So far, there has been little hope for paraplegics of being able to walk independently again despite a severed spinal cord. However, thanks to technological advances such as implantable electrodes, this is gradually changing. Researchers have now presented a technology for the first time with which those affected can take their first steps directly after the implantation of the “spinal cord pacemaker” – albeit initially in a carrying frame. After five months of training, the three test patients were able to walk with a walker. This is made possible thanks to optimized electrodes on the nerve roots of the lower back and signaling optimized by artificial intelligence.
If the nerve lines that transmit the signals between the limbs and the brain are interrupted, permanent paraplegia is usually the result. Because the severed spinal cord has little ability to heal itself and does not grow back together on its own. In the meantime, however, scientists are researching several approaches to circumvent this problem. In addition to molecular and stem cell-based methods, neurostimulation in particular has proven to be successful. For this purpose, a system of electrodes is implanted directly in the spinal cord of the paralyzed patient. Their electrical signals activate the circuits and nerve roots that normally transmit movement signals from the brain to the leg muscles. In 2018, a partially paralyzed patient regained his motor function enough to be able to get out of a wheelchair and use a walker to walk.
Targeted irritation of the nerve roots
So far, however, such electrostimulation therapies have required months of painstaking training before an effect becomes visible. Andreas Rowald from the Swiss Federal Institute of Technology in Lausanne (EPFL) and his colleagues have now succeeded in optimizing the method in such a way that patients can even take the first steps on the first day after the implantation of the electrodes. This was made possible by two key changes compared to previous approaches: First, the team adapted the electrode arrangement so that it acts more specifically on the nerve roots that emerge dorsally from the spinal cord and are responsible for the movement of the legs and lower trunk. “The challenge was to find an arrangement of the 16 electrodes that, despite the individually different topology of the spinal cord, covers the 16 important dorsal nerve roots,” write the scientists. To do this, they developed detailed and lifelike simulations of the neural topography. “Our new implants are placed under the vertebrae directly on the spinal cord,” explains senior author Grégoire Courtine from EPFL. “There they modulate the neurons that control specific muscle groups.”
The next step was to map the individual nerve signals for each test patient. All three patients – men between the ages of 29 and 41 – had suffered a severe spinal cord injury in the chest area as a result of a motorcycle accident between one and nine years previously and were paraplegic as a result. Neither of them could move or feel their legs. In order to adapt the system to the individual neural characteristics of these patients, the researchers stimulated the tendons in their legs and moved the leg joints passively and observed where these signals arrived on the spinal cord based on the oxygen supply to the nerve roots. With the help of an adaptive algorithm, they developed a stimulus profile individually tailored to the patient, which simulated the specific stimulus pattern in certain common movement patterns. “This allows us to activate the spinal cord in a similar way as the brain would normally do to make the patient stand, walk, swim or ride a bike,” explains Courtine.
First steps already on the first day after the operation
After these elaborate preparations, the electrode arrangement was implanted on the spinal cord of the lower back as well as a control unit under the abdominal wall. Immediately after the operation, the neurostimulation in the still bedridden patients was optimized by sending the first signals from a computer interface to the electrode implant. The research team was able to determine whether the individual signal patterns previously developed by the adaptive algorithm matched and carried out final fine adjustments. Then the first test followed: Supported by a suspension that carried a large part of their body weight, the patients tried their first steps – with success: “On the first day all three test subjects were able to take their first steps on a treadmill, although the movement was still not very pronounced was,” reports the team. By the third day, however, the gait pattern had improved to such an extent that all three were able to walk independently in the harness. The patients set the desired form of movement themselves using a tablet, and the corresponding signals were then transmitted to the control system under their abdominal wall.
“All three patients learned to stand, walk, cycle and control their trunk movements within one day,” reports Courtine. “It works thanks to the specific stimulation programs that we wrote for each activity.” Over the next five months, the three patients underwent an intensive training program during which they trained the muscles in their paralyzed legs and learned to use the neuro-pacemaker to stimulate themselves move. After this time, they were able to walk and stand independently without safety belts and only with the help of a normal rollator. The buttons controlling the stimulator were built into the handles of the walker. One of the test patients even managed to walk up and down stairs again with the help of this technique. The three men were also able to swim, canoe and use a recumbent bike again.
“It is important to emphasize that these patients did not regain their natural ability to move,” write Rowald and his colleagues. Because even after months of training, her spinal cord remained severed and the signals came solely from the software-supported stimulation program. “But their rehabilitation was sufficient to allow them to engage in various activities again,” the team said. They now want to work on further improving this therapy so that one day it can also benefit other paraplegics.
Source: Andreas Rowald (EPFL, Lausanne) Nature Medicine, doi: 10.1038/s41591-021-01663-5