You bounce down the block, late for work, holding a coffee, glancing at the headlines on your paper and dodging the other pedestrians who are walking at a maddeningly measured pace. Somehow, the world doesn't bounce around in your eyes like it was being filmed on a hand-held camera. Lucky you.
Some aren't so fortunate. What keeps you balanced and moves your eyes in concert with your gait is a system of loops in your inner ear called the vestibular system, and it can be damaged or disturbed by antibiotics, trauma, viruses, genetic problems and a rare condition called Ménière's Disease.
"When you lose vestibular sensation in both ears every time your heel hits the ground, the world shakes," says Charles C. Della Santina, director of the Vestibular Neuroengineering Laboratory at Johns Hopkins University.
Della Santina says it's like being a little disoriented, a little wobbly and a little seasick all the time. Afflicted people can't drive safely and often have to walk with a cane. Della Santina is one of a handful of researchers working to develop an implant that restores vestibular system function. Call it bionic balance.
The coming vestibular implant is just one of several new, improving and emerging prosthetic organs, limbs and other body parts that are using ever more sensors, processors and motors — they are becoming more bionic. The goal, of course, is to make prosthetics more sophisticated, subtle, and ultimately, more closely interfaced with living human beings and therefore more helpful.
There are three bony loops in your inner ear, called semicircular canals, arranged perpendicular to each other to measure movement in three dimensions. Fluid in the hoops swishes this way and that, moving hair-like cilia on special cells that trigger nerves that send signals to your brain. Your brain triangulates these signals to keep you balanced, and to move your eyes at exactly the same speed (and exactly opposite direction) as your head so the world doesn't look like it's being filmed by an amateur.
The most common cause of the loss of this function in the U.S. is side effects from the powerful antibiotic gentamicin. Worldwide, people treated with streptomycin, an antibiotic used to fight tuberculosis, are also often afflicted. Meningitis and other viruses can also cause loss of vestibular function. (Kids born without vestibular function, however, are able to compensate relatively well.)
For adults who lose this sense, there is not much doctors can do. "The best we can offer them now is having them practice looking at a spot on the wall while shaking their heads," says Della Santina, who is also a doctor. "But the vestibulo-ocular reflex is the fastest reflex in the body. There's nothing you can do to replace that."
In part because there's no treatment, the number of people afflicted is hard to estimate. Della Santina guesses there are 50,000 in the U.S. with serious problems, and many more with less severe symptoms that could also benefit from treatment.
Della Santina has built a vestibular prosthesis that he hopes can someday be implanted in humans much like cochlear implants are now used to restore hearing. Instead of using a microphone to pick up sound, his system uses gyroscopes to measure movement in three dimensions. The measurements, translated to electrical impulses, would be delivered to the three vestibular nerves that emanate from the three semicircular canals, much like audio signals are delivered to the auditory nerve in cochlear implants.
Device being tested in monkeys
Because each semicircular canal has its own nerve, a vestibular implant may actually be easier to perfect than a cochlear implant, which has to deliver many different sound frequencies to a single, bundled auditory nerve. "We have a very good sense of what sort of signal we need to deliver to give an animal or a person the sense they are spinning in a certain direction," Della Santina says.
His device has worked in rodents and monkeys so far, and it is now being tested more extensively in monkeys. He is also working to shrink the device to roughly the size of a cochlear implant and make it less power-hungry.
Across the country, at the University of Washington, Jay T. Rubinstein, a surgeon and professor in the otolaryngology department has developed a more simple vestibular implant that doesn't include sensors. This wouldn't be able to replicate the function of the vestibular system, but it could act like a pacemaker for people who have attacks that make the vestibular system go haywire, like sufferers of Ménière's disease.
He successfully implanted his device in normal rhesus monkeys without disturbing their hearing or sense of balance, and he was able to spark the vestibular system so that the monkeys' eyes moved as if they were spinning. He was then able to stop the movement.
The results were so successful that Rubenstein will apply for approval from the U.S. Food and Drug Administration to start a trial that will allow him to implant the first devices in humans as early as this fall.