People who have lost their sense of balance could one day be fitted with an inner ear implant modelled on the body's own balance organs, say researchers.

Current designs are successful in animals, but two new studies promise a smaller, more accurate device, with a longer battery life - the crucial prerequisites for use in humans.

The sense of balance is controlled by the vestibular portion of the inner ear. It keeps track of the motion and position of the head using three fluid-filled hoops, called semicircular canals. These sit at perpendicular angles to each other. When the head rotates quickly in a certain direction, the fluid in the corresponding hoop pushes against a membrane, bending hair cells that trigger a nerve. The nerve sends the information to the brain which tells the eyes to adjust.

"It's the fastest reflex in the body," says Charles Santina at Johns Hopkins School of Medicine in Baltimore, Maryland, US, who is designing an implant to restore this phenomenon, called the vestibular-ocular reflex, in humans. "Without it, the world looks like you're watching it through a hand-held video camera," he explains.

People lose this reflex when the vestibular hair cells die, usually from genetic disorders, infections or antibiotic poisoning. Hearing loss can also be caused by hair cell death, and since cochlear implants have been successful at restoring partial hearing (see Implant works wonders for deaf babies), scientists reasoned a similar implant could work for balance.

Signal bypass

Cochlear implants use a microphone and processor to code sound and send it directly to the cochlear nerve via electrodes implanted in the inner ear. They completely bypass the dead hair cells. Similarly, a vestibular implant uses tiny gyroscopic sensors to measure head movement and sends that information straight to the vestibular nerve using electrodes.

The first such device was developed by Daniel Merfeld and his team at the Jenks Vestibular Physiology Lab at Harvard in Boston, Massachusetts. That implant, which has shown success in animals, can measure head movement in one plane of movement.

But Santina and his colleagues felt that to really restore the sense of balance, an implant should pick up motion in three dimensions. They built a circuit with three gyros oriented perpendicularly - just like the semicircular canals - and tested it on chinchillas.

"We've been able to restore their vestibular-ocular reflex," says Santina. "We turn their head and measure how their eyes move." The results of those animal studies will be published in the July 2007 issue of IEEE Transactions on Engineering in Medicine and Biology.

Battery life

Before an implant can be made for humans, however, the technology needs some tweaking. In the current system, the motion sensing circuit is mounted to the outside of the chinchillas' head. In a human, it would need to be implanted under the skin behind the ear. But at over 6 millimetres-thick, the circuit is still too large.

The device is also too power hungry. Even if it could be implanted under the skin, the external battery that powers it would need recharging every 5 or 6 hours. Battery life is the biggest challenge facing vestibular implants, says Lawrence Lustig, director of the Douglas Grant Cochlear Implant Center at the University of California in San Francisco, US.

When the battery runs out in a cochlear implant the person simply cannot hear, says Lustig, "but if the battery ran out on someone with a vestibular implant they'd be violently dizzy and vomiting".

Virtual inner ear

To address these problems Santina and his team have designed a prototype for a new motion-sensing system that uses accelerometers instead of gyroscopes. These are much smaller and more energy efficient: A 9-volt battery will power the new device for 24 hours. In comparison, cochlear implants need recharging after 12 hours.

The researchers also have a computer model that shows how the current from the electrodes flows over the vestibular nerve in different situations. "It's a way of creating a virtual inner ear to try out our designs," says Santina.

Lustig says with the progress being made, it is only a matter of time before these implants are being tested in humans. "It's an exciting technology coming down the turnpike," says Lustig. "There is a small but definite number of people who will greatly benefit from this."

The new system will be presented on Wednesday at the Meeting of the Association for Research in Otolaryngology in Denver, Colorado, US.