2Single-Sided Deafness and Asymmetric Loss

FDefining SSD and asymmetric loss

Single-sided deafness (SSD) is severe-to-profound sensorineural loss in one ear with normal or near-normal hearing in the other; asymmetric hearing loss (AHL) describes a poor ear plus a contralateral ear that is impaired but still aidable. Because the better ear hears, these patients fail traditional bilateral candidacy criteria yet still lack the binaural processing that depends on two functioning ears. SSD characteristically impairs speech recognition in noise and sound localisation, and can degrade speech understanding in quiet when the talker is on the side of the deaf ear due to the head-shadow effect. Disabling tinnitus in the deaf ear is common and is frequently a major reason patients seek treatment. The condition is prevalent: epidemiological estimates place the number of people with single-sided hearing loss in the United States in the tens of millions.[2020][2022]

TWhy an implant beats CROS and bone conduction

A contralateral-routing-of-signal (CROS) aid and an osseointegrated bone-conduction device both reroute sound from the deaf side to the better ear, overcoming the head shadow but never delivering sound to the deaf ear itself. Because rerouting devices feed only one cochlea, they cannot restore true binaural cues, so they give little benefit when noise comes from the deaf side or when the signal arrives from the front, and they do not treat tinnitus. A cochlear implant in the deaf ear is the only intervention that restores actual auditory input to that side, re-establishing two independent input channels and therefore genuine binaural hearing. Implanted SSD patients show measurably better sound-localisation accuracy than in the unaided, CROS, or bone-conduction conditions because interaural level and timing differences are restored. Implantation also reduces or abolishes tinnitus in most SSD recipients, an effect rerouting devices cannot reproduce. The trade-off is an intracochlear surgical procedure and a lifelong device on an ear that, by definition, has a perfectly good neighbour, which raises the bar for patient motivation.[2020][2022][2017]

CCandidacy: the short-deafness, intact-nerve rule

The U.S. FDA approved cochlear implantation for SSD and AHL in July 2019 for patients 5 years and older, the first such approval in the United States. The dominant prognostic variable is duration of deafness in the ear to be implanted; a short interval between onset and implantation predicts good integration and daily use, while very long deprivation predicts poorer benefit and a risk of device rejection. In children with congenital SSD the deprivation window is unforgiving: as many as about 50% have cochlear nerve deficiency, so high-resolution MRI to confirm an intact cochlear nerve is mandatory before implantation, and implantation is recommended early. Common adult etiologies, sudden idiopathic loss and labyrinthitis, do well when implanted after a short duration, whereas a post-meningitic ossified or aplastic-nerve ear is a poor candidate. Patient motivation is itself a selection criterion: SSD recipients must commit to consistent device use to retrain binaural processing, and a bothersome tinnitus often increases that motivation. For AHL the rule mirrors SSD, with the added requirement that the contralateral ear be aidable so that the implant supplements rather than replaces the better ear.[2020][2022][2013]

COutcomes and the duration ceiling

SSD recipients reliably improve on speech-in-noise testing when the signal arrives from the side of the implanted ear, and on sound localisation across the frontal field. Benefit for speech in noise when target and noise both arrive from the front is less consistent across studies, an honesty point for counselling. Most recipients report substantial reduction or elimination of tinnitus in the implanted ear, which for many is the single most valued outcome. Hearing thresholds and benefit measured after implantation are generally stable over multi-year follow-up. U.S. labelling has historically capped the indicated duration of deafness in the implanted ear at about 10 years, but newer data suggest that a strongly motivated adult deafened later in life may still benefit, so the ceiling is a guide rather than an absolute bar. Daily wear time is a practical outcome marker: patients who wear the implant during most waking hours achieve the binaural and tinnitus benefits, whereas inconsistent users do not.[2020][2022][2013]