Restoring Binaural Hearing: Cochlear Implantation in Single-Sided Deafness

Advanced-to-profound unilateral hearing loss — widely known in the literature as single-sided deafness (SSD) — describes a sensorineural loss ranging from severe to profound in one ear, with normal or near-normal hearing in the contralateral ear. The definition looks simple on paper; yet the assumption that “the good ear hears anyway” long obscured the auditory, developmental and psychosocial burden of this condition.

How to draw the boundaries of the loss is still debated. In the criterion proposed by Vincent et al. (2015), the better ear should have a pure-tone average (0.5–1–2–4 kHz) of 30 dB HL or better, while the poorer ear carries a severe-to-profound loss¹; Van de Heyning et al. require ≥70 dB HL in the poorer ear, ≤30 dB HL in the better ear, and an interaural threshold difference of ≥40 dB HL². Ramos Macías et al. place the definition on a functional footing: the poorer ear derives no benefit from conventional amplification, and the better ear has an average of 20 dB HL or better³. This diversity of criteria positions SSD as the extreme end that separates it from the broader umbrella of “asymmetric hearing loss”; whereas asymmetric loss covers any degree of interaural difference, SSD sits at the most severe end of that spectrum.

Etiology is the first step in understanding the picture, because it directly affects the benefit expected from the chosen intervention: when the pathology is confined to the cochlea, a good response to cochlear implantation can be expected, whereas outcomes may be limited when it lies in the auditory nerve or central auditory pathways. In adults, the most common cause of acquired SSD is still idiopathic; in congenital/early-onset cases, cochlear nerve deficiency leads, followed by cytomegalovirus (CMV), inner-ear anomalies and auditory neuropathy spectrum disorder⁶. In terms of prevalence, congenital unilateral loss affects roughly 1 in every 1,000 newborns, while SSD in adults is estimated at 12–27 per 100,000⁷. Because some etiologies — such as enlarged vestibular aqueduct syndrome and CMV — can be progressive and threaten the contralateral ear, long-term follow-up is essential.

The arithmetic of two ears — and the cost of one

The advantage of normal two-eared hearing rests on three mechanisms: binaural squelch, binaural summation and the head-shadow effect⁸. By comparing signals that reach the two ears at different times and levels, the brain separates target speech from background noise; it resolves where a sound comes from in space through the interaural time difference (ITD) and the interaural level difference (ILD). When one ear is not functional, these cues disappear: localization is degraded and understanding speech in noisy, reverberant settings becomes markedly harder.

Single-sided hearing loss cannot be dismissed with “the other ear hears anyway”; when the brain cannot use the differences between the two ears, speech in noise and spatial hearing are profoundly affected.

This is not only an acoustic problem; it also has a neurodevelopmental dimension. Maturation of the auditory cortex continues for more than a decade after birth, and during this sensitive period reduced unilateral auditory input leads to morphological and functional changes in the auditory center — unilateral auditory deprivation⁴. Early on, the good ear becomes over-represented in the central pathways; this has been described as the “aural preference syndrome”⁵. Crucially, this reorganization is partly reversible, and the term “dominance” proposed initially was later softened to “preference” — which is largely the rationale for early intervention.

The clinical consequences of these deficits are broad: in adults and children with SSD, localization ability drops markedly, understanding speech in noise becomes difficult, and daily listening effort rises, leading to cognitive fatigue. In children this extends to language development and academic achievement: studies show that roughly one third of children with SSD repeat at least one grade, a substantial proportion need additional educational support, and many experience social-emotional difficulties. Before newborn hearing screening became widespread, these children often went unrecognized until school age.

Rehabilitation options: from CROS to the cochlear implant

Today there are three main rehabilitation routes for SSD: CROS hearing aids, bone-conduction systems and cochlear implants. CROS and bone-conduction devices route sound from the impaired side to the healthy ear, creating a “pseudo-binaural” hearing; but because they do not stimulate the impaired ear, they neither affect tinnitus nor provide true localization, and they can even worsen the head-shadow effect when noise comes from the poorer side. These limitations lead to variability in user acceptance and satisfaction.

The cochlear implant, by contrast, is the only option that directly, electrically stimulates the impaired ear; it therefore carries the potential to restore binaural hearing — without harming the better ear. Studies comparing the CI with CROS/bone-conduction devices in SSD show the CI’s advantage in speech-in-noise and localization, especially when noise comes from the poorer side¹³. In 2019 the FDA markedly expanded the indication by approving cochlear implantation in children aged 5 and older with SSD.

Candidacy and test battery

Preparation for cochlear implantation requires a multi-step evaluation: pure-tone air- and bone-conduction thresholds, immittance and acoustic reflexes, otoacoustic emissions, age-appropriate speech reception and discrimination tests, speech-in-noise and localization assessments, electrophysiological measures, and MRI/CT imaging to confirm the cochlea and the cochlear nerve⁹. The data obtained are discussed by a multidisciplinary team of audiologist, surgeon, speech-language therapist and psychologist.

Two points stand out in this evaluation. The first is the anatomy of the eighth nerve: advanced hypoplasia or aplasia of the cochlear nerve is a contraindication to CI, which is why preoperative high-resolution MRI is critical. The second is the challenge — specific to SSD — of assessing the implanted ear in isolation; to prevent the normal-hearing contralateral ear from contributing, effective masking, plug-and-headphone techniques and the increasingly preferred direct audio streaming are used. After the intervention, unaided thresholds in both ears must be monitored continuously, because some children develop a progressive loss in the contralateral ear.

Cochlear implantation in adults

Unlike in pediatric cases, the decision in adults is shaped by individual autonomy and life context. Some adults adapt remarkably well to one-sidedness while others live with persistent auditory difficulty; identifying the right candidate therefore rests on a comprehensive evaluation weighing the duration and etiology of the loss, device-use history and audiological measures together. Prominent motivations include difficulty hearing in noise and a desire for better localization, treatment-resistant tinnitus and occupational communication needs; in addition, preventive implantation in retrocochlear pathologies such as vestibular schwannoma, and implantation simultaneous with labyrinthectomy in late-stage Ménière’s disease. There is a growing view that a long duration of deafness should not, on its own, be treated as a strict contraindication.

The contribution of cochlear implantation in adults shows itself mainly in four areas. The first is improvement in understanding speech in quiet and in noise; Galvin et al. reported that word-recognition scores in the implanted ear rose considerably within months, with a parallel improvement on speech-in-noise tests¹⁰. Sound localization improves in the configuration of normal hearing in one ear and a CI in the other, and this finding has been consistently replicated in later studies¹¹. Tinnitus suppression is striking: in patients with tinnitus unresponsive to medical and psychological treatment, a marked reduction in severity after implantation has been reported¹². Finally, quality of life assessed with scales such as the SSQ rises significantly¹⁶. Elective non-use of the device is relatively rare; over ten years of follow-up, a non-use rate of about 4% has been reported¹⁷.

Cochlear implantation in children

In children the rationale is largely the window of plasticity. Early implantation reduces the deprivation effect on the developing auditory system; neurophysiological data show that cross-modal cortical reorganization can be reversed after implantation¹⁵. Although the current FDA indication points to a minimum age of 5 and a maximum duration of loss of 10 years, the best outcomes are reported in children implanted before age 4; if intervention does not occur within roughly two years of onset, the risk of disruption in binaural central integration rises. Because cochlear nerve deficiency is the most common etiology in congenital/early-onset SSD, MRI and careful assessment are indispensable in candidacy.

The data from children point the same way. In a systematic review of 119 children, about 80% showed improved speech perception in noise after implantation, a similar proportion in quiet; localization ability increased consistently, and roughly three quarters of the children used their device regularly¹⁴. Congenital loss and late implantation — particularly after age 4 — are, by contrast, associated with weaker outcomes. What is decisive, therefore, is not implantation itself but performing it in the right child at the right time.

Conclusion

Children and adults with SSD can benefit substantially from cochlear implantation: better hearing in noise, genuine localization, tinnitus suppression and improved quality of life. The common condition for these gains is early and timely intervention. Even so, research is still needed to define more precisely the factors — such as etiology, age at implantation and duration of loss — that predict which patients will struggle more after implantation. What today’s evidence makes clear is that “the other ear hears anyway” is no longer a clinical justification.

Author

Alperen Akbulut

Audiologist · Audiology PhD(c) · Founder of İşitme Atölyesi

After completing his undergraduate degree in Audiology at Istanbul University, he earned a master’s degree in the Audiology and Speech Disorders program at Marmara University, where he is continuing his doctoral studies. From 2019 to 2021 he worked as a clinical specialist at Advanced Bionics; since 2021 he has been a lecturer in the Department of Audiology at the University of Health Sciences, Hamidiye Faculty of Health Sciences. His research focuses on music perception, music-related quality of life and the music–memory relationship in cochlear implant users; his work has appeared in journals such as Ear and Hearing and European Archives of Oto-Rhino-Laryngology. He is the founder of İşitme Atölyesi, which aims to communicate audiology in an evidence-based and accessible way.