SSD can be (i) congenital or perinatal, (ii) of sudden acquired causes, or (iii) progressive. The most common causes of SSD in adulthood are sudden and idiopathic.

It has been suggested that SSD affects between 12 and 27 individuals in every 100,000 of the general population (Kitterick et al., 2014). More recently, it has been estimated that 10.4–25.4 individuals per 100,000 are at risk for SSD (Sprinzl and Wolf-Magele, 2016); 5–20 per 100,000 due to sudden sensorineural hearing loss (SSNHL) (Plaza et al., 2011). A more recent study containing information for more than 60 million unique patients estimated the prevalence of SSNHL in the USA to be 27 per 100,000 (Alexander and Harris, 2013). The NHANES epidemiologic study in the USA estimated the prevalence of SSD in adults to be 0.14% (Kay-Rivest et al., 2021); and to be higher in females (0.17%) vs. males (0.11%). The NHANES data also demonstrated that the prevalence of SSD was higher in individuals aged 60–79 years (prevalence of 0.25%) than in younger individuals (0.11% in ages 20–39 years, and 0.11% in ages 40–59 years).

The incidence of SSD in the UK has been estimated at 7,500 new adult cases per year (Baguley et al., 2006). Extrapolating to the 2023 population in the UK of ~67 million (www.ons.gov.uk), the incidence of SSD is ~9,000 new cases per year. Baguley et al. (2006) calculated that the highest incidence of SSD per 100,000 of the adult population per year was due to SSNHL, followed by Ménière's disease, with vestibular schwannoma being the lowest.

Studies have reported central auditory system re-organization in cases of unilateral deafness (Legris et al., 2018; Alzaher et al., 2021). These studies however, pool together data from individuals with various degrees of hearing loss that fall into the highly AHL category, as opposed to an SSD cohort explicitly. In adults, brain reorganization is detectable 5 weeks after onset of SSD (Suzuki et al., 2002), and functional magnetic resonance imaging (fMRI) studies have demonstrated that reorganization plateaus after 1 year (Bilecen et al., 2000). Magnetoencephalography studies of brain activation during performance of auditory syllable sequence reproduction tasks demonstrate that in adult-onset SSD there is both functional and structural alterations to the dorsal temporal and frontal-parietal areas of the brain (Shang et al., 2018). SSD also leads to physiological lateralization of auditory cortical activity, which has an impact on auditory spatial abilities (Karoui et al., 2022). An asymmetry of neuronal activity of the inferior colliculus and primary auditory cortex has been demonstrated using ¹⁸F-FDG PET imaging studies (Speck et al., 2020, 2022). AHL loss has a significant impact on glucose metabolism of the auditory pathway, which in turn can negatively influence audiological performance (e.g., speech recognition in noise) following cochlear implantation (Speck et al., 2022). Speck et al. (2022) enrolled nine participants with either AHL or SSD, with heterogeneous etiology, disease onset, and duration of deafness, so they suggest larger longitudinal studies to be able to confirm their hypothesis.

The disabling effects of SSD (speech, spatial, qualities domains), and impact of these effects on the degree of handicap experienced by the hearing impaired individual, vary considerably (Gatehouse and Noble, 2004; Noble and Gatehouse, 2004).

Good hearing in both ears (binaural hearing) helps us deal with everyday listening tasks (Dwyer et al., 2014; Snapp and Ausili, 2020). Auditory processing of speech in complex environments gives listeners with binaural hearing a benefit of 4–10 dB in processing speech (Hawley et al., 2004) in comparison to monaural (hearing with one ear only). This is known as binaural loudness summation, i.e., sounds presented to two healthy ears is perceived louder than the same level of sound presented to one healthy ear only (Avan et al., 2015). The squelch effect is another ability of a healthy auditory system, which allows listeners to combine signal(s) and competing noise information retrieved from both ears to improve speech perception in noise. Benefits of binaural hearing include understanding speech in noisy or reverberant environments and locating where sounds such as the telephone or car traffic are coming from (Levitt and Rabiner, 1967; Hawley et al., 2004; Snapp, 2019; Snapp and Ausili, 2020; Gallun, 2021).

Sound localization in the horizontal (azimuth) plane relies mainly on interaural time differences (ITDs) and interaural level differences (ILDs) (Agterberg et al., 2012; Rothpletz et al., 2012; Pedley and Kitterick, 2017). In other words, the auditory system helps us judge our positioning in space by dynamically calculating our interaction with signals that are constantly changing in terms of pitch (frequency spectrum), level (intensity), and time (latency) (Akeroyd, 2006; Arndt et al., 2011a; Snapp and Ausili, 2020). The integration of acoustic information from both ears is essential for spatial awareness (Güldner et al., 2013; Karoui et al., 2022), for example, determining where sounds are coming from (Douglas et al., 2007; Pedley and Kitterick, 2017; Snapp, 2019).

Although there can be a degree of adaptation in certain monaural listeners (Slattery and Middlebrooks, 1994; Rothpletz et al., 2012), and possible long-term compensation for loss of binaural cues (Liu et al., 2018; Alzaher et al., 2021), localization abilities can be severely impaired in those hearing monaurally (Wazen et al., 2005; Agterberg et al., 2012; Hoth et al., 2016; Pedley and Kitterick, 2017; Snapp, 2019). A further complication for monaural listeners is introduced by the head shadow effect, where the head acts as an acoustic barrier to signals that travel from one side of the head to the other, which can lead to significantly impaired speech understanding (Akeroyd, 2006; Pedley and Kitterick, 2017; Snapp, 2019). In cases of SSD, speech originating from the poor-hearing side of the head is reduced in intensity by 6.4 dB by the time it reaches the normal-hearing ear; it therefore arrives distorted due to loss of high-frequency information from the speech spectrum (McLeod et al., 2008).

Due to the challenges arising from monaural access to sound, individuals with SSD have difficulties dealing with everyday tasks such as speech recognition (Lieu et al., 2010; Dwyer et al., 2014) and impaired ability to understand speech in the presence of background noise (Welsh et al., 2004; Rothpletz et al., 2012; Firszt et al., 2017; Vannson et al., 2017; Peters et al., 2021; Kitoh et al., 2022). It is estimated that there is a reduction in speech understanding by approximately 3 dB in SNR in cases of SSD (McLeod et al., 2008). Speech understanding is reduced due to reduced signal loudness detected by the monaural listener.

Different features of the conversation context, for example the complexity of the acoustic environment, the type or loudness of the background noise, or the number of people in a group can influence speech perception and impact on individuals' need to modify their communication strategies (Hadley et al., 2021). Adults with unilateral hearing loss are more likely to report a higher level of communication difficulties than normal-hearing adults (Dwyer et al., 2014; Choi et al., 2021). SSD can also impact music appreciation. Music can sound unnatural, unpleasant and indistinct, lack perceptual qualities such as stereo sound, and be confounded by distortion effects and tinnitus (Meehan et al., 2017).

Moreover, SSD is associated to increased listening effort when compared to normally hearing individuals (Dwyer et al., 2014; Lopez et al., 2021). Listening effort is defined as the mental exertion required to attend to and understand an auditory message (McGarrigle et al., 2014). Hearing impaired listeners may experience increased listening effort in challenging listening situations than normally hearing individuals, even if they use hearing aids (Alhanbali et al., 2017). The constant effort applied by a listener with SSD to adjust to their listening environment is fatiguing, and can be unsustainable for many (Snapp, 2019). The real-world impact of increased fatigue is dependent on personal factors and lifestyle (Holman et al., 2019), and can influence social activity level (Holman et al., 2021a). Fatigue could arise due to decreased audibility of sounds, and in part, increased requirement for listening effort (McGarrigle et al., 2014). Other factors such as related challenges in auditory processing, and increased listening effort required in demanding listening environments have been proposed (Hornsby et al., 2016; Ohlenforst et al., 2017; Peelle, 2018). Associations to work, social, or physical activity levels, and wellbeing are also relevant and have implications on daily-life fatigue in people with hearing loss (Holman et al., 2021b). Objective measures of pupil dilation as an indicator of listening effort during listening tasks demonstrate that the individual's motivation is a factor that can influence objective measures of fatigue (Wang et al., 2018). Qualitative studies interviewing people with hearing loss identified factors such as lifestyle, personality, situational control, the relationship with those in conversation and the attribution of blame are key to individual emotional experiences (Holman et al., 2022).

The psychological impact of SSD in adulthood has been well-documented in the literature, including worry about losing the hearing in the other ear, embarrassment related to the social stigma attached to hearing loss, and reduced confidence and belief in one's own abilities to participate (Sano et al., 2013b; Lucas et al., 2018; Choi et al., 2021). Individuals with unilateral hearing loss are at a disadvantage in social and emotional situations. They report being upset, anxious, frustrated and isolated due to their hearing handicap secondary to monaural listening (Araújo et al., 2010; Sano et al., 2013a; Lucas et al., 2018). It has also been reported that SSNHL is associated with anxiety and depression (Arslan et al., 2018). Furthermore, increased stress levels related to the need to find an optimal position in social settings, that will help with speech perception and participation, have been reported in interview studies (Lucas et al., 2018). Those who acquired SSD secondary to a vestibular disorder (e.g., labyrinthitis, Ménière's disease) could be at risk of chronic anxiety which could precede depressive states (Hilber, 2022). Analysis from the 2008 National Health Interview Survey, which included ~18 million people with vestibular vertigo in the USA, suggested that cognitive impairment (memory loss, difficulty concentrating, confusion) and psychiatric diagnoses (depression, anxiety, panic disorder) are comorbidities in those with vestibular deficiencies (Bigelow et al., 2016), which can also be linked to difficulties remembering in 32% of individuals. In addition, individuals diagnosed with SSD report decreased self-esteem when in places with background noise, which can leave them feeling frustrated and isolated (Lucas et al., 2018). They also report increased stress levels and exhaustion related to their constant attempts to maximize their abilities to hear and participate in complex social situations (Wie et al., 2010; Kuppler et al., 2013). Associated feelings of frustration, annoyance, helplessness, embarrassment, and depressive symptoms have been reported in multiple studies (Giolas and Wark, 1967; Gatehouse and Noble, 2004; Wie et al., 2010; Sano et al., 2013b; Lucas et al., 2018; Choi et al., 2021).

QoL is defined as “a patient's general wellbeing, including mental status, stress levels, sexual function, and self-perceived health status” (Farlex Partner Medical Dictionary, 2012). Population based studies evaluated the risks of adverse hearing and wellbeing outcomes (including self-reports on depression, health rating, satisfaction with health, happiness and loneliness), in 113,804 UK Biobank participants aged 40–69 years who self-reported unilateral hearing loss (Pierzycki et al., 2020). Participants with unilateral hearing impairment are significantly more likely to report poor health, dissatisfaction with health, and loneliness than those with normal hearing (Dawes et al., 2014; Pierzycki et al., 2020). The multi-dimensional burden of SSD on overall health is indicated by reductions in health-related QoL in individuals with a diagnosis of SSD, despite use of hearing-assistive devices for SSD (Arndt et al., 2011a; Kitterick et al., 2015; Vannson et al., 2015; Pierzycki et al., 2020). One study reports that the impact of SSD on QoL can exceed that reported by listeners with bilateral hearing loss (Sano et al., 2013a). This Japanese study included 167 adult participants with idiopathic SSNHL and 134 participants with bilateral hearing loss to act as controls. They measured health-related QoL with the Japanese version of the Short-Form Health Survey (SF-36) (Fukuhara et al., 1998). The term “health-related quality of life” (HRQoL) narrows QoL to aspects relevant to health (de Wit and Hajos, 2013). This study concluded that there was reduced mental functioning in those with idiopathic SSNHL, compared to averages in the Japanese population, which was similar to their participants with bilateral hearing loss. The psychosocial impact has also been documented, with annoying tinnitus and remaining vertigo after SSNHL to be the strongest predictors of negative effects on QoL (Baguley et al., 2006; Carlsson et al., 2011). QoL can be affected in those who have vestibular schwannomas surgically removed, as indicated by lower scores yielded on the SF-36 survey instrument (Ware and Sherbourne, 1992), in all categories, but more significantly in physical ability, social functioning, emotional status and vitality (Nicoucar et al., 2006).

Individuals with SSD report feeling excluded in conversations with multiple speakers, have reduced wellbeing in social settings, and a preference to avoid social gatherings in which they thought significant background noise would be present (Wie et al., 2010; Chang et al., 2020). Qualitative studies indicate that coping strategies of individuals with SSD include withdrawal from within a situation and in some cases, from the social situation completely (Lucas et al., 2018). The impact of SSD on communication can also affect intimate relationships (Hétu et al., 1993; Lucas et al., 2018). SSD can impact individual's vocational activities such as business negotiations, customer service, and meetings, contribute to absences or days away from work, and early retirement (McLeod et al., 2008; Härkönen et al., 2015; Marx et al., 2019; Snapp, 2019). These studies disagree with Colletti et al. (1988) who found no difference between monaurally and binaurally hearing individuals on educational, social, and employment achievement. Their participants however were aged 30–55 years, and had SSD since childhood (Colletti et al., 1988), rather than acquired in adulthood like the other studies. However, longitudinal studies in the USA with older adults with age-related hearing loss report that hearing loss may be associated with reduced engagement in physical and mental activities (Goman et al., 2021; Kuo et al., 2021). They also report higher odds of those with hearing loss reporting loneliness than those reporting excellent hearing, after adjusting for comorbidity index, functional and cognitive ability, self-reported health, and demographic characteristics (Huang et al., 2021). It is well-documented in recent literature that hearing impairment can impair social engagement, alter social roles, and impede the formation and maintenance of relationships (Barker et al., 2017; Vas et al., 2017; Heffernan et al., 2022).

The aim of SSD device-based treatments is to address the functional difficulties imposed and in turn improve everyday listening and communication. Devices are categorized as either rerouting or restoring.

Hearing aid and auditory implant cost-effectiveness studies have become increasingly important (Theriou et al., 2019; Neve et al., 2021; Caspers et al., 2022). The device purchasing cost varies from a few hundred pounds for the rerouting hearing aid solutions to ~£20,000 for the restoring implants. A formal cost-effectiveness analysis for BCD devices in a prospective case-control study of 70 pathways found that there was limited data for cost effectiveness calculations for BCD devices (Monksfield et al., 2011). They presented total costs from initial evaluation, surgery, ongoing annual evaluation and maintenance, and processor upgrades after 5 years to the newest model for an estimated life expectancy of the individual patient (Monksfield et al., 2011). The Health Utilities Index (HUI) questionnaire (Horsman et al., 2003) was used in conjunction with life expectancy estimations to derive Quality-Adjusted Life Year (QALY) and Incremental Cost-Effectiveness Ratio (ICER) ratios. There is limited QoL data available for patients living with an osseointegrated implant. As a result, the cost-effectiveness of the osseointegrated implant, compared to conventional hearing aid devices remains unclear (Crowson and Tucci, 2016).

Amin et al. (2021) performed a retrospective case series analysis with a longitudinal economic analysis. They concluded that the mean total cost per patient of the MED-EL middle ear implant was significantly higher than percutaneous BCD at 1-year post-implantation. However, by 5-years post-implantation this difference was no longer statistically significant. Unfortunately, cost-effectiveness evaluations were limited by the lack of usable data on QoL and device usage (Caspers et al., 2022). Based on evidence of moderate quality, cochlear implantation and BCD improve functional and patient-important outcomes in adults and children with SSD (Ontario Health, 2020b). The Ontario health technology assessment report highlighted that among people with SSD, cochlear implantation may be more cost-effective than no intervention, whereas BCDs are unlikely to be although are acceptable to patients who cannot use CI.

Existing literature has highlighted inconsistencies in what benefits and risks (side-effects) are measured when evaluating hearing aid(s) and auditory implant interventions for SSD (Kitterick et al., 2016). The challenge of synthesizing evidence for ENT and audiological interventions from trials, and the importance of utilizing valid measurement instruments that effectively measure the intended audiological outcomes has been highlighted in the case of SSD (Hall et al., 2019). Choosing the appropriate intervention for adults with SSD presents a clinical dilemma (Sin Wai and Chua Wei De, 2021; Underdown and Pryce, 2022).

Researchers investigating SSD intervention outcomes have measured a plethora of outcomes, such as speech understanding in quiet, or speech understanding in the presence of noise (Niparko et al., 2003; Firszt et al., 2012). When assessing speech outcomes in the presence of noise, various configurations are used. For example, Niparko et al. (2003) chose the conditions of (i) noise-front, (ii) noise-to-normal-ear, and (iii) noise-to-deaf-ear to compare BCD and CROS devices, using the Hearing in Noise Test (HINT) (Nilsson et al., 1994) that includes noise filtered to match the long-term average spectrum of sentences. In another example, Arndt et al. (2017) compared speech outcomes in noise with CROS, a BCD device, and CI in three conditions; (i) speech and noise from the front, (ii) speech from the hearing side/noise from the deaf side, and (iii) speech from the deaf side/noise from the hearing side, using the adaptive Oldenburger Sentence Test (OlSa) (Arndt et al., 2011a,b). Härkönen et al. (2015) compared CI outcomes in SSD using a speech-in-noise test that included phonetically balanced bisyllabic Finnish words at a level of 65 dB SPL from the loudspeaker at 0° of azimuth, and unmodulated artificial noise presented from four loudspeakers. Finally, a study assessing the masked speech recognition in 16 participants with SSD and a CI suggest a revised test battery to the Van de Heyning et al. (2017) recommendations to ensure binaural hearing abilities are captured (Anderson et al., 2022). Anderson et al. suggest presentation of the target from the front speaker and the masker co-located with the target, 90° toward the implanted-ear, and 90° toward the normally-hearing-ear, to incorporate real-world situations when the listener often faces the speaker of interest. This selection of studies demonstrates the diversity of test configurations chosen by researchers in the field to demonstrate the benefits and harms of SSD interventions.

The question of what outcome domains are important and relevant to individuals with SSD when deciding whether an intervention works has yet to be addressed. One attempt to harmonize evaluation of SSD interventions was made in 2017; it was based on two discussions among professional experts in CI (Van de Heyning et al., 2017) and was intended for adoption in clinical practice. Recommendations for a minimum set of outcome measures were made including daily device use, pure tone audiometry, free-field testing of speech perception in noise, and sound localization. Recommended instruments were the Speech, Spatial, and Qualities of hearing (SSQ) questionnaire (Noble et al., 2013), the HUI (Horsman et al., 2003) and, if applicable, the Tinnitus Functional Index (TFI) (Meikle et al., 2012). This consensus work by Van de Heyning et al. (2017) focussed on CI as a treatment for SSD and so the expert panels comprised professionals from CI centers. Furthermore, the recommendations included measurement instruments that were readily available in the hearing clinic (e.g., pure tone audiometry, standard audiometric and validated sentence test, binaural effect measures), and there was lack of patient involvement in decision-making. Therefore, it is unclear whether the recommended measures assess outcome domains that are most important or meaningful to patients (e.g., impact on individual's wellbeing, social identity) (Lucas et al., 2018; Underdown and Pryce, 2022). There has been no rigorous scrutiny of outcome reporting for rerouting or restoring interventions, no systematic patient involvement, and no specific consideration of what should be used in clinical trials. Consequently, investigators adopt markedly different methods when assessing the clinical benefit of rerouting and restoring interventions for SSD.

A systematic review and meta-analysis on the use of hearing instruments for SSD in adults has demonstrated ambiguity in the absolute benefit and efficacy of the SSD treatment options (Kitterick et al., 2016). For example, the meta-analysis showed that there was a statistically significant benefit (mean benefit: 2.5 dB) to speech perception in noise for devices that reroute speech signals from the poor-hearing ear to the better-hearing ear using either air or bone conduction. However, rerouting devices also significantly degrade speech understanding (mean deficit: 3.1 dB) when noise gets rerouted from the poor-hearing ear to the better-hearing ear. In relation to sound localization, there was inconsistency in the outcomes chosen by clinical researchers, precluding the synthesis of evidence across studies. Finally, HRQoL was measured in two studies (out of 27 included in the review), and the findings were inconclusive. In summary, Kitterick et al. (2016) concluded that inconsistent measurement of outcomes and observational biases amounted to a low quality of evidence. Kitterick et al. (2016) also concluded that outcome selection was biased toward assessing functional impairments for which measures are readily available and widely used, such as tests of speech perception in noise, with limited measurement of domains that are meaningful to patients such as HRQoL (Lucas et al., 2018). Another systematic review of outcomes of CI in patients with SSD focused on assessing (i) sound localization, (ii) speech perception, (iii) tinnitus, and (iv) QoL outcomes (Cabral Junior et al., 2016). The authors discovered a large variation in choice of outcomes in the included studies, and highlighted the need for high quality studies. Encouragingly, Mertens et al. (2022) have recently developed a consensus classification system for the reporting of sound localization testing results in the field of cochlear implantation. This builds on the Van de Heyning et al. (2017) recommendations, and application of this classification system will allow multi-center studies comparisons and improved meta-analysis in this field (Mertens et al., 2022).

Difficulties that SSD imposes can also affect the individual's psychological and social wellbeing (Carlsson et al., 2011; Sano et al., 2013a; Lucas et al., 2018), and therefore outcomes that assess the impact on an individual's overall health and wellbeing are also relevant and potentially as important (Kitterick et al., 2015). With respect to HRQoL, there is inconsistency in the choice of measurement instruments in trials assessing the benefits of SSD interventions (Kitterick et al., 2016). The authors highlight the need for consist use of patient-reported outcome measures that are sensitive to the impact of devices used by those with SSD, such as the HUI (Horsman et al., 2003). Often, it is unknown what plays a role in decision making and identifying better candidates for specific SSD interventions (Kosaner and Urban, 2014). Qualitative studies demonstrate that patients express uncertainty about choice of treatment options for SSD mainly due to a lack of clarity about their benefit (Underdown and Pryce, 2022), and they seek clinical advice when they need to make a decision. However, due to the varied evidence for different treatment options, clinicians may not know which option is ideal and for whom (Hall et al., 2019).

To address the inconsistency and diversity of outcome measure use in the field of SSD research, the Core Rehabilitation Outcome Set for Single Sided Deafness (CROSSSD) initiative set out to develop a core outcome set for SSD interventions (Katiri et al., 2020, 2022). The CROSSSD initiative developed an international consensus by including opinions of both healthcare users and professionals working in the field. Including a variety of stakeholders in core outcome set development, including the general public, can help demonstrate the impact of interventions on patients' lives (Dodd et al., 2023). The recommended CROSSSD study core outcome set included three outcome domains: spatial orientation, group conversations in noisy social situations, and impact on social situations. Adoption of the CROSSSD core outcome domain set will promote consistent assessment and reporting of outcomes that are meaningful and important to all relevant stakeholders. This consistency will in turn enable comparison of outcomes reported across clinical trials comparing SSD interventions in adults and reduce research waste (Chalmers and Glasziou, 2009; Tunis et al., 2016). The next step entails determining how these outcomes should be measured (Prinsen et al., 2014, 2018; Mokkink et al., 2016). It is important to choose measurement instruments that are comprehensive and sensitive to treatment-related change (Prinsen et al., 2016) as well as inclusive and equitable to ensure they incorporate the diversity of all patients being assessed with the condition of interest (Calvert et al., 2022). Strides are being made to identify available instruments to measure the three outcome domains in the core outcome domain set (Katiri et al., 2022).