Permanent acquired unilateral severe-to-profound hearing loss, or single-sided deafness (SSD), has been estimated to affect between 12–27 individuals in every 100,000 of the general population with the majority of losses being sudden and idiopathic; that is, a cause has not or cannot be determined . Other aetiologies which may give rise to SSD include Vestibular Schwannoma (and associated surgery) and Ménière’s disease. Despite having normal or near-normal hearing in one ear, individuals with SSD report difficulty when listening in many everyday situations [2, 3]. In particular, patients report disability when localising sounds and listening to sounds on the side of the impaired ear . Compatible with these self-reported difficulties, individuals with SSD show little or no ability to localise sounds [5–8] and display a poor ability to understand speech in noise when the speech is on the impaired side of the head [5, 6]. Both impairments reflect a lack of access to binaural cues such as inter-aural time and level differences, while the latter impairment also reflects the acoustic shadow cast by the head which attenuates the high-frequency components of sounds at the ear contra-lateral to their source.
In the UK National Health Service (NHS), the standard of care for SSD is a Contra-lateral Routing of Signals (CROS) hearing aid system. A CROS system picks up sounds arriving at the impaired ear using a remote microphone and presents those sounds to the non-impaired ear through a wired or wireless link . The primary function of the system is therefore to overcome the acoustic shadow cast by the head and, in doing so, to improve access to sounds on the impaired side. The use of a CROS system has been found to improve the perception of speech in noise compared to the unaided condition when the most favourable signal-to-noise ratio is available at the impaired ear; i.e. with speech on the impaired side and the noise from the front [6, 10, 11] or speech from the front and noise on the non-impaired side [7, 12, 13]. Use of a CROS system has also been associated with a reduction in self-reported difficulty with background noise, communication, and reverberation . However, the indiscriminate routing of sounds from the impaired ear to the non-impaired ear can produce undesirable results. For example, the perception of speech in noise can degrade with CROS use compared to the unaided condition when a background noise is located on the impaired side [6, 11, 13]. The use of a CROS has also been found to have no effect on localisation accuracy [6, 10, 11] which is compatible with the fact that the device is not designed to restore access to the binaural cues which underpin the ability to locate sounds in space. Finally, patients also report dissatisfaction with the requirement to wear a hearing aid device on their non-impaired ear [12, 14].
Alternatives to a CROS include a Bone-Anchored Hearing Device (BAHD) which also transmits sounds arriving on the impaired side to the non-impaired ear but achieves this by conduction through the cranial bones. There is some emerging evidence that BAHD may provide benefits to speech perception in noise over a CROS system although more high quality trials are needed [1, 15]. BAHD is currently only commissioned in the NHS in cases of profound unilateral sensorineural hearing loss where conventional acoustic aiding (such as with a CROS) is not possible or contra-indicated . Like the CROS, a BAHD does not restore binaural hearing in individuals with SSD for whom the sensorineural component of the hearing loss is severe-to-profound. Poor localisation ability has been cited by patients as a factor which contributes to their decision not to receive a BAHD, as have cosmetic concerns about the placement of a permanent bone-anchored abutment through the skin .
An alternative treatment for SSD which does have the potential to restore access to binaural cues is cochlear implantation. In cases of SSD where the auditory nerve is intact, impairments of the middle and inner ear can be bypassed and the auditory nerve stimulated electrically. Like CROS and BAHD devices, cochlear implantation in SSD has been found to improve access to sounds on the impaired side and has been associated with a reduction in self-reported difficulties with listening to speech in noise [5, 18, 19]. Unlike CROS and BAHD devices, cochlear implantation has also been found to improve the accuracy with which sounds can be localised [5, 18, 19]. Thus, the restoration of input to the impaired ear has the capacity to provide access to the inter-aural cues which support localisation and to restore useful aspects of binaural hearing. The intervention therefore has the potential to alleviate the high-levels of difficulty that individuals with SSD report with understanding speech in many everyday situations containing background noise  and, in doing so, may improve their health-related quality of life .
In the NHS, cochlear implantation is currently commissioned for adults and children with bilateral severe-to-profound deafness  and is not commissioned routinely for individuals with SSD. Providing a cochlear implant in cases of SSD and supporting its effective use throughout the lifespan is likely to incur a substantially greater cost to the NHS compared to the provision and maintenance of the current standard of care, a CROS system. A cochlear implant is also a lifetime commitment and involves a surgical procedure whereas a CROS system is no more permanent or invasive than a conventional acoustic hearing aid. For these two reasons, it was anticipated that cochlear implantation would be unlikely to be considered as an alternative to CROS in the NHS. Rather, it may be considered as an intervention for SSD in cases where a CROS system has been found to offer insufficient benefit as in these cases no other treatment options are currently available on the NHS. The current trial was therefore designed to evaluate whether cochlear implantation provides benefit to those patients who report receiving insufficient benefit from a CROS system.
This within-subjects longitudinal trial will determine whether cochlear implantation provides significant benefit in patients who have failed to receive sufficient benefit from a Contra-lateral Routing of Signals (CROS) system.
Does a cochlear implant significantly improve sound localisation compared to a CROS hearing aid in patients who report insufficient benefit from a CROS?
Does a cochlear implant significantly improve speech perception in noise compared to a CROS hearing aid in patients who report insufficient benefit from a CROS?