Every year, more than one million people in North America suffer from some form of spinal cord injury (SCI), with an annual cost of more than $7 billion to treat and rehabilitate these patients. The medical community has made incredible strides in mitigating, if not reversing, the effects of paralysis over the past quarter century, including advances in pharmacology, stem cell technologies, neuromodulation, and external prostheses. Electrical stimulation of the spinal cord has already shown particularly promising results in helping patients with spinal cord injuries to rehabilitate, improving not only extremity function, but also spasticity, bladder and blood pressure control. However, in a study published in Nature On Tuesday, therapy start-up SCI Onward Medical announced that it had helped improve the walking gait of a former paraplegic through the use of an implanted brain-computer interface (BCI) and a new “digital bridge” that spans the space where the spine was severed.
We’ve been zapping the spines of paraplegic patients with low-voltage jerks as part of their physical rehabilitation for years in a process known as Functional Electrical Stimulation (FES). The electrodes are placed directly over the nerves they are meant to excite – bypassing their own disrupted neural pathways from the outside – and, when activated, cause the nerves below them to activate and their muscles to contract. . Researchers have used this method to restore hand and arm movement in some patients, the ability to stand and walk in others, and, for a lucky few, exocombinations! The resulting limb movements were decidedly unsightly, however, resulting in heavy arm movements and gaits that looked more like drags.
Onward’s previous research on epidural electrical stimulation has shown it to be effective in targeting nerves in the lower back that can be used to trigger leg muscles. But therapy at the time was hampered by the need for wearable motion sensors and “participants…limited ability to adapt leg movements to changing terrain and volitional demands.” Onward addressed this issue in Tuesday’s study by incorporating a “digital bridge” to monitor command impulses from the brain and transmit them, wirelessly and in real time, to a stimulation pack implanted in the patient’s lower back. .
Clinicians have used these systems for the better part of a decade to help improve upper extremity control and function after spinal cord injury – Onward’s own ARC EX system is designed to do just that – although this study was the first to apply the same theories to the lower limbs. ends.
Onward’s patient was a 38-year-old man who had suffered “cervical incomplete spinal cord injury (C5/C6)” a decade earlier and had undergone a five-month neurorehabilitation program with “targeted epidural electrical stimulation of the spinal cord”. in 2017. “This program allowed him to regain the ability to walk with the aid of a walker,” the research team noted in the Nature study. “Despite continued use of stimulation at home, for about three years he had plateaued in neurological recovery.”
In addition to the EX, Onward Medical has also developed an internally mounted electrical stimulation therapy, the ARC IM. According to the company, it is “specifically designed to be placed along the spinal cord to stimulate the dorsal roots”, to help improve blood pressure regulation in patients with IBS. The system used in Tuesday’s study used ARC IM as its base and married it to a WIMAGINE brain computer interface.
On the medical front
Onward’s team first had to install the BCI inside the patient’s skull. Technically, it was a pair of 64-lead electrode implants, each mounted in a 50-millimeter circular-shaped titanium case that sits flush with the skull. The WIMAGINE “is less invasive than other options while still providing enough resolution to drive the walk,” Dave Marver, CEO of OnwardMedical, told Engadget via email. “It also has five-year data that demonstrates stability in the clarity of the signals produced.”
Two external antennas rest on the scalp, the first feeding the implants via inductive coupling, the second to shun the signal to a portable base station for decoding and processing. The processed signal is then transmitted wirelessly to the ACTIVA RC implantable pulse generator located atop the patient’s lumbar region where 16 additional implanted electrodes shock the appropriate nerve groups to move their legs. Together, they form a Brain Spine Interface (BSI) system, according to Onward.
The entire setup is designed to be used independently by the patient. The assist walker houses all the BSI elements while a tactile feedback interface helps them position the helmet correctly and calibrate the predictive algorithm.
In order for the BCI and pulse generator to work together seamlessly, Onward leveraged an “Aksenova/Markov-switched multilinear algorithm that linked ECoG signals to control of epidural electrical stimulation parameters”, which seems so obvious in hindsight. Basically, this algorithm predicts two things: the likelihood that the patient intends to move a specific joint based on the cues they are monitoring, and the magnitude and direction of this presumed predicted movement. These predictions are then transferred to an analog controller which translates them into code commands which are, in turn, transmitted to the pulse generator every 300 milliseconds. In total, the latency between the patient’s thought “I should walk over there” and the system decoding those thoughts is only 1.1 seconds.
Calibrating the system for the patient was an equally quick process. The patient had figured out how to properly “activate” his hip muscles to generate enough torque to swing his legs within the first two minutes of the test – and did it with 97% accuracy. During rehabilitation, the patient managed to control the movements of each joint of his leg (hip, knee and ankle) with an average precision (in the sense that the BSI did what the patient had intended) of approximately 75%.
“After only 5 minutes of calibration, the BSI allowed continuous monitoring of hip flexor muscle activity,” the team continued, “which allowed the participant to experience a five-fold increase in muscle activity per compared to attempts without the BSI”. Unfortunately, these gains were wiped out as soon as the BCI was turned off, instantly losing the ability to walk, they explained. “The march resumed as soon as the BSI was turned back on.”
It’s not just that the patient was able to switch from walking with a front-wheeled walker to crutches through this procedure – their walking gait also improved significantly. “Compared to stimulation alone, BSI resulted in walking with gait characteristics significantly closer to those quantified in healthy individuals,” the Onward team wrote. The patient was even able to use the system to traverse unpaved terrain while on his crutches, a feat that still proves routinely dangerous for many bipedal robots.
In all, the patient underwent 40 rehabilitation sessions with the BCI – a mixture of standard physio-rehabilitation with balance, gait and movement exercises activated by the BCI. The patient saw moderate gains in his sensory scores (light touch), but a whopping 10 point increase in his WISCI II scores. WISCI II is the Gait Index for Spinal Cord Injury, a 21-point scale measuring a patient’s ambulatory ability ranging from 20, “can walk without assistance”, to 0, “bedridden”. Onward’s patient went from 6 to 16 with the help of this therapy.
“As the participant had previously reached a recovery plateau after intensive rehabilitation using only spinal cord stimulation, it is reasonable to assume that the BSI triggered a reorganization of neural pathways that was responsible for the additional neurological recovery,” said writes the Onward team. “These results suggest that establishing a continuous link between the brain and the spinal cord promotes the reorganization of the residual neural pathways that connect these two regions under normal physiological conditions.”
Although the results are promising, there is still a lot of work to be done. The Onward team says future iterations will require “miniaturization of the base station, computing unit and imperceptible antennas”, faster data rates, “versatile stimulation parameters, direct wireless control from portable computing unit” and “a single low-power integrated circuit”. incorporating a neuromorphic processor with self-calibrating capability that autonomously translates cortical activity into updates to stimulation programs.
Despite the significant technical challenges, “the BCI system described in the Nature the publication could hit the market in five to seven years,” predicted Marver. “It is possible and realistic that BCI-augmented spinal cord stimulation therapy will be on the market by the end of the decade.”
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