ANN ARBOR, Mich. - The younger deaf and hearing-impaired children are when a cochlear implant awakens their hearing, the better they will do on speech recognition tests later in life, according to the new results of the largest and most carefully designed study of its kind.
In fact, University of Michigan Health System researchers report, the positive effect of early implantation is evident even in comparisons of younger and older children who have had their implants for the same length of time, despite the older children's maturity advantage.
The new evidence, published in the January issue of the journal Otology and Neurotology (formerly the American Journal of Otology), adds to the growing proof that eligible hearing-impaired children should receive cochlear implants as early as possible if they cannot benefit from hearing aids.
"We found a significant difference in speech recognition between those who got their implants between the ages of two and four years, during the critical language development period, and those who received them later," says Paul Kileny, Ph.D., lead author and professor and head of audiology at UMHS. "We also found that the longer children had had their implants, the better they did, though the effect was still largest in those who were implanted earliest."
The study looked at test results from 101 children who received the same model of cochlear implant at UMHS between the ages of 2 and 14 years. The group's size and the identical management of the patients make the study's results more relevant than those of past studies of the implanted electronic hearing devices. The children represent one-third of all pediatric patients ever treated in the UMHS program, one of the oldest and largest in the country.
The children were divided roughly in half, to allow for two analyses that could help isolate the effect of age at implantation on speech perception. One group of 48 children had their speech-recognition skills tested when they turned 7, regardless of when they got their implants. The other group of 53 children of various ages was tested at three years after implantation, to isolate the effects of age at implantation. All children took a battery of six standard tests to measure their ability to recognize sounds, words and sentences. Overall, the results showed a strong combined effect of age at implantation and the length time the children had had their implants.
Cochlear implants are electronic devices that transform speech and other sounds into electrical impulses that stimulate auditory nerve terminals in the inner ear. They can restore hearing and improve communication for children and adults with severe to profound sensorineural hearing loss in both ears who get little or no benefit from hearing aids.
Kileny and his co-authors, audiologists Teresa Zwolan, Ph.D., and Carissa Ashbaugh, M.A., collected the data over several years from routine tests taken by UMHS cochlear implant recipients at regular intervals. The children were between ages 5 ½ and 17 ¾ when evaluated.
The researchers grouped the children in each arm of the study into four subgroups. Children tested at three years post-implant were divided according to age at implantation, and those tested at age 7 were grouped according to time since implantation.
The differences between the groups were clearest - and most statistically significant - in the 7-year-olds who had had their implant for four or more years, and the children whose three years of implantation had begun between the ages of 2 and 4. But even those 7-year-olds who had only had their implants three years scored significantly better than those who had had them one or two years. And children who first heard between the ages of 5 and 7 did better than those who got their implants between the ages of 10 and 13.
Not all speech-perception tests produced results that showed clear differences among the groups. But Kileny notes that some tests required sophisticated linguistic and vocabulary skills, which the youngest patients may not have developed no matter what their implant age.
This effect of better speech perception performance with longer and earlier use of an implant is probably linked to the effect that cochlear implants have on the developing auditory nervous system, Kileny explains. Basic research has shown that the nerve cells involved in the auditory system require early and constant stimulation in order to develop important connections and patterns of activation necessary for speech perception. And other clinical studies have shown that both children and adults keep improving their speech perception with time, though adults tend to plateau after a while and children continue to improve over a period of years.
Thus, according to Kileny, the "wait and see" approach in the case of newly diagnosed young children may be detrimental in the long run. "I have seen several youngsters whose parents have opted to wait for new technological developments, in some cases for two to four years. These patients, though they began with great potential, ended up deriving minimal benefit from their cochlear implant, having missed crucial speech and language development milestones."
This "plasticity", or ability to keep learning new things, adds still more fuel to the argument that children with hearing problems who do not benefit from amplification with hearing aids should get cochlear implants as early in life as possible. Researchers in childhood development have consistently found that toddlers and young children reach crucial milestones in speech and language development early in life. The new study's result may clearly point to the risk of missing or delaying those milestones because of hearing impairment.
The team is still looking at the long-term effects of cochlear implantation before the age of 2. The U.S. Food and Drug Administration has approved implants for use in children as young as a year, and clinical trials in 12- to 24-month-olds are now under way at UMHS and elsewhere. Initial data on these youngest patients are already showing a difference from those who got their implants later in life, Kileny says, but it will take time to accumulate a large number of patients with several years' experience to allow the type of analysis done in the current study.