In-person communication helps facilitate the majority of daily activities. Significant single-sided hearing loss can severely impact this. This is true of all ages, from grade school, during professional/work life and in retirement. Between 12 and 27 out of 100 000 people are affected by sudden or idiopathic unilateral severe-profound sensorineural hearing loss (SNHL) . Single-sided deafness (SSD) and asymmetric hearing loss (AHL) can occur for a variety of reasons and both terms are used to describe a person who has no serviceable hearing in one ear. The difference is that, SSD describes a situation in which the contralateral ear has normal hearing, while AHL describes a situation in which the contralateral ear is not normal but is still serviceable (pure tone average between 30 and 60 dB) with benefit from hearing amplification .
Roughly 18.1 million people suffer from SSD/AHL in the United States. The lack of hearing in one ear presents a number of challenges to these individuals. When sound information is processed by only one ear (monaural) rather than two (binaural), this results in a loss of summation and squelch, causing significant impairment in speech perception in all listening conditions  and especially in complex environments . In addition, the loss of the head shadow effect results in the patient needing to constantly adjust head position in these difficult listening conditions in an attempt to better direct the sound to the ‘good’ ear [1,4,5▪]. Tinnitus frequently accompanies SSD/AHL and further impairs sound localization and speech recognition in noise . The association of untreated hearing loss with increased stress, depression, anxiety, and feelings of exclusion in social settings makes it important for providers to be aware of the best treatment options available [6–9].
Treatment options for single-sided deafness/asymmetric hearing loss
Contralateral routing of signal (CROS) hearing aids, bone conduction devices (BCDs), and cochlear implants are currently the primary treatment choices for SSD/AHL. Until the last decade, CROS and BCDs have been the main rehabilitative options. Both of these options provide ‘bilateral’ hearing since the better ear is given sound information from both sides of the head, but neither option provides ‘binaural’ hearing which requires two functional ears to provide sound to the central pathways. For both devices, the microphone is on the patient's deaf side. CROS hearing aids route the acoustic signal to the contralateral better hearing ear and BCDs transmit the signal to the contralateral ear via bone conduction [5▪]. Both CROS and BCDs improve understanding when speech is presented to deaf side [2,5▪]. However, because both of these devices are stimulating the contralateral better-hearing ear and therefore do not provide binaural hearing, users continue to suffer from reduced understanding of speech in noise, poor sound localization, and lack of tinnitus relief [1,2,5▪]. Cochlear implants are a more recent treatment option for SSD/AHL. Although cochlear implant does require an invasive procedure and adaptation may take longer, it is considered a safe and effective procedure [5▪]. One of the first studies to compare CROS/BCD to cochlear implant as a treatment for SSD was performed by Arndt et al. in 2010. They found that not only did patients do better with a cochlear implant for their SSD, but also that they performed better with the cochlear implant than with traditional CROS/BCD options. The promising results from this study generated significant enthusiasm in the field toward the application of cochlear implant for SSD/AHL .
BENEFITS OF COCHLEAR IMPLANTS FOR SINGLE-SIDED DEAFNESS/ASYMMETRIC HEARING LOSS
Cochlear implants were initially developed to help patients with bilateral deafness. More recently, the effectiveness of cochlear implant to improve speech perception in cases of unilateral deafness (SSD/AHL) has been investigated. Arts et al. found a statistically significant improvement in speech perception in quiet approximately 2 months after beginning cochlear implant use in 10 adult patients. Many other studies have corroborated these findings of improved speech perception in quiet, not only in adults but also in children [4,5▪,12–16]. cochlear implant for SSD/AHL can also improve understanding of speech in more difficult listening conditions [14,17,18]. In 2017 with 48 patients, Finke et al. found that speech perception in quiet and noise both significantly improved. Döge et al. published in 2018 that there was a tendency of improved speech perception in complex noise for most patients (eight out of 11 patients). Other studies went on to investigate how these patients perform when the speech and/or noise was presented from various locations. Arts et al. studied this in 10 patients in 2018 and found that speech perception in noise statistically significantly improved when speech was presented on the cochlear implant side or in front. Similarly, Buechner et al. found improved speech understanding for three out of five patients when noise was presented to the normal ear and speech from the front. Also important to note, speech perception in noise improved in all tested configurations with cochlear implant on versus cochlear implant off . Legris et al. found that when speech and noise were spatially separated, speech understanding improved after cochlear implant in postlingually deafened patients. Lastly, Arndt et al. looked at cochlear implant versus CROS/BCDs when directing sound or noise to the aided versus unaided ear. Speech perception was significantly better in the cochlear implant group when compared with the CROS and osseointegrated device groups for both the Sci/Nhe (Sci = Sound to deaf ear, Nhe = noise to hearing ear) and She/Nci (She = sound to hearing ear, Nci = noise to deaf ear) test conditions . The cochlear implant was better than unaided when Sci/Nhe but there was no difference when She/Nci . Although these are promising studies, the variability of test parameters among investigators makes direct comparison difficult, which has been acknowledged in other articles and recent systematic reviews [1,4,21].
Improved sound localization is a key advantage with cochlear implant over the traditional treatment options of CROS hearing aids and BCDs. Localization requires binaural hearing along with central pathways capable of processing the information to give the listener a sense of in which the sound is coming from. Localization error is commonly used when looking at effectiveness of cochlear implant versus other amplification options (traditional aids, CROS, and BCDs) to assess sound localization . After having a cochlear implant for SSD/AHL, localization is generally improved but the degree of improvement varies from study to study [5▪,17]. One complicating factor is that the number of speakers, placement of speakers, spacing between speakers, and other details differ between studies. In the study by Arndt et al., seven speakers were placed in a semicircle 2 m in front of the patient between 90 and +90 degrees. They found significant improvement in the binaural condition with the cochlear implant compared with all other conditions . Döge et al. spaced out seven speakers in front of the patient between −60 and +60 degrees. They found a statistically significant decrease in the median localization error from 26 degrees unaided to 13.1 degrees with the cochlear implant (aided). Galvin et al. used a 12 speaker setup in which the speakers were evenly spaced 15 degrees apart centered behind the patient. They studied localization at 1, 3, and 6 months postactivation and found that localization did not improve until 6 months [5▪].
Sullivan et al. studied a very heterogeneous group of patients. Of the 60 patients studied, 27 had idiopathic sudden SNHL, 22 had a labyrinthectomy for Meniere's disease, five were tumor patients and six lost their hearing for a variety of reason including cholesteatoma or poststapes surgery. Although there was no statistically significant improvement of sound localization, the scores tended to improve throughout the postoperative course . Firszt et al. found improvements in sound localization in bimodal conditions (cochlear implant and hearing aid in better ear) when compared with hearing aid in better ear alone in five out of seven postlingually deafened patients.
Tinnitus frequently accompanies hearing loss and its impact on daily activity covers the entire spectrum. Some patients are not aware of any tinnitus while for others it severely impacts their daily activities. Patients with shorter duration of deafness (<1 year) were more likely to have tinnitus (80%) than patients with longer duration of deafness (>2 years only 38% had tinnitus) . Patients suffering from incapacitating tinnitus in the setting of unilateral SNHL can benefit from cochlear implant in terms of tinnitus reduction [14,24]. Also, SSD/AHL patients with incapacitating tinnitus might benefit more from cochlear implant than other SSD/AHL subpopulations . Several different instruments have been used to assess tinnitus and its impact on the patients including the visual analog scale (VAS), Tinnitus Functional Inventory (TFI), Tinnitus Handicap Index (THI), Tinnitus Reaction Questionnaire (TRQ), and Subjective Tinnitus Severity Scale (STSS).
A number of authors have examined tinnitus suppression with cochlear implants. One of the first studies to examine this was by Van de Heyning et al.. Of their cohort of 21 patients that had SSD and severe, incapacitating tinnitus, 20 had reduction of tinnitus while the device was turned on, some with complete suppression of the tinnitus. This effect remained stable at 2 years follow-up. Buechner et al. found that three out of five patients had significant suppression of tinnitus while wearing the device and the other two patients had tinnitus reduction in certain situations. Arndt et al. found that of the 10 patients in their study with tinnitus, significant reduction of tinnitus as measured by VAS was seen in eight patients, with complete suppression of tinnitus when the cochlear implant processor was on in five of those eight patients. Mertens et al. compared cochlear implant-on and cochlear implant-off and found the VAS ratings of tinnitus were higher with the cochlear implant-off than with the cochlear implant-on. Kleine Punte et al. found significant reduction in tinnitus as measured by VAS and tinnitus questionnaires in seven cochlear implant patients with SSD compared with controls. Galvin et al. measured tinnitus severity with VAS and TFI for the 10 patients in their study. They found that tinnitus was significantly less severe with the cochlear implant-on than with the cochlear implant-off at 1, 3, and 6 months and that tinnitus was significantly less with the cochlear implant-on at 1, 3, and 6 months when compared with pre-cochlear implant baseline [5▪]. Poncet-Wallet et al. found that after cochlear implant, 23 out of 25 patients had reduction in tinnitus with significant improvement at 1 month for THI, at 2 months for VAS and TRQ, and at 4 months for STSS. They also found progressive tinnitus reduction until the last follow-up at 13 months without any plateau . In summary, cochlear implantation is an excellent option to help improve tinnitus in patients with SSD, as study after study has demonstrated that cochlear implants provide tinnitus relief for the vast majority of patients.
Another question that has been examined relates to the duration and durability of tinnitus suppression. In 2015, Távora-Vieira et al. found a gradual, continuous, and significant decrease in tinnitus disturbance when following 28 patients for 24 months as measured by the TRQ. Holder et al. found that 11 out of 12 patients had subjective reduction in tinnitus following cochlear implant, and this reduction was maintained for at least the first year. Sladen et al. found that the majority of patients had a self-reported reduction in tinnitus after implantation; while this was more often the case with the device on, many patient reported continued improvement even with the device off. In contrast to the Sladen et al. study, however, Galvin et al.[5▪] found that tinnitus can return to baseline levels as quickly as a few minutes after the device is turned off. Overall, most patients benefit from durable tinnitus suppression at long-term follow-up while the cochlear implant is being used.
In conclusion, tinnitus suppression appears to be a durable result and is at least stable years after implantation . There is a growing body of evidence supporting the effectiveness of cochlear implant as a viable treatment for tinnitus in 80–100% of cases [9,28,29], although there is a minority of patients who do not have improvement [24,29]. Tinnitus from SSD is also negatively correlated with sound-source localization . One theory is that in addition to providing binaural cues to help with sound localization and speech recognition, cochlear implant may also improve these measures by reducing tinnitus . For this reason, there is some suggestion that SSD patients with tinnitus may benefit more from cochlear implant than SSD patients without tinnitus .
Quality of life
Quality of life is also a very important metric for SSD/AHL patients as both unilateral hearing loss and tinnitus can negatively impact a patient's life. Patients experience significant disability which affects their communication and social interaction . Challenging situations include those with background noise, in poor acoustic settings, and with limited access to speech-reading/direct listening . The perceived difficulty across different listening scenarios was significantly reduced with the cochlear implant turned on [5▪,12]. Several different instruments were used to help quantify quality of life (QOL) changes after cochlear implant, including Speech, Spatial, and Qualities of Hearing scale, Abbreviated Profile of Hearing Aid Benefit, Glasgow Hearing Aid Benefit Profile, and the Njimegen Cochlear Implant Questionnaire. SSD patients with cochlear implant has significant improvement in subjective perception of hearing performance and QOL [18,30]. Some of these patients were also able to regain some of the benefits of binaural hearing including squelch and summation [9,10,14]. At least 50% of patient with SSD have tinnitus. Tinnitus can negatively impact quality of life and lead to severe psychosocial problems like anxiety and depression [2,3,9]. The improvement in tinnitus in SSD patient after cochlear implant was discussed previously.
Although not a direct measure, the fact that a patient continues to use their cochlear implant long-term indicates that they find it useful. Mertens et al. reported that all 23 patients in their study used their cochlear implant 7 days a week with long-term follow-up (mean follow-up was 8 years). Vermeire and Van De Heyning  found that all 20 patients used their cochlear implant daily for the entire day and found substantial benefit in the binaural condition compared with listening with acoustic hearing only. Távora-Vieira et al. had 34 patients and found they had long-term (4–10 years of cochlear implant use) benefits with speech understanding, sound localization, and QOL.
Possible improvement in better hearing (non-cochlear implant) ear
In SSD/AHL the cochlear implant is implanted in the worse hearing ear, but there is an interesting effect seen in several articles where the hearing improves in the contralateral ear. Mertens et al. explained that tinnitus in the deaf ear can significantly reduce speech reception in noise in the better hearing ear and speech recognition improved in the better hearing ear when the cochlear implant was turned on in the deaf ear even when no acoustic signal was provided to the processor. Firszt et al. also found some patients had improved scores in their better hearing ear in hearing aid-alone condition post-cochlear implant. They went on to hypothesize that while this finding may have been an artifact of the testing metric (due to increased familiarity with the tasks/test environment), it may also be a genuine effect resulting from partial restoration of binaural processing, as some of these patients did demonstrate a bimodal benefit .
Several studies have also looked at cochlear implant for SSD/AHL in a pediatric population. Although these are smaller cohorts, this is an important patient population to study because there are roughly two to five per 1000 school-aged children with profound unilateral hearing loss . Like adults, children with SSD/AHL are able to understand speech in quiet settings with one speaker, but difficult listening environments like a classroom can be extremely challenging for the child . This unilateral hearing loss can negatively impact these patients in school as 35% of children with permanent unilateral hearing loss fail one or more grades in school . Multiple studies have shown the benefit of cochlear implant in the unilateral hearing loss pediatric population [23,33,35–37]. One question is if pre/peri/postlingual deafness affects outcomes. The answer is unclear, as some studies found that congenitally deaf (prelingual) patients did well with their cochlear implant [33,35–37], while other studies found that patients with pre/peri-lingual deafness did worse that those with postlingual deafness [23,38].
SSD and AHL can cause significant problems in communication not only from the loss of binaural hearing but compounded also by the presence of tinnitus. Although CROS and BCDs provide some benefit to patients with SSD/AHL, this article has touched on several reasons that cochlear implant may provide additional benefits to such patients and potentially be the best option from an auditory standpoint. Cochlear implantation is the only rehabilitative option at present time that can potentially restore hearing to the deaf ear, thereby providing the patient access to binaural hearing along with reduction of tinnitus. That being said, it is important to counsel prospective cochlear implant patients that it can take some time for the binaural benefits to develop and consistent cochlear implant use and auditory rehabilitation for years may be necessary for maximal improvement .
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Conflicts of interest
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Papers of particular interest, published within the annual period of review, have been highlighted as:
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