About half of the increased risk appears to be due to one model of implant that was taken off the market a year ago. Nevertheless, those with other brands are still about 16 times more likely than usual to have bacterial meningitis.
The increased risk first came to surgeons' attention in the spring of 2002. It was quickly linked to an implant made by Advanced Bionics, one of three companies that sell the devices in the United States.
Nearly 10,000 U.S. children with severe hearing loss have cochlear implants, which are inserted into the inner ear to activate nerves, allowing sound to be transmitted to the brain.
Last year, the Food and Drug Administration reported 52 cases of meningitis after implants, including five deaths. The FDA and the Centers for Disease Control and Prevention investigated the magnitude of the problem and reported the findings in Thursday's New England Journal of Medicine.
They studied 4,264 children who received cochlear implants between 1997 and 2002 and found that 26 had gotten bacterial meningitis.
Overall, they found 138 cases of meningitis caused by Streptococcus pneumoniae, the most common germ, for every 100,000 years of life, which is 30 times the incidence among children the same age in the general population.
The researchers said they cannot be sure how much of this is caused by the implants themselves and how much from other factors, such as the ear abnormalities that underlie the children's deafness. Meningitis itself can cause deafness, and those who lose their hearing this way may be prone to later episodes.
The CDC's Jennita Reefhuis, principal author of the study, said the results do not mean cochlear implants are too risky for the young.
"It's important for people to remember that cochlear implants can do wonderful things for children," she said. "Those who are deaf can learn to sing songs and to speak."
Among the study's recommendations:
After concern about the infections emerged last year, Dr. Noel Cohen of New York University surveyed the 401 U.S. centers that implant the devices and found several cases that had not been reported to the manufacturers.
"The incidence of meningitis turned out to be greater than any of us had realized, and while we don't really know all that went into it," one factor appeared to be the design of the Advanced Bionics implant, he said.
Unlike other implants, it contained a small wedge that pushed an implanted electrode against the wall of the inner ear. How this increased the risk is unclear, though possible factors were the size of the device, trauma it caused to the inner ear and the presence of an additional foreign body.