Scientists, using epidural stimulation over the lumbar spinal cord, have enabled four completely paralyzed men to voluntarily move their legs.
Kent Stephenson is one of the four. This stimulation experiment wasn’t supposed to work for him; he is what clinicians call an AIS A. This is a measure of disability, formally the American Spinal Injury Association Impairment Scale (AIS), that rates impairment from A (no motor or sensory function) to D (ability to walk). Kent, a mid-thoracic paraplegic, has what is considered a “complete” injury. Kent’s doctors told him it was a waste of time to pursue any therapy; per the dogma, A’s don’t get better. Well, the young Texan, who was hurt five years ago on a dirt bike, didn’t get the message. He likes to cite a fortune cookie he got shortly after his injury. It said, “Everything’s impossible until somebody does it.”
Kent had the stimulator implanted. A few days later they turned it on. No one expected it to do anything. Researchers were only looking for a baseline measurement to compare Kent’s function later, after several weeks of intense Locomotor Training (guided weight supported stepping on a treadmill).
Kent tells the story: “The first time they turned the stim on I felt a charge in my back. I was told to try pull my left leg back, something I had tried without success many times before. So I called it out loud, ‘left leg up.’ This time it worked! My leg pulled back toward me. I was in shock; my mom was in the room and was in tears. Words can’t describe the feeling – it was an overwhelming happiness.”
Kent was the second of the four. Rob Summers, three years ago, was the first to pioneer the concept that complete doesn’t mean what it used to; epidural stimulation could make the spinal cord more receptive to nerve signals coming from the senses or the brain. Seven months after he was implanted with a stimulator unit, he initiated voluntary movements of his legs. The other two subjects, Andrew Meas and Dustin Shillcox, also started moving within days of the implant. Summers probably could have initiated movement early on too, but the research team didn’t test for it – they had no reason to believe he could do it.
Here’s lead author of the Brain paper, Claudia Angeli, Ph.D., to explain. She is a senior researcher at the Human Locomotor Research Center at Frazier Rehab Institute, and an assistant professor at the University of Louisville’s Kentucky Spinal Cord Injury Research Center (KSCIRC).
“First, in the Lancet paper [regarding the first stimulation subject] it was just Rob, just one person. Yes, it was proof of concept, yes it went great. But now we are talking about four subjects. That’s four out of four showing functional recovery. What’s more, two of the four are categorized as AIS A – no motor or sensory function below the lesion level, with no chance for any recovery.”
The other two patients are classified AIS B: no motor function below the lesion but with some sensory function.
How does this work? The epidural stimulation supplies a continuous electrical current, at varying frequencies and intensities, to specific locations on the lower part of the spinal cord. A 16-electrode spinal cord stimulator, commonly used to treat pain, is implanted over the spinal cord at T11-L1, a location that corresponds to the complex neural networks that control movement of the hips, knees, ankles and feet.
The leg muscles are not stimulated directly. The epidural stimulation apparently awakens circuitry in the spinal cord. “In simple terms,” says Dr. Angeli, “we are raising the excitability or gain of the spinal cord. Let’s say you have an intent to move. That signal originates in the brain and gets through to the spinal cord but the cord is not aware enough or excited enough to do anything with that intent. When we add the stimulation, the spinal cord networks are made a little more aware, so when the intent comes through, the cord is able to interpret it and movement becomes voluntary.”
The theory behind spinal cord stimulation is that these spinal cord networks are smart: they can remember and they can learn. The current work builds on decades of research. Susan Harkema, Ph.D. (University of Louisville) and V. Reggie Edgerton, Ph.D. (University of California Los Angeles) have led the effort. Dr. Harkema is Principal Investigator for the epidural stimulation projects and Director of the Christopher & Dana Reeve Foundation’s NeuroRecovery Network. Dr. Edgerton, a member of the Reeve Foundation’s International Research Consortium on Spinal Cord Injury, is a basic scientist whose work attempts to understand human locomotion and how the brain and spinal cord adapt and change in response to various interventions, including activity, training and stimulation.
Dr. Harkema says plans are in place to implant eight more patients in the next year. Four will mirror the first group, matched by age, level of injury, time since injury, etc. (Gender, by the way, is not a factor; men with spinal cord injury happen to outnumber women four to one.) Another four patients will be stimulated specifically to control heart rate and blood pressure. Dr. Harkema said one of the first four had issues with low blood pressure. When the stimulator was on, though, the pressure was raised, even without contracting any muscles. They want to assess that sort of autonomic recovery in greater detail.
The research team is aware that epidural stimulation can enhance autonomic function in paralyzed subjects; indeed, the first four subjects report improved temperature control, plus better bowel, bladder, and sexual function. Data is being collected to present that part of the stimulation story in another paper.
Does this mean anyone with a spinal cord injury with an implanted stimulator can move? Not necessarily, says Dr. Harkema. “But what I want people to know about this study is that we need to change our attitude about what a complete injury is, challenge the dogma that in AIS A patients there is no possibility of recovery. The view is that it is not a worthwhile investment to offer even intense rehabilitation to people with complete injuries. They’re not going to recover. But the message now is that there is a tremendous amount available. These individuals have potential for recoveries that will improve their health and quality of life. Now we have a fundamentally new strategy that can dramatically affect recovery of voluntary movement in those with complete paralysis, even years after injury.”
Image credit: Video and image both used with permission from the Christopher and Dana Reeve Foundation.