Prognosis after cochlear implantation

Children benefit the most as do many adults

Multichannel cochlear implant systems were approved by the Food and Drug Administration for adults in 1985 and for children in 1990. NHS funding became available in the early 1990s. About 4000 patients have received implants in the United Kingdom (50 000 worldwide). Children now outnumber adults by 2:1. Cochlear implants are reliable, and cochlear implant surgery is safe despite recent concerns regarding a risk of meningitis. The numbers of suitable candidates is rising as selectioncriteria change, and it is timely to consider the benefits and risks of the technique.

A cochlear implant takes the place of the damaged organ of Corti and stimulates the spiral ganglion cells directly. Acquired causes of hair cell loss include infection such as rubella, cytomegalovirus infection, mumps, measles, meningitis, and middle ear infection, drug toxicity, trauma, and autoimmune disease, as well as Menière's disease and cochlear otosclerosis. Congenital hair cellloss may be due to recessive inheritance or may be the result of failure of normal intrauterine development of the inner ear due to causes of which some are known, some as yet unidentified.

An individual deafened after the critical period of language acquisition will have had exposureto the normal sounds of his or her own language. The central auditory pathways will have been stimulated normally, and neural networks will be in place that provide the phonetic, lexical, semantic, and syntactical basis of language. In such individuals cochlear implantation has a good chance of success even after an interval as long as 10-20 years after the hearing loss. A child born deaf or deafened before or around the time that language is acquired (prelingually or perilingually deafened), however, has only a short critical period during which stimulation with a cochlear implant can be expected to result in the acquisition of speech and language. The plasticity of the auditory system is currently thought to be at its maximum below the age of 2 years. It is lessening by the age of 4 years, and by the age of 8-10 years has decreased to such an extent that most implant teams would be reluctant to consider such a child a candidate for implantation.

The Medical Research Council's report Cochlear Implantation in the UK 1990-1994 showed that more than 97% of profoundly postlingually deafened adults with implants could identify some environmental sounds.1 Ninety five per cent identified more words using the implant with lipreading than using lipreading alone. Fifty per cent were able to recognise 10% of words without lipreading, and 15% were able to recognise 50% of words without lipreading. Thirty five per cent of individuals demonstrated some ability to understand telephone questions. The report also showed that in terms of cost effectiveness cochlear implantation compared favourably with other high tech interventions such as coronary artery bypass grafting, peritoneal dialysis, and haemodialysis. Since that report progressive improvements in speech processing technology have led to markedly better outcomes: the majority of postlingually deafened adults are now able to use the telephone to some extent, some effortlessly. Deaf blind adults (who cannot lipread) are now among those in whom implantation is undertaken most successfully.2 Pilot studies on bilateral implantation have shown an increased understanding of speech in background noise and improved directional accuracy.

It is children, however, who show the greatest benefits. Govaerts et al are the most recent of many teams to have shown the effectiveness of cochlear implantation in congenitally deaf or prelingually deafened children.3 They also show the influence ofthe critical period for speech and language development. All children who had received implants upto the age of 6 years derived some benefit. By using a scale of categories of auditory performance(CAP), with scores ranging from 1 (no awareness of environmental sound) to 7 (able to use the telephone with a familiar talker), the researchers showed that children who received implants at an age younger than 2 years achieved normal scores that were appropriate for their age within only three months of implantation: 90% were estimated to enter mainstream education. Children who had received implants at age 2-4 years took two years to reach CAP 5 and four years to reach CAP 7: 60% of them entered mainstream education. Only 20% of children who had received implants after age 4 achieved normal CAP scores, and only 33% integrated into mainstream education by the age of 7. Heightened professional and parental awareness of the importance of the critical period together with universal neonatal screening should ensure early referral.

Earlier this year concerns about a possible association between cochlear implantation and meningitis were raised by the Food and Drug Administration and by the Medical Devices Agency in the United Kingdom.4 Ninety one cases of meningitis (mainly pneumococcal) in patients with implants have been reported worldwide. Most have been in children, and 17 people have died. It is not clear if this figure exceeds expectations for the profoundly deaf population without implants, given that many deaf children with implants have inner ear malformations (such as Mondini dysplasia) that in themselves predispose to meningitis. The suggestion of a possible association with one particular implant design has led to its withdrawal. Acting on recommendations from the Department of Health all centres now advise vaccination against pneumococcus, especially in groups at risk, and prompt treatment of otitis media in people with implants.

New developments allowing implantation with the preservation of residual hearing open the door to the possibility of combined electroacoustic stimulation by using a cochlear implant for high frequency information combined with a conventional hearing aid for low frequency amplification.5