Implant infection answer a winner

The following article was published in The Australian, on June 21st.

The puzzle of why people with cochlear implants are more susceptible to meningitis has been discovered by a Victorian researcher. Bianca Nogrady reports | June 21, 2008
THE brain is an incredibly fragile organ. Evolution certainly thinks so -- it has surrounded this mass of nerves and cells with a solid case of bone to guard against physical trauma, and lined its blood vessels with an almost impermeable membrane to guard against chemical and biological threats. As long as these defences remain unbreached, the brain is relatively safe.

But sometimes they have to be breached. Cochlear implants bypass damaged hearing systems to directly stimulate the auditory nerves, but to enable this, surgeons must drill through the bone and implant electrodes deep within the inner ear, where they come into direct contact with the nerves.
It is testament to the careful design of implants and the skill of the surgeon that this procedure and the device are so safe. But in June 2002, something went wrong. The US Food and Drug Administration began getting reports of bacterial meningitis in children who had received a cochlear implant. Their investigation revealed a 30-fold increase in the risk of bacterial meningitis compared to the general population, especially in children with a particular US-designed cochlear implant that included a "positioner" -- a tiny wedge that held the implanted electrode against the wall of the inner ear. The discovery led to swift withdrawal of that design of implant.
Around the same time, a young Taiwan-born Australian doctor, Benjamin Wei, was taking his first steps on the path to becoming an ear, nose and throat surgeon. But unusually, he also hankered for the laboratory.
"At the time, meningitis and cochlear implants were very topical and I was very interested and I wanted to have some experience in research," says Wei, an ENT surgical registrar and scientist at The Bionic Ear Institute in Melbourne.
Earlier this month, Wei, 31, received the 2008 Victorian Premier's Award for Health and Medical Research for his investigation into the link between implants and pneumococcal meningitis. What he discovered has implications not just for cochlear implants, but for any device implanted in the brain.
Pneumococcal meningitis is the medical term for inflammation or infection of the meninges, the membranes surrounding and protecting the brain and spinal cord, most commonly caused by the bacterium Streptococcus pneumoniae, or pneumococcus. Infection can be treated with antibiotics if caught early, but one in five patients will die and up to half will experience long-term complications including deafness, paralysis and mental retardation.
The FDA's investigation highlighted several risk factors that appeared to predispose cochlear implant recipients to bacterial meningitis, but left many questions unanswered.
"It was hard to say if (infection) was due to cochlear implants because a lot of patients had pre-existing risk factors, such as a malformed inner ear or had a previous history of meningitis because they were immuno-suppressed," says Wei. "No one knows exactly what caused it, whether it was the implants or underlying risk factors."
Wei's first step was to study the effect of a cochlear implant in the absence of these other risk factors. With assistance from the departments of otolaryngology, and microbiology and immunology at the University of Melbourne, he conducted an experiment using healthy rats where some received a cochlear implant and others simply had the operation to insert a cochlear implant without actually getting the device implanted. The rats were then exposed to pneumococcus via the usual routes that infect humans.
This simple study revealed that the presence of the implant lowered the amount of bacteria needed to trigger meningitis, making implant recipients more vulnerable.
"In essence, any time when you have a foreign body in any part of the body we increase the chance of infection," says Wei. The presence of a foreign body seems to impair the activity of immune cells, but there was more to the picture.
A second experiment of a similar design included a group of rats who underwent surgery that caused considerable trauma to the inner ear. It showed that the more trauma inflicted on the inner ear during implantation, the greater the risk of meningitis.
"When you have trauma in the inner ear, it opens more direct communication from the inner ear to the brain, a more direct pathway," Wei says. This offered some explanation as to why the US implant withdrawn in 2002 was associated with increased incidence of infection -- its extra components made it bulkier and the design did not adequately consider the very small dimensions of the inner ear, says Wei.
The end result of these studies was the realization that while any implant can increase the risk of meningitis, poor surgical technique and a more traumatic implant design considerably amplify that risk.
This new understanding has helped shape implant design in Australia, according to doctor Rob Shepherd, director of The Bionic Ear Institute.
"We've done a lot of work both in Melbourne and the cochlear group in Sydney to make sure the electrode array for a cochlear implant is designed for very safe insertion so it doesn't cause any trauma," Shepherd says. "Ben's work doubly emphasised the importance that any new design of any device needs to take into account design that absolutely minimises any trauma."
But Wei's work was not over.
Having identified the route and cause of infection, Wei's next move was to prevent it. The FDA had recommended that children receiving implants should be vaccinated against pneumococcus, but the agency made this recommendation without having clear evidence that immunisation would be as protective in patients with implants as it was in the general population. So Wei went back to his rats, immunised then implanted them, and found strong evidence that vaccination prevented meningitis.
Thanks to what Shepherd described as "elegant and rigorous" research, scientists now have a clearer understanding of what leads to meningitis in cochlear implant recipients, and how to avoid it. Those who stand to gain the most are the patients -- people such as retired high school teacher Joav Niran. He lost hearing in one ear after an explosion in his earlier army days, then seven years ago the hearing in his other ear began to fail after a bout of influenza.
"I couldn't communicate before -- people had to speak to me directly in front of me, sometimes I had to ask people to write things down because I just couldn't understand them," says Niran. His implant changed all that. "Basically, I'm able to communicate ... I'm functioning."
While Australian cochlear implants do not have the same design issues as the American implant withdrawn from the market, Niran, like all implant recipients, was also vaccinated against pneumococcus.
The application of Wei's research also goes beyond the ear to include other devices that by their very nature compromise the brain's defences.
"There are devices such as brain stimulation for treating Parkinson's tremor and potentially also shunt devices that take fluid from the brain in areas where there's build-up of fluid," Shepherd says. A bionic eye, also being developed in Australia, would operate on similar principles to the bionic ear in terms of direct stimulation of the optic nerve, and will therefore also benefit from Wei's findings.
"I think the most important message to come across is that the research is paving the way for better health, making implants even safer, reducing the chance of infection and making infection risk very low," says Wei.
"Our research basically ensures current and future cochlear recipients that the chance of acquiring meningitis is no greater than a person without implants."

Here is the direct link to the article. (The Australian.)

K.L.