One ear or two
A cochlear implant gives sound to an ear that a hearing aid can no longer help. The next question is whether the other ear should have an implant too, keep a hearing aid, or be left alone. The answer depends less on the audiogram than on what two ears do together, set out on binaural hearing and spatial hearing.
What a second implant adds
With one implant, all sound arrives through one side. In quiet that is often enough; in a restaurant, a meeting or a car it often is not. A second implant brings back three things that depend on two ears:
- The head shadow. When a voice is on one side and noise on the other, the ear nearer the voice hears it with less noise. With one implant, the voice is sometimes on the wrong side.
- Squelch. The brain compares the two ears and uses the differences to suppress noise from another direction. This takes time to develop after implantation.
- Localisation. Knowing where a sound comes from — a voice calling, a car, a phone — needs a comparison between the ears. With one implant it is close to impossible.
What a second implant does not do is make a single voice in quiet much clearer. If daily life is mostly one-to-one conversation in quiet rooms, the gain may be small; if it is full of noise and several speakers, it can be large. Hearing in noise through one side also costs effort; see listening effort.
What the trials show
Funders long hesitated over second implants for lack of good trials. The author’s Utrecht group, with four other Dutch university centres, randomised adults to one implant or two, tested with separated sound sources rather than a single loudspeaker. Binaural benefit appeared where the physics predicts it: speech in noise and sound localisation; stable at two years. The trial included 38 adults and was reported between 2016 and 2019 (see research).
Two findings matter. When speech and noise both came from straight ahead, one and two implants performed alike; the benefit appeared only when sound sources were placed apart, as in real life. And people with two implants could locate sounds, which people with one implant largely could not. Their own reports of hearing in daily situations matched the measurements; general quality-of-life questionnaires showed the difference less consistently.
A Dutch systematic review of the wider literature, with the author among its co-authors, reached a similar conclusion: a localisation benefit in every included study, a benefit for speech in noise under certain conditions, and usually none for speech in quiet.
Trial design and numbers
Five Dutch tertiary centres (Utrecht, Maastricht, Groningen, Leiden, Nijmegen) randomised postlingually deafened adults aged 18–70, with hearing loss of less than 20 years' duration and marginal hearing-aid benefit, to unilateral or simultaneous bilateral implantation (19 per arm; NTR1722). The primary outcome, speech in noise from straight ahead, did not differ at one year (9.1 versus 8.2 dB, P = .39), nor did CVC words in quiet. Speech in noise from spatially separated sources and localisation differed markedly: correct localisation at 60° speaker separation was a median 50.0% with one implant against 96.7% with two. Results did not change between one and two years. Direct costs were about €44,000 for one implant and €88,000 for two; annual costs about €3,400 and €6,900.
Is it worth the cost?
A second implant roughly doubles the cost of the device and its lifelong maintenance. A systematic review of cost-utility studies, published before the trial with the author as co-author, found adult estimates ranging from about $38,000 to $132,000 per quality-adjusted life year, largely because studies assumed different quality-of-life gains; costs fall when both implants are placed in one operation or the second device is discounted. In the trial’s own analysis, a second implant became cost-effective after five to ten years.
Simultaneous or sequential?
Two implants can be placed in one operation (simultaneous) or in two operations months or years apart (sequential).
One operation means one anaesthetic, one recovery and one period of rehabilitation. Both ears start together, and the brain learns to combine them from the beginning. It is also cheaper.
Two operations allow the result of the first ear to be known before the second is touched. Useful hearing in the second ear can be used with a hearing aid in the meantime, and balance function on the first side can be checked before the second is operated.
In a randomised comparison by the same Dutch group, adults who received their implants two years apart did as well, one year after the second implant, as those who received both at once; while they had only one implant, they had the limitations of one ear. A systematic review of cohort studies suggested that a second implant can still help after a long interval, although the evidence is of poor quality. A long gap is a reason for careful assessment, not automatic refusal.
Bimodal hearing: implant plus hearing aid
The main alternative to a second implant is a hearing aid in the other ear: bimodal hearing, electrical on one side and acoustic on the other. In most studies it gives better speech understanding in quiet and noise, and better localisation, than the implant alone.
It works because the aided ear often carries low-pitched sound that an implant conveys poorly: the pitch of a voice, the melody of music, cues that help separate one speaker from another. It depends on how much residual hearing the aided ear has and on careful fitting of the hearing aid alongside the implant. While the aided ear still contributes, bimodal hearing is often the arrangement of choice. When it has stopped contributing — speech through it is no longer understood, or the aid stays in the drawer — a second implant becomes the relevant question.
Who is a candidate
In general terms, a second implant is considered for adults who:
- have severe to profound sensorineural hearing loss in both ears;
- get limited speech understanding from a hearing aid in the second ear;
- have inner-ear anatomy and an auditory nerve suitable for implantation on that side;
- are prepared for the rehabilitation that follows, with realistic expectations of what two ears add.
How long the second ear has been without useful sound matters, because long deprivation can limit what an implant achieves. Fitness for anaesthesia and the person’s own priorities count too.
Funding differs between countries and changes over time. Some health systems provide two implants for adults routinely, some provide one, and some a second only in particular situations; an implant centre can say what applies locally. Children are a separate subject.
Risks of a second operation
A second implant carries the risks of any cochlear implant operation: wound infection, problems with healing over the device, dizziness, a change in tinnitus or in taste on one side, and rarely facial nerve weakness. Over the years a small proportion of devices fail and need replacement. Most centres advise vaccination against pneumococcal infection, because meningitis, although rare, is a recognised risk after implantation.
Two risks are specific to the second ear. The first is the loss of its residual hearing. The second is balance: implantation can reduce balance function in the operated ear. With one side affected the brain usually compensates; with both, unsteadiness in the dark or on uneven ground can be more lasting. Some centres therefore test balance function in both ears before the second operation.
After the operation, the advice on when to contact your surgeon applies: facial weakness, severe or spinning vertigo, a sudden drop in hearing, signs of infection spreading beyond the ear, or painful red swelling over the implant need attention the same day.
Preserving residual hearing in the second ear
The principles of cochlear hearing preservation apply to implant surgery as much as to middle-ear surgery. Modern implantation aims to keep low-frequency hearing: thin flexible electrodes, a gentle opening through the round window, slow insertion and, in many centres, steroids around the time of surgery. These measures often preserve some hearing, but not always, and the amount retained cannot be predicted for the individual.
That uncertainty shapes the decision. If the second ear still contributes clearly with a hearing aid, the question is whether the expected binaural gain outweighs what may be lost. If it no longer contributes, there is little to lose. Device options are compared on hearing rehabilitation.
Second-ear decision points
Usually decisive: aided word recognition and measured bimodal benefit in the contralateral ear; duration of deafness in that ear; imaging (cochlear patency, nerve); vestibular function on both sides (calorics or video head impulse test, cVEMP), since bilateral vestibular loss is poorly compensated; and, where low-frequency thresholds allow, hearing-preservation insertion and electric-acoustic stimulation. Post-meningitic deafness, with a risk of labyrinthine ossification, is the classic indication for prompt bilateral implantation. For bimodal users a standard prescription rule (NAL-NL1, NAL-NL2 or DSL) is a reasonable starting point; frequency lowering has not shown benefit.
Sources
- Smulders YE, van Zon A, Stegeman I, et al. Comparison of bilateral and unilateral cochlear implantation in adults: a randomized clinical trial. JAMA Otolaryngol Head Neck Surg. 2016;142(3):249-256. PubMed
- van Zon A, Smulders YE, Stegeman I, et al. Stable benefits of bilateral over unilateral cochlear implantation after two years: a randomized controlled trial. Laryngoscope. 2017;127(5):1161-1168. PubMed
- Smulders YE, van Zon A, Stegeman I, et al. Cost-utility of bilateral versus unilateral cochlear implantation in adults: a randomized controlled trial. Otol Neurotol. 2016;37(1):38-45. PubMed
- Kraaijenga VJC, Ramakers GGJ, Smulders YE, et al. Objective and subjective measures of simultaneous vs sequential bilateral cochlear implants in adults: a randomized clinical trial. JAMA Otolaryngol Head Neck Surg. 2017;143(9):881-890. PubMed
- Lammers MJ, Grolman W, Smulders YE, Rovers MM. The cost-utility of bilateral cochlear implantation: a systematic review. Laryngoscope. 2011;121(12):2604-2609. PubMed
- Smulders YE, Rinia AB, Rovers MM, van Zanten GA, Grolman W. What is the effect of time between sequential cochlear implantations on hearing in adults and children? A systematic review of the literature. Laryngoscope. 2011;121(9):1942-1949. PubMed
- van Schoonhoven J, Sparreboom M, van Zanten BG, et al. The effectiveness of bilateral cochlear implants for severe-to-profound deafness in adults: a systematic review. Otol Neurotol. 2013;34(2):190-198. PubMed
- Vroegop JL, Goedegebure A, van der Schroeff MP. How to optimally fit a hearing aid for bimodal cochlear implant users: a systematic review. Ear Hear. 2018;39(6):1039-1045. PubMed
Common questions
Is a second cochlear implant worth it?
For many adults with severe to profound hearing loss in both ears, yes, but the gain is specific. In a randomised trial, two implants improved understanding when speech and noise came from different directions and made it possible to tell where sounds come from. In quiet, or with speech and noise from the same direction, one implant did about as well. Whether that gain matters depends on how much of your life is spent in noisy, multi-speaker places.
Is it better to have both cochlear implants at the same time?
Not necessarily. In a randomised comparison of adults, having both implants in one operation and having them two years apart gave similar results one year after the second implant. A single operation means one anaesthetic and one recovery; two operations allow the first ear to be judged, and any useful hearing in the second ear to be used in the meantime. The choice depends on your hearing, your balance function and local practice.
What is bimodal hearing?
Bimodal hearing means a cochlear implant in one ear and a hearing aid in the other. The implant gives clarity; the hearing aid adds low-pitched natural sound, which can help with voices, music and noise. Studies show a benefit over the implant alone in most users. It works best when the aided ear still has useful hearing, and it is often the arrangement used before a second implant is considered.
Will a second implant destroy the hearing I still have in that ear?
It may reduce it or remove it. Modern electrodes and gentle insertion techniques aim to keep low-frequency hearing, and they often succeed in part, but preservation cannot be guaranteed. If the second ear still gives useful sound with a hearing aid, that hearing is worth weighing carefully before it is exchanged for an implant. Ask what is likely to remain in your case.
Is a second cochlear implant covered by health insurance?
It depends on the country and changes over time. Some health systems fund two implants for adults; others fund one, or a second only in particular circumstances. Trials such as the Dutch randomised study were set up partly because funding decisions lacked good evidence, but the rules themselves are set locally. Your implant centre will know what applies where you live.
What are the risks of a second cochlear implant operation?
They are those of any cochlear implant operation: infection, wound problems, dizziness, a change in tinnitus or taste, and rarely facial nerve weakness or device failure needing replacement. Two points are specific to the second ear: the loss of any residual hearing in that ear, and the small chance of reduced balance function on both sides, which is harder to compensate for than on one.
This page provides general educational information. It cannot replace an individual assessment, which depends on a full history, examination, audiometry and, where relevant, imaging. If you have sudden hearing loss, severe pain, facial weakness, severe dizziness or discharge with fever, seek medical care promptly.