain is able to separate noise and speech from different locations using distinct interaural timing, spectral cues and level, thus, refining intelligibility. A third possible advantage is related to the summation effect, responsible for improved speech perception through the identification of identical signals arriving in both ears.6

The search in PubMed, Cochrane Library, and Lylacs retrieved a total of 228 articles, but only 17 met the inclusion criteria and were included in the study. Next, the respective studies were appraised, according to evidence-based guidelines of categorization of medical studies (►Table 2), and systematically analyzed. None of the studies were conducted as a randomized controlled trial and only one evaluated a control group.7 Furthermore, blinding was not observed in any study selected. Only prospective comparative studies and case series were to be analyzed in this review.

The operated patients’ demographics and audiometric data were carefully examined to avoid double counting of
cases. Three studies presented data which were also showed in more recent articles, including this review8–10;
thus, they were discarded. Two studies scored low in patient population and did not provide suitable follow-up
(patients had missed follow-up)11,12. Some studies presented incomplete data13 and were excluded for further