Cranking it up to 11 Proves Ineffective When Tapping Spinal Nerves
October 29, 2013 6:00 AM
(Source: Embassy Pictures)
Researchers make major progress in lower spine stimulation but are slowed by lack of equipment
The U.S. Food and Drug Administration
(FDA) is both receiving praise and criticism for its handling of research related to the exciting new field of electric stimulation of the lower spine. These new techniques promise to give
the ability to walk again -- a key therapeutic goal.
The most intensive multi-patient study to date -- conducted by Professor Susan Harkema of the University of Louisville, Kentucky -- is currently concluding and the medical community is carefully eyeing its results. But recent interviews indicate frustration among biomedical engineers crafting the next generation of stimulation devices. They feel that the FDA's restrictions are making it difficult to apply bleeding edge electronics to this pressing problem.
I. Do Your Legs Have a Mind of Their Own?
The roots of electrical stimulation trace back at least to the start of the twentieth century. Medical textbooks from a hundred years ago or more indicated a basic understanding that walking and other limb motions in humans and animals was a combination of both control by the brain and by automatic responses (reflexes).
But it was not until the 60s and 70s that researchers discovered that automatic responses might be far more dominant than the brain, a contradiction of the assumption that the brain controls the body.
Published research from this period showed increasing evidence that nerve bundles in animals could be stimulated to produce locomotion even after they were disconnected from the brain. For example,
this 1972 study
on walking in crabs showed that gait resulted from current from a combination of muscle contraction and muscle stretching sensors in the limbs.
This study hinted that while the brain (or similar nerve cluster) controlled learning of movement (so called "motor memory"), and sent triggers to initiate a specific kind of movement, from there local clusters of nerves took over and drove motion.
These clusters were sometimes labeled a "cluster pattern generator" (CPG). Some may recall learning about how dinosaurs had a "second brain" in their tail/lower back. That "brain" is actually thought to have been a CPG -- no different that the pattern generators found in modern animals ranging from crabs to cats.
A crucial piece of the proof necessary to convince the medical community that walking in animals was mostly automatic came in the form of
a 1987 study
Dr. Serge Rossignol
(first author) and
Professor Jean Barbeau
(senior author) of the
Université de Montréal
(Univ. of Montreal, UdeM), published in the journal
. In the study house cats were fully spinalized (sorry, cat lovers) using a spinal cord-cutting procedure similar to
the one outlined in this newer study
[PDF] by Prof. Rossignol.
When put on treadmills, walking reflexes were observed in paralyzed cats in a salient 1987 work.
[Image Source: J. Neurosci.]
Remarkably the cats were able to be trained to walk again via being placed on a treadmill during "therapy" sessions. The cats were unable to stand on their own, but once supported showed off a remarkable ability to learn to walk despite having no communication between the brain and leg nerves. A key here was that the spinal column was intact, showing that the three crucial necessities to locomotion were balance (partially brain-derived), limb feedback (quasi-automatic, after learning), and mechanical support (derived by the integrity of muscles and the spinal column. In this case the spine was undamaged, so two out of the three necessary traits were fulfilled, thus cats were able to walk with help on the balance issue.
This study taunted the medical establishment. Could walking in humans follow a similar pattern? Could our legs essentially have a "mind of their own" and be only loosely under the control of our brain?
It seemed highly probable that if felines -- a relatively advanced mammal -- had CPGs and automatic gait, humans
have them as well. But finding evidence of a CPG in primates, much less humans, proved infuriatingly elusive.
II. Humans Aren't So Special After All; We Have Pattern Gen. Just Like the Next Animal
A half decade later and that frustration turned to elation.
The breakthrough came in 1993 when
Professor Hans Hultborn
(first author) and
Professor Jens Bo Nielsen
(senior author) of the
Univ. of Copenhagen
presented the first evidence of a CPG in a species of marmoset (
an annual meeting
Society of Neuroscience
in Washington, D.C.
Evidence of a CPG in primates was first observed in Marmosets. [Image Source: Flickr/L. Leszczynski]
The work set the medical research community ablaze with excitement. Within a year
The Miami Project
-- a group of researchers from the
University of Miami
seeking a "cure" for human complete and incomplete paralysis -- had
published a new study
in the journal
showing that electrical stimulation to the lower spine could trigger involuntary walking motion in paralzyed humans.
The work by
Professor Blair Calancie
(first author; now at the
State Univ. of New York
Dr. Barth Green
(senior author), was followed by several other studies in the late 1990s.
The Miami Project (Chairman Dr. Green is pictured left) was crucial in showing early evidence of a human CPG. [Image Source: Miami Project]
(BU) in Houston, Texas in 1998
published a study
Annals of the N.Y. Academies of Science
showed further evidence of a human CPG. Using an electric generator,
Dr. Milan Dinitrijevic
(first author) (BU) and
Dr. Michaela Pinter
Ludwig Boltzmann Institute for Restorative Neurology and Neuromodulation
in Vienna, Austria (senior author), applied currents of frequency of 25 to 60 Hz and an amplitude of 5-9 V to the L2 vertebrae (the second lumbar) of completely paralyzed subjects and observed walking motions.
The human CPG is thought to be located around the L1-L2 vertebrae.
[Image Source: PVA, modifications: Jason Mick/DailyTech LLC]
Other studies around this time showed that similar autonomous nerve centers in the lower back controlled the mictration (urination) and ejaculation (sexual function) in vertebrates, including primates.
III. Jump Starting the "Second Brain"
Now researchers had a seemingly clear path to rerouting the circuitry of the human body and giving victims of paralysis everything they have lost -- the ability to urinate, experience sexual encounters, stand, and walk -- all without assistance. But in practice this clear road quickly devolved into a string of disappointments.
Researchers were able to stimulate walking motion in some cases, but were unable to figure out how to consistently and reliable use this physiological parlor trick to reliably restore the abilkity to walk and stand in paralysis victims. In 2001
Professor Susan Harkema
University of California
wrote a paper
in the journal
detailing these struggles and the potential of so-called "locomotor training" -- trying to trick the human CPG into relearning how to walk in paralysis victims.
A couple years later she moved to the
University of Louisville
, Kentucky and began
to expand the scope of locomotor training and lower spine stimulation. It was not easy to find the equipment to do her work with as she was literally inventing a new field.
She found a potential fit in the
, a 16 electrode spinal stimulation device by
, Inc. (
) which was FDA approved, but marketed as a means of managing extreme pain. But its range -- 2 to 100 Hertz and 0 to 10.5 volts -- seemed ideal as it was in the realm of what past researchers observed was necessary to overcome a threshold and stimulate motion.
Armed with the FDA approved device, Professor Harkema found an ideal patient in
, a former top college pitcher who was tragically paralyzed below the neck by a hit-and-run driver. Project Walk was born.
For six hours a day, Mr. Summers would push through a mentally exhausting regiment of physical therapy. In December 2009, Professor Harkema obtained the FDA's permission to implant electrodes from a RestoreAdvanced unit into Mr. Summers' lower back. And it wouldn't take long for that procedure to pay off.
Defying the odds, Mr. Summers stood within three days of the implant surgery. And within nine months Mr. Summers took his first steps since becoming a paraplegic. Better still, Mr. Summers' therapy restored his control over bowel, bladder, and sexual function -- allowing him freedoms that most of us take for granted, but which he feared he'd never again enjoy.
Professor Harkema (first author) and UCLA
Prof. Reggie Edgerton
published an account of this terrific success
in the June 2011 edition of one of medicine's most prestigious peer-reviewed journals --
A key to this success was not overdoing it with the voltage and frequency. The results of the work and others since hint that pushing too much voltage into the lower spine actually interferes with the human CPG, scrambling its signals and preventing triggering walking or other useful motions. The researchers saw their greatest success in inducing locomotor (walking) patterns at around 30-40 Hz and 7 V. Comments Prof. Edgerton to
a recent interview
, "[Before this work] everyone, including us, was hung up on the idea that you have to stimulate at this high level to induce the movement."
Professor Susan Harkema (L) and Reggie Edgerton (R) pose with patient Rob Summers at an awards ceremony. [Image Source: Getty Images]
In other words, when it came to tapping into spinal nerves, it wasn't always best to "turn it up to 11", so to speak.
Already an expert in the field Prof. Harkema found that the results with Mr. Summers opened many new doors. She comments, "I have no problem asking for help now."
IV. Round II: Electrostimulation Trials Expand
Professors Harkema and Edgerton's next goal was to test the procedure on more patients. As Professor Edgerton stated, "The next big question was, Will you ever see these things in more than one subject?"
The pair last year moved ahead in trying to answer that question. The pair successfully obtained FDA approval to test the therapy on four more patients. Candidates were encouraged to submit their names to a pool; winnners were announced in July 2012. Among the participants was Wyoming native Dustin Shillcox who became a paraplegic at age 26, when he was driving a work van for his family business and flipped the vehicle.
Dustin Shillcox is Professor Harkema's fourth patient in her electrostimulation trials and was able to stand. [Image Source: IEEE Spectrum]
The first two participants in the second round trial -- known as Patients 2 and 3 -- were able to stand, according to the recent
report. Likewise in February of this year Mr. Shillcox stood for the first time since being paralyzed.
The feat required a Luke Skywalker-turn-off-your-targeting-computer sort of moment, as researchers only successfully stimulated standing after releasing Mr. Shillcox from a supportive tether that was depriving his legs from the weight-bearing feedback that proved critical in allowing the CPG -- his spinal nerve cluster -- to order his legs to stand. Sometimes less is more; with less support the CPG received received more feedback and with the help of the electrostimulation allowed Mr. Shillcox to stand with minimal balance support from his helpers.
Dustin Shillcox does physical therapy exercises with Professor Harkema. [Image Source: IEEE Explore]
While the announced preliminary results make it clear that Professor Harkema has manged to consistently stimulate standing motion, it's unclear whether Mr. Shillcox or the other patients have been able to walk or regained control over bowel, bladder, and sexual function like Mr. Summers.
Even as the medical community is carefully watching these benchmarks, Mr. Shillcox says he's doing his best to keep his expectations realistic. He comments, "I don’t want to be too optimistic, and I’m trying to be prepared for no results at all. I hope that whatever they find from this research will at least benefit other people."
V. Progress Stymied by Crude Tools
At this point Professor Harkema's research is primarily geared towards physical therapy -- preventing the chronic atrophying of leg muscles in patients with spinal cord injuries (SCIs). She would love to give patients the tools to walk again, but she acknowledges that with current generation hardware that may be impossible.
The RestoreAdvanced stimulator has 4.3x10^7 combinations. Each setting requires the stimulator to entirely power off then cycle back on. A 75 minute trial may only allow 10 test configurations. Thus in the 170 or so trials involved in a single-patient evaluation as few as 1.7x10^3 configurations may be explored -- or about 4 thousandths of a
of the total combinations.
So Prof. Harkema has to curb her expectations and guess smart when it comes to settings. She particularly bemoans the shutdown/startup requirement, commenting, "You can get really close, and you think the person is almost standing independently, and if you could just shift the field a little you would have it. But you can’t. You have to go to zero. And then everything starts over. It’s a left-to-right problem. If we get the right leg to step, the left is doing nothing."
Professor Harkema's current hardware has 43 million configurations. It takes up to 10 minutes to test a single configuration during a therapy session. [Image Source: Harkema Lab]
While stem cells therapies --
regrowing spinal tissue to replace damaged tissue
in a patient's spinal cord -- is the most promising route to a full "cure" of paralysis, those efforts remain in the very crudest of stages still, focusing on getting cells to differentiate.
One key problem is finding a source of stem cells.
Embryonic stem cells
are the most promising source in some ways, but they also raise ethical issues and may be attacked by a patient's immune system, potentially requiring a patient to go on caustic immune-blocking regiments. The alternative -- stem cells created by
tricking skin cells or other tissues into reverting to stem cell form
-- have their own issues. They can be nearly as flexible as embryonic cells, but they frequently transform into cancer. Rates of tumorogenesis are so high, that they're not ready for human use -- not yet.
Stem cells are a potential long-term full cure, but currently suffer from a number of ethical and technical challenges. [Image Source: NewsOne]
A stop-gap measure would be to create a better spinal simulator that can learn better and trigger actions more quickly via faster output switching. Such an advanced stimulator could be used on its own, or
in combination with an exoskeleton
, perhaps allowing a person's limbs to do part of the work, and having a machine exoskeleton assist with balance and a bit of extra effort.
Vanderbilt's new exoskeleton is lighter than its rivals and bakes in new capabilities.
[Image Source: Vanderbilt/Parker Hannifin]
Center for Intelligent Mechatronics
and medical equipment maker Parker Hannifin Corp. (
) recently unveiled an exoskeleton designed at exactly such an application. It's important to recgonize that these two solutions -- electrostimulation and exosuits -- are each imperfect solutions, which ideally should be able to be combined for better results.
VI. Towards a Next-Generation Stimulator
California Institute of Technology
(CalTech) mechanical engineering
Professor Joel Burdick
is working on the firmware/software side, developing algorithms to better guess which combinations of parameters will trigger certain actions [see, for example,
this 2008 work
on Bayesian sampling and
this 2010 publication
on when to stimulate the CPG). Via machine learning, he hopes to cut the search space by orders of magnitude, allowing researchers to pinpoint signals that trigger the CPG into useful actions.
CalTech electrical engineering
Professor Yu-Chong Tai
is designing a very complex stimulation tool. One prototype featuring 40 electrodes in a 4 x 10 configuration is being tested at stimulating the CPG in mice. The array stretches along about 2 centimers of the rodent spine. A human version woulde be about 5 centimers long -- long enough to stretch across the key L1 and L2 vertebrae -- and would feature around 125 electrodes. That's about the same surface area as the current RestoreAdvanced implant covers, but it's almost a ten-fold increase in electrodes.
The CalTech stimulator prototype
Those electrodes give finer control -- which Profesor Tai believes may be crucial to trigger complex movements like walking. But it also expands the necessary search space by orders of magnitude. A
summary of this work
was recently published in the
Journal of Neuroengineering and Rehabilitation
At Prof. Harkema's home base -- the University of Louisville, electric engineering
Professor John Naber
is looking for a milder hardware hack, working on an improved version of the Medtronic stimulator, which would offer greater independent control of the 16 electrodes and the ability to switch states without powering off. The problem, he says, is that the device may never get to patient tests due to bureaucratic red tape.
While many SCI victims would be happy to sign whatever waivers were necessary to try the device, it must first be approved by the FDA. And the FDA calls for a rigorous, slow, and expensive testing process for implants. He comments to
, "It’s not like a commercial integrated circuit or product, because of the FDA requirements for human implants."
The new generation of spinal implants are being slowed by red tape at the FDA. [Image Source: Miami Project]
The holdup in human trials of these prototypes is yet another source of criticism for the FDA -- a federal agency often accused of obstructing potential cures to severe diseases like cancer and spinal injuries.
But red tape or not, the work of researchers around the world -- ranging from Denmark to Kentucky to Miami to California -- has given spinal patients hope of healthier lives even if a full cure still awaits. So that's something to celebrate. And it's important to acknowledge the contributions -- both of Professor Harkema and her predecessors -- that got the medical community here.
University of Louisville
"There is a single light of science, and to brighten it anywhere is to brighten it everywhere." -- Isaac Asimov
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