For most of the history of cochlear implantation, a normal contralateral ear was a reason not to operate. Implants were developed for people who gained nothing from hearing aids in either ear, and the signal they deliver, a coarse electrical code of the sound envelope spread across a limited number of electrodes, was assumed to be of little use beside natural hearing and possibly a distraction from it. Unilateral deafness was managed with reassurance or with devices that pick up sound on the deaf side and present it to the good ear.

That view changed after Van de Heyning et al. (2008) implanted adults with single sided deafness (SSD) whose principal complaint was severe tinnitus in the deaf ear and observed a marked and lasting reduction in its loudness. Reports that the same patients heard better in some listening situations followed (Vermeire and Van de Heyning, 2009), and within a few years groups in Europe, Australia and North America were studying implantation for SSD as a hearing intervention in its own right. In 2019 the United States Food and Drug Administration approved a cochlear implant system for SSD and asymmetric hearing loss in adults and in children aged five years and older. This review considers who is regarded as a candidate, what benefits have been shown, and what remains uncertain.

The article is a narrative review of the published literature. It is organised around the three themes of its title and relies on the early clinical series, the prospective trials, the systematic reviews and the consensus papers that have shaped practice. No formal search protocol was applied and no pooled estimates are offered. Where studies are cited for a finding, the intention is to indicate the direction and quality of the evidence, and readers who need effect sizes are referred to the original reports.

Two ears give the auditory system three advantages when speech and noise come from different directions. The head casts an acoustic shadow, so that one ear always has the more favourable signal-to-noise ratio. The brain can compare the inputs at the two ears and partly cancel the noise, an effect known as binaural squelch. And the same signal presented to both ears is perceived as louder and clearer, which is binaural summation. Localisation in the horizontal plane depends on differences between the ears in the time of arrival and the level of a sound. A person with SSD has none of these, apart from the head shadow when the talker happens to be on the better side.

The practical effects are well described. Adults report difficulty in groups, in traffic and in open offices, and questionnaires such as the Speech, Spatial and Qualities of Hearing Scale were designed in part to capture disabilities of this kind that a pure-tone audiogram does not show (Gatehouse and Noble, 2004). In children, unilateral loss is associated with delays in speech and language and with poorer progress at school (Lieu, 2004). Experimental work adds a developmental concern: when one ear is deaf during an early sensitive period, the auditory cortex reorganises in favour of the hearing ear, and the representation of the deaf ear weakens (Kral et al., 2013). Gordon et al. (2015) have called the result an aural preference syndrome and argue that it sets a time limit on effective treatment.

The established alternatives are the contralateral routing of signal (CROS) hearing aid, which transmits sound from a microphone on the deaf side to a receiver in the hearing ear, and the bone conduction device, which sends vibration through the skull to the hearing cochlea. Both lift the head shadow for sounds on the deaf side. A systematic review by Kitterick et al. (2016) found that they improve speech perception when the talker is on the deaf side, can make matters worse when noise is on that side, and do not restore localisation, as would be expected of devices that deliver everything to one cochlea. Table 1 compares the options.

Comparative data are limited but point in a consistent direction. Arndt et al. (2011) tested patients with a CROS aid and a bone conduction device before implantation and with the implant afterwards, and found localisation and speech perception in several configurations to be better with the implant. A randomised trial from Utrecht assigned adults with SSD to implantation or to an initial trial of the two rerouting devices and reported short-term advantages for the implant group in speech perception in noise and in localisation (Peters et al., 2021).

Agreement on who counts as having SSD came late. A consensus group proposed that the term be reserved for a pure-tone average of 70 dB HL or worse in the poorer ear with 30 dB HL or better in the other, and that asymmetric hearing loss describe the same poorer ear paired with a better ear between 30 and 55 dB HL (Vincent et al., 2015). The distinction matters because a patient with asymmetric loss already depends on a hearing aid in the better ear and stands to lose more if that ear deteriorates.

The reported benefits fall under four headings. They are not equally well supported, and Figure 1 sets them beside the elements of candidacy and the questions discussed later in this review.

The first open question is how the brain combines a degraded electric signal with normal acoustic hearing. Electrode arrays do not reach the apex of the cochlea, so a given sound may excite a place on the implanted side that corresponds to a higher pitch than on the acoustic side, and the two signals also arrive with different delays. Whether fitting the processor to reduce these mismatches improves binaural benefit, and how much the listener adapts unaided, are active areas of research with no settled clinical protocol.

Rehabilitation is a related and practical uncertainty. Because the normal ear dominates everyday listening, the implanted ear gets little practice on its own, and many programmes ask recipients to train it in isolation, by streaming audio directly to the processor or by plugging and masking the hearing ear. Whether such training hastens or enlarges the eventual benefit, and how much of it is needed, has not been established in controlled studies, and practice differs from centre to centre.

The second concerns selection. Duration of deafness, cause, age and tinnitus are all thought to influence outcome, yet no study has been large enough to weigh them against one another, and the adult who has been unilaterally deaf for decades cannot be given a reliable estimate of benefit. In children the corresponding question is how late is too late for congenital SSD. Animal and developmental data imply a narrow window (Kral et al., 2013; Gordon et al., 2015), but a threshold age has not been defined from clinical outcomes.

Third, comparison between studies is hampered by their methods. Loudspeaker arrangements, noise types and the way the hearing ear is masked when the implant is tested alone differ from centre to centre. A consensus paper has proposed a common framework of tests for SSD studies (Van de Heyning et al., 2016), and its adoption would make future pooling of data more meaningful. Fourth, long-term use is poorly documented. An implant that is worn for two years and then left in a drawer has not succeeded, and non-use has been reported, particularly in late-implanted children (Arndt et al., 2015). Finally, the device and its lifelong maintenance are costly, the benefit is an improvement in hearing that is already serviceable, and economic evaluations specific to SSD are few. Funding bodies in many countries have yet to decide the matter.

Cochlear implantation is the only treatment that returns sound to the deaf ear in SSD, and the studies published since 2008 show with reasonable consistency that it suppresses tinnitus in most recipients, improves localisation and helps with speech in noise in configurations where two-sided input matters. Adults with acquired, recent deafness and young children with a normal cochlear nerve are the candidates for whom the case is strongest. The uncertainties are substantial: the evidence base is small and largely uncontrolled, predictors of individual outcome are weak, the integration of electric and acoustic hearing is imperfectly understood, and long-term use and cost-effectiveness have barely been examined. Candidates deserve counselling that conveys both the promise and these limits.

To be declared by the author(s) before publication.

To be declared by the author(s) before publication.