Next-generation spinal implants help people with severe paralysis walk, cycle, and swim

2022-02-07

Three men paralyzed in motorcycle accidents have become the first success stories for a new spinal stimulation device that could enable faster and easier recoveries than its predecessors. The men, who had no sensation or control over their legs, were able to take supported steps within 1 day of turning on the electrical stimulation, and could stroll outside with a walker after a few months, researchers report today. The nerve-stimulating device doesn’t cure spinal cord injury, and it likely won’t eliminate wheelchair use, but it raises hopes that the assistive technology is practical enough for widespread use.

“This [result] I would call a big deal,” says Vivian Mushahwar, a biomedical engineer and neuroscientist at the University of Alberta, Edmonton, who was not involved in the work. “This adds a level of refinement that allows for these approaches … to make it to the clinic and hopefully help a large number of people.”

When trauma severely damages the bundle of nerves that make up a person’s spinal cord, the brain’s electrical signals no longer reach the body’s muscles, resulting in paralysis. But epidural stimulation devices, thin sheets of electrodes implanted beneath the vertebra of the lower spine, can re-create those commands beyond the injury site and trigger leg movements. When such stimulation is turned on, even some patients with “complete” paraplegia—no movement or sensation in the lower body—have been able to walk after extensive training and with assistance from supportive devices or a therapist.

But spinal cord stimulators, developed in the 1980s to treat chronic pain, weren’t designed with spinal cord injury in mind, says Grégoire Courtine, and neuroscientist at the Swiss Federal Institute of Technology, Lausanne. One problem with existing implants is their shape: They consist of a narrow silicone strip that targets the center of the spinal cord to disrupt pain signals ascending to the brain. To trigger leg and torso movements, researchers need to stimulate the dorsal roots, pairs of thick sensory fibers extending from either side of the spinal cord. Existing electrode strips are also too short to reach the dorsal roots that control the trunk and enable bending and straightening the torso, Courtine says.

So he and his colleagues designed a longer and wider implant, roughly the size of a pointer finger. To position electrodes along its surface so they would precisely stimulate the dorsal roots, the researchers studied cadavers and images of healthy spines. Once they had the new design, they used computer models to predict the ideal position of the implant on each patient’s spinal cord.

Finally, the team designed software to activate the electrodes in set patterns that produce movements such as standing up and stepping. Typical epidural implants deliver uniform, repetitive pulses of electricity, says Peter Grahn, a neuroscientist at the Mayo Clinic. Patterned stimulation might help retrain damaged networks of nerves in the spinal cord to better receive and interpret signals descending from the brain that are preserved after spinal cord injury, he says. But just how the electrical stimulation interacts with spinal networks, and in turn the relative advantages of the two approaches, aren’t clear yet, he adds.

In 2018, this patterned stimulation approach got a big test: People with spinal cord injuries who had some residual leg sensation or movement were able to walk and cycle. But the participants in the new study had more severe, complete injuries, all of which occurred at least 1 year before their enrollment. With the new, larger implant and custom-positioned electrodes, all three could take steps on a treadmill within the first day after the stimulation was turned on—albeit with harnesses that supported more than half of their weight, the team reports today in Nature Medicine.

“That is remarkable to see within 1 day with a severe injury like this,” says Megan Gill, a research physical therapist at the Mayo Clinic who was not involved in the study. Previous studies have shown leg movements early on for people with complete paralysis, but this is the first time Gill has heard of such patients stepping their legs in an upright, “loaded” position in the first day of therapy.

After 4 to 6 months, all three participants were able to walk across the ground using only a walker for stability. It took participants in previous studies more than 1 year to achieve overground stepping, Mushahwar notes. “Intense therapy for a year and a half is a little bit impractical under current health care systems, at least in the U.S.” she says. The new work makes such therapy “meaningful from a health care management perspective.” And such daily movement is valuable to patients with spinal cord injuries: Even short walks can lead to better cardiovascular function, more bowel and bladder control, increased bone density, and less risk of pressure injuries from prolonged sitting.

Using different stimulation patterns, the participants in the new study could swim, cycle, and do leg presses and sitting forward bends. One patient was even able to climb a staircase. But with the stimulation off, their abilities remain limited. One regained some ability to activate leg muscles, but not to make functional movements. And two participants in a previous study who had incomplete paralysis could eventually stand without stimulation. How much ability spinal cord stimulation can restore long-term is unclear, Courtine says. It may depend on severity of the original injury and how soon after that injury the device is implanted, he adds.

For now, sending commands to the device is cumbersome. Users must select their desired movement on a tablet, which sends Bluetooth commands to a transmitter worn around the waist. That device must be positioned next to a “pulse generator” implanted in the abdomen, which then activates electrodes along the spine. Setting up to use the stimulation takes 5 to 10 minutes, Courtine says.

But the next generation of devices should allow users to activate the pulse generator by giving voice commands to a smartwatch, says Courtine, who is also chief scientific officer of the medical technology company ONWARD. In 2024, the company plans to test this newer mobility system in a multisite clinical trial of 70 to 100 participants that the team hopes will lead to U.S. regulatory approval.

science.org, 07 February 2022
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