David (shown in a composite image) is one of the three study participants who benefited from the treatment.Credit: EPFL Jamani Caillet

Electrical stimulation has helped three people with spinal-cord injuries to regain control over their leg muscles and improve their walking.

It even enabled one of them who couldn’t previously walk to walk with assistance.

“It’s a tremendous step forward,” says rehabilitation-medicine researcher Chet Moritz of the University of Washington in Seattle, who was not involved in the research.

Importantly, all of the participants retained some improvement in muscle movement even after the stimulation therapy had stopped, and two retained improvements in walking ability.

﻿But researchers caution that the technique, called epidural electrical stimulation, is in its early stages and that it is not clear for what proportion of injured people it could work. So far, it has been demonstrated only in people who retained some level of motor function below their injuries.

The work was published on 31 October in Nature1.

David can walk with assistance after a combination of electrical stimulation and therapy. Credit: EPFL Hillary Sanctuary

It has been a “breakthrough” time for spinal-cord-injury research, says Moritz, after two other groups also reported patients walking after electrical stimulation of the spinal cord.

In a paper published on 24 September, physical therapist Megan Gill at Mayo Clinic in Rochester, Minnesota, and her colleagues described2 how a person who had been completely paralysed below their injury could walk on a treadmill after 43 weeks of training and electrical stimulation.

And on 27 September, spinal-cord researcher Claudia Angeli at the University of Louisville, Kentucky, and colleagues reported3 that two out of the four people given continuous epidural stimulation were able to walk with assistive devices after 15 and 85 weeks of training, respectively.

But Grégoire Courtine, a neuroscientist at the Swiss Federal Institute of Technology in Lausanne who led the latest effort, says that his team’s approach of precisely timed stimulation could work better than continuous stimulation, which might be blocking some residual signals travelling from the legs back to the brain4.

Disrupted communication

Injuries to the spinal cord disrupt the connections between the brain and spinal-cord neurons, creating motor and sensory deficits in areas of the body below the injury, and sometimes causing paralysis.

In most cases, there are still some connections between the brain and the motor neurons in the spinal cord below the injury, but these might not be sufficient to allow a person to move.

Courtine’s team used electrical stimulation to give these motor neurons extra excitation, boosting the signals received from the remaining connections with the brain.

The research team investigating electrical stimulation to help people walk.Credit: EPFL Hillary Sanctuary

The researchers first mapped out which areas of the spinal cord are involved in each movement required for walking, such as flexing the hip or extending the ankle.

They then implanted electrical stimulators in three people with varying levels of motor impairment in their legs owing to spinal-cord injury. Having worked out which parts of the spinal cord are involved in walking, the team was able to program a sequence of electrical pulses that would stimulate the spinal cord at the correct time and location to facilitate those movements.

Intention into action

This electrical stimulation didn’t produce motion by itself — it worked only when the study participants attempted to make movements themselves. “It really works as an amplifier,” says Courtine. “It’s not that we’re taking over control of the leg. The patients — they have to do it.”

Courtine says that after just two days, this became almost natural to them. Within a week, the participants were able to walk with the assistance of devices that held some of their body weight.

This included one person who had previously had no movement in his legs, and one whose left leg had been completely paralysed. The third person had more motor ability in his legs, but had been unable to lift his legs when attempting to walk.

Over five months of rehabilitation alongside stimulation, the participants improved further: eventually, the team saw improvement in mobility even when the extra stimulation was turned off. Two participants were able to walk independently with crutches; one could even take a few steps without any assistance. The third, most severely injured, person could move his previously paralysed legs while lying down.

This suggests that the electrical stimulation is strengthening the connections between the brain and neurons in the spinal cord, says Moritz, who wrote a News & Views in ﻿Nature Neuroscience to accompany the paper. A basic tenet of neuroscience is that “cells that fire together will wire together”, so it makes sense that augmenting the interaction between brain and spinal-cord neurons will strengthen those connections, he says.

Tempered hopes

The work is really exciting, says Jennifer French, executive director of the Neurotech Network in St Petersburg, Florida, a non-profit organization that educates people with neurological conditions about neurotechnologies.

But she points out that the participants still required body support to move around.

Kim Anderson, a clinical researcher in spinal-cord injuries at Case Western Reserve University in Cleveland, Ohio, adds that the technique might not be able to help everyone with such injuries. The study participants retained some level of motor function below their injury before the stimulation began, whereas most people with spinal cord injuries have ‘motor complete’ injuries, with no residual movement ability.

Courtine’s team has also developed technology that enabled the participants to use the epidural electrical stimulation outside the lab. This includes wearable sensors that trigger the stimulation, and an app that runs on a voice-controlled watch, allowing users to pick the exact form of stimulation needed.

These devices are still in development, says Courtine, but the participants have used them for walking and even, in one case, riding an arm-and-leg-powered tricycle. Over the next three years, Courtine says, he aims to optimize the technique and validate its safety and efficacy.

Simone Di Giovanni, neurologist at Imperial College London, is optimistic that the technology could one day be used more widely. It remains unclear how well it will work in people with more-severe injuries, he says, and that will require further experiments.