THE world looked on as 22-year-old Nan Davis took her first tremulous steps
after four years confined to a wheelchair. She had been paralysed from the waist
down after a car crash on the day of her high school graduation. But electrodes
attached to the outside of her legs were stimulating her muscles and enabling
her to 鈥渨alk鈥� once again.
A harness supported most of her weight and her legs moved unsteadily. But the
technology gave new hope to many disabled people with spinal injuries. The aim
was simple, said Jerrold Petrofsky, the scientist who led the work at Wright
State University in Ohio. He wanted implanted electrodes to control motion and
balance so that disabled people could walk with as much control and fluidity as
an able-bodied person within the next few decades.
That was in 1982. But if the story sounds familiar, you shouldn鈥檛 be
surprised. Last month similar claims were made when, in front of the world鈥檚
media, Marc Merger, a French financial consultant, got to his feet for the first
time in a decade. But despite all the fanfare, the plain fact, say most expert
observers, is that Merger was not much closer to the goal of useful walking than
Davis had been nearly twenty years earlier.
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Some critics are unimpressed with the progress in two decades. They ask
whether the aim of restoring people鈥檚 ability to walk can be justified when it
raises hopes out of all proportion to our technical capabilities.
Gerald Loeb, a biomedical engineer at the University of Southern California
in Los Angeles, and one of the pioneers of the cochlear implant, believes that
scientists have an unhealthy obsession with trying to make people walk again.
鈥淔or the vast majority of people it鈥檚 too expensive, too cumbersome and involves
too much surgery for what they are likely to get from it,鈥� he says.
Besides, he says, stimulating leg muscles to contract is not the same as
walking. The act of walking is one of the most complicated actions we could
attempt to reproduce artificially. Inherently unstable, the movement boils down
to people falling forward and catching themselves on the next foot, before
repeating the process over and over.
Patients鈥� groups are also concerned. 鈥淭here have been no real advances,鈥� says
Peter Mansell of the Royal Association for Disability and Rehabilitation (RADAR)
in London, when you look at things from the point of view of a disabled person.
Merger may have electrodes stimulating muscles inside his body and sophisticated
electronics controlling these like an artificial nervous system, but ultimately,
this still doesn鈥檛 add up to walking.
Like a number of others, Mansell even doubts whether we should be trying to
make people walk again. Besides raising false hopes, it鈥檚 a drain on important
resources. Mansell believes the sight of Christopher Reeve vowing publicly that
he鈥檒l be able to walk again by 2005 is a sexy topic that attracts
grants鈥攖o the detriment of funding for more prosaic work that is more
likely to get results, such as research into electrically stimulated
bladders.
鈥淧eople in our field have been very naive on the whole about this,鈥� says Nick
Donaldson, a biomedical engineer at University College London, who has worked
extensively on functional electrical stimulation (FES), as it has become known.
FES is not particularly well suited to locomotion, he says. At best it can help
people stand so they can reach objects or make it easier to get from a
wheelchair into a car. A broader range of activities would need a large number
of electrodes capable of surviving inside the human body for years without
breaking down or harming the patient.
Wet and acidic, with rampaging immune cells ready to pounce on anything
artificial, the human body is a hostile environment, says Michael Craggs, also
at University College London, who has worked on FES to give paraplegics control
of their bladders and bowels. As a result, implant scientists have spent a lot
of time in the last two decades making implants more durable鈥攚ith a fair
amount of success.
Implants may also cause infections in patients, and particular problems arise
from the mechanical stress walking places upon some electrodes. In part,
researchers have tried to reduce this by limiting the amount of equipment
actually exposed to the body. Instead of trying to position electrodes at the
muscles鈥攚hich means running long leads over a number of joints, making
them more susceptible to failure鈥攔esearchers have tried to stimulate
bunches of nerves at the spinal column. The electrodes are better protected
here.
Unfortunately, it is more difficult to select specific nerves for activation
from this part of the central nervous system, says Craggs. For example, the
nerves that produce some of the muscle contractions for walking also make the
legs fold up to the torso. Another reason for this lack of discrimination is the
size of electrodes, says Donaldson. 鈥淭he nerves we are stimulating are tens of
microns in diameter while our electrodes are millimetres,鈥� he says.
A further obstacle has been spasticity and muscle spasms. These exaggerated
reflexes are built into our nervous system as a control feature, says Craggs.
Normally, small muscles working antagonistically enable us to make controlled
movements and not overshoot a target. But in the case of spinal injury the
absence of an inhibitory nerve signal can produce extreme and undesirable
jerkiness.
But perhaps the biggest hurdle facing Petrofsky and those with similar
ambitions is that smooth, natural walking requires sensory feedback. Despite
some limited successes, research in this field is still in its infancy. At the
moment implants can鈥檛 receive and respond to information coming back from muscle
tissue. This means people 鈥渉elped to walk鈥� can鈥檛 feel pain or sense changes in
orientation.
To restore sensation to someone with a spine injury will most likely mean
bypassing the damaged part of the spine and using a computerised control system
to feed signals directly into the brain. But such control systems are a long way
off. This was demonstrated earlier this year when the Dobelle Institute in New
York presented its findings on an artificial vision experiment. In a bid to
restore his vision, a patient known as 鈥淛erry鈥� has had electrodes in his visual
cortex and wires hanging out of the back of his head for the past 22 years
(快猫短视频, 22 January, p 6).
That these electrodes have stayed in place for so long without failing or
causing infection is a testament to the engineering skills of William Dobelle.
Yet it has taken this long to develop software that could turn video images into
crude pseudo-biological signals that the brain can recognise. And the artificial
vision system is very crude. In comparison, the feat of biological engineering
needed to channel enough feedback from the limbs to the brain is daunting
indeed.
The bottom line is that we still know very little about how to tap in or tap
out of the nervous system. Practical walking really is a very long way off.
Donaldson believes that the true value of FES is therapeutic鈥攅xercising
muscles, for example. Without adequate exercise, muscles can atrophy and cause
circulatory problems. These are very real and very immediate goals that FES can
assist with in the short term. But Donaldson is concerned about the way some
findings are presented to the public. While the press is certainly to blame for
its uncanny ability to pounce on headline-grabbing words like 鈥渂reakthrough鈥�, it
isn鈥檛 solely responsible for raising expectations.
It鈥檚 part of human nature that some of the biggest technological challenges
are irresistible鈥攁nd reversing paralysis, one of the medicine鈥檚 Holy
Grails, is no exception, says Thomas Sinkjaer of the Center for Sensory-Motor
Intervention in Aalborg, Denmark. And there remain many researchers, such as
Merger鈥檚 doctors and others, committed to the goal.
Nonetheless, Loeb and others like him say a dose of realism is needed. People
forget that the wheelchair is an extremely efficient way of getting about,
despite its limitations, he says. Quite simply, a quantum leap in
neuro-prosthetic implant technology will have to occur before it has any chance
of competing with the humble wheelchair.