The cochlear implant is one of the great triumphs of biomedical and surgical collaboration, enabling more than 219,000 severely-to-profoundly deaf people around the world to hear. 

For congenitally deaf children who, in earlier decades, would have lived in perpetual silence, this small, complex electronic device has opened their world to sound and speech. Some even develop the lilting accents of their home counties. 

Post-lingually deaf adults also experience measurable benefits from cochlear implants. Research published in February this year added to mounting evidence that cochlear implants improve speech and significantly enhance quality of life in adults with severe hearing loss.¹ 

Over the past 20 years in particular, this implantable prosthetic neurostimulator – sometimes called the ‘bionic ear’ – has revolutionised the management of sensorineural deafness. Today, surgical rehabilitation by cochlear implantation has become a widely accepted, routine procedure for people who are profoundly deaf or severely hard of hearing where conventional hearing aids are of little or no benefit. Despite being referred to as ‘nerve deafness’, sensorineural hearing loss is caused by damage to the inner ear or cochlea, not the hearing nerve. This damage can be congenital or acquired. 

“Hearing aids amplify the sound so the cochlea and the hearing pathway can try to make the best use of sounds that are louder, whereas a cochlear implant works like a Morse code machine: it bypasses the cochlea, completely removes it from the equation, and electrically stimulates the auditory nerve directly to produce a sensation of sound,” explains Mr Peter Walshe, one of two consultant ENT surgeons heading up the National Cochlear Implant Programme (NCIP) in Dublin’s Beaumont Hospital. His colleague, Ms Laura Viani, NCIP director, founded this national service in 1995. In that year, two adults and one child received cochlear implants. In 2012, approximately 90 children, adolescents and adults in Ireland had some level of hearing restored with cochlear implants. 

“We’ve come a long way since the 1990s,” Laura remarks. “Peter joined myself and the team in 2009, and over the years we’ve built up a team of audiological scientists, speech and language therapists, teachers of the deaf, clinical psychologists and administrators. We’re very much multidisciplinary and we are probably the only service in this country that is totally public; we don’t do any private work at all, which means you can be a king or a pauper, it doesn’t matter to us.”

So far, more than 700 patients have received cochlear implants in Ireland, primarily at the national centre in Beaumont Hospital but some children have also had their surgery in Temple Street Children’s Hospital, where Laura and Peter run a consultant-delivered service. Children referred for a unilateral implant (in one ear) are seen within three months, and usually undergo surgery by seven months. The adult unilateral implant wait is up to two years. 

How it works

There are two recognisable components of a typical cochlear implant system: the implanted electrode array with associated microcircuitry, which is placed just under the skin, behind the ear; and the external component that refines the raw signal before delivering it to the implanted electrode. This component ‘sits’ over the implant, attached to the internal part by a magnet. The external component comprises a microphone that picks up sounds from the environment and sends them to the ‘brain’ of the system – the speech processor. There are many different strategies employed by the various manufacturers, but the aim of speech processing is to manipulate the raw signal so that the most important features necessary for speech recognition are preserved and delivered to the ear.

According to the various bands of spectral (or frequency) energy in speech, low-frequency information corresponds to the vowel sounds and contributes to the prosodic patterns of speech (the patterns of stress and intonation in a language), while the higher frequencies convey consonant information, which is essential for speech recognition. The cochlea is one of the mechanisms used by our auditory system for encoding these and other sound frequencies, therefore the cochlear implant attempts to replace that damaged portion of the ear. 

The external speech processor captures sound waves and converts them into digital code. This digitally coded sound is transferred from the processor through the intact skin by a process of ‘inductive coupling’ – an external coil is held magnetically over an internal coil that is part of the implanted component of the system. A microchip decodes the incoming waveform and directs it to the appropriate intracochlear electrodes depending on the frequency of the sound, and at a rate determined by the specification of the individual processing programme. Electrodes near the base of the cochlea are stimulated with high-frequency signals, while electrodes near the apex are stimulated with low-frequency signals. 

The cochlear implant is based on the idea that there are enough auditory nerve fibres left for stimulation in the vicinity of the electrodes. Once these nerve fibres are stimulated, they fire and propagate neural impulses to the brain, which interprets them as sounds.

While commercial cochlear implant systems have only been available since the 1980s, the idea of using electrical rather than acoustic stimulation to activate the auditory system in individuals with profound sensorineural hearing loss dates back to 1800 when Alessandro Volta first reported that electrical stimulation to metal rods inserted in his ear canal created an auditory sensation. In 1964, Blair Simmons at Stanford University implanted several patients with a six-channel device. Recipients could still not understand speech through the implant but, importantly, it demonstrated that stimulating different areas of the cochlea produces different pitch percepts.These observations fuelled the push toward the development of functional, permanent cochlear implant systems. Prof Graeme Clark made a major breakthrough in 1978 when he and his small team performed the first multi-channel cochlear implant operation, which allowed Rod Saunders, who’d lost his hearing at 46, to hear again. The Australian surgeon recalled: “When he heard his first words I was just so overcome I went into the next-door laboratory and wept for joy.”

Through the 1990s, clinical and basic science investigations resulted in progress in implant technology and in clinical approaches to cochlear implants. Ongoing advances in the miniaturisation of digital signal processors and electrodes now produce coding strategies that are associated with successively higher performance levels. 

A different kind of hearing

But it is important to understand that cochlear implants do not restore ‘normal’ hearing. They are tools that allow sound and speech to be processed and sent to the brain. 

“I’ve asked a few adults, who have had normal hearing, what it sounds like to them and they said that in the beginning it sounds robotic but then over time it sounds normal, so the brain adapts,” says Laura. “We don’t really know what prelingually deaf children are hearing but it can’t be that robotic because if children are getting Cork or Limerick or Donegal accents, this means they must be hearing to quite a good level with their implant. It must be more finely tuned than we actually realise.” 

However, not all people with hearing impairment are suitable candidates for cochlear implantation. “The audiological criteria vary a little between children and adults. For children, they must be profoundly deaf, over 90 decibels. We have a detailed assessment process that involves speech and language, various scans and medical assessments. With older children the outcome is less good. If, for example, a seven-year-old came to us having spent all her life signing and had never used hearing aids, it would be too late to use a cochlear implant because the auditory pathways diminish if they’re not used.”

“The earlier you tap into that neural plasticity the quicker and more effectively the brain adapts,” Peter says. Early implantation provides exposure to sounds that can be helpful during the critical period when children learn speech and language skills. Research has shown that those that have their cochlear implants early are often on a par with their hearing peers when then enter mainstream school, and maintain that parity throughout their school years.² “It’s very different for the adults we treat,” he adds. “They have acquired speech and those centres of the brain are formed and secluded off for hearing purposes, they just need to be re-activated. They may get significant benefit from a cochlear implant.”

Laura points out that the cochlear implant process does not end with the surgery. “Our service is lifelong. The external speech processor is fitted and switched on a month after surgery and the implant will be tuned. This little computer has to be programmed lifelong. We give each child a five-year full rehabilitation with us here, and three years for adults. Some people may need less or more after that, but we don’t lose patients, they are with us for life.”

Among studies comparing one versus two implants, the majority found participants had better hearing, significantly improved quality of life, and were better at communicating and localising sound when they had a cochlear implant in each ear. Remarkably, research published in March this year found that prelingually deaf children with bilateral implants were able to develop the ability to accurately identify emotion (ie. happiness or sadness) in speech and music.³

“As a single national unit, we are affiliated with the British Cochlear Implant Group and abide by the NICE guidelines, as such the gold standard of treatment is bilateral simultaneous implants from close to one year of age for congenital deaf children, and then sequential implants for other children who are born congenitally deaf as well,” explains Peter.

Laura adds: “We have been seeking funding to introduce this gold standard of bilateral cochlear implants since 2009, and our CEO here in Beaumont, Liam Duffy, has been really helpful in this regard.” In January, parents of children from the national cochlear implant unit launched a campaign called ‘Happy New Ear’, which aims to tackle the funding problem and bring Ireland on a par with the rest of Europe.  

References

1. Gaylor J M, Raman G, Chung M, et al. Cochlear Implantation in Adults: A Systematic Review and Meta-analysis. JAMA Otolaryngol Head Neck Surg 2013; First Online. doi:10.1001/jamaoto.2013.1744
2. Geers A, Sedey A. Language and Verbal Reasoning Skills in Adolescents with 10 or More Years of Cochlear Implant Experience. Ear Hear 2011; 32(1 suppl): 39S-48S.
3. Volkova A, Trehub SE, Schellenbery EG, et al. Children with bilateral cochlear implants identify emotion in speech and music. Cochlear Implants Int 2013; 14(2): 80-91